Embryology: Cavitieso Formation of Body Cavities

Clefts appear in lateral plate mesoderm Parietal (Somatic) Visceral (Splanchnic) Space between parietal and visceral = primitive body cavity

Entire gut is suspended by dorsal mesentery connecting the parietal and visceral layer

Separation of Pericardial & Peritoneal Cavities Septum transversum

o Thick plate of mesoderm that incompletely divides the primitive body cavity

Between thorax and abdomeno Still communication between two cavities via

pericardioperitoneal canalso Liver will grow hereo Forms central tendon of diaphragm

Separation of Pericardial & Pleural Cavities Lung buds

o Grow into pericardioperitoneal cavitieso Bud off foregut

Pleuropericardial foldso Separate off pleural cavities from pericardial cavityo Forms the fibrous pericardium

Phrenic nerve will sit on fibrous pericardium Diaphragm

Septum transversum forms central tendon to heart Pleuroperitoneal folds close the pericardioperitoneal canals

o Complete closure of pleural and peritoneal cavities Mesentery of esophagus form crura of diaphragm

o The crura of the diaphragm (singular: crus) are tendinous structures that extend inferiorly from the diaphragm to attach to the vertebral column.

Muscular part formed by myoblasts from C3-C5 somites C3,C4,&C5 spinal nerves invade the diaphragm Clinical Correlation

o Diaphragmatic hernias Failure of one pleuroperitoneal membrane closing 90% on posterior left side because liver is on right side

o Respiratory system Respiratory diverticulum (lung bud)

Outgrowth from floor of foreguto All epithelium in respiratory tree is from endodermo Surrounding tissue is splanchnic (visceral) mesoderm

Opening into the diverticulum is the laryngeal orifice Tracheoesophageal ridges grow inward to separate esophagus from

tracheao Clinical correlation

Tracheoesophageal fistulas 90% will be fistula between distal esophagus

and trachea with a blind proximal esophagus Mostly caused by excess amniotic fluid

(polyhydramnios) Tissues from pharyngeal slits IV&VI form larynx

Respiratory system Lung buds elongate to form trachea and bifurcates into primary bronchi Primary bronchi divide

o Right – 3 secondary bronchio Left – 2 secondary bronchio Further division form tertiary bronchi

Lungs expand into pericardioperitoneal canals and eventually fill the pleural cavities

Maturation of lungso Further division of bronchi through 7 monthso Cuboidal cells change

Type I alveolar – squamous for gas exchange Type II alveolar – secrete surfactant

Clinical correlationo Respiratory distress syndrome

Insufficient surfactant production

Treated by intratracheal surfactant

Alveolar cells establish close ties with capillarieso Stages

Pseudoglandular: 5-15 weeks Canalicular: 16-26 weeks Terminal sac: 26-weeks to birth Alveolar Period: childhood

Embryology: Cardiovascularo Cardiogenic Field

Splanchnic layer of lateral plate mesoderm surrounding head of embryo

Cardiac myoblasts Blood islands

o Unite to dorm U-shaped endocardial tube surrounded by cardiac myoblasts

Intraembryonic cavity over the tube will form the pericardial cavityo Cephalocaudal Folding

Endocardial tube is brought into neck region of embryo Pericardial cavity accompanies movement

o Lateral folding Two parts of the endocardial tube are brought together in the midline to form a

single heart tube Two pericardial cavities also join into a single cavity Heart tube suspended from dorsal body wall by dorsal mesocardium 3 layers of heart tube

o Endocardiumo Myocardiumo Epicardium

o Cardiac Loop Cephalic end bends ventrally, caudally, and to the right

Bulbus cordis and ventricle Caudal end bends dorsally, cranially, and to the left

Atrium Divisions

Bulbus Cordiso Primitive right ventricleo Conus cordis

Outflow tract of the ventricleso Truncus arteriosus

Aorta and pulmonary trunk Ventricle

o Primitive left ventricle Atrium

o Primitive right & left atriao Sinus venosus located here

o Venous Drainage Sinus venosus blood return

Vitelline veinso From yolk sac

Umbilical vein o From placenta

Cardinal veins

o From embryoo Sinus venosus

Entrance of sinus into atrium shifts to the right Right vitelline vein becomes inferior vena cava Right common cardinal becomes superior vena cava Left sinus horn becomes coronary sinus Right sinus horn becomes smooth part of right atrium

o Atrial septation Septum primum

Ostium (opening) primumo First free opening that disappears

Ostium secundumo Forms by cell death

Septum secundum Covers over ostium secundum Never completely divides

o Opening is foramen ovale Upper part of septum primum disappears

Rest becomes the valve of the foramen ovaleo Atrial Development

Right atrium Absorbs right sinus horn forming the smooth part of the right atrium

with the openings of the superior and inferior vena cavas Left atrium

Atrium absorbs proximal part of the pulmonary vein forming the smooth part of left atrium with 4 openings

Primitive atria become auricles in adult Clinical Correlation

Atrial septal defectso Left-to-right shunt is acyanotico Right-to-left shunt is cyanotic

o Division of the Atrioventricular Canal Endocardial cushions (mesenchyme) divide common atrioventricular canal into

right and left canals AV valves form by selective degeneration of surrounding myocardium

o Ventricular Septation Right and left sides grow. In the process a muscular interventricular septum is

created Membranous part of septum is made from conotruncal septum to finally

separate the two ventricleso Conotruncal Septum

Pair of opposing ridges form in both truncus arteriosus and conus arteriosus Conotruncal septa

Spiral around each other and fuse dividing the common truncus and conus into the aorta and pulmonary trunk

Septum fuses with the muscular IV septum forming membranous part of interventricular septum

Neural crest cells from the pharyngeal arches contribute to the endocardial cushions of conotruncal septum

Clinical Correlation Ventricular septal defects

o Types Membranous

Conotruncal septa Muscular

Ventricular growth Tetralogy of Fallot

o Four defects Pulmonary stenosis VSD Overriding aorta

Aorta overrides pulmonary trunk Right ventricular hypertrophy

o Cyanotico Major Arteries

Aortic arches Arch of aorta Carotids Pulmonary arteries

Dorsal aorta Descending aorta

Vitelline Celiac trunk Superior and inferior mesenteric arteries

Umbilical arteries Medial umbilical ligaments

o Major veins Umbilical vein (left)

Ligamentum teres hepatis Vitelline veins

Hepatic portal system Cardinal veins

Superior and inferior vena cavao Changes at birth

Three shunts Ductus venosus

o Shunt in liver that bypasses liver and go right into the inferior vena cava to enter right atrium

o Goes through right atrium and through the foramen ovale and straight into the left atrium

o Blood entering right atrium goes to right ventricle which is pumped into the ductus arteriosus and into the aortic arch. Some goes to the lungs

o First breath decreases pulmonary resistance Foramen Ovale

o Clamping of the maternal blood causes an increase in pressure in the left atrium to close the foramen ovale

Lungs are able to give oxygenated blood to left atrium Ductus arteriosus

o Increase left pressure causes reverse flow through the ductus arteriosus

o Oxygenated blood inhibits prostaglandin production and causes muscle contraction

o Degenerates and eventually becomes the ligamentum arteriosum

If it does not close can treat with prostaglandin inhibition

o Clinical correlation Patent Ductus Arteriosus

Ductus arteriosus does not close Small – asymptomatic Large

o Significant left-to-right shunt Embryology: Head and Neck I

o Skull Arise from two sources

Paraxial mesoderm (In red)o Somitomereso Occipital somites

Neural crest (In blue)o Pharyngeal arches

Dividing line is sella turcica Bone formation

Membranous ossificationo Flat bones of neurocranium and most viscerocranium

Endochondral ossificationo Base of skull

Newborn skull Fontanelles

o Bones not fused together and create soft spotso Head is largest part of body and during birth process the bones

can move to allow passage and then realign later (molding)o Usually close around 18 months

o Pharyngeal Arches Swellings of mesenchyme covered by ectoderm

Swellings = pharyngeal arches Separations = pharyngeal clefts

Appear in neck region during 4-5 weeks Outpocketings of foregut (endoderm) also form in the same areas as the

pharyngeal cleft (pharyngeal pouches) Each pharyngeal arch

Covered by ectoderm Lined by endoderm Contains

o Mesenchyme Neural crest – bones; skeletal segments of each arch Paraxial mesoderm – muscles, dermis Lateral plate – laryngeal cartilages, CT

o Cranial nerve Any muscle that derives from a certain arch is

innervated by the cranial nerve of that archo Aortic arch

4 archeso 1st

o 2nd

o 3rd

o 4th & 6th

Pharyngeal arches – skeletal (neural crest cells) 1st arch

o Meckel’s cartilage Part of Maxilla Mandible

o Inner ear ossicles Malleus Incus

2nd archo Styloido Stapeso Stylohyoid ligamento Lesser horn of hyoid bone

3rd archo Body of hyoid boneo Greater horn of hyoid bone

4th arch o Thyroid cartilage

6th archo Cricoid cartilage

Pharyngeal arches – Muscles 1st arch

o Muscles of mastication, mylohyoid 2nd arch

o Muscles of facial expression 3rd arch

o Stylopharyngeus 4th/6th arch

o Muscles of pharynx and larynx Muscles of tongue

o Formed from occipital somites Pharyngeal Arches – Nerves

o Pharyngeal arches – nerves 1st arch

Trigeminal 2nd arch

Facial 3rd

Glossopharyngeal 4th/6th

Vagus nerve and cranial part of accessory o Clinical Correlation

Treacher Collins Syndrome Malformed external ear, mandibular, and malar hypoplasia, conductive

hearing loss Malformed development of 1st pharyngeal arch

o Pharyngeal Pouches and Clefts 1st pharyngeal pouch

Auditory (eustachian) tube 2nd pharyngeal pouch

Palatine tonsil 3rd pharyngeal pouch

Inferior parathyroid glands Thymus

4th pharyngeal pouch Superior parathyroid glands Parafollicular cells of thyroid (C cells)

o Secrete calcitonino Tongue

Forms from 1st and 3rd pharyngeal arches Anterior 2/3 from 1st arch

o General sensory – lingual nerve (CN V)o Taste CN VII (chordae tympani hitchhiking onto lingual)

Posterior 1/3 from 3rd archo General sense and taste from CN IX

o Thyroid Gland Forms from a diverticulum of endoderm between 1st and 2nd arches Descends into neck

Connection is called the thyroglossal duct

Original point of invagination becomes foramen cecum Embryology Head and Neck II

o Face Formed

Frontonasal prominenceo Mesenchyme cranial to pharyngeal arches

V1 First pharyngeal arch

o Maxillary prominence – V2o Mandibular prominence – V3

Nasal placodeso Medial nasal prominence

Fuse to form intermaxillary segment –philtrum Dimple on upper lip is from fusion of medial nasal

prominence Then fuses with the maxillary prominence to form

upper lipo Lateral nasal prominence

Nasolacrimal groove (wings on side of nose) Lacrimal sac Nasolacrimal duct

o

o Palate Primary palate is intermaxillary segment (philtrum)

Forms with 4 incisors Secondary palate

Formed by the palatine shelves from maxillary prominenceo Separates oral from nasal cavities

Point of junction is incisive foramen

o Clinical correlation

Cleft lip Males > Females Picture B

Cleft Lip and palate Females > Male Picture C

Median cleft palate Picture E Rare

Median cleft palate and lip Picture F Most rare

Problems with sucking with this disorder

o Derivatives of Germ Layers

Most sense organs derive from ectodermo Nasal Cavity

Forms from nasal placode which invaginates to form nasal pits Placode = thickening of ectoderm

Olfactory epithelium forms from olfactory placode (ectoderm) Source of 1st cranial nerve

Pits deepen and are separated from oral cavity by oronasal membrane Oronasal membrane breaks down Final separation of nasal cavity from oral cavity is secondary palate Paranasal sinuses develop from diverticula from nasal cavity

o Ear Middle Ear

Tympanic membrane Ear ossicles

o Stapes Carries vibrations to inner ear by vibrating perilymph

o Malleus Sits on tympanic membrane

o Incus Carries vibrations from malleus to stapes

Inner Ear Cochlear

o Contains perilympho Endolymph is contained within endolymph ducts

Sacculeo Horizontal sensation

Semilunar canalso Angular sensation

o Inner ear Thickenings of the ectoderm near rhombencephalon form otic placodes Otic placode invaginates to form otic vesicles Otic vesicles components

Ventral componento Sacculeo Cochlear duct

Doral componento Utricleo Semicircular canalso Endolymphatic duct

o Middle ear Stapedius

Connects neck of stapes to stiffen the stapes to help dampen the vibration of stapes

o To help withstand loud sounds Tympanic cavity comes from 1st pharyngeal pouch

Connects to nasopharynx and remains as auditory tube Ossicles from neural crest

Trigeminal innervates the muscles that connect to malleus and incuso 1st arch – malleus and incus

Innervation of stapes is from facial nerveo 2nd arch – stapes

o External Ear External auditory meatus

1st pharyngeal cleft Tympanic membrane

Externally – ectodermal lining Internally – endodermal lining

Auricle Mesenchyme from 1st and 2nd pharyngeal arches

o 6 auricular hillockso Eye

Wall of eye from 3 primary layers Inner layer – retina Middle layer – choroid (pia mater of brain) Outer layer – sclera (dura mater of brain)

o Optic cup & lens Outpocketings from the developing forebrain from optic vesicle Come into contact with ectoderm which induces lens placode which forms lens

vesicle Optic vesicle invaginate to form two layered optic cup

Choroid fissure allows hyaloid artery into cup Mouth of the cup forms the pupil

o Retina, Iris, & Ciliary Body Optic cup

Posterior 4/5thso Outer layer forms the pigmented layer

Colored layer of iriso Intraretinal space

Typical site for detachment of retina (separation of an embryonic union)

No longer functions of photo receptiveness o Inner layer forms the neural retina

Rods, cones, and ganglionic cell layer Anterior 1/5

o Inner layer of iriso Ciliary body

Lens Cells of posterior wall elongate

Lumen disappearso Choroid, sclera, cornea & optic nerve

Surrounding mesenchyme Posteriorly

o Inner layer forms choroid (highly vascular and pigmented)o Outer layer forms sclera

Anteriorlyo Anterior chambero Outer layer of iriso Cornea

Vitreous body formed from mesenchyme that invades optic cup Axons from ganglionic layer invade stalk forming the optic nerve

o Hyaloid artery becomes the central artery of retina which runs through optic nerve