This is a very general overview of embryology for 1st year medical school anatomy (in Cambridge) describing what becomes what.

1. Embryology overview C. Riedinger 1
Overview of what becomes what.
 Epiblast  Ectoderm
 Mesoderm
Embryoblast
 Hypoblast  Endoderm
Ectoderm
- Overall : epidermis, nervous system
- neural crest cells  glial and schwann cells
 melanocytes
 parts of meninges
 parts of teeth
 dorsal root, cranial, enteric, autonomic ganglia
 connective tissue of face
 bones of skull
 adrenal medulla
- neural tube  brain + spinal cord
Mesoderm
- overall: skeletal, muscle, connective
- axial  notochord  nucleus pulposus of vertebrae
- paraxial  somites  myotome, scleratome, dermatome
- intermediate  urogenital
- lateral plate (continuous with extra-embryonic mesoderm of
amniotic cavity  parietal/somatic layer
and yolk sac  visceral/splanchnic layer)
 blood, peritoneum
 intraembryonic coelum  pericardial, pleural and
peritoneal cavities
Endoderm
- overall:  gut tube, lining of GI tract and respiratory system (epithelium)
- pharyngeal arches
- thyroid gland (from foramen caecum)
- lung (from respiratory diverticulum in foregut)
- liver (hepatic diverticulum in duodenum)
- pancreas
- foregut: bucopharyngeal membrane to midgut
- midgut: foregut to hindgut, initially connected to yolk sac
- hindgut: midgut to cloacal membrane, connecting stalk
(later to urogenital openings, allantois= “überhindgut”)
- parenchymal cells of liver, pancreas, parathyroid
Yolk sack  vitelline duct  meckel’s diverticulum (if not obliterated)

2. Embryology overview C. Riedinger 2
Pharyngeal arches
Pharyngeal Pharyngeal Pharyngeal Pharyngeal
arches cartilages pouches (inside) clefts (outside)
1 Vc: Aliphenoid Tympanic cavity External
Mastication malleolus incus Eustachian tube auditory
MATT Meckel’s meatus
cartilage
2 VII: Stapes Palatine tonsils
facial expression styloid process
POSS stylohyoid
ligament Cervical sinus
lesser cornu of (obliterated)
hyoid
3 XI: Greater cornu of Inferior parathyroid
stylopharyngeus hyoid thymus
body of hyoid
4 Sup. laryngeal: Thyroid cartilage Superior
cricothyroid cricoid cartilage parathyroid
Middle + inferior (could also be 6) ultimo-branchial
constrictor body (parafollicular
calcitonin-
producing cells)
6 Recurrent
laryngeal:
larynx
Cross-talk between epithelium (endoderm) and mesenchyme
(mesoderm)
- mesenchyme = undifferentiated loose connective tissue derived mainly from
mesoderm (some from neural crest cells though which are ectodermal)
- parenchyma = functional parts of an organ in the body
- stroma = structural tissue (connective, supportive)
- differentiation of endodermal epithelium dictated by signals from
mesoderm (mesenchyme)
- stomach: gastric glands
- intestines: villi
- liver: hepatic cords
- pancreas:
- lung: branching morphogenesis
branching (in bronchi) or inhibition of branching (trachea)
- kidney: dichotomous branching
ureteric bud induces mesoderm to become metanephric blastema
/mesenchyme, which in turn induces further buds

3. Embryology overview C. Riedinger 3
Endodermal derivatives
Lung
- lung diverticulum (from gut endoderm) grows into splanchnopleuric/visceral
mesoderm
- branching morphogenesis: guided by FGF10, antagonist: sonic hedgehog
- stages of lung growth: embryonic, pseudoglandular, canalicular, sacuular,
alveolar
Stomach
- thickening of foregut tube (differential growth)
more on left  greater curvature
less on right  lesser curvature
- 90* clockwise rotation so that:
left vagus  ant
right  post
ventral mesentery  right  lesser omentum
dorsal mesentery  left  greater omentum
- pylorus rises, this makes duodenum C-shaped
- duodenum is half foregut half midgut
Liver
- diverticulum from duodenal endoderm
- pushes into septum transversum  ventral mesentery
- gall bladder = ventral outpouching
-
Pancreas
- outgrowth of hepatic diverticulum
- dorsal bud  accessory pancreatic duct / minor papilla
- ventral bud  uncinate process, manjor papilla along with bile
Small intestine
- rapid enlongation of midgut causes physiological umbilical hernia
- 1* rotation, then another 90*, another 180*, all anticlockwise
Bladder
- at cloacal membrane (no mesoderm) urogenital septum grows in to divide
hindgut from allantois
- urogenital septum  perineum (?)
- widening of gut on allantoic side = urogenital sinus  bladder, urethra
male: only prostatic and membranous urethra
female: entire urethra
- allantois  urachus  median umbilical ligament

4. Embryology overview C. Riedinger 4
Mesodermal derivatives
Development of heart
♥ from angiogenic cell clusters in extra-embryonic mesoderm
♥ Two heart tubes form single tube during folding
♥ Truncus arteriosus  spiral septum  aorta + pulmonary trunk
♥ Bulboventricular groove
♥ Bulbis cordis  conus cordis  RV / infundibulum
♥ Ventricle  LV (trabeculated part)
♥ Atrioventricular groove  atrioventricular valves
 endocard. cushions sept. intermedium
(septum intermedium between right and left AV canal)
spiral septum eventually fuses with septum intermedium and muscular
ventricular septum
♥ Atrium  auricles
♥ Sinus venosus (right sinus horn)  RA
♥ Left sinus horn  coronary sinus
♥ Septum primum has osteum primum (which closes) and then osteum
secundum, septum
♥ Right/left directionality determined by nodal gene
Fetal circulation:
♥ 3 shunts:
o ductus venosus: closure within 5 days
o foramen ovale
o ductus arteriosus: closure within 10 days
♥ changes at birth:
o lungs inflate, blood enters them and returns to the LA
o p in LA > p in RA
o foramen ovale shuts
o prostaglandin levels decrease as no more flow from umbilical vein
♥ umbilical vein  ligamentum teres
♥ ductus venosus  ligamentum venosum
♥ foramen ovale  fossa ovalis
♥ ductus arteriosus  ligamentum arteriosum (left recurrent laryngeal
winds around it)
Blood vessels:
- vasculogenesis: differentiation from within a cell mass
- angiogenesis: invasion of tissue from existing blood vessels

5. Embryology overview C. Riedinger 5
Septum transversum
- thickened sheet of mesoderm between cardiogenic area and cranial margin of
disc, later caudal and anterior to gut tube
- septum transversum  central tendon of diaphragm
- septum transversum also makes VENTRAL MESENTERY for caudal portion
of foregut: liver, stomach, spleen
- complete diaphragm develops from:
o septum transversum
o somatic mesoderm from body wall
o mesentery of oesophagus
o pleuroperitoneal membrane
o myoblasts from cervical somites
Kidney
- from intra-embryonic intermediate mesoderm
- nephric part or urogenital ridge
- pronephros  regresses early, non-functional
- mesonephros  functional, regresses
- metanephros  definite kidney
- duct from pronephros through mesonephros to urogenital sinus = mesonephric
duct (Wolffian duct)
- mesonephric duct outpouching/metanephric diverticulum
 ureteric bud  metanephros

6. Embryology overview C. Riedinger 6
Urogenital system
- same origin as kidney, from from intra-embryonic intermediate mesoderm
- gonadal part of urogenital ridge
- migrating primordial germ cells enter and induce sex-specific differentiation =
end of indifferent stage (germ cells originate from epiblast?)
germ cells  spermatogonia / oocytes
- SRY (XY gene product), SOX9 crucial for development of testes
male:
- mesonephric duct  vas deferens
 epididymis
 seminal vesicle
- paramesonephric duct  regresses to prostratic utricle, appendix of
the testes, ejaculatory duct
- mesonephric mesenchyme  Leydig cells (make androgens!)
- making testosterone requires 5-alpha reductase
- sex cords  sertoli cells (Muellerian inhibitory substance
to suppress formation of femal genitalia!) +
seminiferous tubules (spermatogenesis)
- gubernaculum guides descent of testes  gubernaculum
 scrotal ligament
- genital tubercle / urogenital folds  penis
 corpora cavernosa
 corpus spongiosum
- labioscrotal swellings /folds  scrotum
female:
- mesonephric duct  regresses to Gartner’s cyst in wall of vagina
- paramesonephric duct  fallopian tubes
 uterus
 top of vagina
(inf end of vagina develops from urogenital sinus (sinovaginal bulb))
- mesonephric mesenchyme  thecal cells (make corpuls luteum to make
progesterone but also androgens)
- sex cords break up and condense around germ cells  primary follicles
- gubernaculum  round ligament of ovary and uterus
- genital tubercle  clitoris
 corpus cavernosa
 bulbospongiosum
- urethral folds  labia minora
- labioscrotal swellings  labia majora