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Embryology: Cavities
o 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 abdomen
o Still communication between two cavities via
pericardioperitoneal canals
o Liver will grow here
o Forms central tendon of diaphragm
 Separation of Pericardial & Pleural Cavities
 Lung buds
o Grow into pericardioperitoneal cavities
o Bud off foregut
 Pleuropericardial folds
o Separate off pleural cavities from pericardial cavity
o 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)
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Outgrowth from floor of foregut
o All epithelium in respiratory tree is from endoderm
o Surrounding tissue is splanchnic (visceral) mesoderm
 Opening into the diverticulum is the laryngeal orifice
 Tracheoesophageal ridges grow inward to separate esophagus from
trachea
o 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 bronchi
o Left – 2 secondary bronchi
o Further division form tertiary bronchi
 Lungs expand into pericardioperitoneal canals and eventually fill the
pleural cavities
 Maturation of lungs
o Further division of bronchi through 7 months
o Cuboidal cells change
 Type I alveolar – squamous for gas exchange
 Type II alveolar – secrete surfactant
 Clinical correlation
o Respiratory distress syndrome
 Insufficient surfactant
production
 Treated by intratracheal
surfactant
 Alveolar cells establish close ties with capillaries
o Stages
 Pseudoglandular: 5-15 weeks
 Canalicular: 16-26 weeks
 Terminal sac: 26-weeks to birth
 Alveolar Period: childhood
Embryology: Cardiovascular
o Cardiogenic Field
 Splanchnic layer of lateral plate mesoderm surrounding head of embryo
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o
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 cavity
Cephalocaudal Folding
 Endocardial tube is brought into neck region of embryo
 Pericardial cavity accompanies movement
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 Endocardium
o Myocardium
o Epicardium
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 Cordis
o Primitive right ventricle
o Conus cordis
 Outflow tract of the ventricles
o Truncus arteriosus
 Aorta and pulmonary trunk
 Ventricle
o Primitive left ventricle
 Atrium
o Primitive right & left atria
o Sinus venosus located here
Venous Drainage
 Sinus venosus blood return
 Vitelline veins
o From yolk sac
 Umbilical vein
o From placenta
 Cardinal veins
o
o
o
o
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From embryo
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
Atrial septation
 Septum primum
 Ostium (opening) primum
o First free opening that disappears
 Ostium secundum
o 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 ovale
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 defects
o Left-to-right shunt is acyanotic
o Right-to-left shunt is cyanotic
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
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 ventricles
Conotruncal Septum
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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 Cyanotic
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
Major veins
 Umbilical vein (left)
 Ligamentum teres hepatis
 Vitelline veins
 Hepatic portal system
 Cardinal veins
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 Superior and inferior vena cava
o 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 Somitomeres
o Occipital somites
 Neural crest (In blue)
o Pharyngeal arches
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Dividing line is sella turcica
Bone formation
 Membranous ossification
o Flat bones of neurocranium and most viscerocranium
 Endochondral ossification
o Base of skull
 Newborn skull
 Fontanelles
o Bones not fused together and create soft spots
o 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
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
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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 arch
o Aortic arch
 4 arches
o 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
nd
 2 arch
o Styloid
o Stapes
o Stylohyoid ligament
o Lesser horn of hyoid bone
rd
 3 arch
o Body of hyoid bone
o Greater horn of hyoid bone
th
 4 arch
o Thyroid cartilage
th
 6 arch
o Cricoid cartilage
Pharyngeal arches – Muscles
 1st arch
o Muscles of mastication, mylohyoid
nd
 2 arch
o Muscles of facial expression
rd
 3 arch
o Stylopharyngeus
th th
 4 /6 arch
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o
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o Muscles of pharynx and larynx
 Muscles of tongue
o Formed from occipital somites
 Pharyngeal Arches – Nerves
Pharyngeal arches – nerves
 1st arch
 Trigeminal
 2nd arch
 Facial
rd
 3
 Glossopharyngeal
th th
 4 /6
 Vagus nerve and cranial part of accessory
Clinical Correlation
 Treacher Collins Syndrome
 Malformed external ear, mandibular, and malar hypoplasia, conductive
hearing loss
 Malformed development of 1st pharyngeal arch
Pharyngeal Pouches and Clefts
 1st pharyngeal pouch
 Auditory (eustachian) tube
nd
 2 pharyngeal pouch
 Palatine tonsil
rd
 3 pharyngeal pouch
 Inferior parathyroid glands
 Thymus
th
 4 pharyngeal pouch
 Superior parathyroid glands
 Parafollicular cells of thyroid (C cells)
o Secrete calcitonin
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 arch
o General sense and taste from CN IX
Thyroid Gland
 Forms from a diverticulum of endoderm between 1st and 2nd arches
 Descends into neck
 Connection is called the thyroglossal duct
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 Original point of invagination becomes foramen cecum
Embryology Head and Neck II
o Face
 Formed
 Frontonasal prominence
o Mesenchyme cranial to pharyngeal arches
 V1
 First pharyngeal arch
o Maxillary prominence – V2
o Mandibular prominence – V3
 Nasal placodes
o 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 lip
o Lateral nasal prominence
 Nasolacrimal groove (wings on side of nose)
 Lacrimal sac
 Nasolacrimal duct
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Palate
 Primary palate is intermaxillary segment (philtrum)
 Forms with 4 incisors
 Secondary palate
 Formed by the palatine shelves from maxillary prominence
o Separates oral from nasal cavities
 Point of junction is incisive foramen
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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
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Derivatives of Germ Layers
 Most sense organs derive from ectoderm
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
Ear
 Middle Ear
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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 perilymph
o Endolymph is contained within endolymph ducts
 Saccule
o Horizontal sensation
 Semilunar canals
o Angular sensation
Inner ear
 Thickenings of the ectoderm near rhombencephalon form otic placodes
 Otic placode invaginates to form otic vesicles
 Otic vesicles components
 Ventral component
o Saccule
o Cochlear duct
 Doral component
o Utricle
o Semicircular canals
o Endolymphatic duct
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 incus
o 1st arch – malleus and incus
 Innervation of stapes is from facial nerve
o 2nd arch – stapes
External Ear
 External auditory meatus
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Eye
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o
 1st pharyngeal cleft
Tympanic membrane
 Externally – ectodermal lining
 Internally – endodermal lining
Auricle
 Mesenchyme from 1st and 2nd pharyngeal arches
o 6 auricular hillocks
Wall of eye from 3 primary layers
 Inner layer – retina
 Middle layer – choroid (pia mater of brain)
 Outer layer – sclera (dura mater of brain)
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
Retina, Iris, & Ciliary Body
 Optic cup
 Posterior 4/5ths
o Outer layer forms the pigmented layer
 Colored layer of iris
o 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 iris
o Ciliary body
 Lens
 Cells of posterior wall elongate
 Lumen disappears
Choroid, sclera, cornea & optic nerve
 Surrounding mesenchyme
 Posteriorly
o Inner layer forms choroid (highly vascular and pigmented)
o Outer layer forms sclera
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Anteriorly
o Anterior chamber
o Outer layer of iris
o 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