Embryology Overview

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)


Pharyngeal arches

Pharyngeal arches

Pharyngeal cartilages

Pharyngeal pouches (inside)

Pharyngeal clefts (outside)

1 Vc: Mastication MATT

Aliphenoid malleolus incus Meckel’s cartilage

Tympanic cavity Eustachian tube

External auditory meatus

2 VII: facial expression POSS

Stapes styloid process stylohyoid ligament lesser cornu of hyoid

Palatine tonsils

3 XI: stylopharyngeus

Greater cornu of hyoid body of hyoid

Inferior parathyroid thymus

4 Sup. laryngeal: cricothyroid Middle + inferior constrictor

Thyroid cartilage cricoid cartilage (could also be 6)

Superior parathyroid ultimo-branchial body (parafollicular calcitonin-producing cells)

Cervical sinus (obliterated)

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


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


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


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


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

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Embryology Overview