Eric Holland OMS-I · ex-software engineer

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Embryology: how the pattern gets made

The five ANI development lectures in one place — weeks 1–4, the neural tube and spinal cord, the vertebral column, the limbs, the lungs and diaphragm, and the heart — told as stories first, then drilled as what becomes what, when, and what goes wrong. Embryology is anatomy's pattern: learn it and you understand instead of memorise.

Weeks 1–4: fertilisation → gastrulation → folding · ANI 1.2 Introduction to Embryology (Klein)

Weeks 1–3 in one breath

  1. Week 1 — fertilisation in the ampulla; cleavage to a morula inside the zona; blastocyst with trophoblast (outside) and embryoblast (inside); zona shed; implantation on the upper posterior uterine wall begins day 6–7.
  2. Week 2 — 'the week of twos': trophoblast → syncytio- + cytotrophoblast; embryoblast → epiblast + hypoblast — a BILAMINAR disc; two cavities (amniotic, yolk sac); extraembryonic mesoderm appears OUTSIDE the disc and is split by the chorionic cavity into a somatic layer (lining the trophoblast and amnion) and a splanchnic layer (wrapping the yolk sac); syncytiotrophoblast makes β-hCG and opens lacunae → primitive uteroplacental circulation.
  3. Week 3 — 'the week of threes': the primitive streak appears caudally; epiblast cells migrate through it to become definitive endoderm and then INTRAembryonic mesoderm, and the epiblast left behind is ectoderm — a TRILAMINAR disc; the node sends the notochord cranially; the notochord induces the neural plate; three germ layers, three body axes.
  4. Don't let the two mesoderms blur: week 2 gives extraembryonic mesoderm, which is membrane tissue only (chorion, amnion, yolk sac, connecting stalk) and never becomes part of the embryo. The embryo proper has no mesoderm until gastrulation in week 3 — that one is intraembryonic, and it is what becomes somites, heart, kidneys and gonads. Bilaminar + extraembryonic in week 2; trilaminar + intraembryonic in week 3.
  5. Week 4 — the flat disc folds: lateral folds close the ventral body wall, cephalocaudal folding drops the heart into the thorax and puts the septum transversum where the diaphragm will be; the neuropores close (25, 27); limb buds and the lung bud appear.

Take home: Weeks 3–8 are organogenesis — the teratogen window. The fetal period is growth.

Signals here:FGF8NodalHNF-3βGoes wrong as:Ectopic pregnancyHydatidiform mole (complete)Sacrococcygeal teratomaAmniotic band syndrome

Neurulation, CNS & spinal cord · ANI 1.0.1 Development of the CNS and Axial Skeleton (Hinojosa)

Neurulation and the spinal cord

  1. Notochord + paraxial mesoderm induce the ectoderm above them → neural plate → groove → folds, which zip shut from the cervical region outward; rostral neuropore closes day 25, caudal day 27.
  2. Cells that pinch off at the fusing crest are neural crest — they go everywhere (DRG, autonomic ganglia, Schwann cells, meninges, conotruncal septum).
  3. The tube wall is a pseudostratified neuroepithelium. Its cells divide at the lumen (ventricular zone), move out to the intermediate zone as neuroblasts (→ grey matter), and their processes fill the marginal zone (→ white matter). Neurons first, then glia (astrocytes inside, oligodendrocytes outside), then ependyma; microglia come in later with the blood vessels.
  4. The sulcus limitans splits each wall into an alar plate (dorsal, sensory; TGF-β/BMP from the roof) and a basal plate (ventral, motor; SHH from notochord and floor plate). The lateral horn (sympathetic, T1–L2/3) is basal plate too.
  5. At week 8 the cord fills the canal. The column then outgrows it: conus at L2–L3 in a neonate, L1–L2 in an adult — which is why a lumbar puncture below L2 misses the cord.
  6. Motor roots myelinate before sensory; oligodendrocytes wrap ~50 CNS axons each, Schwann cells one axon each.

Take home: Neural tube defects are failures of the neuropores in week 4: rostral → anencephaly, caudal → spina bifida (occulta → meningocele → meningomyelocele).

Signals here:Sonic hedgehog (SHH)TGF-β / BMPGoes wrong as:AnencephalyCraniorachischisisSpina bifida occultaSpina bifida with meningoceleSpina bifida with meningomyelocele

Vertebral column, meninges & back muscles · ANI 1.0.1 Development of the CNS and Axial Skeleton (Hinojosa)

Building a vertebra

  1. Paraxial mesoderm segments into somites at the end of week 3. Each somite has a ventromedial sclerotome (vertebrae, ribs) and a dorsolateral dermomyotome (dermis + myotome → muscle).
  2. Sclerotome cells surround the notochord, the neural tube and spread into the body wall. Each sclerotome has a loose cranial half and a dense caudal half; the dense caudal half of one joins the loose cranial half of the next → one vertebral body. This resegmentation makes the vertebra intersegmental, so the segmental myotome muscles and spinal nerves span two vertebrae.
  3. The notochord degenerates inside the bodies but swells between them → nucleus pulposus; the anulus fibrosus comes from surrounding mesenchyme. Neural arch from the mesenchyme around the tube.
  4. Cartilage in week 6 (two chondrification centres per centrum, more in the arch). Bone: three primary centres by week 8 (centrum + each half-arch); at birth three bony pieces joined by cartilage; arch halves fuse at 3–5 years (lumbar first), arch to centrum at 3–6 years (neurocentral joints); five secondary centres at puberty (spinous tip, two transverse tips, two annular epiphyses); done by 25.
  5. Myotomes split at week 5 into epaxial (→ intrinsic back extensors, dorsal ramus) and hypaxial (→ body wall and limbs, ventral ramus); the nerve splits to match at week 7.

Take home: Klippel–Feil = failed cervical segmentation. Chordoma = leftover notochord.

Signals here:Goes wrong as:Klippel–Feil syndromeChordoma

Limbs · ANI 2.2 Development of the Limbs (Veazey)

How a limb is built and why the flexors face forward

  1. Late week 4: limb buds bulge from the lateral body wall — a core of somatic lateral-plate mesoderm (bones, connective tissue) under ectoderm; hindlimb 1–2 days behind forelimb.
  2. Three signalling centres pattern three axes: AER (ectodermal ridge at the tip, induced by BMPs, secreting FGFs) → proximal-distal outgrowth; ZPA (posterior mesoderm, SHH) → anterior-posterior, i.e. thumb vs pinky; dorsal ectoderm Wnts → dorsal-ventral, knuckles vs palm. HOX genes decide where along the body a limb forms.
  3. Week 6: hand and foot plates; constrictions make wrist/ankle then elbow/knee; apoptosis carves the digits by week 8. Hyaline cartilage models appear (week 6), joints where chondrogenesis is arrested, ossification endochondral — diaphysis first, epiphyses still cartilage at birth.
  4. Muscles come from hypaxial myotomes (C5–T1 upper, L4–S3 lower), split into flexor and extensor masses then into individual muscles — so most muscles are multi-segmental. Ventral rami follow them in; motor axons in week 5, sensory following to their dermatome.
  5. Vessels: one intersegmental branch of the dorsal aorta per bud becomes the axial artery (upper limb: brachial → anterior interosseous → deep palmar arch; lower: internal iliac + bits of popliteal/fibular). Everything else sprouts by angiogenesis.
  6. Week 7 rotation: upper limb rotates LATERALLY (thumb lateral, palm anterior, flexors anterior); lower limb rotates MEDIALLY (big toe medial, flexors posterior). That is why dermatomes spiral down the limbs.

Take home: Limb defects: 1–4/1000, upper 3× lower, thalidomide the historic teratogen. '-melia' = the whole limb; '-dactyly' = digits.

Signals here:HOX genesFGFsBMPsSHHWntsVEGFFGFR3Goes wrong as:AchondroplasiaRadial polydactyly (ZPA duplication)PolydactylySyndactylyEctrodactyly (cleft hand/foot)BrachydactylyTransverse limb deficiencyAmelia / meromelia / phocomelia / micromeliaDevelopmental dysplasia of the hipClubfoot (talipes equinovarus)ArthrogryposisHolt–Oram syndromeOsteogenesis imperfectaMarfan syndrome

Lungs & diaphragm · ANI 5.4 Development of the Lungs and Respiratory Diaphragm (Klein)

Partitioning the trunk and growing a lung

  1. THE PROBLEM. Week 3: lateral plate mesoderm splits into a somatic (parietal) layer stuck to the ectoderm and a splanchnic (visceral) layer stuck to the endoderm. The space between them is the intraembryonic coelom — and it is ONE continuous horseshoe-shaped cavity running from thorax to pelvis. It has to end up as four: two pleural, one pericardial, one peritoneal. Everything that follows is partitions. That same split is also why parietal pleura is somatic and hurts sharply, while visceral pleura is not pain-sensitive at all.
  2. PARTITION 1 — the incomplete floor. The septum transversum is a plate of visceral mesoderm that first forms opposite somites C3–C5, up in the future neck. Cephalocaudal folding swings it down between heart and liver, and it drags its nerve supply with it — that is the whole reason the phrenic nerve is C3, C4, C5 and has the longest intrathoracic course of any nerve you'll dissect. But it does NOT reach the dorsal body wall. The gaps it leaves either side of the foregut are the PERICARDIOPERITONEAL CANALS, and they are the future pleural cavities.
  3. PARTITION 2 — PLEUROPERICARDIAL folds: pleural from pericardial. These are sheets of body-wall mesoderm on the lateral walls, and they carry the phrenic nerves and the common cardinal veins inside them. As the lungs expand they are stretched thin into pleuropericardial MEMBRANES, which swing medially and fuse with the mesenchyme at the root of the lung. Two consequences worth more than the fact itself: the membranes become the FIBROUS PERICARDIUM, and because the phrenic nerve was trapped in the fold, it runs on the fibrous pericardium ANTERIOR to the lung root in the adult — while the vagus passes POSTERIOR to it.
  4. PARTITION 3 — PLEUROPERITONEAL folds: pleural from peritoneal. Crescents appear on the DORSOLATERAL body wall and grow into the open pericardioperitoneal canals. As the lungs push in, the folds are stretched into pleuroperitoneal MEMBRANES, which by WEEK 7 have fused with each other, with the dorsal mesentery of the oesophagus, and with the dorsal edge of the septum transversum. Thorax and abdomen are finally separate. The right canal closes earlier — and the liver plugs it — so when this fails it fails on the LEFT.
  5. THE DIAPHRAGM, FOUR SOURCES (the most reliably tested list in the lecture): septum transversum → CENTRAL TENDON. Pleuroperitoneal membranes → the dorsolateral diaphragm and the connective-tissue SCAFFOLD. Paraxial mesoderm of the C3–C5 myotomes, arriving through the body wall → the MUSCLE. Dorsal mesentery of the oesophagus → the CRURA. Don't swap the central tendon and the crura. Note the order: muscle comes LAST and from the PERIPHERY, onto a tendon that already exists — which is exactly why a congenital hole is posterolateral and not central.
  6. INNERVATION follows the same logic. Motor to the entire diaphragm, crura included, is the phrenic nerve (C3–5), because the myoblasts came from cervical myotomes and brought their nerve down with them. Sensory splits: phrenic centrally (central tendon and the pleura/peritoneum next to it), intercostals T5–T11 and subcostal T12 at the rim. So central irritation — blood, abscess, gallbladder — refers to the C4 dermatome at the SHOULDER TIP (Kehr sign), while peripheral irritation refers to the thoracoabdominal wall. A cut phrenic nerve gives an elevated hemidiaphragm that moves paradoxically UPWARD on inspiration.
  7. NOW THE LUNG. Week 4 (day 22–26): the respiratory diverticulum buds from the VENTRAL foregut endoderm just caudal to the pharyngeal pouches — ventral, which is why the trachea ends up anterior to the oesophagus. Endoderm makes epithelium and glands only, all the way down to the type I and type II pneumocytes. Splanchnic mesoderm makes everything else: cartilage, smooth muscle, connective tissue, vessels, visceral pleura — and it INSTRUCTS the branching, FGF10 from the mesenchyme acting on the endodermal tip. No mesenchyme, no branches. Tracheo-oesophageal ridges fuse by week 6 into a septum: trachea ventral, oesophagus dorsal.
  8. HOW THE LUNG GETS ITS PLEURA: the bud grows laterally into the pericardioperitoneal canal and invaginates its lining — a fist pushed into an inflated balloon. The layer draped on the lung is VISCERAL pleura (visceral mesoderm), the layer left on the wall is PARIETAL pleura (somatic mesoderm), and the potential space between them is the pleural cavity, a leftover piece of coelom. The lungs eventually fill the canals so completely that parietal pleura lines the whole thoracic wall.
  9. BRANCHING, BY THE NUMBERS. Week 4: right and left PRIMARY bronchi. Week 5: SECONDARY (lobar) bronchi — three on the right, two on the left, so the lobes are set this early. Week 16: the CONDUCTING ZONE is complete, meaning the tree has reached the TERMINAL BRONCHIOLES. Month 6: 17 generations of divisions. After birth: 6 more, giving the adult ~23. And the trachea bifurcates at TV4. Embryonic (0–5 wk): trachea, main and segmental bronchi. Pseudoglandular (5–16): down to TERMINAL BRONCHIOLES, looks like an exocrine gland, nothing that can breathe. Canalicular (16–26): respiratory bronchioles, ducts, capillary invasion — the first blood–air barrier, hence viability around 24 weeks. Saccular (26–36): terminal sacs, barrier thins, real surfactant — the IRDS window. Alveolar (36 wk onward): septation multiplies alveoli, and only about a sixth to a third of the adult 300 million are there at birth. 'Every Person Can Say Alveoli.'
  10. The adult airway inherits that timeline. Conducting zone = trachea through terminal bronchioles, no gas exchange, ~150 mL of anatomic dead space; bronchi have cartilage and glands, bronchioles don't. The ACINUS is everything distal to a terminal bronchiole — respiratory bronchioles (first gas exchange), alveolar ducts, sacs, alveoli. Pseudoglandular built the conducting part, canalicular onward built the acinus.
  11. Larynx: endodermal epithelium; cartilage and muscle from arches 4 and 6 (vagus — superior laryngeal from 4, recurrent laryngeal from 6), epiglottis from arches 3–4; the lumen occludes with epithelium and recanalises at week 10.

Take home: Three partitions, three jobs: pleuropericardial = pleural from pericardial (→ fibrous pericardium, phrenic in front of the lung root); pleuroperitoneal = pleural from peritoneal (→ week 7, Bochdalek if it fails); septum transversum = the central tendon between them. CDH is 85–90% left and posterolateral; the killer is pulmonary hypoplasia. IRDS is a surfactant problem — steroids to mother, surfactant to baby.

Signals here:FGF10Goes wrong as:Congenital diaphragmatic hernia (Bochdalek)Oesophageal atresia with tracheo-oesophageal fistulaPyloric stenosisInfant respiratory distress syndrome

Heart & great vessels · ANI 7.1 Development and Congenital Defects of the Heart (Klein)

From a tube to four chambers

  1. Cardiac progenitors in splanchnic lateral-plate mesoderm form a horseshoe (primary heart field); blood islands fuse into an endothelial tube wrapped in myoblasts; folding brings the two limbs together as one tube in the pericardial cavity, hung by the dorsal mesocardium. Layers: endocardium, cardiac jelly, myocardium. The secondary heart field adds the RV, outflow tract, atria and sinus venosus.
  2. The tube has dilations, venous pole to arterial pole: sinus venosus → primitive atrium → primitive ventricle → bulbus cordis (proximal, conus cordis, truncus arteriosus). Looping days 22–24 puts the ventricles in front and the atria behind.
  3. Derivatives: primitive ventricle = LV; proximal bulbus = RV; conus = smooth outflow tracts (conus arteriosus, aortic vestibule); truncus = aorta + pulmonary trunk; primitive atrium = rough auricles; sinus venosus right horn = smooth RA (sinus venarum), left horn = coronary sinus; pulmonary vein incorporated = smooth LA.
  4. Septation happens in three places. AV canal: endocardial cushions fuse → mitral and tricuspid orifices. Atria: septum primum grows down (ostium primum below it), the cushions close ostium primum while apoptosis opens ostium secundum above; septum secundum grows to its right, leaving the foramen ovale, which the primum flap covers as a one-way valve. Ventricles: muscular septum grows up, leaving an interventricular foramen closed by bulbar ridges + cushions = membranous septum.
  5. Outflow: neural-crest conotruncal ridges spiral 180° and fuse into the aorticopulmonary septum — aorta to LV, pulmonary trunk to RV. Fail to form → persistent truncus; fail to spiral → transposition; displaced anteriorly → tetralogy.
  6. Arches: six pairs of aortic arch arteries off the aortic sac; 3 → carotids, 4 → arch of aorta (L) and proximal right subclavian (R), 6 → pulmonary arteries and the ductus arteriosus (L), 1 and 2 mostly vanish, 5 never counts.

Take home: The membranous septum is where VSDs live (~80%). Neural crest failure = conotruncal defects.

Signals here:Goes wrong as:Tetralogy of FallotVentricular septal defectPersistent truncus arteriosusTransposition of the great arteriesAtrial septal defect

Fetal circulation · ANI 7.1 Development and Congenital Defects of the Heart (Klein)

Fetal circulation and what closes at birth

  1. Oxygenated blood leaves the placenta in the umbilical vein → ductus venosus bypasses the liver → IVC → RA → most crosses the FORAMEN OVALE to the LA → LV → aorta → brain and coronaries first.
  2. Deoxygenated SVC blood tends to go RA → RV → pulmonary trunk, but the lungs are collapsed and high-resistance, so it crosses the DUCTUS ARTERIOSUS into the descending aorta → umbilical arteries → placenta.
  3. At birth: the lungs inflate, pulmonary resistance falls, so pulmonary-trunk blood finally goes to the lungs; more blood returns to the LA, LA pressure rises and slams the septum primum shut over the foramen ovale; the ductus arteriosus constricts (oxygen up, prostaglandins down); umbilical arteries and ductus venosus close.
  4. Remnants: ductus arteriosus → ligamentum arteriosum; foramen ovale → fossa ovalis; ductus venosus → ligamentum venosum; umbilical vein → ligamentum teres hepatis; umbilical arteries → medial umbilical ligaments.

Take home: Three shunts: ductus venosus, foramen ovale, ductus arteriosus. Prostaglandin E keeps the ductus open when a baby needs it; indomethacin closes it.

Signals here:Goes wrong as:Patent ductus arteriosusPatent foramen ovale

Sources: ANI 1.2, 1.0.1, 2.2, 5.4, 7.1 (Klein, Hinojosa, Veazey); blanks in the student slides filled from Langman 15e / Moore 11e.

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