Organogenesis Part 2 V. Lateral Plate Mesoderm VI. Endoderm VII. Development of the Tetrapod Limb...

Organogenesis Part 2

V. Lateral Plate Mesoderm

VI. Endoderm

VII. Development of the Tetrapod Limb

VIII. Sex Determination

V. Lateral Plate Mesoderm

chordamesodermparaxial mesoderm

intermediate mesoderm

lateral plate mesoderm

Lateral Plate Mesoderm

Terminology: - Somatopleure: somatic mesoderm plus ectoderm - Splanchnopleure: splanchnic mesoderm plus endoderm - Coelom: body cavity forms between them

Lateral Plate Mesoderm

• The Coelom:

– eventually left and right cavities fuse into one

– runs from neck to anus in vertebrates

– portioned off by folds of somatic mesoderm• pleural cavity: surrounds the thorax and lungs• pericardial cavity: surrounds the heart• peritoneal cavity: surrounds the abdominal organs

Figure 12.1 Mesodermal development in frog and chick embryos (Part 3)

Heart Development

• The heart is the first organ to function in the embryo and the circulatory system is the first functional system.

– heartarteriescapillariesveinsheart

• Before the embryo can get very big it must switch from nutrient diffusion to active nutrient transport

Heart Development

1. Tube Formation2. Looping3. Chamber Formation

human timeline

Anatomical Stages:

inflow

outflow

Heart Development: Tube Formation

presumptive heart cells are specifiedbut not determined in the epiblast

migrate through together near node

outflow forming cells (red)migrate in first, inflow second

“the heart field”


The cardiogenic mesoderm migrates out of the mesodermal layertowards the endoderm to form endocardial tubes on either side.

At the same timethe endoderm isfolding inward


The endoderm continues folding inward until it forms its own tube,which drags the two endocardial primordia close to each other.

The endocardial tubesare surrounded by myocardial progenitors

When the endocardialtubes get close enough,they fuse together


If you mess withendoderm migrationor signaling, you endup with two hearts


• Heart Tube Cell Biology

– Splanchnic mesoderm cells express cadherins and form an epithelial sheet for their inward migration - MET

– The presumptive endocardial cells undergo EMT to migrate away from the sheet and another MET to form tubes

– The cells in the original mesodermal sheet form the myocardium

– The myocardial epithelium fuses first and the two endocardial tubes exist together inside for a while before fusing

– Both the rostral end (outflow) and caudal end (inflow) remain as unfused double tubes

– The heart beat starts spontaneously as myocardial cells express the sodium-calcium pump - before fusion is even complete

Heart Development: Looping and Chamber Formation

anterior

posterior leftright

left-right asymmetry is due to Nodal and Pitx2

Looping requires: cytoskeletal rearrangement extracellular matrix remodeling asymmetric cell division


valve formation

septation

heart valves keepthe blood from flowing back intothe chamber it wasjust ejected from

The septa separatethe two atria and the two ventricles


The truncus arteriosis, or outflow tract, also becomes septated allowing one greatartery to flow from right ventricle to lungsand the other from left ventricle to the body.


• The tricuspid valve is between the right atrium and right ventricle.

• The pulmonary or pulmonic valve is between the right ventricle and the pulmonary artery.

• The mitral valve is between the left atrium and left ventricle.

• The aortic valve is between the left ventricle and the aorta.


Steps: 1. Endocardial cushions form and fuse 2. Septa grow towards cushion 3. Valves form from myocardium

In utero, the foramen ovale allows right left shunting of blood

Embryonic circulatory systems

All of the blood must circulate outside of theembryo for oxygenation

Redirection of human blood flow at birth

Blood Vessel Development

• The vessels form independently of the heart

• They form for embryonic needs as much as adult– Must get nutrition before there is a GI tract– Must circulate oxygen before there are lungs– Must excrete waste before there are kidneys– They do these through links to extraembryonic

membranes


• The vessels are constrained by evolution– Mammals still extend vessels to empty yolk sac– Birds and mammals also build six aortic arches as if we

had gills, eventually settling on a single arch

• The vessels adapt to the laws of fluid dynamics – Large vessels move fluid with low resistance– Diffusion requires small volumes and slow flow– Highly organized size variance controls volume– And superbranching smaller vessels control speed

Embryonic circulatory systems


Vasculogenesisis the de novodifferentiationof mesoderminto endothelium

It is followed bythe endotheliumrecruiting smoothmuscle cell coat


Starts in theextraembryonicmesoderm aswell as in thelarge embryonicblood vessels


Angiogenesisis the growthand remodelingof the 1st vesselsin response toblood flow andtissue-derivedrecruitment signals


1. PEO forms from splanchnicmesoderm overlying the liver

2. PEO contacts the ventricleand migrates as epicardium

3. Subset of epicardial cellsdelaminate towards myocardium

4. These undergo MET toform coronary endothelium

Secondary Vasculogenesis

5. Coronary arteries then pluginto the aorta where nerves are


It is a commonphenomenon forarteries and nervesto form together

Less so for veins....


• Lymphatic drainage forms from jugular vein

– Sprouts as lymphatic sacs by angiogenesis

– Continues to form secondary drainage system

– Major conduit for immune cells

Where do the hematopoietic stem cells of the adult bone marrow come from?

Splanchnic mesoderm of aorta-gonad-mesonephros (AGM) region in embryo

Hemogenic endotheliumfrom sclerotome

Hemogenicendotheliumfrom many sites

Wherever they come from....

WOW!

Development of the Endoderm

• The Digestive Tube– Anterior endoderm forms anterior intestinal portal– Posterior endoderm forms posterior intestinal portal– Midgut goes through expansion and contraction to yolk– Each end has ectodermal cap, then forms an entrance

• The Derivatives– 4 pharyngeal pouches form head and neck structures– Floor between 4th pair buds out to form respiratory tube– Gut tube forms esophagus, stomach, SI, LI, rectum– Gut tube buds out to form liver, gall bladder, pancreas


Human Timeline


The cranial neural crest cells migratethrough this endoderm and contributecomponent structures around them


Localized Wnt/B-Catenin and retinoic acid cause budding


Normal-time birth issignaled from the lungs


Anterior-Posteriorspecification of thegastrointestinal tract


Reciprocal InductionSimultaneous

Anterior-Posteriorspecification of both endoderm and mesoderm


Mesoderm alsoinduces liver bud

The Extraembryonic Membranes

• Adaptation for development on dry land– As the body starts to develop epithelium

expands to isolate embryo within them

• Four sets of extraembryonic membranes– Somatopleure forms amnion and chorion– Splanchnopleure forms yolk sac and allantois


Somatopleure forms amnion and chorionSplanchnopleure forms yolk sac and allantois


The amnion folds up to cover the embryo and keep it from drying out

The cells of theamnion secrete water


The chorion surrounds the entire embryo and controls gas exchange

In birds and reptiles it lines shellIn mammals it forms the placenta


The yolk sac expands to surround yolk (even if you don’t have any)


The allantoic membrane creates a space for waste storage

Bird and reptileeggs gotta’ have it

We don’t use it forwaste but it contributesto our umbilical cord

Organogenesis Part 2 V. Lateral Plate Mesoderm VI. Endoderm VII. Development of the Tetrapod Limb...

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