Embryology Flashcards
Embryogenesis week 1
Fertilisation -> zygote (fertilised ovum)
12 hours post ovulation at ampulla of fallopian tube
2nd meiotic division post fertilisation, then 1st mitotic complete 30 hours later
Zygote = 2 cell, at day 2
Morula = 16 cell, day 3, to uterine cornu
Blastocyst at day 4, to uterine cavity
Implantation at day 6
Blastocyst
Inner cell mass - embryoblast
Outer layer - trophoblast to make placenta and extra-embryonic tissues
- before gastrulation this will differentiate into outer syncytuym (syncytiotrophoblast), and inner cytotrophoblast
70-100 cells
Fluid-filled cavity is blastocoele
Week 2
Day 8 - decidual reaction (thickened endometrium), formation of embryonic bilaminar disc, amniotic cavity in epiblast
Day 9 - lacunar stage, hypoblast (ventral germinal layer) forms exocoelomic membrane lining yolk sac
Day 12 - lacunae from syncitiotrophoblast communicate with endometrial sinusoids
Day 13 - implantation complete, cytotrophoblast forms primary chorionic villi, secretes hCG and uteroplacental circulation begins
(hCG tells corpus luteum to keep making progesterone so that endometrium doesn’t shed)
Nutrients coming from trophoblastic digestion of uterine mucosa and diffusion from contents of yolk sac
Week 3
Day 15 - primitive streak caudal end, primitive node cephalic end, ectodermal cells migrate to streak and detach and spread to form intra-embryonic mesoderm
- intra-embyronic mesoderm is between ectoderm and endoderm everywhere except two places - the prochordal plate (=buccopharyngeal membrane), cloacal plate - will break down at weeks 4/7
Day 16 - allantois (diverticulum forming posterior wall of yolk sac and extending to connecting stalk)
Day 18 - neuralation (neural plate for brain/spinal cord)
Day 20 - primitve heart tube
Day 21 - first heart beat
Week 4
Embryo folding starts day 21-24
Longitudinal (enlargement of cranial end)
Transverse (enlargement of somites)
Gastrulation
Formation of 3 germ layers
In third week
Ectoderm - epidermis, nervous system
Mesoderm - muscles, skeletal system, connective tissues
Endoderm - GIT, resp tract, endocrine glands, auditory, urinary system
Mesoderm
Paraxial mesoderm (most medial, by notocord) - segmental, forms somites in pairs up to 44 pairs
- differentiate to dermatomes (will become spinal meninges, SC tissue and dermis), myotomes (will become muscle, epiaxial and hypaxial), sclerotomes (will become vertebrae, IV discs and ribs)
Intermediate mesoderm - forms kidneys + ureters, and gonads + ducts (testes + epididymis + vas deferens, or ovaries + fallopian tubes + uterus)
Lateral plate mesoderm - forms somatic (lining amnion with ectoderm, will make parietal layers) and splanchnic (lining yolk sac with endoderm, will make visceral layers) mesoderm
+ splanchnic makes adrenal cortex, lymph nodes, spleen, smooth muscle of GIT, helps development of CVS
+ somatic makes sternum + bones and cartilage oflimbs
MESODERMAL CELLS pneumonic
Myeloid stem cells
Erythroid stem cells
Spleen
gOnads
Dermis
Entire trunk
Renal system
Meninges
Adrenal cortex
Lymphoid stem cells/lymph nodes
Cardiovascular system
Endothelial lining of heart + vessels
Lining of body cavities (visercal and parietal)
Limbs
Smooth muscle of GIT
Decidua
= endometrium
3 layers:
- basalis - where implantation takes place and the basal plate is formed
- capsularis - overlies chorion
- parietalis - covering the rest of endometrial cavity
(C+P will come together and obliterate the uterine cavity at 4months gestation)
Placenta
Maternal parts from decidua basalis - basal plate
- if deficient, placenta accrete (placenta adherent to myometrium)
Fetal from villi of chorion frondosum - chorionic plate
Villi are to maximise area of interchange with maternal blood (from uterine vessels), each contain capillary plexus supplied by branches of umbilical vessels
Form day 12-28
At 4 months, septa divide placenta to 15-20 lobes
Placental membrane will become thinner as villous maturation
Umbilical cord
Should get to 50-60cm long, 1.5-2cm diameter
Two umbilical arteries (1:100 have only one)
One umbilical vein
Wharton’s jelly
- mucopolysaccharides, from extra-embryonic mesoderm, to protect umbilical blood vessels
Usually inserts centre of placenta, but can be:
- eccentric - up to 2cm from edge
- marginal - within 2cm of edge
- velamentous - cord into membranes, exposed vessels not protected by WJ
Fetal membranes
= structures from blastocyst that do not contribute to embryo
4 components:
- Amnion - inner side from ectoderm, outer from mesoderm, no blood vessels/lymphatics/nerves
- Chorion - double-layered membrane, formed by trophoblast and extra-embryonic mesoderm
- Yolk sac - will be incorporated into GIT
- Allantois - diverticulum of the yolk sac, will become attached to bladder
Twins
DCDA - separation at morula, by day 3 (70% twins)
MCDA - separation at days 4-8
MCMA - separation after amnion formed, days 9-12
Best to determine chorionicity before week 14
Molar pregnancy
Complete mole
- diffuse swelling of villous tissue and trophoblastic hyperplasia, NO embryonic tissue
- if empty egg is fertilised by 1 or 2 normal spermatozoa so all nuclear genes are paternal
- diploid, 46XX or XY
Partial mole
- focal swelling of villous tissue and focal trophoblastic hyperplasia WITH embryonic tissue
- if normal haploid egg is fertilised by 2 or 3 sperm
- triploid, 69XXY, 69XXX or 69XYY
Mostly benign, 15% will develop into invasive moles (persistent trophoblastic disease), 2% into choriocarcinoma
Placenta accreta/increta/percreta
Accreta - placenta morbidly adherent to myometrium, due to deficient deciduas basalis
Increta - placenta invades myometrium but not breached to uterine serosa
Percreta - placenta breaches serosa of uterus
Formation of diaphragm
From septum transversum, somatic mesoderm, mesentery of oesophagus, pleuroperitoneal membrane,cervical somites
- innervation from nerves C3-5 - which will fuse to form phrenic nerve
Twin-to-twin transfusion syndrome
Due to abnormal connecting vessels in twins’ placenta, so imbalanced flow of blood from one twin to another
Quintero classification system based on 1 (amniotic fluid volume discrepancy) to 5 (fetal demise)
15% risk in monochorionic twins (MCDA>MCMA)
Donor twin - oligohydramnios (<2cm), anaemia, smaller
Recipient twin - polyhydramnios (>8cm) at same time of measurement, heart strain/hydrops
Treat with fetoscopic laser ablation of interconnecting blood vessels
Mesenchyme
Will give rise to MSK system
Derived from mesodermal cells of somites, somatopleuric layer of lateral plate mesoderm, and neural crest cells
Can migrate and differentiate into many different cell types
Condensation of mesenchymal cells -> bones
Ossification
Membranous - no cartilage, for flat bones (skull, clavicle)
Endochondral - replacement of hyaline cartilage, for long bones
Vertebral column
3 stages:
1. mesenchymal condensation around notochord
2. cartilaginous transformation
3. ossification - starting at 6w gestation, continuing until 25yo adulthood
Vertebral arch from sclerotome interaction with surface ectoderm
- gives rise to costal and transverse processes, fuses to form spinous process
IV disc has outer annulus fibrosus (from sclerotomes), inner nucleus polposus (from notochord)
Muscles development
Skeletal muscle from myoblasts from somites (paired paraxial mesoderm)
Head musculature from pharyngeal arches and neural crest mesenchyme
Each myotome divides
- ventral hypomere - muscles of anterior and lateral wall
- dorsal epimere - muscles of posterior wall
Respiratory system
Endoderm makes epithelial lining, splanchnic layer of lateral plate mesoderm makes cartilage muscle etc
Larynx - endoderm from pharyngeal pouches 4+6
- mesoderm from pharyngeal pouches 4+6
Trachea and lungs - endoderm from foregut
- mesoderm from LPM
Larynx
Pharyngeal arches 4+6 (there is no 5!)
Week 5 - laryngeal orifice forms by endoderm invasion between 4+6, will develop into inlet then vocal cords
Week 6 - swellings around orifice - epiglottic swelling superior to orifice, arytenoid swelling inferior
Endoderm makes pseudostratified ciliated columnar epithelial tissue, and vocal cords
Mesoderm makes laryngeal cartilage - thyroid, cricoid etc, and muscles
Vagus nerve also derived from arches 4+6 - supplies larynx
Week 12 - mature larynx
Trachea
Week 4 - foregut develops lung bud, with tracheoesophageal groove/ridge
Bronchial bud when bifurcates, and TO groove forms septum between foregut and bud, called tracheoesophageal septum
Week 5 ends with lung bud anteriorly, foregut (oesophagus) posteriorly
Endoderm invades to form epithelium (cuboidal initially then thinning to squamous) and glands
LPM forms muscle, connective tissue and cartilage
Lungs
Week 5-16 - pseudoglandular stage
- bronchial buds continue to grow out and down - primary bronchi, then further bifurcation (3 for right, 2 for left)
- continue to terminal bronchioles
- bronchial tree division not complete until after birth
Week 16-26 - cannalicular stage
- divide into respiratory bronchioles, then alveolar ducts, then primitive alveoli
Lung epithelium
Initially cuboidal, then thins with ageing
Gas exchange not possible until thinned sufficiently to squamous - at 26 weeks
Type 1 pneumocytes - line alveoli
Type 2 pneumocytes - produce surfactant, appear around 24 weeks. (remain cuboidal)
Formation of heart tube
Starts as angiogenic cell clusters in extra-embryonic mesoderm lining yolk sac, influenced to differentiate by VEGF released by endoderm
Merge to form cardiogenic area
Form paired heart tubes
Dorsal aortae on each side of midline, connect to heart tubes, then they fuse with each other as embryo folds (lateral and cranio-caudal)
Dorsal mesocardium connects heart tube to pericardial cavity
Starts beating day 21
Heart tube elongates more than available volume, so bends and forms cardiac loop at day 23
Heart tube now contains
- single atrium, single ventricle, an atrioventricular canal
- AV valve in AV canal, formed by endocardial cushion
- bulbus cordis between primitive ventricle and atrial outflow, proximal 1/3rd becomes right ventricle (BC becomes truncus arteriosus which will become pulmonary trunk and aortic arch)
Heart development from week 4
Atrial septation
- septum primum in roof of common atrium, grows towards endocardial cushions. Ostium primum is opening between these, will close as grows. Ostium secundum is small hole in septum primum that will OPEN as the ostium primum closes.
- septum secundum to right of septum primum, grows over SP but does not completely divide the atrua, leaving the foramen ovale as an opening
Foramen ovale
- functional closure after birth when pressure in left heart > right heart with the start of breathing and pulmonary circulation
Ventricular septation
- muscular part from floor of ventricle towards endocardial cushions, leaving a gap
- membranous part from endocardial cushions, containing AV conducting bundle
Spiralling of aorticopulmonary spetum at week 5 develops ‘heart shape’ in cardiac looping
Venous drainage of heart
Sinus venosum (bottom end) empties into common atrium by single opening
- left horn becomes coronary sinus, right horn becomes dominant horn - proximal part of IVC and SVC
- vein pairs include:
– common cardinal veins - drain head and body
– vitelline veins - form sinus venosus and hepatic vessels
– umbilical veins - will break down leaving just left distal umbilical vein, connects with ductus venosum to bypass liver, after birth becomes ligamentum teres and lies in falciform ligament
Arterial system of heart
- aortic sac gives vessels to pharyngeal arches, 5 or 6 pairs of aortic arches
- aortic arches join dorsal aorta on each side
- aortic sac divides to form right and left dorsal aorta (will later fuse and become descending aorta)
Ductus arteriosus - becomes ligamentum arteriosum after birth, closure by bradykinin
Aortic arches and resultant arteries
- Maxillary artery, external carotid
- Disappears
- Common carotid, prox part of internal carotid
- RIGHT - prox part of right subclavian, LEFT - arch of aorta
- Disappears
- RIGHT - distal part of R subclavian, L PROX - pulmonary artery, L DIS - ductus arteriosus
** therefore recurrent laryngeal nerve path differs on each side - on left nerve hooks under ductus arteriosus, on right nerve hooks under R subclavian
Vitelline artery
Supplies yolk sac
Paired
Fuse to form coeliac artery, SMA, IMA
Umbilical arteries
Supply deoxygenated blood from fetus to placenta
Arise from common iliac artery
After birth - distal portion obliterates to form medical umbilical ligament, proximal portion persists as internal iliac and vesicular arteries
Longitudinal venous channels
Formed by week 5
Paired
- azygos veins
- supracardinal veins
- subcardinal veins
Become progressively asymmetrical so Rsided dominance by week 7, leading to:
- IVC - from supra and subcardinal veins
- hemiazygos vein - from left azygos
So left to right shunt
GI tract origins
Endoderm
- foregut
- midgut (continuous with remaining yolk sac, connected by vitello-intestinal duct)
- hindgut
- liver
- pancreas
Splanchnopleuric lateral plate mesoderm
- connective tissue
- smooth muscle
- serosal layer of gut tube
(intraperitoneal if not completely invested in LPM)
Mesenteries
Dorsal mesentery (foregut)
- formed by serosal membrane, suspends gut tube in body of embryo attached to post wall
- from lower oesophagus to cloaca
- with growth, lost to duodenum, ascending and descending colon
(everything not suspended by this is retroperitoneal)
Dorsal mesogastrium
- dorsal mesentery of stomach, becomes greater omentum
Ventral mesentery (foregut)
- from caudal part of foregut from septum transversum
- covers stomach, terminal oesophagus, initial part of duodenum
- liver develops within and divides into two - falciform ligament to connect to ventral wall and lesser omentum between stomach and liver
Transverse mesocolon is dorsal mesentery of transverse colon, fuses with anterior 2 layers of greater omentum so appears attached
All mesenteries double layered, so nerves and blood vessels lie within them
Foregut
Oesophagus, stomach, 1/2 duodenum, pancreas, liver
Coeliac artery supplies
Innervation from T5-9
Cranial portion forms the pharynx, assoc with pharyngeal arches
Caudal portion is the rest
OESOPHAGUS
- caudal
- gives rise to respiratory diverticulum
- lengthens with growth of thoracic organs
STOMACH
- 90deg rotation clockwise to the right, pulling dorsal mesentery over to left, this forms lesser sac
Pancreas
- dorsal wall
- develops from endodermal lining of duodenum
4 weeks, develop 2 pancreatic buds
- dorsal in dorsal mesentery, larger, main pancreatic duct
- ventral in ventral mesentery
As duodenum rotates right, ventral bud moves dorsally to lie superior, entrapping superior mesenteric vessels between
Both ducts fuse entering medial wall of duodenum - forms major duodenal papilla
Tail of pancreas within double layer of lienorenal ligament (between spleen and dorsal wall)
Duodenum
From foregut (1st half) and midgut, so dual arterial supply
- bile duct at junction between
- lumen initially hollow, then solidifies at 2months
- in posterior abdominal wall
- gives rise to pancreas in mesoduodenum
- posterior leaf of mesoduodenum regresses so attaching duodenum to dorsal wall, sealing off lower sac
Liver
Hepatic diverticulum
- arises on ventral wall of duodenum just above ventral pancreatic duct
- liver sinusoids lined by kupffer cells (bone marrow derived)
Gallbladder
- arises from ventral outpouching of hepatic diverticulum
- duct initially solid
Peritoneum covers all of liver except bare area in contact with diaphragm
Liver starts producing blood cells at 10weeks
Midgut
From midpoint of duodenum until 2/3 along transverse colon (so inc ileum, jejunum, ascending colon)
Superior mesentery artery
Midgut rapidly elongates but not enough room in peritoneal cavity, so herniates into umbilical cord at 6 weeks (rotating 90deg anticlockwise) then returns at 10 weeks (rotating 180deg anticlockwise)
- rotation means caecum moves from subhepatic to RIF
Innervation from T10-11
Abnormal midgut herniation
Omphalocoele
- indirect herniation
- via umbilicus
- covered in layer of amnion
- associated with eg Edward’s syndrome/structural chromosomal abnormalities
Gastroschisis
- direct herniation, due to abnormal involution of umbilical vein
- usually to right of cord insertion site
- via abdominal wall
- inside amniotic sac, so not covered
- no chromosomal association
Meckel’s diverticulum
Remnant of vitello-intestinal duct
Contains gastric and pancreatic mucosa
Can get inflamed and mistaken for appendicitis
2 inches long
2ft from ileocaecal valve
2% population have
Hindgut
Distal 1/3 transverse colon, descending colon, rectum, upper half of anal canal
Initially opens into primitive cloaca (communicating with allantois)
Also forms epithelial layer of bladder and urethra
Innervation from T12/L1
Urorectal septum
Band of mesenchymal tissue, dividing primitive cloaca into
- urogential sinus anterior
- rectum posterior
Will become perineal body when reaches cloacal membrane, which divides perineum into
- urogenital triangle
- anal triangle
(cloacal membrane breaks down at week 7)
Allantois
Non functional
Connects primitive cloaca to umbilicus
Becomes urachus, which will become mediaN umbilical ligament
Anal canal
Upper half - mucosa from endoderm (hindgut)
Lower half - mucosa from ectoderm
Pectinate line demarcates the two, at base of anal columns
- change in blood supply, nerve supply, epithelial lining
Anal canal lumen occluded at 7 weeks by ectoderm, then recannalised at 9 weeks
Primary vs secondary retroperitoneal organs
Primary
- abdominal aorta, IVC, oesophagus, kidneys, adrenals, ureters, bladder, lower rectum
Secondary
- 2nd half duodenum, ascending colon, descending colon, head and body of pancreas
Urinary system origins
Mostly from intermediate intra-embryonic mesoderm (nephrogenic cord underlying urogenital ridge)
- develops in trunk, craniocaudal
Pronephros - non-functional, temporary
Mesonephros - produces urine
- cavities appear, becoming tubules (Bowman’s capsules), join laterally to form mesonephric duct which drains into urogenital sinus, forms bladder
+ ductus deferens + efferent testes ductules in males, or Gardner’s ducts in females
Metanephros - produces urine, becomes the kidney
- ureteric bud grows from mesonephric duct becomes metanephric duct, forms ureter, renal pelvis, calyces, collecting ducts
- metanephric blastema is condensation of nephrogenic cord tissue around ureteric bud, forms nephrons - functional from 10weeks and continues to develop to 32weeks
Bladder arises from urogenital sinus after cloaca divides
Indifferent gonad
From intermediate mesoderm
On gonadal ridge (medial part of urogenital ridge)
Primordial germ cell migration
- from week 6, migrate from wall of yolk sac to genital ridge via dorsal mesentery of hindgut, induced formation of primitive sex cord
- primitive sex cord derived from mesonephros and overlying coelomic epithelium
Default is for gonad to develop into ovary
Testis
To change gonad from default ovary - sex-determining region of Y chromosome (SRY gene)
- produces testis-determining factor
- differentiation of gonad mesenchymal cells into interstitial Leydig cells (make testosterone from week 8)
- primitive sex cords penetrate medulla to make testicular cords, anastamose to make rete testis, then differentiate into Sertoli cells (make anti-Mullerian hormone from week 8) and become seminiferous tubules
Female genitalia formation
Absence of testis-determining factor (SRY gene)
- sex cords in medulla degenerate, form vascular medullary stroma
- surface epithelium to cortical cords
- at 4 months, primordial germ cells invest in secondary cords
-> ovary
In absence of testosterone (Leydig) and anti-Mullerian hormone (Sertoli)
-> female internal genitalia
Genital ducts
Mesonephric duct (aka Leydig’s)
- loses urinary function as mesonephros -> metanephros
- persists in males (to make ductus deferens, epididymis, seminal vesicles, prostatic utricle, trigone then opens into urogenital sinus), regresses in females
- differentiation due to testosterone production in week 8
Paramesonephric duct (aka Mullerian)
- persists in females, regresses in males due to anti-mullerian hormone in week 8
- lateral to mesonephric duct
- longitudinal invagination of coelomic epithelium overlying urogenital ridge
- cranial end opens to peritoneal cavity, becomes fimbriae of tubes
- caudal ends fuse in midline to bring together peritoneal folds and form broad ligament and uterovaginal canal
Formation of uterus and vagina
Uterovaginal canal inc uterus + first half vagina, from paramesonephric duct
- fuses with sinovaginal bulb (swelling on urogenital sinus)
Sinovaginal bulb gives rise to lower half of vagina, initially solid later cannalised
Testicular descent
- guided via gubernaculums towards labioscrotal swelling
- in females, gubernaculum -> ovarian and round ligaments
- processus vaginalis is outpouching of peritoneum into labioscrotal swelling, forms inguinal canal as it pushes through 3 layers of abdominal wall, then becomes tunica vaginalis
- Independent phase - testis reaches deep inguinal ring by 7months
- Hormone dependent phase - 7-9months
Determined by elongation of fetal trunk, increase in intra-abdominal pressure, regression of gubernaculum
External genitalia
Undifferentiated until week 9
Default to female
DHT (dihydrotestosterone) stimulates external genitalia virilisation in male
At week 5 - cloacal fold on either side of cloacal membrane, fuses anteriorly to give genital tubercle, forms labia minora or penile urethra
- genital swellings lateral to cloacal fold - forms labia majora or scrotum
- genital tubercle elongates to form clitoris or penis
Ambiguous genitalia at birth
1:4000
Either virilisation of genetically female, or under-masculinisation of genetically male:
- XY under-masculinisation - partial/complete androgen insensitisation, 5alpha-reductase deficiency (can’t testosterone -> DHT), gonadal dysgenesis
- XX virilisation - CAH, placental aromatase deficiency, in-utero exposure to maternal androgens
- ovotesticular disorder of sexual development - true hermaphroditism
Severity according to Prader stages 1-5 (1 is isolated clitoromegaly, 5 is looking like male)
Androgen insensitivity
Partial or complete
XY under-masculinisation
X-linked in 65%
- normal testicular production of androgens with ABNORMAL androgen receptors
- so incomplete virilisation of external genitalia
- testes usually in inguinal canal
Nervous system
From ectoderm, around 3 weeks
Day 18 - neural plate
- ectodermal thickening, widest at cranial end
Day 20 - neural folds
- lateral to neural plates on each side, form neural groove
Neural crest - beneath surface ectoderm and migrating laterally
- brain, nerves, cells eg melanocytes, odontoblasts, C cells, and other eg adrenal medulla, dermis of face, skull cap bones
Day 22 - neural tube
- cranially brain, caudally spinal cord
- neuropore at each end, anterior closes at day 24, posterior closes at day 26
- lined with neuroepithelium
Neural tube defects
Spina bifida occulta - unfused vertebral arch
Spina bifida cystica - neural tube and its coverings protude through vertebral arch, assoc with hydrocephalus
i) Meningocoele - neural tube in normal position, but cyst formed by protruding subarachnoid membrane
ii) Meningomyelocoele - neural tube lies ectopically in cystic space
Rachischisis - no neural tube, neural tissue is fused with skin
Spinal cord
3 zones - neuroepithelial (ventricular) layer, mantle zone (grey matter), marginal zone (white matter, axons entering and leaving mantle zone)
2 plates divided by sulcus limitans
- alar - dorsal plate, sensory region
- basal - ventral plate, motor region
Extends whole length of vertebral column until 3rd month, where it ends at L2
Neural tube development
3 dilatations at cranial end of neural tube
- prosencephalon - forebrain
– telencephalon - bilateral portions become cerebral hemispheres
– diencephalon - thymus + hypothalamus
- mesencephalon - midbrain
- rhombencephalon - hindbrain
– will become metencephalon - pons + cerebellum
– and myelencephalon - medulla
Tube lumen becomes ventricular system
Pituitary gland
From 2 sources
- downgrowth from floor of diencephalons for posterior pituitary (neurohypophysis)
- upgrowth from ectodermal oral cavity, Rathke’s pouch for anterior pituitary (adenohypophysis)
Pharyngeal arches
From week 4
Arise from neural crests
5 pairs - 1, 2, 3, 4, 6
(5th pair is rudimentary and disappears)
Each arch consists of:
- mesenchyme core, differentiating into cartilage, muscle, aortic arch artery
- inner endoderm, to become pharyngeal pouches
- outer ectoderm, to become pharyngeal clefts
(only first pair of clefts contributes to form external auditory meatus)
Each arch supplied by a cranial nerve
1st pharyngeal arch
Supplied by trigeminal nerve (V)
Muscles of mastication - mylohyoid, ant belly of digastric, tensor veli palatine, tensor tympanii
Bones/ cartilage - malleus, incus, sphenoid
2nd pharyngeal arch
Supplied by facial nerve (VII)
Muscles of facial expression - buccinator, stapedius, stylohyoid, post belly of digastric
Bones/cartilage - stapes, styloid process, lesser horn of hyoid bone
3rd pharyngeal arch
Supplied by glossopharyngeal nerve (IX)
Muscles - stylopharyngeus
Bones/cartilage - greater horn (lower part) of hyoid bone
4th pharyngeal arch
Supplied by vagus nerve (X)
Muscles - pharyngeal, cricothyroid
Bones/cartilage - thyroid cartilage, cricoid cartilage
6th pharyngeal arch
Supplied by vagus nerve (X)
Muscles of larynx, oesophagus
Bones/cartilage - arytenoid cartilage
Pharyngeal pouches
Derived from endoderm of foregut growing laterally as pockets either side of pharynx
1st - tubotympanic recess, middle ear, auditory tube, tubal tonsil
2nd - palantine tonsil
3rd - dorsal - inferior parathyroid glands, ventral - thymus gland
4th - dorsal - superior parathyroid glands, ventral - ultimobranchial body
5th - ultimobranchial body, parafollicular cells
Tongue
From epithelium of floor of pharynx
Muscles invade in 2nd month from occipital myotomes, bringing hypoglossal innervation
Anterior 2/3 - from 3 swellings from 1st pharyngeal arch (2 lateral lingual, + tuberculum impar)
- innervation from lingual branch of mandibular (trigeminal) and chorda tympani (facial)
Posterior 1/3 - from hypobranchial eminence, from 3rd and 4th pharyngeal arch
- innervation from vagus and glossopharyngeal
Face development
From neural crest as 5 swellings
- Frontonasal prominence - forehead, nose, philtrum, primary palate
- PAIR of maxillary prominences - cheek, maxilla, zygoma, lateral upper lip, secondary palate
- PAIR of mandibular prominences - cheek, lower lip, mandible
Nasal cavity
Nasal placodes invaginate to form nasal pit
Nasal pit -> nasal sac (=anterior nares)
Nasal sac grows upwards, separated from oral cavity by oronasal membrane (will break down week 7)
Nasal septum at week 9, grows towards palate
Eye development
Placode - thickened areas of ectoderm in head region, due to interaction between neural tube and overlying ectoderm
- become columnar and invaginate, then migrate deep to surface ectoderm
Optic vesicles by week 4, as approach the surface ectoderm they induce development of lens placode
Lens placode detaches from surface ectoderm and sinks into optic vesicle
Optic vesicles begin to fold and form two walled optic cup
Hyaloid artery is branch of ophthalmic artery, supplies lens and retina, runs along choroidal fissure
Eyelids are folds of ectoderm, fused initially then separate around month 5-7
Retina
Formed by 2 layers of optic cup
- outer layer - forms pigmented layer
- inner layer - forms rods and cones
At the rim of the optic cup, retina gives rise to ciliary body and iris
Ear development
Develops from 3 regions:
- external ear from 1st pharyngeal cleft
- middle ear from 1st pharyngeal pouch
- inner ear from otic placode
Otic placode is close to hindbrain, starts to develop at day 22 then invaginates as otic vesicle
Otic vesicle
- dorsal vestibular and ventral cochlear portions
