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Germ Layers & Early Embryology

Name: Germ Layers & Early Embryology
Creator: Veli Bakalov, MD

Three layers, the neural crest wildcard, and the first eight weeks

Anatomy & Embryology·Embryology·5 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Gastrulation (week 3): epiblast cells migrate through the primitive streak to form ectoderm, mesoderm and endoderm; the notochord induces the neural plate and persists as the nucleus pulposus (remnant → chordoma).
Surface ectoderm forms the epidermis, lens, inner ear, anal canal below the pectinate line and the anterior pituitary (Rathke pouch); neuroectoderm forms the CNS, retina, pineal gland and posterior pituitary.
Neural crest forms the PNS ganglia, Schwann cells, adrenal medulla, melanocytes, C cells, enterochromaffin cells, pia and arachnoid, odontoblasts, skull bones and the aorticopulmonary septum.
Failed neural crest contribution to the aorticopulmonary septum causes transposition, tetralogy of Fallot and persistent truncus arteriosus.
Endoderm forms gut and airway epithelium, liver, pancreas, lungs, thymus, parathyroids and thyroid follicular cells; mesoderm forms muscle, bone, blood, kidneys, gonads and the adrenal cortex.
Teratogen timing: before week 3 is all-or-none, weeks 3–8 (organogenesis) cause structural malformations, and after week 8 causes growth and functional defects.
Monozygotic twins: split before day 4 = di/di, days 4–8 = mono/di (most common), days 8–12 = mono/mono, after day 13 = conjoined; dizygotic twins are always di/di.
The syncytiotrophoblast secretes hCG and lacks MHC I; the cord has two umbilical arteries (deoxygenated) and one vein (oxygenated), and a single umbilical artery suggests congenital anomalies.
Open neural tube defects (week-4 closure failure, linked to folate deficiency) raise maternal serum and amniotic AFP and amniotic acetylcholinesterase.

The first weeks: from blastocyst to three layers

Week 1Fertilization, cleavage to morula and blastocyst; implantation around day 6; hCG secretion begins
Week 2 ("rule of 2s")Bilaminar disc (epiblast + hypoblast), two cavities (amniotic cavity, yolk sac), two trophoblast layers
Week 3 ("rule of 3s")Gastrulation: epiblast cells migrate through the primitive streak to form ectoderm, mesoderm and endoderm; notochord forms
Week 4 ("rule of 4s")Neural tube closes; the heart begins to beat; limb buds appear
Weeks 3–8Embryonic period: organogenesis

Gastrulation. In week 3, epiblast cells migrate through the primitive streak and form all three germ layers: ectoderm, mesoderm and endoderm. Every germ layer comes from the epiblast.

Notochord. The notochord induces the overlying ectoderm to become the neural plate (neurulation). Sonic hedgehog signaling from the notochord then patterns the ventral neural tube. The notochord later persists as the nucleus pulposus of the intervertebral disc. A notochord remnant can give rise to a chordoma, classically at the clivus or sacrum.

Germ-layer derivatives

What each layer builds

Layer	Derivatives	Exam hooks
Surface ectoderm	Epidermis and adnexa (sweat, sebaceous and mammary glands); lens and corneal epithelium; inner ear; olfactory epithelium; anal canal below the pectinate line; parotid gland; anterior pituitary	Adenohypophysis comes from Rathke pouch
Neuroectoderm	CNS neurons, ependymal cells (line the ventricles, make CSF), oligodendrocytes, astrocytes, pineal gland, retina and optic nerve (CN II), posterior pituitary	Neurohypophysis is neuroectoderm; microglia are not (they are mesodermal)
Neural crest	PNS (dorsal root and autonomic ganglia, Schwann cells), adrenal medulla, melanocytes, parafollicular C cells, enterochromaffin cells, pia and arachnoid, odontoblasts, skull bones, aorticopulmonary septum	The "great migrators"; see the next section
Mesoderm	Muscle, bone, connective tissue, dermis, serous linings (peritoneum, pleura), heart, vessels and blood, lymphatics, spleen, kidneys and ureters, gonads, adrenal cortex, nucleus pulposus (from notochord)	Mesodermal defects: VACTERL association
Endoderm	Gut-tube epithelium and its outgrowths (liver, gallbladder, pancreas); lungs; thymus; thyroid follicular cells; parathyroids; lining of the GI and respiratory tracts; middle ear and eustachian tube; bladder and urethral lining	Thyroid follicular cells are endoderm, but C cells are neural crest

The anterior pituitary is surface ectoderm (Rathke pouch), and the posterior pituitary is neuroectoderm. The adrenal cortex is mesoderm, and the adrenal medulla is neural crest.

VACTERL. Mesodermal defects cluster as Vertebral defects, Anal atresia, Cardiac defects, Tracheo-Esophageal fistula, Renal defects and Limb defects. A teratoma, by contrast, is a tumor containing tissue from all three germ layers.

Neural crest and its clinical correlates

The wildcard layer. Neural crest cells arise at the edges of the neural folds and migrate throughout the body. That is why one population builds pigment cells, the adrenal medulla and part of the heart. Failed migration and tumors of crest derivatives therefore span many organ systems.

Neural crest derivatives and what goes wrong

Derivative	Disorder or tumor
Aorticopulmonary septum	Conotruncal defects: transposition of the great arteries, tetralogy of Fallot, persistent truncus arteriosus
Adrenal medulla (chromaffin cells)	Neuroblastoma, pheochromocytoma
Melanocytes	Melanoma
Parafollicular C cells	Medullary thyroid carcinoma (calcitonin)
Enterochromaffin cells	Carcinoid tumors
Enteric ganglia	Hirschsprung disease: failed migration leaves the distal colon without ganglion cells
Schwann cells	Schwannoma

Teratogenic windows and neural tube defects

Timing determines the damage.

Before week 3: the "all-or-none" period. The embryo either dies (spontaneous abortion) or recovers completely, because its surviving undifferentiated cells can replace the damaged ones.
Weeks 3–8 (embryonic period): organogenesis, the window most sensitive to teratogen-induced structural malformations. An exposure at week 5 causes major structural defects, whereas the same exposure at week 2 would cause all-or-none loss.
After week 8 (fetal period): teratogens mainly cause growth restriction and functional defects rather than malformed structures.

Neural tube defects. Failure of neural tube closure in week 4 causes anencephaly (cranial end) and spina bifida (caudal end). These defects are linked to folate deficiency, and periconceptional folic acid lowers the risk. Open defects raise maternal serum and amniotic alpha-fetoprotein (AFP) and amniotic acetylcholinesterase (AChE).

Elevated amniotic AChE together with AFP confirms an open neural tube defect. A closed defect covered by skin does not leak these markers.

Twinning, placenta and umbilical cord

Twin membranes depend on timing. Dizygotic twins come from two eggs and two sperm and are always dichorionic/diamniotic. The membranes of monozygotic (MZ) twins depend on when the embryo splits.

Monozygotic twinning

Split timing	Stage	Membranes
Before day 4	Morula	Dichorionic, diamniotic
Days 4–8	Blastocyst	Monochorionic, diamniotic (most common MZ pattern)
Days 8–12	After the amnion forms	Monochorionic, monoamniotic
After day 13	Embryonic disc	Conjoined twins

One placenta with two amniotic sacs means a split at days 4–8. A shared placenta also carries a risk of twin-twin transfusion syndrome.

Placenta. The fetal side has two layers. The inner cytotrophoblast "makes cells". The outer syncytiotrophoblast secretes hCG, which maintains the corpus luteum in early pregnancy. It also lacks MHC class I, which shields the semi-allogeneic fetus from maternal immune attack. The maternal component is the decidua basalis.

Umbilical vessels. The cord normally holds two umbilical arteries, which carry deoxygenated blood from the fetus to the placenta, and one umbilical vein, which carries oxygenated blood to the fetus. A single umbilical artery is associated with congenital and chromosomal anomalies and warrants evaluation.

Umbilical vein → ligamentum teres hepatis (in the falciform ligament).
Umbilical arteries → medial umbilical ligaments.
Allantois → urachus → median umbilical ligament. A patent urachus leaks urine from the umbilicus.
Vitelline duct → normally obliterates. Incomplete closure causes a Meckel diverticulum.