Carlson Embryology Notes

Aaron Geller

May 18, 2008

1 Gametogenesis

1. 4 phases of gametogenesis

(a) extraembryonic origin of germ cells & migration into gonads

• germ cells arise in yolk sac around day 24

• they are large and high in ALP

• they move into the hindgut & arrive at primordia of gonads

• if they get lost they can develop into teratomas (tumors withmultiple germ layers) in oral region, mediastinum, sacrococ-cygeal region

(b) mitotic proliferation of germ cells

• in females,

– oogonia go through 1-time proliferation period at 5th monthof gestation

– after this a lot of oogonia degenerate (atresia)

– number of active oogonia diminishes continually until menopause

• in males, spermatogonia continue to proliferate in seminifer-ous tubules for whole lifespan

(c) ploidy reduction by meiosis

• meiosis vs. mitosis

i. mitosis:

– cells start 2n and 4c: 2 copies of each chromosome, withchromosomes replicated → 4 total chromosomes

– at anaphase, each replicated chromosome is split at thecentromere, so you get 2 genetically identical 2n, 2cdaughter cells

1

ii. meiosis

– meiosis also starts with 2n, 4c cells

– the chromosomes line up as homologous pairs of repli-cated chromosomes, so you get tetrads with crossingover

– at anaphase I the replicated homologs are pulled apart,so after 1st division you have non-identical n, 2c daugh-ter cells

– after meiosis II you have n, c daugter cells

• meiosis in females

i. as mentioned, before entering meiosis, oogonia cells pro-liferate by mitosis

ii. oogonia descend from germ cells, but they are surroundedby flat epithelial cells, called follicular cells, which aren’t

iii. before or at birth, all non-atretic oogonia enter meiosis I,becoming 1◦ oocytes

iv. they stop at the diplotene (lacy chromosomes after initialcrossing over, but before metaphase) phase of meiosis I

– many species (esp. lower vertebrates) intensively syn-thesize RNA & proteins here in preparation for ovula-tion

– arrested state maintained by oocyte maturation in-hibitor (OMI)

– the oocytes stay this way indefinitely, until some ofthem are allowed to mature after onset of puberty

• in males

– as in females, you have germ cells → spermatogonia sur-rounded by supporting cells→ sustentacular or Sertolicells

– no synchronized transition to meiosis like in females, manyspermatogonia stay mitotic for whole life

– type A spermatogonia replicate several times, finally be-coming type B spermatogonia at last pre-meiotic division

– each type B spermatogonium gives 2 1◦ spermatocytes

– meiosis II of a 2◦ spermatocyte produces 2 spermatids

– prophase I takes several weeks, meiosis II hours

– cytokinesis is incomplete so the different generations arejoined by cytoplasmic bridges (S Fig 2.22)

2

(d) maturation of gametes into eggs & spermatozoa

• females:

i. primordial follicle: a 1◦ oocyte + its follicular cells

ii. at puberty, some primordial follicles transition to 1◦(orpre-antral) follicles

– flat follicular cells become cuboidal, becoming granu-losa cells

– basement membrane separates granulosa from outsidecells, which are called theca folliculi; stimulated byGDF-9

– zona pellucida: layer of glycoproteins secreted byoocyte & granulosa cells as oocyte ECM (importantfor transport into oocyte)

iii. as fluid accumulates between granulosa cells, a cavity isformed between oocyte called the antrum, at which pointthe follicle becomes an 2◦ (or antral) follicle

iv. follicular cells surrounding the oocyte in the follicle arethe cumulus oophorus

v. during ovulation

A. surge in luteinizing hormone (LH) causes one 2◦ follicleto complete meiosis I

B. one of the daughter cells receives most of the cytoplasm,becoming the 2◦ oocyte; the other becomes the firstpolar body

C. 2◦ oocyte enters meiosis II but stops at metaphse 3hbefore ovulation

D. meiosis II is only completed if fertilization occurs

(e) males:

• as in females, stimulated by LH

i. bound by Leydig cells which make testosterone

ii. testosterone bound by Sertoli cells, stimulates spermato-genesis

• FSH also important because it triggers upregulation of andro-gen receptor genes

• final maturation: spermiogenesis

i. acrosome

ii. condensation (picnosis) of nucleus

3

iii. neck, middle piece & tail

iv. jettisoning of cytoplasm

• sperm become motile in the epididymis

4 Molecular Basis

1. two categories of molecules guide development:

(a) transcription factors

(b) signalling molecules (cytokines)

• category includes growth factors

• bind to membrane receptors & trigger signal transduction cas-cade

• Reznick lecture 1: distinguish between paracrine & juxtacrine

2. transcription factors

• two kinds:

(a) helix-loop-helix

(b) Zn finger

• homeodomain proteins (helix-loop-helix family)

– contain 60 AA stretch which provides helix-loop-helix, calledthe homeodomain

– the 180 nucleotides which code for the homeodomain are calledthe homeobox

– general family called Hox

– Hox genes are expressed in sequential order along thechromosome, from 3’ to 5’, and determine the cranial-caudaltraits of the tissue expressing them (68).

– loss-of-function (LOF) of a givne Hox gene →neighboring anterior segment will be “stretched” into segmentwith LOF mutation; vice-versa for GOF

– subfamilies: Pax, Sox (complexes with other TFs; see fig. 4-10), Lim, T-box

3. paracrine signalling molecules

(a) overview

4

• growth factors– most important families:

i. transforming growth factor β (TGF-β)

ii. fibroblast growth factor

• other GFs: nerve, epidermal, insulin-like

• hedgehog proteins

(b) typical interaction: regulators compete to exert positive or nega-tive effects:chordin induces neurulation by inhibiting action of BMP, which isinhibitory (74)

(c) hedgehog pathway

i. pre-hedgehog protein produced in ER with non-signalling Cend intracellular signal at N end

ii. (intracellular) signal cleaved off of N end

iii. C end cleaves itself off

iv. remaining segment covalently bonds to cholesterol and is se-creted from cell

v. shh protein (with cholesterol) binds to Ptc (patched) receptoron target cell

vi. Ptc stops inhibiting smo (smoothened) membrane protein,which (via intermediate steps) activates a transcription factor

(d) membrane receptor proteins can either have intrinsic kinase ac-tivity (e.g., FGF and TGF-β receptors) or use second-messengersystems which activate kinases downstream

(e) signalling cascade: G-protein sequence can activate transcriptionfactors

(f) retinol (vitamin A) can interfere with development if missing orpresent in excess

• retinol is taken up by special membrane proteins

• in the cell it isdoubly-oxidized to retinoic acid

• in the nucleus it complexes with proteins to make a transcrip-tion factor RARE (retinoic acid response element)

• RARE acts as an enhancer → extra limbs on tadpole whengrown in vitamin A solution

4. juxtacrine (close contact) signalling; 3 modes

(a) protein on cell A interacts with receptor on cell Be.g. Notch receptor provides lateral inhibition for cell differen-tiation

5

i. one cell decides its own fate and is “dominant”

ii. it signals neighboring cells (via Delta or Jagged proteins) notto differentiate into the same kind of cell

iii. Notch receptor on “submissive” cells transduces the signal tochoose a different path:

iv. when it binds its ligand, a protease cleaves off the intracellulardomain

v. the intracellular domain complexes with other proteins whichenhance transcription of a transcription factor which supressesexpression of the “dominant” phenotype

(b) secreted ligands in ECM interact with receptors on neighboringcells: fibronectin, laminin interact with integrins, affecting actincytoskeleton

(c) gap junctions in epithelia

5. cancer can arise in 2 ways:

(a) GOF in a proto-oncogene

(b) (double) LOF in a tumor-suppressor gene (ex: patched inhibitionof smoothened)

6. histogenesis depends on CAMs (cell adhesion molecules; R 2)

(a) 1◦

• N

• L

(b) 2◦

• Ng

• Cell CAM 105 (hepatocytes)

• I (epithelia)

5 First Week of Development

1. ovulation (S 31)

• hypothalamus cyclically secretes gonadotrophin-releasing hormone(GnRH)

• this stimulates the pituitary to secrete LH & FSH

6

(a) FSH

– “rescues” 15-20 of the current pool of 1◦ follicles (prevent-ing atresia)

– stimulates maturation of granulosa cells

– granulosa & theca cells produce estrogens which cause

i. endometrium to enter proliferative phase2 layers: basal, spongy

ii. secrete thinner cervical mucus (help sperm move)

iii. stimulate pituitary to secrete more LH (positive feed-back)

(b) LH spike results (36h before ovulation), causing

– secretion of maturation promoting factor (MPF) topromote meiosis

– oocytes to complete meiosis I

– follicular stromal cells to secrete progesterone (and notestrogen)

– follicular rupture

i. follicle bulges, with avascular area, the stigma

ii. LH increases collagenase activity, breaking down ovar-ian wall

iii. prostaglandins trigger muscular contractions, which ejectoocyte together with some cumulus oophorus cells (whichbecome the corona radiata)

2. corpus luteum (S 32)

• remainder of follicle stuck in ovary becomes vascularized

• under influence of LH, they become yellow & develop into corpusluteum

• corpus luteum secretes progesterone

• progesterone + estrogens trigger uterine progestational (secre-tory) phase3 layers: basal, spongy (intermediate), compact (superficial)

3. motion of fimbriae and contractions of uterine tubes suck the oocyteinto the tube; takes 2-3 days to reach lumen (S 32-3)

4. if no fertilization:

7

• corpus luteum degenerates & turns into scar tissue: corpus albi-cans

• progesterone levels fall off, triggering menstruation

5. if yes fertilization:

• embryo’s syncitiotrophoblast secretes human chorionic gonadotrophin(hCG), which keeps corpus luteum from degenerating

• corpus lutem becomes corpus luteum of pregnancy

– gets bigger

– keeps making progesterone until 4th month

– after that, placenta takes over progesterone secretion

• instead of menstrual phase, uterus is in gravid phase (S 42)

6. fertilization

• without ovulation, sperm lose motility in the isthmus of the fal-lopian tube

• with ovulation, they are able to use chemoattraction to swim toampulla, where the action happens

• prerequisites for fertilization

(a) capacitation:

– removal of glycoproteins & plasma proteins from mem-brane over acrosome

– allows penetration of the corona radiata

(b) acrosome reaction:

– release of enzymes for penetration of the zona pellucida(acrosin, trypsin-like substances)

– triggered by zona

• phases

(a) penetration of corona radiata

(b) penetration of zona pellucida:as soon as first sperm penetrates, the zona reaction causesa change in permeability which prevents further penetrations

i. sperm penetration raises intracellular Ca2+

ii. in response, exocytosis of cortical granules occurs

(c) fusion of sperm & oocyte cell membranes

8

– similar to zona reaction, penetration by sperm causesoocyte membrane to become impenetrable

– meiosis II is completed, creating a second polar body andthe definitive oocyte

– the resulting chromosomes of the definitive oocyte arrangethemselves in a the female pronucleus

– sperm loses its tail & leaves the male pronucleus

– both karyotypes lose their nuclear envelopes & replicatechromosomes

– fertilization is now complete; mitosis takes over from here

7. blastomeres

• mitotic divisions increase number of cells (called blastomeres) &decreases their volume

• after 3rd division (8 cells), compaction occurs

– cell outlines become indistinct; cells are connected via tightjunctions, maximizing cell-cell contact

– inner cells (connected via gap junctions) differentiate fromouter cells

• next divsion (day 3) produces morula (16 cells)

(a) inner cells → embryo

(b) outer cells → trophoblast

8. blastocyst

• fluid penetrates intercellular space

• as cells proliferate space coalesces into a single cavity: blasto-coele

• zona disappears, allowing implantation

9. implantation

(a) initial capture

• similar mechanism to capture of WBCs from bloodstreamonto endothelium

• trophoblast cells express L-selectin, uterine wall a carbohy-drate receptor

(b) anchoring & penetration through endometrium

9

• further attachment uses integrins, laminin, fibronectin

• matrix metalloproteases (MMPs) for ECM breakdown in stroma

6 Formation of Germ Layers (Gastrulation)

1. early in 2nd week: cells of inner cell mass are arranged in two layers:

(a) epiblast (on top=dorsal)

(b) hypoblast (underneath=ventral)

2. epiblast develops amniotic cavity, and hypoblast cells spread aroundcytotrophoblast to make primitive yolk sac (cf. Sadler p. 43)

3. some cells of yolk sac differentiate extraembryonic mesoderm, andproliferate around inner wall of cytotrophoblast, squeezing between epi-blast and cytotrophoblast.

4. primitive streak (visual proof of craniocaudal differentiation) forms andbecomes primitive groove

5. epiblast cells migrate through streak (changing shape and characterfrom epithelial to mesenchymal/fibroblastic) and become

(a) embryonic endoderm

(b) embryonic mesoderm

(c) extraembryonic mesoderm

(d) prechordal plate (induced formation of forebrain)

(e) notochord

• The cells need hyaluronic acid and fibronectin to migrate success-fully.

• at cranial end, there’s a region without mesoderm (endo meetsectoderm) called buccopharyngeal membrane (aka oropharyn-geal)

• induction of mesoderm (p. 91)

– experiment: isolate “animal pole” (epiblast) of blastula

– on its own, remains ectoderm (makes keratin)

– when stuck directly on vegetal pole (endoderm), becomesmesoderm (makes actin)

10

– on its own + added activin, becomes mesoder (makes actin)

6. notochord development:

(a) first, it’s a tube (closed at the cranial end, open at the caudal end)extending cranially from primitive pit (anterior end of primitivegroove)

(b) bottom of caudal end tube and the endoderm it sits on erode, soamniotic cavity communicates with the yolk sac (connection is theneurenteric canal

(c) the tube fills in anterior to the canal so it is a solid cylinder ofcells

• sacrococcygeal teratomas are thought to result from retention ofpart of the primitive streak, which has germ cells for all layers

• pp. 88-89 notochord laid out by regression of primitive streak ?

7. induction of nervous system

• experiment: notochord mesoderm required for induction of never-ous system; shown in salamanders where removal of notochord-analogue prevents foormation of nervous system, and transplan-tation of an extra notochord-analogue results in extra nervoussytem (89-90).

• signalling molecules: noggin, follistatin and chordin trigger induc-tion by blocking BMP-4

• fig 5-14: regressing primitive streak coincides with developmentof neural plate

8. box: ciliary currents may be mechanism for establishing left-right axis(95-96)

• nodal expressed selectively on left side of disc

• lefty is expressed on left side of primitive streak and may keeplateralization molecules from diffusing to right side

9. Cell Adhesion Molecules (CAMs)

• before gastrulation, both epiblast and hypblast exress N-CAMand L-CAM (=E-cadherin)

11

• in the process of migrating though primitive streak, cells stop ex-pressing both CAMs; when they arrive at their destination, CAMsare re-expressed

• in neural plate, only N-CAM is expressed (also N-cadherin)

• in non-neural ectoderm, only L-CAM is expressed

7 Establishment of Basic Body Plan

1. steps in neurulation

(a) thickening of neural plate

(b) narrowing & lengthening

(c) folding up along neural groove

(d) apposition and fusion of folds & separation of neural crest cells

2. segmentation

(a) anterior region segments into 3 vesicles of brain

i. forebrain (prosenchephalon)

ii. midbrain (mesencephalon)

iii. hindbrian (rhombencephalon)

(b) forebrain and hindbrain subdivide into neuromeres and rhom-bomeres

(c) anterior visceral endoderm (near buccopharyngeal membrane)and prechordal plate induce the differentiation of forebrain/midbrainfrom hindbrain/spinal cord (establishing the rhombenchephalicisthmus; Otx is transcription factor for forebrain/midbrain, Gbx-2 for hindbrain/spinal cord (106).

(d) transcription factors trigger individual rhombomeres (107)

(e) segmentation of SC is induced by and organized by paraxial meso-derm, not neural tissue (107-8)

3. ectodermal placodes develop concurrently with neurulation and willteam up with neural crest cells to become peripheral ganglia and cranialnerves

4. fig 6-6

(a) as neural tube is closing, paraxial mesoderm segments into somites

12

(b) at the lateral edge of the mesoderm (where embryonic mesodermmeets extraembryonic mesoderm), coelomic vesicles fuse causingbifurcated of mesoderm into somatic mesoderm and splanch-nic mesoderm

(c) the bifurcation creates intraembryonic coelom (continuous withextraembryonic coelom)

(d) at this point there are paired aortas

(e) somites start to form (around day 20; after somitomeres)

(f) lateral ectoderm and somatic mesoderm droops down (2 layerstogether = somatopleure)

(g) when neural tube is closed, part of endoderm closes into canal,rises dorsally & is surrounded by splanchnic mesoderm (2 layerstogether = splanchnopleure) → gut

5. fig 6-7

(a) regressing primitive streak stimulates development of somitomeresin mesoderm as it moves

(b) after 7 cranial and 13 caudal pairs have formed, somites developout of the first caudal pair and

(c) development into somites proceeds caudally, as does somitomeredevelopment

6. somite development

(a) complex induction mechanism causes some mesodermic mesenchymeto become epithelial sphere (enclosing somitocoel)

• Notch, WNT

• retinoic acid in rostrocaudal gradient

• FGF8 in caudorostral gradient (S 75)

(b) hedgehog signal from notochord & neural tube causes:

i. mitosis

ii. loss of adhesion molecules

iii. reversion to mesenchymal shape, becoming the sclerotome→ spinal column & rib cage (bones & cartilage)

(c) remaining dorsal half of somite becomes dermatome → dermis& subcutaneous skin

13

(d) dorsomedial somite cells migrate ventrally, underneath dermatome,to form myotome (S 76) → epaxial (back) muscles

(e) dermatome + myotome = dermomyotome

(f) “Each myotome and dermatome retains its innervation from itssegment of origin, no matter where the cells migrate. Hence, eachsomite forms its own sclerotome (the tendon cartilage & bonecompartment), its own myotome (providing the segmental musclecompartment), and its own dermatome, which forms the dermisof the back.” (S 76)

(g) signalling for somite development (S fig 6.11)

structure signal target result

notochord, Shh, ventromedial somite → sclerotomeneural tube floor noggin

sclerotome PAX1 sclerotome cartilage, bone differentiationdorsal neural tube WNT dorsal somite → dermomyotome

dorsal somite PAX3 ” ”dorsal neural tube WNT dermomyotome DML → epaxial muscle

DML dermomyotome MYF5, ” ”MYOD

lateral plate BMP4 dermomyotome VLL → limb, body wallmesoderm muscleepidermis WNT ” ”

VLL dermomyotome MYF5, ” ”MYOD

• DML = dorsomedial lip, VLL = ventrolateral lip

• Shh, WNT, BMP are diffusable paracrine factors aka growthand differentiation factors (GDFs; S 8)

• PAX1, MYF5, MYOD are transcription factors (S 143)

(h) fig 6-10

i. cells migrate from sclerotome to notochord to make spinalcolumn

ii. anterior (rostral) cells from sclerotome n + 1 coalesce withposterior (caudal) cells from sclerotome n to make a vertebra

iii. since sclerotomes (not vertebrae) make spinal muscles, you getspinal muscles which span each sequential pair of vertebrae

14

7. early development of circulatory sytem

(a) epiblast cells which migrate through caudal part of primitive streakbecome cadiogenic mesoderm & arrange in ∩-shaped pattern

• N-cadherin positive → cardiac myocytes

• N-cadherin negative → endocardial cells

(b) cardiogenic plate

• first recognizable precardiac mesoderm

• special area anterior to buccopharyngeal membrane

• adjacent is forerunner of pericardial cavity

(c) pericardiac splanchnic mesoderm thickens, becoming myocardialprimordium

(d) nearby mesoderm vescicles start fusing to make up endocardialprimordia (future interior of heart)

(e) myocardium is formed by fusion of its primordia from the sidestowards the center

(f) epicardium is formed from its primodium (??)

(g) endocardium is fused by fused of its primordia, surrounded bycardiac jelly (ECM)

(h) by day 22 heart starts beating

8. blood & blood vessels

• blood & blood vessels start growing in extraembryonic splanchno-pleuric mesoderm of yolk sac (induced by yolk sac endoderm), asblood islands which contain hemangioblasts

• centrally located hemangioblasts → hemocytoblasts

• peripherally located hemangioblasts → endothelial lining cells

• vessels fuse & grow towards embryo, where they eventually dumpblood into developing heart & embryonic vessels

9. endoderm development

(a) together with lateral folding which brings splanchnopleure downaround endoderm, ventral folding pushes mesoderm in around yolksac.

(b) ventral folding (driven caudally by bulging of head) separates thecavity of the gut from the yolk sac by working from the ends:

15

• foregut formed from rostral to caudal

• hindgut from caudal to rostral

• midgut where gut flows straight into yolk sac (connectioneventually narrows to become the vitelline duct)

(c) at rostral & caudal ends, gut meets ectoderm directly and bilayerwill break down

• buccopharyngeal membrane rostrally

• proctodeal membrane (caudal to allantois) caudally; aka cloa-cal plate

10. at week 4, 3 circulatory arcs

(a) intraembryonic:heart → ventral aorta → pharyngeal arches → dorsal aorta →rest of body

(b) vitelline: goes to yolk sac

(c) allantoic/umbilical

8 Placenta & Extraembryonic Membranes

1. overview

• fetal/maternal interface (placenta & chorion) derived from tro-phoblast

• mebranes derived from inner cell mass:

(a) amnion (from epiblast/ectoderm)

(b) yolk sac (from endoderm)

(c) allantois (from endoderm)

(d) extraembryonic mesoderm

2. amnion

• 2 periods of fluid production, corresponding to keratinization offetal skin (becomes keratinized at week 21)

• initially transudate of maternal blood plasma, increasing contri-butions from fetal urine, filtration from fetal vessels (120)

• fetus drinks it late in pregnancy

• amniotic fluid problems

16

– hydramnios (excessive fluid): often results from fetal inabil-ity to swallow, like in esophageal atresia or anencephaly

– oligohydramnios (not enough fluid): bilateral renal agenesis(fetus not urinating) or rupture of amniotic membrane

• amniocentesis: examine cells (chromosomes) & proteins in fluid

– α-fetoprotein indicative of neural tube defect

– monitor Rh-factor responses

– monitor lung maturity with lecithin/sphingomyelin ratio

3. yolk sac

• progressive narrowing and eventual closure of connection with gut

• small percentage have vestige of gut/yolk sac connection: Meckel’sdiverticulum

4. allantois

• vestigial excretory route

• embedded in umbilical cord

• turns into urachus: ligament from bladder to umbilicus

5. chorion & placenta

(a) chorionic plate: outermost extraembryonic mesoderm and tro-phoblast wrapping; villi emerge from this

(b) 1◦ villus: cytotrophoblast aggregates sticking into syncitiotro-phoblast (end of 2nd week); do not touch maternal decidua

(c) 2◦ villus: mesenchyme pushes into 1◦ villus

(d) 3◦ villus: embryonic vessens penetrate 2◦ villus (end of 3rd week)

(e) at tips of villi, just (cyto) trophoblastic cells: cytotrophoblasticcell column

(f) cell columns grow and reach through syncitiotrophoblast to ma-ternal decidua; villi that reach maternal cells are anchoring villi

(g) proliferating distal cytotrophoblast cells from neighboring villimeet and form the cytotrophoblastic shell

6. uteroplacental circulation

• invasive cytotrophoblastic cells which extend from anchoringvilli perforate maternal spiral arteries

17

• by secreting special ECM they cause structural changes, includingwidening to reduce local BP

• layers of interface:

– syncitiotrophoblast

– its basal lamina

– early on: cytotrophoblast (disappears)

– capillary basal lamina (sometime 2 laminae are consolidated)

– capillary epithelium

7. gross relations

• decidua: stroma cells swollen from influx of glycogen & lipid(days after implantation)

– decidua capsularis: decidua covering chorionic vesicle (theinner bump of “a”)

– decidua basalis: decidua underlying chorionic vesicle (thedownstroke of “a”)

– decidua parietalis: decidua not touching chorionic vesicle(top curve of “a”)

• increasing asymmetry in villi development

– more develop where body stalk connects embryo (embryonicpole) → chorion frondosum

– less at abembryonic pole → chorion laeve

– chorion frondosum + neighboring decidua = placenta

• eventually, decidua capsularis disappears/fuses with parietalis andchorionoic laeve is covered by parietalis

8. villus structure

• inside: mesensenchyme + Hofbauer cells (macrophages)

• outside: syncitiotrophoblast

– covered with many microvilli to increase surface area & fa-cilitate diffusion

– stress causes increase in microvilli

– arranged into specialized territories (? 140)

– (maternal ?) placental surface lacks histocompatibility anti-gens, keeps mom from rejecting baby

18

9. materials transported

(a) gases: O2, CO2, anesthetics

(b) H2O, electrolytes

(c) fetal wastes: urea, creatinine, bilirubin

(d) glucose but not fructose

(e) AAs, fatty acids, vitamins

(f) steroid hormones but generally not peptide hormones

(g) IgG → passive immunity

(h) Fe via transferrin carrier

10. placental hormone synthesis (142)

• syncitiotrophoblast peptide hormones

(a) Human Chorionic Gonadotropin (HCG)

– secretion begins before implantation → use in pregnancytest

– tells corpus luteum to keep making progesterone & estro-gen

– production peaks at 8th week; by end of 1st trimester pro-gesterone placenta is progesterone-independent and canmake estrogen with adrenal gland & liver

(b) chorionic somatomammotropin (aka human placental lacto-gen)promotes growth, lactation, lipid & carbohydrate metabolism

(c) human placental growth hormone

– closely resembles normal GH

– helps regulate maternal glucose levels

• cytotrophoblast peptide hormone: homologue of Gonadotropin-releasing hormone (GnRH); stimulates release of HCG from synci-tiotrophoblast

11. placental immunology: why the fetus is not rejected as foreign is anopen question; partial answers/theories:

• lack of antigens from fetus

• inactivation of maternal immune systemReznick:

19

(a) immunusuppressive cytokines secreted by syncitiotrophoblast

(b) HLA-G (human leukocyte antigen G) class IB histocompati-bility complex protein

• interference with immunity signal or response at the decidua

• inactivation of antibodies by fetus

12. multiple pregnancies

(a) if implanted far from each other (as in dizygotic or early-splitmonozygotic), separate chorions & placentas

(b) normal monozygotic: partially fused chorions & placentas

(c) late-split monozygotic: shared chorion & placenta, separate am-nions

• vascular systems can be separate or fused

• if fused, one twin can siphon from the other, causing poten-tially fatal stunting of growth

(d) conjoined twins or (rarely) dizygotic, can share chorion, placenta& amnion (why not monozygotic?)

13. placenta problems

• placenta previa: placenta covers cervical opening (os of uterus)& obstructs fetus’ exit; can cause fatal bleeding

• velamentous insertion of umbilical cord: cord stuck in thinchorionic membrane, not in placenta

• hyatidiform moles: grape-like deformation of villi associatedwith non-viable fetus; results from double paternal dose of DNAwith nothing maternal

9 Skeletal System

1. skull/skeletal defects (S 129)

(a) cranioschisis: cranial vault fails to close

(b) craniosynostosis: premature closure of a suture

• sagittal → sacphocephaly (long skull)

• coronal → acrocephaly (tower skull)

• coronal & lambdoid → brachycephaly (short skull)

20

(c) achondroplasia: autosomal dominant

(d) possible molecular mechanism: mutations in FGFRs (see S table9.1)

• they are tyrosine kinase receptors

• FGFR1, FGFR2 (Resnick: TGFβ also) are expressed in pre-bone & cartilage regions (important for proliferation & differ-entiation, respectively) & are implicated in craniosynostoses

• FGFR3 is expressed in long bones & is implicated in achon-droplasia

2. limb growth

• at week 4 (Reshef: week 5), limb buds emerge from ventrolateralbody wall; 2 kinds of tissue:

(a) ectoderm: at distal border, apical ectodermal ridge forms

– it induces adjacent tissue to proliferate and not to dif-ferentiate

– as growth happens & AER moves out, tissue behind itdifferentiates

(b) mesenchyme from somatic layer of lateral plate meso-derm; 2 components:

i. progress zone

ii. “the rest”

(c) somite mesoderm develops into limb muscles

• finished by week 8

3. limbs develop along 3 axes:

(a) proximodistal (hand goes at end of arm)

(b) dorsoventral (hair goes on back of hand)

(c) anteroposterior (thumb goes on lateral side of hand; for AP ori-entation, think of pectoral fins of fish)

4. hand development

• hyaline cartilage models are precursors for bones, including digits

• separation between digits occurs by apoptosis between the con-densing cartilage

5. chicken wing is homologous to fly wing!

21

• same embryonic tissues

• same signalling pathways

6. proximodistal development: AER & progress zone

• the longer the cell stays in the progress zone, the more distal itwill travel

• experiments:

(a) removal of AER at increasingly late stages in developmentleaves longer (less shortened) limbs

(b) removal of AER and replacement with a bead soaked in FGFproduces a normal limb (wing)

(c) – removal of AER and replacement with 1 FGF bead re-stores part of growth

– removal of AER and replacement with 2 successive FGFbeads restores more growth

(d) – extra AER produces double wing

– replace wing mesoderm with leg mesoderm produces legin place of wing

– replace wing mesoderm with non-limb mesoderm causesregression of AER & stopping of limb development

(e) FGF-soaked beads can induce ectopic limbs

• signalling

– FGF-8 signals initial budding of limb

– radical fringe is expressed dorsally

– → serrate 2 (Ser-2) at the apical ridge, which induces theAER

– ventrally, engrailed-1 inhibits expression of radical fringe,keeping AER region restricted to ridge

– absence of radical fringe ventrally and high concentration dor-sally both suppress Ser-2 in those regions

– FGF2, FGF4 also involved

• also controlled by nested expression of Hoxa genes, e.g.:

– only Hoxa-9 proximally

– Hoxa-9 through Hoxa-11 in between

– Hoxa-9 through Hoxa-13 distallyHoxa-13 mutations implicated in hand-foot-genital syndrome

22

7. anteroposterior development: ZPA & Shh

• ZPA is located posteriorly; it secretes a morphogene which has aconcentration gradient: high concentration posteriorly, low con-centration anteriorly

• experiments:

(a) add an additional ZPA anteriorly and you get bilaterally sym-metrical limb-ends (hand/foot/wing)

(b) FISH identified Shh in ZPA

(c) Shh expressing cells can cause the same abnormal symmetryas an extra ZPA (retinoic acid can also do this)

• cases of polydactyly may result from insufficient Shh gradient

• also controlled by Hoxd genes

– Hoxd-9 most anteriorly

– Hoxd-9 through Hoxd-11 in between

– Hoxd-9 through Hoxd-13 posteriorlyHoxd-13 mutations implicated in polysyndactyly

8. dorsoventral development: WNTs

• WNT-7a expressed in dorsal ectoderm

• LMX1 expressed in dorsal mesoderm

• engrailed expressed in ventral ectoderm

9. signalling for limb bud initiation

• some signal from intermediate mesoderm (S 135: FGF10, TBX4/5)induces ectoderm to start to bulge & secrete FGF8

• FGF8 causes

– proliferation of lateral-plate mesoderm

– induction of ZPA & Shh secretion

• Shh induces secretion of FGF4 in ectoderm, which (together withFGF8) signal ZPA to keep making Shh → positive feedback

• FGF4, FGF8 maintain (lateral plate) mesoderm proliferation

23

10 Muscular System

1.

type of muscle originskeletal paraxial mesoderm (somites)cardiac splanchnic mesoderm near heart tubesmooth splanchnic mesoderm near gut

2. skeletal muscle

• 2 origins:

(a) ventrolateral lip (VLL) of dermomyotome→ hypomeric aka hypaxial muscles (limbs & body wall)

(b) dorsomedial lip (DML) of dermomyotome→ epimeric aka epaxial muscles (back)

• see “signalling for somite development”

• “Patterns of muscle formation are controlled by connective tissueinto which myoblasts migrate” (S 143)

– head: neural crest cells

– neck: somite mesoderm

– body wall/limbs: somatic mesoderm

• limb musculature

– at first, the limbs are innevated by a dorsal branch and aventral branch from each segment

– the dorsal branches from each segment merge and the ven-tral branches from each segment merge to give large dorsal &ventral nerves

– ex: radial nerve (innervates extensor) is collection of dorsalbranches, ulnar & median (innervate flexor) are collectionsof ventral branches

11 Nervous System

1. proliferation within neural tube

(a) once neural tube is formed, cells organized into an epithelium(234-5)

(b) DNA synthesis occurs in lateral neural tube cells– external lim-iting membrane (=marginal zone)

24

(c) cells migrate towards neural tube lumen

(d) if cell splits vertically (metaphase plate perpendicular to NT innersurface): both daughter cells migrate back to marginal zone &repeat cycle

(e) if cell splits horizontally:

• lateral daughter cell

i. migrates laterally

ii. has high dose of Notch receptor

iii. stops dividing & is a postmitotic neuroblast → be-comes neuron

• medial daughter cell

i. slowly migrates away

ii. continues proliferating

2. cell lineages (236-7)

(a) multipotential stem cells (mitotic) → bipotential stem cells

(b) bipotential stem (=bipolar progenitor) cells (express nestin) →i. neuronal progenitor cells (express NFs)

ii. glial progenitor cells (express glial fibrillary acidic protein)

(c) neuronal progenitor cell (non-mitotic) → unipolar neuroblast →multipolar neuroblast

(d) glial progenitor cell (mitotic) → 3 lines, including radial glial cells

(e) radial glial cells act as guide wires for migrating neurons; neuronsprevent proliferation and final differentiation of radial glial cellsinto final form

3. neural tube cross-sectional organaziation

(a) most medial: marginal zone– mitotic cells; becomes ependyma

(b) intermediate zone– cell bodies and radially developing processes;becomes grey matter of SC

• sulcus limitans separates IZ into basal plate (ventral) and alarplate (dorsal)

• basal plate → motor neurons & interneurons

– differentiation controlled by shh gradient

– netrin attracts axons from dorsal region which descend &decussate (commisural axons)

25

– repels axons of trochlear nerve

• alar plate→ sensory interneurons & autonomic system (IMLcolumns)

• floor plate: main receiver for notochord signals; importantfor organizing exiting axons into ventral and dorsal roots (ex-periments with removed notochord and extra notochord)

• roof plate

(c) marginal zone–only neuronal processes (axons); becomes whitematter of SC

4. patterns in rhombomere development (240-2)

5. isthmic organizer induces dorsal midbrain and cerebellum (243)

6. forebrain

• neuromereric organization (numbered caudal to rotral)

(a) p1–p3: diencephalon

(b) p4–p6: telencephalon + optic vesicles

• ventral forebrain induced by shh, secreted by midline axial struc-tures; insuffient shh→ fusion of optic vesicles & holoprosencephaly

7. pns

• unmylinated neurons are still wrapped by Schwann cells, just notwith spiral

• axons communicate with Schwann cells via neuregulins, which tellthe Schwann cells not to apoptose and whether to myelinate.

• growth cone overview (245-7)

• development of neuromuscular junction (247-8)

• presence of a target (e.g. a limb) increases # of axons headingin its direction; apoptosis of neurons if they don’t get trophicfactors (249)

8. ans overview

• sympathetic & para (249-250)

• some neural crest cells need to migrate a long way to destinationsin abdomen (where they become postganglionic neurons); gutfacilitates by inducing mitosis of the cells that do make it (251)

26

• aganglionic megacolon (Hirschsprung’s disease): failure ofparasympathetic neural crest cells to colonize colon (252)

• neural maturation

(a) start off able to become either symp or para, then commit

(b) by default, they are both noradrenergic, then choose whichtransmitter (get signal from target tissue; see fig 11-21)

9. cns structural development

(a) overview

• migration

– cells start near ventricle

– postmitotic cells migrate out along radial glial cell “guidewires”→ grey matter is on outside of brain, unlike SC

– for most of brain, accumulation is inside-out: 6th layer(innermost) arrives first, then 5th pushes through that &on top of it, etc.

– exceptions:

i. migration parallel to surface (in cerebellum)

ii. outside-in development (3 layers of hippocampus & su-perior colliculi)

• cortex as a whole may be organized by columnar unit

(b) spinal cord: during gestation column grows more than SC, socaudal nerve roots need to elongate inside of column; in adult SCterminates at L2 (253-4)

(c) rhombencephalon →i. myelencephalon → medulla

• transitional between SC and brain

• has general layout of SC (fig 11-6), but roof plate stretchesout laterally & becomes very thin, overlying an enlargedcentral canal (→ 4th ventricle)

• 3 pairs of ventral (motor) and dorsal (sensory) nuclei

• cells from alar plate migrate ventrally and become ventralolivary nuclei (fig 11-25)

ii. metencephalpon → cerebellum & pons

• like medulla, has expanded & thinner roof plate over bigcentral canal (= 4th ventricle)

27

• like medulla, has 3 pairs of motor & sensory nuclei

• cerebellum develops from anterior rhombic lips (dorso-lateral bulges); posterior rhombic lips migrate ventrally &become pontine nuclei (cf. olivary nuclei)

• cells migrate anterolaterally to form a temporary layercalled external granular layer, then migrate back in &form the (inner) granular layer; in 2nd phase purkinjecells migrate in opposite direction

(d) mesencephalon

i. basal plate→ tegmentum→ efferent nuclei for cranial nervesIII, IV (eye)

ii. alar plate → tectum → corpora quadrigemina = superior &inferior colliculi

iii. cerebral peduncles

(e) diencephalon

• from here anteriorly, hard to see basal/alar plate organization;most structures probably come from ala

• central canal → 3rd ventricle

• thalamus develops surrounding 3rd ventricle & impinges onit from both sides, eventually meeting in middle (interthala-mic adhesion aka massa intermedia)

• ventral to thalamus: hypothalamus

• caudal to thalamus: epiphysis (=pineal gland); secretesmelatonin for circadian rhythm

• hypophysis (=pituitary gland) (259)made from 2 parts: infundibular process & Rathke’s pouch

i. infundibular process: ventral growth from floor of di-encephalon, posterior to hypothalamus → neural lobe ofhypophysis

ii. in stomodeum, anterior to buccopharyngeal membrane,Rathke’s pouch invaginates dorsally & wraps aroundinfundibulum (fig 11-31)

iii. outer wall of pouch thickens & becomes glandular→ parsdistalis of hypophysis

iv. inner wall → pars intermedia, separated from from an-terior part by residual lumen

v. connection of internal pouch with stomodeum degeneratesand canal regresses

28

• optic cups

(f) telencephalon

• base of each telencephalic vesiscle becomes basal ganglia(=striatum + glubus pallidus):

– lentiform nucleus (ball-shaped): composed of globuspallidus medially & putamen laterally, separated by in-ternal capsule; sits anterolateral to thalamus

– caudate nucleus (C-shaped elongation)

– see Netter p. 110

• general developmental pattern:

i. migration of cell bodies

ii. outgrowth of axons to appropriate targets

• lamina terminalis:

– thick anterior wall of 3rd ventricle (fig 11-32)

– site of development of important interhemispheric connec-tions:

i. anterior commisure

ii. hippocampal commisure (=fornix)

iii. corpus callosum

• rhinencephalon (=archicortex) small layer immediatelycovering lateral ventricles, primarily for olfaction (receives in-puts from olfactory bulbs)

10. CSF circulation (264-5)Arnold-Chiari malformation: herniation of cerebellum into foramenmagnum, blocking CSF flow and causing hydrocephalus

11. cranial nerves

• 3 categories

(a) extensions of brain tracts:

i. olfactory (I)

ii. optic (II)

(b) pure motor nerves:

i. oculomotor (III)

ii. trochlear (IV)

iii. abducens (VI)

iv. hypoglossal (XII)

29

(c) mixed nerves mapped to specific pharyngeal arches

i. trigeminal (V, arch I)

ii. facial (VII, arch II)

iii. glossopharyngeal (IX, arch III)

iv. vagus (X, arch IV)

(d) others (?)

i. auditory (VIII)

ii. accessory (XI)

• sensory part of nerves from category c + VIII dervied both fromneural crest and placode ectoderm cells

12. development of function

(a) generally, reflexes appear in craniocaudal order (267-272); firststep: neural differention to put components of reflex arc in place

(b) reflex arc goes online

(c) then, vertical (& supplementary horizontal) connections are made;these aren’t completed till early adulthood

(d) during postnatal development the intersegmental connections arerefined and myelinated

(e) myelination begins in SC at week 11 and continues craniocaudally,motor neurons first

(f) in brain, sensory neurons myelinated first

13. defects (268-71)

(a) cranioschisis: failure of neural tube closure at the brain

(b) rachischisis: failure of neural tube closure at the SC

(c) spina bifida occulta: failure of spinal column to close aroundSC at 1 or more vertebrae

(d) meningocele: protrusion of arachnoid out through defect in spinalcolumn or skull; only minor neurological problems (figs 11-42, 44)

(e) myelomeningocele: protrusion of SC through spinal column de-fect; severe neurological problems

(f) meningoencephalocele: protrusion of brain through skull de-fect

(g) meningohydroencephalocele: protrusion of brain + part ofventricle through skull defect

(h) lissencephaly: smooth cortices due to failure of cell migration

30

12 Neural Crest

1. origins of neural crest cells:

• lateral edges of neural plate

• they need to transition from epithelial to mesenchymal structure;they lose CAMs

• they migrate, and migration is controlled by ECM of basal laminasof neural tube and surface ectoderm and, of somites

• they selectively penetrate anterior parts of somites, since posteriorpart is rich in repellent chondroitin sulfate

• earliest-migrating cells have most possibilities for differentiation

• different environmental cues trigger different phenotypes (279-80)

2. trunk neural crest: 3 migratory paths

(a) dorsolateral: between ectoderm & somites → pigment cells

(b) ventral: via space between anterior halves of somites & neuraltube, ventral to aorta → sympathoadrenal cells

(c) ventrolateral: to space between anterior halves of somites & neuraltube → sensory ganglia

3. circumpharyngeal neural crest (282-4)cells migrate behind 6th pharyngeal arch then turn rostrally, with 2turn-offs:

(a) cardiac crest → parts of aorta, heart, thyroid, parathyroid, thy-mus; DiGeorge syndrome (due to chromosomal deletion) occurswhen this flow of cells is deficient

(b) vagal crest → parasympathetic innervation of gut

4. cranial neural crest

• may be “morphological substrate for evolution of vertebrate head”

• outflow of rhomobomeres 2, 4 and 6 flow into (and constitue bulkof) pharyngeal arches 1, 2 and 3, respectively

• mesenchyme at r3 and r5 repulses crest cells and causes them tojoin flow from the neighboring rhomobomeres (fig 12-8)

• fig 12-9: pattern of Hoxb expression in pharyngeal arches reflectsrhombomere expression

31

5. defects

(a) DiGeorge syndrome: see above

(b) CHARGE association

i. coloboma

ii. heart disease

iii. atresia of nsal choanae

iv. retardation of development

v. genital hypoplasia

vi. ear anomalies

(c) Waardenburg’s syndrome

• mutation in transcription factor Pax-3 (Hox relative; 69, fig4-9)

• pigmentation defects, deafness, cleft palate, (in Type I) hy-poplasia of limb muscles

(d) neurofibromatosis: tumors of neural crest descendents

(e) frontonasal dysplasia: facial abnormalities

6. differences between cranial and trunk crest cells

(a) cranial cells can form skeletal, muscle & connective components

(b) more morphogenetic information encoded in cranial cells (e.g.,craniocaudal level)

14 Face & Neck

1. 2 parts of skull (S 125)

(a) neurocranium; 2 parts:

i. membranous part, made of flat bones, which form vault ofbrain

• derived from neural crest cells & paraxial mesoderm

• made by membranous ossification (no cartilage interme-diate)

• growth happens by adding bone on outside & resorptionon inside

• flat bones connected at flexible sutures, which intersect atfontanelles → allow molding of head during delivery

32

ii. chondrocranium, cartilaginous base of skull; 2 parts (S fig9.5)

A. prechordal chondrocranium

• anterior to end of notochord (level of sella turcica)

• derived from neural crest

B. chordal chondrocranium

• posterior to end of notochord

• derived from paraxial mesoderm

(b) viscerocranium

2. as evolution progresses, 1st pharyngeal arch becomes

(a) upper & lower jaw

(b) bones of inner ear

3. anterior face vs. viscerocranium

• viscerocranium is segmented and controlled by Hox

• anterior face is from NC & begins (anatomically) at anterior endof notochord

4. in addition to NC cells, paraxial and prechordal mesoderm migrate intohead/neck region, contributing to

(a) connective tissue

(b) muscle

(c) skeleton: anterior/posterior distinction (S fig 16.1)

• occipital bones of skull from mesoderm (parietal, occipital,petrous temporal)

• anterior bones of skull from neural crest cells (frontal, sphe-noid, squamous temporal, hyoid)

• laryngeal from endoderm

5. pharynx structure overview (fig 14-4)

(a) pharyngeal pouches: 4 pairs of pouches off of pharyngeal foregut

(b) pharyngeal arches:

• ectoderm bulges over each pair of pouches, appear in weeks 4and 5

33

• components (S fig 16.6):

i. artery (medial)

ii. cartilage (intermediate)

iii. nervous (lateral)

iv. muscle

(c) pharyngeal grooves: grooves between arches

(d) each arch fed by aortic arches flowing dorsoventrally; weird num-bering: 6th arch follows 4th (?)

6. primordial face

• frontonasal prominence medially

• nasomedial & nasolateral processes make U-shaped primor-dial nose

• maxillary processes grow toward midline from both sides ofstomodeum

• nasolacrimal groove runs between maxillary process & nasalprominence, from nasolateral processes to each eye

– near eye, develops into lacrimal sac

– becomes nasolacrimal duct, drains tears into nasal cavity

• as face develops, frontonasal prominence recedes (fig 14-5)

7. development of frontonasal prominence depends on retinoic acid/shhsignalling between forebrain & overlying ectoderm (323)

8. similar Hox expression pattern for limb growth & facial development(324)

9. phylogenetic history of TMJ (325-6)

10. palate

• separates nasal and oral cavities

• forms between weeks 6 and 10

• Y-shaped intersection of 3 primordial shelves:

(a) medial palatine process anteriorly

(b) paired lateral palatine processes

• shelves first grow down, then rotate up so they meet in middle

34

• incisive foramen of palate marks the Y intersection

11. pharyngeal arches (figs 14-21, 22)

(a) 1st

• includes maxillary process dorsally & mandibular prominence/processventrally (S 259)

• contributes to face & ear; its cartilage strut (Meckel’s carti-lage) is primordial jawbone, but ultimately resorbed

i. maxillary process → quadrate cartilage

ii. mandibular process → Meckel’s cartilagequadrate Meckel

dorsal zygomatic, part of temporal malleus, incusventral premaxilla, maxilla mandible

• trigeminal nerve & the muscles it innervates (mastication)

• no Hox genes expressed

(b) 2nd (hyoid arch)

• muscles of facial expression

• Richert cartilage, becomes:

– stylohyoid ligament

– styloid process of temporal bone

– stapes

– part of hyoid bone

• Hoxa expressed; knockout results in mirror image of arch 1

(c) 3rd → rest of hyoid bone

(d) 4th → cartilages of larynx

12. pharyngeal grooves (clefts)

• 1st groove becomes external acoustic meatus

• 2nd & 3rd grooves covered by growth of 2nd arch & become thecervical sinus (340)

• 2nd arch meets forward projection of 4th arch, obliterating cervi-cal sinus (hence neck is smooth)

13. pharyngeal pouches (340-2)

(a) 1st: tubotympanic recess becomes eustachian tube, connect-ing pharynx & middle ear

35

(b) 2nd shrinks, becomes supratonsillar fossae

(c) 3rd disappears, sends the thymic & parathyroid tissue which de-velops there caudally (fig 23), becoming thymus and inferiorparathyroid glands.

(d) 4th: similar to 3rd, with thymic (but not in humans) & parathy-roid components; becomes superior parathyroid glands

14. thyroid (342)

• starts as thickening just caudal to tongue bud, between 1st and2nd pouches

• migrates caudally, growing as it goes in the thyroid diverticu-lum

• arrives at front of esophagus

• connection to its origin, the thyroglossal duct regresses by 7thweek, but part near thyroid can persist as pyramidal lobe ofthyroid

• point of origin persists as foramen cecum depression

15. tongue

• formation (fig 14-25, S 16.17)

(a) all 4 arches contribute, but component from 3rd arch over-grows the component from the 2nd part

(b) from inferior 1st arch, get paired lateral lingual swellingsand tuberculum impar in middle

(c) lateral swellings grow anteriorly & push together, with tuber-culum impar growing behind, in middle

(d) anterior 2/3, aka body (component from 1st arch) ends atterminal sulcus which runs through foramen cecum

(e) posterior 1/3 aka root (from 3rd & 4th arches) → epiglottis

• innervation (S 269-270)

(a)

general sensory special sensorybody lingual branch of chorda tympani branch of

mandibular division of trigeminal facial nerveroot glossopharyngeal & vagus glossopharyngeal

(b) motor: hypoglossal

16. defects

36

(a) facial clefts

i. cleft lip: lack of fusion of nasomedial & maxillary processes

ii. cleft palate: lack of fusion of palatal shelves

iii. oblique facial cleft: lack of fusion of nasolateral & maxillaryprocesses

iv. macrostomia: lack of fusion of maxillary & mandibular pro-cesses

v. medial cleft lip: lack of fusion of nasomedial processes

(b) holoprosencephaly

• failure to develop midline structures, in extreme cases result-ing in cyclopia

• factors:

i. heredity (esp. shh mutation, e.g. Meckel’s syndrome),

ii. shh malfunction from retinoic acid

iii. maternal alcoholism, or diabetes

(c) frontonasal dysplasia:

• too much tissue in nose region: broad nasal bridge, hyper-telorism

• can include lack of fusion of nasomedial processes → separateexternal nares, median cleft lip

(d) first arch related (jaw hypoplasias)

i. Pierre Robin

ii. Treacher Collins (autosomal dominant gene)

iii. agnathia - no jaw

(e) lateral cysts, sinuses, fistulas

i. from cervical sinus → appear in front of sternocleidomastoidall 3 possible (?)

(f) from ridges on 1st 2 arches: preauricular sinuses & fistulastrue cervicoaural fistulas come from 1st pharyngeal groove

(g) ectopic thyroid tissue– anywhere along migratory route

16 Urogenital

16.1 Urinary

1. overview: recurring themes

37

(a) interconnectedness of urinary & genital systems

(b) recapitulation in kidney ontogeny of phylogenetic kidney history

(c) epithelial-mesenchymal interactions

(d) sexualization of structures

2. pronephros

• homologue of lowest vertebrate kidney

• starts developing around day 21, disappear by day 28

• only in cervical region (S fig 15.1, 15.2)

• nephrotomes: mediolateral tubules; in pronephric system theyremain segmented (discrete)

3. mesonephros

• homologue of fish/amphibian kidney (no need for water retention)

• nephrotomes connect with longitudinal primary nephric ducts,which grow caudally towards cloaca

• as PNDs grow they stimulate growth of new mesonephric tubules(different from nephrotomes??)

• mesonephric unit:

(a) glomerulus

(b) glomerular capsule surrounding glomerulus

(c) glomerular capsule feeds into mesonephric tubule

(d) mesonephric tubules dump into 1◦ nephric duct (becomesmesonephric [Wolffian] duct)

• also, there are external glomeruli where blood filtrate dumpsinto embryonic coelom (S fig 15.1)

• by end of week 4 mesonephric ducts attach to cloaca

• near attachment to cloaca duct outgrowth occurs→ ureteric bud(→ metanephros, future kidney)

• as metanephros matures, mesonephric duct regresses, but in males,testis attaches to it and it becomes vas deferens (sperm conduitto urethra; fig 16-2)

4. metanephros

38

• metanephrogenic blastema: mesenchyme cells in vicinity ofbud from which metanephros arises

• 2 parts of kidney development (396)

(a) branching of ureteric bud

– signalled by mesenchyme via GDNF (glial-derived neu-rotrophic factor), HGF (hepatocyte growth factor)

– GDNF and HGF genes turned on by WT1 transcriptionfactor

– signal transduced in bud epithelium by c-RET and MET

(b) condensation of new tubules from metanephrogenic blastema

– proliferation signalled (in response to part a) by epithe-lium via FGF-2, BMP-7, LIF (leukemia inhibitory factor)

– WT1 enables mesenchyme response

– Pax-2, Wnt-4 signal epithelialization of mesenchyme &cause differentiation into excretory tubules

Action is reciprocal; without blastema bud doesn’t branch, with-out bud blastema doesn’t make new tubules.

• 3 kinds of mesoderm in a nephron

(a) ureteric bud epithelium (→ excretory/renal tubule)

(b) metanephrogenic blastema (→ collecting tubule)

(c) vascular endothelial cells

• mesenchyme development

(a) metanephric duct branches (happens 15 successive times)

(b) 1st condensation of mesenchyme makes renal vesicle (S fig15.6)

(c) renal vesicle → renal tubule: distal part (becomes proximal,from glomerulus) with epithelium (podocytes)

(d) renal tubules & collecting tubules establish continuity (S fig15.6C); renal cell carcinomas can occur at the interface

(e) slit forms beneath future podocyte cells

(f) precursors of vascular endothelial cells grow into the slit

(g) glomerulus forms from interface of blood vessels (glomeru-lar endothelium) and podocyte epithelium; filtration done byspecial basement membrane between the 2

5. epithelium polarization (fig 16-6)

39

(a) cells adhere to laminin in basement membrane, but not each other

(b) cells adhere to each other via E-cadherin

(c) cells adhere to each oterh via tight junction & develop brush bor-der

6. mature kidney

• calices: penultimate confluences of collecting tubules

• pelvis: confluence of calices, last stop before ureter

• kidneys move up from pelvis and rotate 90 degrees (from facingfront to facing inwards) & “land” on suprarenal glands

7. bladder (399)

8. defects

(a) renal agenesis

• if unilateral, kidney grows to compensate

• fatal if bilateral

• bilateral renal agenesis symptoms:

i. oligohydramnios (less amniotic fluid)

ii. flattened Potter face from mechanical pressure (due tooligohydramnios)

iii. tapered, fat fingers

(b) renal hypoplasia: small kidney

(c) polycystic disease of kidney

(d) urachal cyst/sinus/fistula

(e) exstrophy of bladderfrequently with epispadias

16.2 Genital

1. overview (401)

• indifferent stage: until 7th week, only way to determine sex isby looking for Barr bodies (no sex phenotype)

• sexualization starts in gonads & spreads from there

• genetic gender can be overridden by environment

40

• example: maleness depends on virilizing action of testes; if malesex hormones not secreted or receptors not active, female pheno-type occurs despite genotype

2. migration of germ cells

(a) primordial germ cells start as epiblast cells which migrate throughprimitive streak

(b) they wind up as endoderm, lining hindgut near yolk sac

(c) they migrate from there to genital ridges (see next)

(d) if they land in the wrong place, they become oogonia, regardlessof genetic sex; they normally degenerate when this happens but ifthey don’t they can become teratomas

3. origin of gonads: elongated region of steroidogenic mesoderm alongborder of mesonephros

(a) rostral end → adrenocortical primordial

(b) caudal end → genital ridges

i. coelomic epithelium (medial part of roof of embryonic coelom)

ii. mesonephric ridge (lateral part)

4. testes

(a) development

i. Sry triggers testes by inhibiting Dax-1, which prevents testes

ii. 5th week: coelomic epithelium thickens, PGCs arrive

iii. 6th week: columns of cells grow from epitheliun inward →primitive sex cords

iv. myoid cells (become interstitial cells) migrate from mesonephros

v. sex cords thicken (precursors of Sertoli cells)

• distal part of sex cords → seminiferous tubules

• proximal part → rete testis

• rete flow into efferent tubules

vi. tunica albuginea: connective tissue separating testis fromepithelium, develops

(b) Sertoli secretions

i. cause migration of mesonephric mesenchyme into testis

41

ii. prevent male germ cells from entering meiosis

iii. induce Leydig cells

iv. mullerian inhibiting substance (see ducts)

v. androgen-binding factor

(c) Leydig cells

• appear during 8th week

• secrete androgens

i. testosterone

ii. androstenedione

5. ovaries

• in contrast to testes, need PGCs in order to develop

• like testes, have coelomic & mesonephric cell descendants

• PGCs become oogonia

– in medulla (??) oogonia divide mitotically but some entermeiosis → oocytes

– in cortex only mitosis

6. duct system

(a) indifferent

i. mesonephric (wolffian) ducts

ii. paramesonephric (mullerian) ducts

• ventral to mesonephric ducts

• appear between 44 & 48 days

• fate depends on gender

(b) male

• because of mullerian inhibiting substance (secreted by Sertolicells), mullerian ducts degenerate

• under influence of testosterone, mesonephric ducts developinto ductus (vas) deferens

• mesonephric tubules can persist near testis as paradidymis

• development of accessory sex glands (414)

• Hoxa & Hoxd specify layout (414)

(c) female

42

• with no testosterone (with ovaries or without), mesonephrictubules regress, paramesonephric tubules develop

– as in male, depends on Hoxa and Hoxd genes directed byWnt-7a (fig 16-30)

– cranial parts → uterine tubes

– caudal parts go to midline & meet

– region of fusion becomes vagina

7. descent of testes

• begins at 7th month

• 3 phases

(a) release/degradation of cranial suspensory ligament

(b) transabdominal descent: testis arrives at inguinal ring

(c) transinguinal descent: traversal of inguinal canal

17 Cardiovascular

1. intro–functions of the heart

• drive flow of blood through fetus & to placenta

• take over job of oxygenation postnatally (pulmonary circulation)

• spare load on underdeveloped fetal lungs by shunting

2. hematopoiesis

• starts in yolk sac, but by 28th day starts inside embryo in paraaor-tic clusters in aorta/genital ridge/mesonephros (AGM) region

• then main site moves to liver

• secretion of cortisol causes switch from liver to bone marrow

• cell lines: puripotent stem cells become

(a) myeloid SCs

(b) lymphoid SCs

3. 2 mechanisms of vascular development (S 78, 180)

(a) vasculogenesis

• vessels arise by coalescence of (hem)angioblasts

43

• only major vessels develop this way

• pathway: (S fig 6.13)factor receptor transformationFGF2 FGFR mesoderm → hemangioblastVEGF VEGF-R1 hemangioblast → epithelial cellVEGF VEGF-R2 epithelial cell → tube

(b) angiogenesis:

• vessels arise from pre-existing ones

• all other vessels

4. early heart devcelopment

(a) cardiogenic field: epiblast cells which migrate through the prim-itive streak, land anterior to buccopharyngeal membrane

(b) development begins around day 18-19, when coelomic vesicles oflateral mesoderm (see fig 6-6B) are fusing, causing separation ofsomatic and splanchninc mesoderm

(c) heart originates underneath space which develops but doesn’tfuse with the extraembryonic coelom (like the space formed at thesame level along the sides of the embryo; fig 6-6C, 6-14A)

• the space is (as mentioned) anterior to buccopharyngeal mem-brane

• the space becomes the pericardial cavity

• the splanchnic mesoderm underneath it becomes the myocar-dial priomordium

• the growth of the brain pushes the mesoderm with cardiacprimordia ventrally then caudally, rotating it 180◦

(d) the lateral curling of the embryo pushes ∩ shape in in the middle

i. caudal ends become venous inflow

ii. top part broadens, grows arterial outflow

(e) initially, heart tube is suspended from dorsal roof of pericardialcavity by mesoderm called mesocardium, but this deterioratesso heart is “floating” in the pericardial space

(f) septum transversum: semicircular horizontal shelf which sepa-rates heart from liver (382)

(g) proepicardium starts forming on septum transversum and spreadover the hear to form epicardium (S 161) aka visceral peri-cardium

44

5. formation of loop (446, S 162-3)

• outflow tract arises from:

– cephalic paraxial & lateral mesoderm (endothelium)

– cranial neural crest (wall)

• atrial portion twists rostrally, dorsally and to left so that it residesin pericardium; driven by HAND-1, 2

• bulbus cordis: connection of ventricle to aorta (446)

– middle part: conus cordis

– distal part: truncus arteriosus→ proximal aorta & pulmonaryartery

• narrowings:

(a) atrioventricular junction → atrioventricular canal

(b) truncus arteriosus

(c) 1◦ interventricular foramen, between ventricle and bulbus;marked on outside by bulboventricular sulcus

• trabeculation (S 163)

– proximal bulbus cordis becomes trabeculated→ primitive RV

– whole ventricle becomes trabeculated → primitive LV

6. aortic arches (436-8)

(a) fig 17-7: phylogenetic history: instead of pulmonary circuit, bloodused to flow

• from common ventricle → ventral aorta

• → branchial arches for oxygenation

• → paired dorsal aortas

(b) in humans, never observe all arches at once, instead see cradio-caudal progression

(c)

arch right left1–3 carotid a’s4 right subclavian arch of aorta5 vestigial6 right pulmonary pulmonary trunk,

left pulmonary,ductus arteriosus

cf. table 17-3

45

(d) asymmetrical development of 6th arch derivatives reflected in tra-jectories of R & L recurrent laryngeal nerves (438)

i. right: curves under 4th arch → subclavian

ii. left: stays under ductus arteriosus → ligamentum arteriosum

7. skipping 438–444 since not on syllabus

8. 444-6 lymphatic system

9. valves (446-7, S 171)

(a) atrioventricular cushions: dorsal & ventral thickenings formedat AV junction (thickening also happens between ventricle & out-flow)

(b) at the cushions, cardiac jelly (mesenchyme) protrudes into canal

(c) induced by myocardium, some endocardial cells transform fromepithelium to mesenchyme & migrate into jelly (of AV cushion-?)

(d) blood flow hollows out the mesenchyme & (as mesenchyme re-placed by connective tissue) valves form (S 171)

(e) definitive valves come from epicardium

i. right AV canal: tricuspid

ii. left AV canal: bicuspid aka mitral

iii. they are connected to venetricular trabeculae called papil-lary muscles by the chordae tendinae

10. partitioning (448-451, fig 17-20)

(a) atria

i. interatrial septum primum grows down from ceiling ofheart to split common atrium into right & left

ii. before it fuses with the AV cushion the atria are connected bythe interatrial foramen (ostium) primum, which shuntsblood away from pumonary circuit to systemic

iii. don’t want to shunt all blood away from R ventricle, though,because it would be hypoplastic; it is allowed to pump at70-80% capacity, & its output shunted away from lungs byductus arteriosus (451)

iv. as interatrial septum primum fuses with AV cushion, perfo-rations appear at top (cephalic) end, and coalesce to becomeinteratrial foramen secundum, to keep shunting going

46

v. septum secundum grows out from dorsal atrial wall, rightto the right of the septum primum

vi. the two septa with the foramen make the foramen ovale,which acts as a valve, preventing backflow from L to R atrium

(b) ventricles:

• originally (day 30), blood flows through AV canal→ primitive LV→ 1◦ interventricular foramen→ primitive RV→ bulbus cordisI.e., flow is sequential from L to R, and is deflected L bybulboventricular flange (S fig 12.8)

• by day 35, AV canal enlarged to the right & flange has re-tracted enough so blood flows into both RV and LV simulta-neously (S fig 12.17)

• muscular interventricular septum grows up from (caudal)floor of common ventricle towards AV cushion

• membranous part of septum contributed by AV cushion;when it meets the conus septum (see next), the interventric-ular foramen is completely closed (S fig 12.22)

(c) outflow

i. truncoconal ridges appear & grow spirally

• distally, superior & inferior truncus swellings (S 173-5)

• proximally, dorsal & ventral conus swellings

ii. they fuse to make a spiral partition, the aorticopulmonaryseptum

iii. RV reaches around ventrally to access its outflow, LV dorsally,the R & L outflow tubes slide past each other (fig 17-22)

iv. depends on NCC migration, potential defect source

(d) outflow (semilunar) valves:

• from cranial NCC & cardiac mesoderm

• protect backflow of blood into ventricles

i. aortic valve

ii. pulmonary valve

11. sinus venosus starts in middle, but with twisting of heart, winds updumping exclusively into RA (5th week); its left horn becomes coronarysinus (also dumps into RA)

47

• confusing terminology: flaps around right input each called valves,right and left venous valves (S 167)

• as the orificie shrinks, wall develops along line of fusion: septumspurium

• left valve & septum spurium fuse with developing atrial septum

• right valve: superior part disappears, inferior part → valve of IVC& valve of coronary sinus

12. pulmonary vein (S 170)

• starts as outgrowth of left atrial wall & meets up with developingveins of lung buds

• atrial attachment grows & becomes smooth-walled part of atrium;displaces original trabeculated part (which remains as appendage)

13. innervation (452)

• sympathetic fibers grow from trunk neural crest (adrenergic, ex-citatory)

• parasympathetic fibers grow from crania neural crest (choliner-gic, inhibitory)

• cardiac ganglia: migrate from cardiac neural crest; synapse withparasympathetic

• sensory fibers: from nodose ganglion

• parasympathetic & sensory carried by vagus nerve

14. electrical conduction system

• as sinus venosus becomes RA, SA node develops in RA.

• AV node develops in interatrial septum

• AV node communicates with ventricles via the AV bundle

• AV bundle extend Purkinje fibers into ventricles

• cellular aspects:

– conducting bundles are specialized myocytes, with high glyco-gen content

– coronary arteries induce conducting bundles by secreting endothelin-1

– responding cells express engrailed-2

48

15. initiation of function

• ductus venosus allows blood from placenta to skip liver & pro-ceed directly to IVC

• there may be a sphincter which enables/disables ductus venosusshunt, to prevent backup of “bypassed” blood from liver

• in RA:

(a) high pressure blood from IVC (oxygenated) goes through fora-men ovale to LA

(b) low pressure blood from SVC (deoxygenated) goes throughtricuspid valve to RV

16. defects: heart defects are most common birth defect (1% of live births,S 172-3)

• cardiovascular teratogens (?): rubella virus, thalidomide

• risk factors: maternal diabetes, hypertension

• genetic problems

– holt-oram syndrome

– heart-hand syndrome

– tetralogy of Fallot

• endocardial cushion formation: since neural crest implicated as-sociated with craniofacial defects (S 168)

(a) AV septal defects

(b) transposition of great vessels

(c) tetralogy of Fallot

• issues at atrial septum

(a) atrial septal defect (ASD): patent foramen ovale

(b) absence of atrial septum: cor triloculare biventriculare

(c) premature closure of foramen ovale: hypoplasia of left side ofheart

(d) issues at AV canal

i. persistent AV canal

ii. ostium primum defect??

iii. tricuspid atresia →A. patent foramen ovale

49

B. ventricular septal defect (see next)

C. hypertrophy of LV & hypoplasia of RV

• ventricular septal defect (VSD): most common

– usually restricted to membranous part of septum

– causes more blood to pulmonary circuit than systemic

• issues from conotruncal septum

(a) tetralogy of Fallot: unequal division of conus; conotruncalseptum displaced anteriorly so aorta gets more volume; notfatal

i. pulmonary infundibular stenosis

ii. defective interventricular septum

iii. aorta originating right on top of septal defect

iv. RV hypertrophy, since higher pressure

(b) persistent truncus arteriosus:

– conotruncal rides don’t fuse

– always defective interventricular septum

(c) transposition of great vessels:

– septum doesn’t spiral

– usually patent ductus arteriosus

18 Intro:Reznick lecture 1

1. paracrine

19 Birth Defects (Sadler 8, Reznick lecture

2)

1. definitions

• ectrodactyly: missing digits

• amelia: missing limbs

• meromelia: partial absence of limb

• phocomeila: flipper limb (phoco=seal)

2. frequency of congenital malformations

50

• 2% of all births

• 1% are minor

• .5% are lethal

• .5% are serious (only partially treatable) but nonlethal

3. components

• genetic: 5% mutations, 5% chromosomal defects

• 10% purely environmental

• 80% combined environment & genetics

4. see tables in lecture notes

5. environmental factors: especially powerful during weeks 3–8 (organo-genesis); see S table 8.1

(a) infection: bacteria, viruses, parasites, protozoa

• rubella

• cytomegalovirus: mother is often asymptomatic

• herpes simplex

• HIV

• varicella- maternal infection has 25% defect rate

(b) chemicals

• thalidomide

• valium

• alcohol: most common cause of mental retardation (S 115)

• aminopterin (chemotherapy)

(c) hormones

• progestins administered to prevent abortion cause masculin-ization of female genitalia

• diethystilbestrol (synthetic estrogen) given for same reasoncarcinogenic for female fetuses later in life

(d) malnutrition

(e) hypoxia → CNS damage

(f) ionizing radiation: kills rapidly proliferating cells

6. preventing congenital malformations

51

(a) genetic counseling

(b) environmental factors:

• avoid mutagens & infections, especially during critical period

• good nutrition

7. prenatal diagnosis

trimester method1 CVS 1-3 % risk to fetus2 ultrasound2 fetoscopy2 amniocentesis .5% risk2 chordocentesis fetal blood sample2 α-fetoprotein2 nuchal translucency test for trisomy 21

8. syndrome vs. association (111)

• syndrome: group of co-occurring symptoms whose cause is known

• association: group of co-occurring symptoms whose cause is un-known

52