· 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)
· 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
· 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
o 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 Endocardium
o Myocardium
o 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 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
o Venous Drainage
§ Sinus venosus blood return
· Vitelline veins
o From yolk sac
· Umbilical vein
o From placenta
· Cardinal veins
o From embryo
o 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) 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
o 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
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 ventricles
o 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 Cyanotic
o 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 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
·
·
§ 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
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 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
· 2nd arch
o Styloid
o Stapes
o Stylohyoid ligament
o Lesser horn of hyoid bone
· 3rd arch
o Body of hyoid bone
o Greater horn of hyoid bone
· 4th arch
o Thyroid cartilage
· 6th arch
o 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 calcitonin
o 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
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 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
o
o 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
§
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 ectoderm
o 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 perilymph
o Endolymph is contained within endolymph ducts
· Saccule
o Horizontal sensation
· Semilunar canals
o 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 component
o Saccule
o Cochlear duct
· Doral component
o Utricle
o Semicircular canals
o Endolymphatic duct
o Middle ear
§ Stapedius
· Connects neck of stapes to stiffen the stapes to help dampen the vibration of stapes