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Embryology Overview

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

Embryology Overview

  1. 1. Embryology overview C. Riedinger 1 Overview of what becomes what.  Epiblast  Ectoderm  MesodermEmbryoblast  Hypoblast  EndodermEctoderm - 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 cordMesoderm - 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 cavitiesEndoderm - 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, parathyroidYolk sack  vitelline duct  meckel’s diverticulum (if not obliterated)
  2. 2. Embryology overview C. Riedinger 2Pharyngeal 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: larynxCross-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. 3. Embryology overview C. Riedinger 3Endodermal derivativesLung - 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, alveolarStomach - 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 midgutLiver - 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 bileSmall intestine - rapid enlongation of midgut causes physiological umbilical hernia - 1* rotation, then another 90*, another 180*, all anticlockwiseBladder - 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. 4. Embryology overview C. Riedinger 4Mesodermal derivativesDevelopment 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 geneFetal 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. 5. Embryology overview C. Riedinger 5Septum 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 somitesKidney - 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. 6. Embryology overview C. Riedinger 6Urogenital 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 testesmale: - 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  scrotumfemale: - 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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