Embryology and Paediatric Anatomy

  • 1st trimester (1-12/40); 2nd (13-26/40), 3rd (27/40-birth)
  • 4/40 gestational sac visible
  • 5/40 yolk sac visible
  • 5-6/40 embryo and cardiac activity
  • 6-8/40 placenta
  • 8/40 head, body, limb buds
  • 12/40 lateral ventricles, ossification femur
  • 14/40 closure umbilical hernia; bladder, stomach, kidneys, penis
  • 15-16/40 small intestine
  • 18-20/40 major organs

Blastocyst Period

  • Öocyte + sperm = zygote
  • blastula (2-16cells)
  • morula (~16 cells)
  • blastocyst (fluid invades morula)
  • outer cell mass = trophoblast (outer layer; cytotrophoblast > chorionic membrane, syncytiotrophoblast > fetal placenta)
  • blastocyst cavity
  • inner cell mass = embryoblast (>embryo, amnion, cord, secondary yolk sac)
  • implantation (1st week, 3/40)
  • embryoblast
  • bilaminar embryonic disk (epiblast and hypoblast)
  • primary/primitive yolk sac (hypoblast covers cytotrophoblast)
  • amniotic cavity (adj to epiblast)
  • extraembryonic mesoderm (<epiblast; somatic > lines cytotrophoblast, visceral > lines yolk sac)
  • chorionic cavity (betw layers of extraembryonic mesoderm)
  • secondary yolk sac (shrunk primary sac)
  • gestational sac (5/40; chorionic cavity)

Embryonic Period

  • cloacal membrane
  • primitive streak
  • notochord
  • primitive streak (in epiblast
  • prochordal plate (>>mouth)
  • cardiogenic area
  • gastrulation (epiblast cells move through primive streak to between epiblast and hypoblast and migrate)
  • ectoderm (epiblast after gastrulation)
  • surface ectoderm (>lens, epidermis outer ear, skin, anterior 2/3 tongue, lower anus, distal penile urethra, parotid, mmamary glands, adenohypophysis)
  • neuroectoderm (>CNS neurons, astrocytes, oligodendrocytes, ependymocytes, choroid plexus, retina, pineal gland, neurohypophysis)
  • neural crest (>postganglionic neurons, sensory neurons, Schwann cells, pia and arachnoid, adrenal chromaffin cells, melanocytes, head bones, aorticopulmonary septum, ciliary m, carotid body)
  • mesoderm (cells between epiblast and hypoblast)
  • paraxial mesoderm (35 pairs of somites >axial mm, vertebrae, ribs, occipital bone, dermis, dura)
  • intermediate mesoderm (>kidneys, testes, ovaries, accessory sex glands/ducts)
  • lateral mesoderm (>limb mm, sternum, appendicular skeleton, GI walls, CVS, lymph, spleen, adrenal cortex)
  • endoderm (epiblast cells that displace hypoblast; > epithelium mid/inner ear, epithelium organs, thyroid/parathyroid, liver/biliary tract, pancreass)

Placenta and Umbilical Cord

  • chorion (extraembryonic somatic mesoderm, cytotrophoblast, synctiotrophoblast)
  • decidua capsularis (materal decidua covering blastocyst)
  • chorion laeve >>chorionic membrane (chorion devoid of villi)
  • chorion frondosum = villous chorion (>fetal placenta)
  • primary chorionic villi (cytotrophoblast)
  • secondary chorionic villi (extraembryonic mesoderm evaginates in)
  • tertiary chorionic villi (umbilical blood vessels in mesoderm)
  • intervillous space
  • placental membrane (synctiotrophoblast, ± cytotrophoblast, fetal capillary endothelium)
  • decidua basalis (>materanal placenta)
  • monozygotic/identical twins (65% monochorionic diamniotic; 35% 2 placentas, dichorionic diamniotic)
  • dizygotic/fraternal twins
  • decidua capsularis fuses with decidua parietalis (uterine lining, obliterating endometrial cavity) at 12/40, capsularis degenerates and chorion fuses with parietalis
  • cotyledons (20-40 functional units)
  • lobes separated by septae (maternal surface)
  • amniotic fluid (from dialysis maternal>fetal blood, fetal urine, removed by fetal swallowing, placenta)
  • umbilical cord (umbilical aa, umbilical v)
  • umbical vein > cord insertion > left portal v > ductus venosus
  • ductus venosis (>IVC/L hepatic v or R portal v; muscular >> ligamentum venosum)
  • internal iliac aa > cord insertion > umbilical aa (2)

Central Nervous System

  • ectoderm overlying notochord -> neuroectoderm -> neural plate -> folds -> neural tube
  • notochord -> nucleus pulposus
  • neural crest (from neural plate folding -> pia, arachnoid, autonomic nervous system
  • neuropores (connect neural tube to amniotic cavity; anterior [->lamina terminalis] and posterior)
  • neural tube fusion starts junction brain/cord, extends both directions, anterior neuropore fuses before post
  • neural tube wall -> neuroblasts (CNS neurons), glioblasts (astrocytes [surround capillaries], oligodendrocytes [make myelin], ependymocytes [line ventricles], tanycytes [line 3rd ventricle, transport CSF->hypothalamic portal system], choroid plexus cells [make CSF], microglia [CNS phagocytes])
  • sulcus limitans (grooves in lateral wall, separate alar/sensory [->dorsal horn] and basal/motor [->ventral horn] plates)
  • primary (3) and secondary (5) vesicles (of cranial neural tube)
  • prosencephalon/forebrain (telencephalon [hemispheres, basal ganglia, lamina terminalis, olfactory bulbs, hippocampus], diencephalon [thalamus, hypothalamus, neurohypophysis, pineal gland, retina, optic n, mamillary bodies])
  • mesencephalon/midbrain
  • rhombencephalon/hindbrain (metencephalon [pons, cerebellum], myelencephalon [medulla])
  • telencephalic, mesencephalic, pontine and cervical flexures (of the neural tube)
  • sulci and gyri form at 20/40
  • Rathke’s pouch -> adenyhypophysis (evagination of ectoderm roof of mouth)

Head and Neck

  • pharangeal apparatus = pharyngeal arches, pouches, grooves, membranes
  • pharyngeal arches (mesoderm and neural crest cells)
    • arch 1 = maxillomandibular arch (CNV)> mm of mastication, maxilla, zygoma, temporal bone, palatine, vomer, mabdible, incus, malleus
    • arch 2 = hyoid arch (CNVII) > mm fascial expression, hyoid, stapes, styloid process
    • arch 3 (CNIX) > stylopharyngeus, hyoid
    • arch 4 (CNX superior laryngeal) > mm of soft palate, pharynx
    • arch 6 (CNX rec. laryngeal) > mm of larynx, upper oesophagus, laryngeal cartilage
  • pharyngeal pouches (evaginations of foregut)
    • pouch 1 > auditory tube, middle ear
    • pouch 2 > palatine tonsil crypts
    • pouch 3 > inferior parathyroids, thymus
    • pouch 4 > superior parathyroids, ultimobranchila body (thyroid)
  • pharyngeal/branchial grooves/clefts (invanations surface ectoderm; 1> external auditory meatus)
  • pharyngeal membranes (junction of grooves and pouches; 1 > tympanic membrane)
  • foramen cecum > thyroid diverticulum > thyroglossal duct > thyroid
  • median and 2 distal tongue buds (arch 1) > anterior 2/3 of tongue
  • hypobranchial eminence (arch 3&4) > posterior 1/3 of tongue
  • frontonasal prominence > forehead, nose, medial upper lip
  • globular prominence (maxillary part of frontonasal prominence)
  • intermaxillary segment (fusion of medial nasal promences) > philtrum of lip, 4 incisors, primary palate
  • maxillary prominences > cheeks, palatine shelves (>secondary palate)
  • mandibular prominence > mandible
  • palate < fusion of 1° and 2° palates at incisive foramen

Cadiovascular System

Heart

  • primitive heart tube (mesoderm; 5 dilatations)
  • truncus arteriosus (>aorta, pulmonary trunk)
  • bulbus cordis (>smooth RV/LV)
  • primitive ventricle (>trabeculated RV/LV)
  • primitive atrium (>trabeculated RA/LA)
  • sinus venosus (>smooth RA, coronary sinus)
  • smooth LA < pulmonary vv
  • aorticopulmonary/AP/spiral septum (<neural crest cells grow in truncal and bulbar ridges in spiral fashion)
  • atrioventricular/AV septum (ventral and dorsal AV/endocardial cusions fuse>R&L AV canals)
  • septum primum (towards AV septum
  • foramen primum (edge septum primum, later fuses with AV septum)
  • foramen secundum (in centre of septum primum)
  • septum secundum (to right of primum)
  • foramen ovale (in septum secundum)
  • interventricular septum (towards AV septum)
  • temporary interventricular foramen
  • membranous IV septum (fusion of AP&AV septi)

Major Arteries

  • branchial/pharyngeal arches (6 pairs) = ‘gills’
  • dorsal aortae (2; distal fusion of arches; R degenerates)
  • 3rd arch > carotid arteries
  • R4th arch > brachiocephalic artery, right subcalvian, proximal aortic arch
  • L4th arch > aortic arch, left subclavian, descending aorta
  • 6th arch > pulmonary arteries, ductus arteriosus

Major Veins

  • cardinal veins (ant<cephalic body; posterior <rest of body)
  • ant&post cardinal veins > common cardinal vv > sinus horns>sinus venosus > atrium
  • ant cardinal vv > L brachiocephalic v
  • right common cardinal v, proximal R anterior cardinal v > SVC
  • subcardinal vv (kidneys), anastamoses between = L renal v
  • L subcardinal v disappears, distal > L gonadal v; R subcardinal v > renal segment IVC
  • sacrocardinal vv (<lower extremities); anastamosis between > L CIV; R sacrocardinal v > sacrocardinal segment of IVC
  • supracardinal vv (<body wall)
  • IVC (hepatic [R vitelline v], renal and sacrocardinal segments)
  • R subcardinal v, posterior cardina l v > azygos v
  • L supracardinal v > hemiazygos v
  • right vitelline v > hepatic vv and sinusoids, ductus venosus, IVC, portal, SMV/IMV/splenic vv
  • left vitelline v > hematic vv and sinusoids, ductus venosus
  • umbilical vv (R degenerates, L > ligamentum teres)

Respiratory System

  • respiratory/tracheobronchial /laryngotracheal groove and diverticulum (<ventral wall forgut)
  • tracheoesophageal septum (divides the two)
  • primary bronchial buds > primary, secondary and tertial bronchi

Breast

  • tubule-acinar type of modified apocrine sweat gland
  • milk line (base forelimb to hindlimb); thoracic section > mammary ridge
  • epithelial cords > laciferous ducts (15-20) >ductules, acinar ducts, simple acini

Gastrointestinal Tract

  • primary gut tube (incorporation of yolk sac)
  • endoderm > epithelial mucosa; mesoderm > wall
  • foregut closed by buccopharyngeal membrane
  • hindgut closed by cloacal membrane
  • proliferation of endoderm occluding lumen an 6/40 with subsequent recanalisation at 9/40
  • meconium (desquamated cells, bile pigment, mucoproteins in distal illeum)
  • septum transversum > diaphragm, ventral mesentery (attaching to stomach)
  • ventral mesentery > lesser omentum, falciform lig, coronary lig, triangular lig
  • dorsal mesentery > greater omentum, mesentery of SB, appx, transverse colon, sigmoid

Foregut

  • stomodaeum (ant ectodermal part of gut)
  • stomach rotates 90°; dorsal mesogastrastrium (dorsal mesentery attached to stomach) > lesser sac
  • spleen <mesenchyme on dorsal mesogastrium
  • liver, GB < hepatic diverticulum from duodenum
  • large dorsal pancreatic bud > head, body and tail, accessory duct
  • small ventral pancreatic bud (from hepatic diverticulum) > uncinate process, main duct

Midgut (distal to ampulla of vater > splenic flexure)

  • primary intestinal loop (cranial limb > ileum, caudal > colon)
  • vitelline duct > umbilicus
  • herniation into umbilical cord at 6/40, with 90° counter-clockwise rotation around vessels axis
  • caecal bud
  • retraction of gut at 10/40, with 180° counter-clockwise rotation and cephalic limb first lying posterior to vessels
  • zygosis = mesenteries of ascending and descending colon blending with posterior peritoneal wall

Hindgut

  • cloaca = anterior urogenital sinus and posterior rectum
  • superior anal canal < distal hindgut
  • inferior anal canal < proctodeum = ectodermal invagination
  • pectinate line < anal membrane separating endodermal and ectodermal portions

Genitourinary System

  • nephrogenic cord (part of urogenital ridge forming pronephros, mesonephros, metanephros)
  • pronephros (regresses)
  • mesophephros > mesonephric/wolffian duct > duct of Gartner, epoöphoron, epididymis, vas deferens, ejaculatory duct, trigone of bladder
  • swelling at termination of mesonephric duct > seminal vesicles
  • metanephric duct/ureteric bud (diverticulum of mesonephric duct > ureter, pelvis, calyces, collecting tubules)
  • metanephros/metanephric mesoderm (cap on duct) > glomeruli, proximal duct system
  • paramesonephric ducts > uterine tubes, uterus, cervix, upper vagina, appx of testes
  • cloaca
  • urorectal septum
  • urogenital sinus (upper > bladder; lower > urethra, vestibule of vagina)
  • allantois (continuation of urogenital sinus; diverticulum of hindgut > umbilicus > urachus)
  • phallus > clitoris, penis
  • urogenital folds > labia minora, ventral penis, penile raphe
  • labioscrotal swellings > labia majora, scrotum, scrotal raphe
  • migration of kidney (and changing blood supply)
  • suprarenal gland (1st mesoderm proliferation > fetal cortex; 2nd > adult cortex; neural crest cells > medulla, chromaffin cells)
  • renal capsule visible at 19/40, fetal lobulation at 24/40
  • renal pelvis (</= 5mm in AP diameter)
  • gubernaculum (fibrous band > ovarian lig, round lig, gubernaculum testes)
  • processus vaginalis (evagination peritoneum >tubica vaginalis)

Coelom/Body Cavities

  • pericardial space
  • pleuropericardial membrane > fibrous pericardium
  • pleural cavity
  • septum transversum > central tendon of diaphragm
  • pleuroperitoneal membrane > diaphragm
  • dorsal mesentery of oesophagus > crura
  • body wall > periphery of diaphragm
  • peritoneal cavity

Paediatric Anatomy

CNS

  • myelination in stepwise progression (hence white matter age)
  • sulci and gyri increase in number with age
  • primary sulci (30/40, single line), secondary (35-40/40, V-shaped terminations) and tertiary (further side-branching)
  • ventricles decrease in size and assymetry with age
  • caudothalamic groove (betw caudate and thalamus)
  • germinal matrices (highly cellular and vascularised area of brain for development; lining floor lateral ventricles ± grooves of 3rd and 4th ventricles; prone to haemorrhage; involutes early gestation)

Chest

  • fat distribution different (paucity in mediastinum and retroperitoneum
  • cardiac and respiratory rates higher
  • airways narrower and more pliable
  • large thymus quadrilateral > triangular in shape
  • lung fissures not seen

Abdomen

  • liver higher proportional size (>left lateral abdo wall anterior to spleen)
  • gastro-oesophageal and vesicoureteric reflux virtually physiological
  • swallowing <18/12 3-5 boluses held in pharynx before swallow triggered
  • stomach and small bowel may disted with air, milk curd appears solid
  • flow in ductus venosus (1-2 x 11-19mm) at 18/7 in 11%
  • portal vein (8.5mm <10y, 10mm 10-20y)
  • CBD (2mm infants, 4mm children, 7mm adolescents)
  • suprarenal gland in neonate 30% of kidney (80% cortex which undergoes haemorrhagic necrosis, losing 30% weigth in 2-3/52)
  • cortex echogenicity increased (similar to liver/spleen; glomeruli occupy 20% of cortex cf 9% in adult)
  • medullary pyramids larger and hypoechoic
  • renal sinuses contain less fat
  • persistent primitive tissue in kidney
  • persistent fetal lobulations
  • septa at ant-sup or post-inf amrgins from fusion of metanephric elements (<6/12)
  • minor upper tract dilatation esp when bladder full

Skeletal

  • epiphysis
  • physis/growth plate
  • metaphysis
  • diametaphysis
  • diaphysis
  • apophysis (accessory ossification area not contributing to bone length)
  • primary ossification (shaft)
  • secondary (epiphyseal/apophyseal)
  • ossification (girls faster than boys)
  • red marrow converts to yellow marrow 1st in appendicular skeleton epiphyses then diaphyses with some preservation of red marrow in metaphysis hymerus and glenoid