Q4 LONG ANSWER 10M ★ High-yield

Describe the derivatives of the three germ layers. Explain the folding of the embryo and add a note on the fetal membranes.

Anatomy · 1st Professional — Pre-Clinical

Subdivision	Derivatives
Surface ectoderm	Epidermis, hair, nails, sweat and sebaceous glands, mammary gland; enamel of teeth; lens of the eye; internal ear; anterior pituitary (Rathke pouch); parotid gland; epithelium of the mouth and lower anal canal
Neuroectoderm	Brain and spinal cord; retina and optic nerve; posterior pituitary; pineal gland
Neural crest	Dorsal root and autonomic ganglia; Schwann cells; suprarenal medulla; melanocytes; pia and arachnoid; odontoblasts; parafollicular C cells; bones and connective tissue of the face; aorticopulmonary septum

All muscle — skeletal, cardiac and smooth (except the muscles of the iris, which are neuroectodermal)
All connective tissue, bone and cartilage (except in the face)
Blood, blood vessels, lymphatics, spleen
Kidney, ureter, gonad and genital ducts — from intermediate mesoderm
Suprarenal cortex
Dermis; serous membranes — pleura, pericardium, peritoneum

Direction	Cause	Result
Cephalocaudal (longitudinal)	Rapid growth of the neural tube, especially the brain	The developing heart and septum transversum are carried from a position in front of the head to the thorax; the buccopharyngeal membrane comes to lie at the mouth
Transverse (lateral)	Growth of the somites	Converts the flat disc into a cylinder; the endoderm is rolled into the gut tube; the body wall closes ventrally; the yolk sac is constricted into the vitelline duct

The gut tube is divided into three — foregut, midgut (which retains its connection with the yolk sac) and hindgut
The septum transversum, carried down by folding, becomes the central tendon of the diaphragm — and it takes its nerve supply, the phrenic (C3, 4, 5), with it

Structure	Function	Fate
Amnion	Encloses the amniotic fluid; protects and allows movement	Fuses with the chorion; forms the inner covering of the cord
Chorion	Chorion frondosum forms the fetal placenta; chorion laeve is smooth	Fuses with the decidua
Yolk sac	Early haemopoiesis; source of primordial germ cells; early nutrition	Vitelline duct; normally disappears
Allantois	Vestigial in man; involved in early blood formation	Urachus, later the median umbilical ligament

Organ	Parts and their origins
Suprarenal gland	Cortex — mesoderm; medulla — neural crest
Pituitary gland	Anterior — surface ectoderm (Rathke pouch); posterior — neuroectoderm
Retina and optic nerve	Neuroectoderm — an outgrowth of the brain, not a peripheral nerve
Anal canal	Above the pectinate line — endoderm (hindgut); below — ectoderm (proctodeum)
Tooth	Enamel — ectoderm; dentine, cementum and pulp — mesoderm and neural crest
Diaphragm	Septum transversum, pleuroperitoneal membranes, dorsal mesentery and body wall — four sources
Iris muscles	Neuroectoderm — the only smooth muscle not of mesodermal origin

Dual origin explains dual behaviour — the two parts of the anal canal differ in epithelium, blood supply, nerve supply, lymphatic drainage and the type of carcinoma they develop, all following the germ layer boundary

Division	Extent	Artery	Derivatives
Foregut	Pharynx to the duodenum at the entry of the bile duct	Coeliac trunk	Oesophagus, stomach, upper duodenum, liver, gall bladder, pancreas, respiratory tract
Midgut	Lower duodenum to the junction of the proximal two-thirds and distal third of the transverse colon	Superior mesenteric	Jejunum, ileum, caecum, appendix, ascending and most of the transverse colon
Hindgut	Distal third of the transverse colon to the upper anal canal	Inferior mesenteric	Distal transverse, descending and sigmoid colon, rectum, upper anal canal, bladder and most of the urethra

The artery follows the embryological division, not the anatomical one — which is why the blood supply changes abruptly in the middle of the transverse colon and at the middle of the duodenum, and why these are watershed zones vulnerable to ischaemia
Referred pain follows the same rule — foregut pain to the epigastrium, midgut to the umbilicus, hindgut to the suprapubic region

Failure of ventral body wall closure — omphalocele (herniation into the cord, covered by amnion, with associated anomalies) and gastroschisis (a defect lateral to the cord, uncovered, usually isolated)
MECKEL diverticulum — persistence of the proximal vitelline duct; the rule of 2s: 2% of people, 2 feet from the ileocaecal valve, 2 inches long, 2 types of ectopic tissue (gastric and pancreatic), often symptomatic before age 2
Urachal anomalies — a patent urachus leaks urine at the umbilicus; a urachal cyst or sinus may become infected
Ectopia vesicae — failure of the infra-umbilical body wall to close, exposing the posterior bladder wall
Amniocentesis at 15–18 weeks samples fetal cells for karyotyping; chorionic villus sampling at 10–12 weeks is earlier but carries a slightly higher risk

All questions in General Embryology