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AN79.1-6 | 3rd to 8th week of development — Self-Directed Learning

Part 1: Primitive Streak — Formation and Fate (AN79.1)

The Primitive Streak

Formation (Day 14–15, 3rd week):
The primitive streak appears as a thickening of the epiblast in the caudal midline of the embryonic disc. It is the first landmark of the 3rd week and establishes the body axes:
• Cranial–caudal axis (head–tail direction)
• Left–right axis (bilateral symmetry)
• Dorsal–ventral axis

Structure:
• Primitive streak — midline thickening in caudal epiblast
• Primitive node (Hensen's node) — cranial thickening at the cranial end of the streak; the organiser of the entire embryo
• Primitive groove — central depression along the streak through which cells invaginate
• Primitive pit — depression at the centre of the primitive node

Gastrulation — The Process:
Cells from the epiblast migrate toward the primitive streak, ingress through the primitive groove, and spread laterally and cranially between the epiblast (ectoderm) and hypoblast (endoderm) to form the intraembryonic mesoderm:

Layer	Origin	Future
Ectoderm	Epiblast cells remaining on surface	Skin, nervous system, sense organs
Mesoderm	Epiblast cells that migrate through the streak	Muscles, skeleton, connective tissue, cardiovascular system, kidneys, gonads
Endoderm	Epiblast cells that replace the hypoblast	Gut epithelium, liver, pancreas, lungs, bladder lining

Thus, gastrulation converts the bilaminar disc → trilaminar disc (ectoderm + mesoderm + endoderm).

Fate of the Primitive Streak:
• The streak regresses caudally as gastrulation proceeds (3rd–4th weeks)
• By the 4th week, only a small caudal remnant remains (Müller's tubercle region)
• By the end of the 4th week, the primitive streak disappears entirely

Clinical Significance:
• Sacrococcygeal teratoma — if the primitive streak fails to regress completely, remnant pluripotent cells persist in the presacral/sacrococcygeal region → form a teratoma (tumour containing derivatives of all three germ layers: hair, teeth, neural tissue, cartilage)
- Most common solid tumour in neonates (1 in 35,000 births)
- Diagnosed on antenatal ultrasound as a large sacrococcygeal mass
- 80% benign at birth; becomes malignant if not excised early → urgent surgical excision in the neonatal period
• Situs inversus — Hensen's node controls L–R asymmetry via cilia-driven nodal flow; defects → situs inversus (heart and liver on opposite sides); associated with Kartagener syndrome (immotile cilia)
• Conjoined twins — incomplete separation of the primitive streak in identical twins

Part 2: Notochord — Formation and Fate (AN79.2)

The Notochord

Formation (Days 16–22, 3rd week):
• Cells from the primitive node migrate cranially in the midline (between ectoderm and endoderm) to form the notochordal process
• The notochordal process initially forms a hollow tube, then becomes solid → the definitive notochord
• The notochord extends from the oropharyngeal membrane (cranially) to the cloacal membrane (caudally)

Functions of the Notochord:
1. Defines the central axis — establishes the longitudinal axis of the embryo; vertebral column forms around it
2. Induces neurulation — induces the overlying ectoderm to form the neural plate (a critical inductive signal)
3. Determines vertebral body formation — the sclerotome (from somites) condenses around the notochord to form vertebral bodies; the notochord becomes enclosed within the vertebral bodies

Fate of the Notochord:
• The notochord is progressively surrounded and incorporated into the developing vertebral column
• Within each vertebral body, the notochord degenerates completely
• At each intervertebral disc, the notochord persists and expands → forms the nucleus pulposus (the gelatinous core of the intervertebral disc)
• The nucleus pulposus is surrounded by the annulus fibrosus (from sclerotome mesenchyme)

Clinical Significance:
• Disc prolapse (Herniated Nucleus Pulposus — HNP / "slipped disc"):
- The nucleus pulposus is the notochordal remnant; it has a high water content (80% water at birth, decreasing with age)
- With aging and mechanical stress → the annulus fibrosus develops radial tears → nucleus pulposus herniates posterolaterally
- Compresses nerve roots (lumbar L4–L5, L5–S1 most common) → sciatica (radiating leg pain)
- Extremely common in India: lumbar disc prolapse causes low back pain + sciatica
- L4–L5 disc prolapse → L5 root compression → foot drop + sensory loss on dorsum of foot
- L5–S1 disc prolapse → S1 root compression → absent ankle jerk + sensory loss on sole
• Chordoma — rare malignant tumour arising from notochordal remnants; occurs at the clivus (base of skull) or sacrococcygeal region (the two ends of the notochordal axis where remnants may persist)

Part 3: Neurulation — Formation of the Neural Tube (AN79.3)

Neurulation (3rd–4th weeks)

Neurulation is the process by which the neural plate folds to form the neural tube — the precursor of the brain and spinal cord.

Steps:
1. Neural plate induction (Day 18): The notochord induces the overlying ectoderm → thickens to form the neural plate (a flat sheet of neuroectoderm, widest cranially)
2. Neural folds (Day 20–22): The lateral edges of the neural plate elevate as neural folds; the central region deepens into the neural groove
3. Neural tube closure (Days 22–26): The neural folds fuse in the midline, starting at the mid-cervical region (future C3–C4) and proceeding simultaneously cranially and caudally (like a zipper)
- Anterior (cranial) neuropore closes on Day 25
- Posterior (caudal) neuropore closes on Day 27
4. Neural crest cells migrate away from the neural folds as fusion occurs

Neural Tube → CNS:

Region	Derivative
Cranial neural tube	Brain (3 primary vesicles: prosencephalon, mesencephalon, rhombencephalon)
Caudal neural tube	Spinal cord

Neural Crest Cells (the 4th germ layer):
• Derived from the neural folds; migrate extensively throughout the embryo
• Form: Dorsal root ganglia, autonomic ganglia, Schwann cells, melanocytes, craniofacial cartilage and bone, adrenal medulla (chromaffin cells), C-cells of thyroid, odontoblasts

Neural Tube Defects (NTDs) — Clinical Significance (AN79.5):

Type	Mechanism	Clinical Presentation
Anencephaly	Anterior neuropore fails to close (Day 25)	Absence of cranial vault and brain; incompatible with life; 50% detected by elevated AFP + ultrasound
Spina bifida occulta	Posterior arch of a vertebra (usually L5/S1) fails to close; spinal cord normal	Dimple/tuft of hair over lumbosacral region; mostly asymptomatic
Meningocele	Posterior neuropore fails to close; meninges herniate through bony defect	Midline cystic swelling, neural elements intact inside canal; better prognosis
Myelomeningocele	Posterior neuropore fails to close; meninges + spinal cord herniate through defect	Exposed neural tissue/sac; leads to paralysis below level, bladder/bowel dysfunction, Chiari II malformation
Encephalocele	Defect in cranial vault; brain + meninges herniate	Midline posterior skull mass; may cause hydrocephalus

Chiari II Malformation (Arnold-Chiari):
• Accompanies most myelomeningoceles
• Cerebellar tonsils herniate through foramen magnum → CSF flow obstruction → hydrocephalus
• "Lemon sign" on US = frontal bone scalloping from CSF pressure; "Banana sign" = cerebellar herniation

Prevention of NTDs:
• Periconceptional folic acid (0.4–5 mg/day) — reduces NTDs by 70%; folic acid is required for neural tube closure
• All women planning pregnancy in India should begin folic acid ≥1 month before conception and continue through the 1st trimester
• NTD prevalence in India: ~3–4 per 1,000 births (higher than Western countries) due to inadequate folic acid intake

Part 4: Somite Development and Intra-Embryonic Coelom (AN79.4)

Somite Formation

Paraxial mesoderm (mesoderm adjacent to the notochord and neural tube) segments into paired blocks called somites starting from the 20th day.

Somite timeline:
• 1st pair: appears at the cranial end on Day 20
• New pairs added caudally at the rate of ~3 per day
• Total: 42–44 pairs (8 occipital, 12 thoracic, 5 lumbar, 5 sacral, 8–10 coccygeal) — number used to estimate embryonic age
• Total somites appears between Days 20–30

Somite Derivatives (3 components):

Component	Location	Derivatives
Sclerotome (ventromedial)	Surrounds notochord and neural tube	Vertebral body, vertebral arch, intervertebral disc (annulus fibrosus), ribs
Myotome (dorsomedial)	Lateral to sclerotome	Skeletal muscles of trunk (back muscles, intercostals, abdominal wall); each myotome → one spinal segment
Dermatome (dorsolateral)	Migrates to surface	Dermis of back skin; each dermatome → sensory supply from one spinal segment

Note: The word "dermatome" in clinical anatomy (a strip of skin supplied by one spinal nerve) derives directly from the embryological dermatome concept.

Resegmentation:
Each vertebra is formed by fusion of the caudal half of one sclerotome + cranial half of the next sclerotome → this places the myotome (muscle) and its spinal nerve OPPOSITE a vertebral body (not beside it) — allowing free movement of muscles across joints.

Intra-Embryonic Coelom (Body Cavity)

Formation:
• The lateral plate mesoderm (lateral to the somites) splits into two layers:
- Somatic (parietal) mesoderm — adjacent to ectoderm
- Splanchnic (visceral) mesoderm — adjacent to endoderm/yolk sac
• The space between them = intraembryonic coelom (appears around Day 21–28)

Partitioning of the coelom:

Final Cavity	Location	Lining
Pericardial cavity	Cranial to septum transversum	Pericardium
Pleural cavities (×2)	Between pericardial and peritoneal	Pleura
Peritoneal cavity	Caudal	Peritoneum

Partitioning occurs through: (1) Septum transversum (develops into the central tendon of the diaphragm), (2) Pleuropericardial folds, (3) Pleuroperitoneal folds (close the pleuroperitoneal canals).

Clinical: Congenital Diaphragmatic Hernia (CDH):
• The pleuroperitoneal canal fails to close completely (usually on the LEFT side at the foramen of Bochdalek → abdominal contents (intestines, stomach, spleen) herniate into the left pleural cavity → compress the developing lung → pulmonary hypoplasia
• Presents at birth with respiratory distress; scaphoid abdomen; heart shifted to right on X-ray
• Mortality ~50% due to pulmonary hypoplasia; treated by surgical reduction of hernia + lung support