Development of spinal cord with diagrams

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Development of the Spinal Cord

Sources: The Developing Human: Clinically Oriented Embryology (Moore & Persaud); THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System

1. Origin and Overview

The spinal cord develops from the caudal part of the neural plate and the caudal eminence. Specifically, the neural tube caudal to the fourth pair of somites gives rise to the spinal cord. The process depends critically on retinoic acid signaling, which governs patterning and neurogenesis from the earliest stages.

2. Neurulation: Formation of the Neural Tube

Before the spinal cord forms, the neural plate folds and closes to create the neural tube. The neural groove folds away from the overlying dorsal ectoderm; cells at the lateral margins of the groove migrate away to form the neural crest on each side.

Neural tube cross-section showing alar plate, basal plate, floor plate, zone of autonomic neurons, and roof plate (THIEME Atlas)

Cross-section of the early neural tube showing its key plate regions - THIEME Atlas of Anatomy

3. Wall Structure and Zones of the Developing Spinal Cord

Initially, the wall of the neural tube is a thick pseudostratified columnar neuroepithelium. As it develops, three distinct zones emerge:

Zone	Former Name	Adult Derivative
Ventricular zone	Ependymal layer	Ependymal cells lining central canal
Intermediate zone	Mantle layer	Gray matter (neuronal cell bodies)
Marginal zone	-	White matter (axonal tracts)

The diagram below shows this progression from ~23 days through 9 weeks:

FIG. 17.5 - Development of the spinal cord from ~23 days to 9 weeks. A: neural tube; B: 6-week cross-section showing sulcus limitans, alar plate, basal plate; C: 9-week cross-section showing dorsal/ventral gray horns; D: neuroepithelial wall; E: three-zone wall structure

FIG. 17.5 - Development of the spinal cord. A (~23 days): neural tube with primordium of spinal ganglion; B (6 weeks): alar plate, basal plate, sulcus limitans; C (9 weeks): dorsal/ventral gray horns forming; D–E: wall zones (ventricular, intermediate, marginal).

As the lateral walls thicken, the neural canal progressively narrows until only the tiny central canal remains by 9-10 weeks.

4. Alar Plate and Basal Plate: Sensory vs Motor Separation

A critical developmental milestone is the formation of a shallow longitudinal groove on each side: the sulcus limitans. This divides each lateral wall into:

Alar plate (dorsal) - gives rise to afferent (sensory) nuclei → dorsal gray horns
Basal plate (ventral) - gives rise to efferent (motor) neurons → ventral and lateral gray horns

As basal plates enlarge, they bulge ventrally, forming the ventral median fissure. As alar plates enlarge, the dorsal median septum forms.

Histological cross-section at Carnegie stage 16 (~40 days) showing roof plate, alar plate, sulcus limitans, basal plate, floor plate, central canal, spinal ganglion, and developing vertebral body

FIG. 17.7 - Transverse section at ~40 days. The ventral root arises from neuroblasts in the basal plate; the dorsal root arises from neuroblasts in the spinal ganglion.

The nuclear column organization:

Nuclear columns of the neural tube: somatosensory, viscerosensory, visceromotor, and somatomotor columns

Nuclear column organization: alar plate = somatosensory + viscerosensory columns; basal plate = visceromotor + somatomotor columns.

5. Histogenesis of CNS Cells

The neuroepithelial cells (ventricular zone) are the progenitors of all neurons and glial cells:

Neuroblasts - form from neuroepithelial cells; migrate into intermediate zone; differentiate into neurons
- Apolar neuroblast → Bipolar neuroblast → Unipolar neuroblast → Multipolar neuron (with dendrites and axon)
Glioblasts (spongioblasts) - form after neuroblast production ceases; migrate into intermediate and marginal zones
- Astroblasts → Astrocytes (protoplasmic or fibrous)
- Oligodendroblasts → Oligodendrocytes
Ependymal cells - form when neuroepithelial cells cease neuroblast/glioblast production; line the central canal
Microglial cells - derived from mesenchyme (bone marrow origin), not neuroepithelium; invade the CNS late in the fetal period via blood vessels

Molecular regulation: SHH and Olig2-bHLH signaling controls proliferation and patterning via GLI transcription factors.

FIG. 17.6 - Histogenesis chart: Neural tube → Neuroepithelium → Apolar neuroblast → Bipolar → Unipolar → Neuron; Glioblast → Astroblast → Astrocytes; Oligodendroblast → Oligodendrocytes; Mesenchyme → Microglial cells; Ependymal cells

FIG. 17.6 - Histogenesis of CNS cells. Neuroepithelial cells give rise to all neurons and macroglial cells. Microglia are derived from mesenchyme.

6. Development of Spinal Ganglia (Dorsal Root Ganglia)

Spinal ganglia are derived from neural crest cells (not from the neural tube)
Initially the neurons are bipolar, but the two processes unite in a T-shaped fashion to become unipolar (pseudounipolar)
Peripheral process - functionally a dendrite (conducts toward cell body); joins spinal nerve
Central process - enters spinal cord as the dorsal root
Neural crest cells also give rise to Schwann cells, satellite cells, melanocytes, and autonomic ganglia

FIG. 17.8 - Neural crest derivatives: spinal cord with dorsal/ventral horns, lateral gray horn; neural crest → spinal ganglion (unipolar neurons + satellite cells + Schwann cells); sympathetic trunk ganglion → celiac ganglion, renal ganglion, intestinal plexus, suprarenal medulla; neural crest also → melanocytes

FIG. 17.8 - Neural crest derivatives including spinal ganglia, sympathetic trunk, and other peripheral structures.

7. Development of Spinal Meninges

Meninges develop from neural crest cells and mesenchyme between 20 and 35 days
These cells surround the neural tube and form the primordial meninges
Outer layer thickens → Dura mater
Inner layer (pia-arachnoid or leptomeninges) splits into:
- Pia mater (innermost)
- Arachnoid mater (middle)
Fluid-filled spaces within leptomeninges coalesce → subarachnoid space
CSF begins forming at week 5

8. Positional Changes of the Spinal Cord

This is a clinically important topic. Early in development, the spinal cord extends the full length of the vertebral canal and spinal nerves exit horizontally. Because the vertebral column and dura grow faster than the spinal cord, the cord's caudal end progressively moves to higher levels:

Stage	Level of cord termination
8 weeks (embryo)	Extends entire length of vertebral canal
24 weeks (fetus)	Level of S1 vertebra
Neonate	L2-L3
Adult	Lower border of L1 (range: T12-L3)

This discrepancy means lumbar and sacral nerve roots run obliquely downward within the vertebral canal, forming the cauda equina ("horsetail"). Below the cord's tip (conus medullaris), the pia mater continues as the filum terminale, anchoring to the periosteum of Coccyx 1.

FIG. 17.10 - Positional changes of spinal cord: A (8 weeks): cord to sacrum; B (24 weeks): ends at S1, medullary cone forms; C (neonate): ends at L2-3, filum terminale visible; D (adult): ends at L1, cauda equina well-formed, dural sac ends at S2

FIG. 17.10 - Positional changes of the spinal cord relative to the vertebral column: 8 weeks (A) → 24 weeks (B) → Neonate (C) → Adult (D). Note the progressive ascent of the conus medullaris and increasing obliquity of nerve roots.

9. Myelination of Nerve Fibers

Feature	CNS	PNS
Myelinating cell	Oligodendrocytes (from neuroepithelium)	Schwann cells / neurilemma (from neural crest)
Timing begins	Late fetal period, continues postnatally	~20 weeks gestation
Motor vs sensory	Motor roots myelinated before sensory roots	Same sequence

Myelin consists of layers of lipid and protein (myelin basic proteins)
β1-integrins regulate myelination; Profilin 1 (PFN1) promotes cytoskeletal changes in oligodendrocytes
Fiber tracts become functional at approximately the time they become myelinated
In the PNS, Schwann cell membrane wraps around the axon (mesaxon elongation); cytoplasm condenses between layers

10. Congenital Defects (Spina Bifida)

Most defects result from failure of fusion of one or more neural arches of the developing vertebrae during the 4th week. Collectively called Neural Tube Defects (NTDs), they affect meninges, neural arches, muscles, and skin.

Type	Description
Spina bifida occulta	Nonfusion of neural arch only; skin intact; clinically silent; tuft of hair may mark it
Meningocele	Neural arch + meninges protrude through defect; spinal cord remains in canal; skin covered
Meningomyelocele	Neural arch + meninges + spinal cord/roots protrude; most common severe form
Myeloschisis (rachischisis)	Most severe; cord open to surface; neural folds fail to fuse

Risk factors include folate deficiency (folate supplementation before conception reduces NTD risk by ~70%). Severe NTDs are often associated with Arnold-Chiari malformation (downward displacement of cerebellum and brainstem).

Summary Timeline

Day 18    Neural plate forms from ectoderm
Day 20-21 Neural folds begin to elevate
Day 22-23 Neural tube closure begins (cervical region first)
Day 25    Rostral neuropore closes → brain develops
Day 27-28 Caudal neuropore closes → spinal cord primordium complete
Week 4    Alar/basal plates distinguish; neural crest migration
Week 5    Sulcus limitans forms; CSF begins
Week 6    Three-zone wall (ventricular/intermediate/marginal) established
Week 8    Cord extends full vertebral canal length
Week 9-10 Central canal narrows; gray/white matter distinct
Week 20+  PNS myelination begins (peripheral nerves)
Late fetal CNS myelination begins; continues through year 1
Neonate   Cord ends at L2-L3
Adult     Cord ends at L1 (lower border)

- The Developing Human: Clinically Oriented Embryology (Moore, Persaud & Torchia), Chapter 17, pp. 1028-1039 - THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System, pp. 93-94

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