06 EMBRYOLOGY
👩⚕️ Embryology is a small but highly rewarding MRCS Part A topic. Examiners return to the same handful of themes: germ layer derivatives, gut rotation, urogenital development, cardiac septation and vertebral formation. Learn these well and the questions become almost predictable.
The three germ layers
Gastrulation (week 3) creates the trilaminar disc. Every organ traces back to one (or more) of these layers.
| Germ layer | Major derivatives |
|---|---|
| Ectoderm | Epidermis and skin appendages; entire nervous system; neural crest (melanocytes, Schwann cells, adrenal medulla, facial bones, aorticopulmonary septum); lens and cornea |
| Mesoderm | All muscle; bone, cartilage, connective tissue; blood vessels and heart; kidneys, gonads, adrenal cortex; spleen; serous membranes |
| Endoderm | Epithelial lining of GI tract, respiratory tract, bladder and urethra; parenchyma of liver, pancreas, thyroid, parathyroids and thymus |
➡ Neural crest is ectodermal in origin but behaves like mesoderm — a classic exam trap.
➡ The trigone of the bladder is the famous exception: bladder is endoderm, but the trigone is mesoderm (mesonephric duct).
Somites and vertebral development
Paraxial mesoderm segments into somites in a cranio-caudal sequence. Each somite splits into three populations:
- Sclerotome ➡ vertebrae and ribs
- Myotome ➡ skeletal muscle
- Dermatome ➡ dermis of the back
The notochord induces vertebral formation but does not itself become bone — it persists only as the nucleus pulposus of the intervertebral disc.
👩⚕️ Hemivertebra = failure of formation or segmentation of a single sclerotome. It is the commonest cause of congenital scoliosis.
Kidney and ureter development
The kidney develops in three successive waves from intermediate mesoderm:
1. Pronephros — rudimentary, regresses.
2. Mesonephros — transient; its duct (Wolffian) persists in males as vas, epididymis, seminal vesicle and ejaculatory duct.
3. Metanephros — definitive kidney. Forms by interaction of the ureteric bud (off the mesonephric duct) and the metanephric blastema.
➡ Ureteric bud ➡ ureter, renal pelvis, calyces, collecting ducts.
➡ Metanephric blastema ➡ nephrons (glomerulus to distal convoluted tubule).
The kidneys ascend from the pelvis to L1 between weeks 6 and 9. If the lower poles fuse during ascent they get caught beneath the inferior mesenteric artery — a horseshoe kidney.
👩⚕️ Failure of ureteric bud induction ➡ renal agenesis. Premature splitting of the bud ➡ duplex ureter.
Bladder, urethra and external genitalia
The cloaca is the common chamber at the caudal end of the hindgut. The urorectal septum divides it into:
- Urogenital sinus anteriorly ➡ bladder, urethra, lower vagina.
- Anorectal canal posteriorly ➡ rectum and upper anal canal.
The allantois is an endodermal diverticulum that connects the urogenital sinus to the umbilicus. Its remnant is the urachus (median umbilical ligament). A patent urachus presents as urine leaking from the umbilicus.
External genitalia develop in parallel in both sexes before diverging under hormonal influence:
| Indifferent structure | Male | Female |
|---|---|---|
| Genital tubercle | Glans penis | Glans clitoris |
| Urogenital folds | Fuse to form penile urethra and ventral shaft | Labia minora |
| Labioscrotal swellings | Fuse to form scrotum | Labia majora |
➡ Hypospadias = failure of urogenital fold fusion ➡ urethral opening on the ventral surface.
➡ Epispadias = defective genital tubercle positioning ➡ urethral opening on the dorsal surface, classically with bladder exstrophy.
Gut development and rotation
The primitive gut tube is divided by blood supply:
| Region | Artery | Structures |
|---|---|---|
| Foregut | Coeliac trunk | Oesophagus to D2; liver, pancreas, spleen (mesoderm) |
| Midgut | SMA | D3 to proximal two-thirds of transverse colon |
| Hindgut | IMA | Distal transverse colon to upper anal canal |
The midgut grows faster than the abdominal cavity can accommodate. The classic timeline:
- Week 6 — midgut herniates into the umbilical cord (physiological herniation).
- Weeks 6–10 — rotates 270° counterclockwise around the axis of the superior mesenteric artery.
- Week 10 — gut returns to the abdomen.
👩⚕️ Out at 6, back at 10. Failure to return ➡ omphalocele (gut covered by peritoneum, midline defect). Failure of abdominal wall closure around returned gut ➡ gastroschisis (gut exposed, defect to the right of umbilicus, no covering).
Incomplete rotation produces malrotation, predisposing to midgut volvulus around a narrow mesenteric base — a neonatal surgical emergency.
A Meckel's diverticulum is a persistent remnant of the vitelline duct. Rule of 2s: 2% of population, 2 feet from the ileocaecal valve, 2 inches long, 2 ectopic tissues (gastric, pancreatic), presents before age 2.
Cardiac development
The heart tube forms from splanchnic mesoderm and undergoes looping and septation between weeks 4 and 8. The five segments and their derivatives:
| Primitive structure | Adult derivative |
|---|---|
| Truncus arteriosus | Ascending aorta and pulmonary trunk |
| Bulbus cordis | Outflow tracts of both ventricles |
| Primitive ventricle | Trabeculated left ventricle |
| Primitive atrium | Trabeculated parts of both atria |
| Sinus venosus | Smooth right atrium, coronary sinus |
Septation is where most congenital lesions arise:
- Endocardial cushions fuse to form the AV septum and contribute to the mitral and tricuspid valves. Failure ➡ AV septal defect, classically in Down syndrome.
- Septum primum and secundum form the atrial septum. Defective fusion ➡ ostium primum or secundum ASD.
- Aorticopulmonary septum (neural crest) spirals through the truncus to separate aorta from pulmonary trunk. Failure to partition ➡ persistent truncus arteriosus; failure to spiral ➡ transposition; malalignment ➡ tetralogy of Fallot.
👩⚕️ Memorise the association: Down syndrome ➡ endocardial cushion defect ➡ AV septal defect. It comes up almost every sitting.
Pharyngeal arches (worth knowing)
Five arches contribute (1, 2, 3, 4, 6 — the fifth regresses). Each carries its own nerve:
- Arch 1 — CN V; muscles of mastication; mandible, malleus, incus.
- Arch 2 — CN VII; muscles of facial expression; stapes, styloid.
- Arch 3 — CN IX; stylopharyngeus; common and internal carotid arteries.
- Arch 4 — CN X (superior laryngeal); cricothyroid; aortic arch (left), right subclavian.
- Arch 6 — CN X (recurrent laryngeal); intrinsic laryngeal muscles; pulmonary arteries, ductus arteriosus.
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Test yourself
Hemivertebra develops from which embryological structure?
- ((Mesoderm::☑️ Vertebrae arise from sclerotome (paraxial mesoderm); failed segmentation causes hemivertebra and congenital scoliosis.))
- ((Ectoderm::Forms skin and the entire nervous system, not bone.))
- ((Endoderm::Forms gut, respiratory and bladder epithelia — not vertebrae.))
- ((Notochord::Induces vertebral formation but persists only as nucleus pulposus.))
- ((Neural tube::Becomes brain and spinal cord; lies within, not as, the vertebral column.))
👩⚕️ Sclerotome → vertebrae, myotome → muscle, dermatome → dermis.
Failure of development of the caudal portion of the metanephros results in which condition?
- ((Horseshoe kidney::Caused by fusion of lower poles trapped beneath the inferior mesenteric artery during ascent.))
- ((Renal agenesis::☑️ The metanephros forms the definitive kidney; failure of induction means no kidney develops.))
- ((Polycystic kidney disease::Genetic tubular disorder, not a defect of metanephric induction.))
- ((Ureter agenesis::Ureter derives from the ureteric bud off the mesonephric duct, not metanephros.))
- ((Double ureter::Premature bifurcation of the ureteric bud, not a metanephric defect.))
👩⚕️ Pro → meso → metanephros. Ureteric bud makes the plumbing; metanephric blastema makes the nephrons.
An atrioventricular septal defect results from a developmental defect in which embryological structure?
- ((Endocardial cushion::☑️ Cushions fuse to form the AV septum and AV valves; strongly linked with Down syndrome.))
- ((Bulbus cordis::Becomes the outflow tracts of both ventricles, not the AV septum.))
- ((Septum primum::Defects here cause ostium primum or secundum ASDs, not AVSDs.))
- ((Truncus arteriosus::Failed partitioning gives persistent truncus arteriosus instead.))
- ((Sinus venosus::Defects cause sinus venosus-type ASD, a distinct entity.))
👩⚕️ AV septal defect plus Down syndrome equals endocardial cushion defect.
Which of the following is the axis of gut rotation during development?
- ((Superior mesenteric artery::☑️ Midgut rotates 270° counterclockwise around the SMA between weeks 6 and 10.))
- ((Inferior mesenteric artery::Supplies the hindgut; not involved in midgut rotation.))
- ((Coeliac trunk::Foregut artery; foregut does not undergo rotation.))
- ((Aorta::Posterior structure; gives off the SMA but is not the rotational axis.))
- ((Vitelline duct::Connects midgut to yolk sac; remnant is Meckel's diverticulum.))
👩⚕️ Malrotation predisposes to midgut volvulus around a narrow mesenteric base.
During fetal development, at what gestational age do the intestines return to the abdominal cavity after physiological herniation into the umbilical cord?
- ((6 weeks::This is when the midgut herniates out, not when it returns.))
- ((8 weeks::Still rotating outside the abdomen at this point.))
- ((10 weeks::☑️ Midgut returns at week 10 having completed its 270° counterclockwise rotation.))
- ((12 weeks::Gut already back; abdominal wall closing.))
- ((16 weeks::Far too late; failure to return by week 10 gives omphalocele.))
👩⚕️ Out at 6, back at 10. Omphalocele is covered; gastroschisis is not.
Hypospadias results from a developmental defect involving which of the following structures?
- ((Urogenital fold::☑️ Failure of urogenital fold fusion gives a ventral urethral opening on the penile shaft.))
- ((Genital tubercle::Forms glans penis or clitoris; defects cause ambiguous genitalia.))
- ((Labioscrotal swelling::Fuses to form scrotum; non-fusion gives bifid scrotum, not hypospadias.))
- ((Urogenital sinus::Forms bladder and proximal urethra, not the distal penile urethra.))
- ((Mesonephric duct::Forms vas, epididymis and seminal vesicles in males.))
👩⚕️ Hypospadias = ventral. Epispadias = dorsal and associated with bladder exstrophy.
A child presents with epispadias and bladder exstrophy. What is the embryological origin of the bladder?
- ((Urogenital sinus::☑️ Bladder is endodermal, derived from the anterior division of the partitioned cloaca.))
- ((Mesonephric duct::Contributes only the trigone — the classic exception to the endodermal rule.))
- ((Metanephros::Becomes the definitive kidney, not the bladder.))
- ((Allantois::Its remnant forms the urachus (median umbilical ligament), not the bladder body.))
- ((Cloaca::Common precursor, but bladder specifically derives from its urogenital sinus division.))
👩⚕️ Bladder body = endoderm (urogenital sinus). Trigone = mesoderm (mesonephric duct).
Revision summary
- Ectoderm ➡ skin, nervous system. Mesoderm ➡ muscle, bone, kidneys, heart. Endoderm ➡ gut, respiratory and bladder linings, liver, pancreas.
- Sclerotome ➡ vertebrae. Hemivertebra causes congenital scoliosis.
- Kidney: ureteric bud ➡ collecting system; metanephric blastema ➡ nephrons. Horseshoe kidney trapped under IMA.
- Cloaca ➡ urogenital sinus (bladder, urethra) + anorectal canal. Urachus = allantois remnant.
- Hypospadias ventral (urogenital fold). Epispadias dorsal (bladder exstrophy).
- Gut: 270° counterclockwise around SMA. Out at 6, back at 10. Omphalocele covered; gastroschisis not.
- Meckel's = persistent vitelline duct (rule of 2s).
- Cardiac: endocardial cushions ➡ AV septum (AVSD + Down syndrome). Aorticopulmonary septum (neural crest) ➡ aorta/PA.
- Bladder = endoderm; trigone = mesoderm — the one exception.