Spinal cord: alar and basal plates and the ascent of the cord !

Embryonic structure

The tail part of the neural tube forms the spinal cord: sensory neurons sit at the back, motor neurons at the front, and its end 'rises' as the spine grows.

Updated 2026-11-01

See it in 3D: Spinal cord · Conus medullaris · Filum terminale · Cauda equina · Dura (dural sac)

What it is

The spinal cord forms from the neural tube caudal to the fourth pair of somites. Its walls organize into a dorsal alar plate (sensory) and a ventral basal plate (motor), separated by the sulcus limitans. Because the vertebral column outgrows the cord, the conus medullaris ends progressively higher: this explains the cauda equina and where a lumbar puncture is done.

Key facts

Wall layers: ventricular (ependymal), mantle (gray matter) and marginal (white matter).
Basal plate → anterior motor horns; alar plate → posterior sensory horns.
Intermediate (lateral) horn: preganglionic sympathetic neurons from T1 to L2.
Roof and floor plates: no neurons; they carry fibers crossing the midline.
Third month: the cord fills the whole vertebral canal; at birth it ends near L3; in adults near L1–L2.
Pia mater continuing past the conus forms the filum terminale; roots below the conus form the cauda equina.

Structure

SHH, secreted by the notochord and then by the floor plate, ventralizes the cord (motor neurons).
BMPs from the ectoderm and roof plate dorsalize the cord (sensory neurons, PAX3/PAX7).
Dorsal root ganglion neurons come from the neural crest, not from the tube.

What it becomes

Basal plate: somatic and visceral motor neurons (ventral roots).
Alar plate: association neurons of the dorsal horn that receive the dorsal roots.
Neuroepithelium: neurons, astrocytes, oligodendrocytes and ependyma; microglia come from mesoderm.
Tube lumen: the central (ependymal) canal.

Step by step

Week 4: the posterior neuropore closes and the caudal tube is complete.
The neuroepithelium proliferates and forms the mantle and marginal layers.
Ventral SHH and dorsal BMP divide the wall into basal and alar plates.
Motor neurons send axons out ventrally; the crest forms the dorsal root ganglia.
From the third month the vertebral column grows faster than the cord.
At birth the conus lies near L3; during childhood it rises to its adult level.

Markers

SHH
BMP4
PAX3
PAX7
NKX2.2
Olig2

Clinical notes

Lumbar puncture is done below the conus (L3–L4 or L4–L5) to avoid the cord.
In tethered cord the filum or a lesion fixes the cord, which is stretched as the child grows: pain, neurogenic bladder, leg weakness, scoliosis.
A hair tuft, dimple or lipoma over the low back can mark occult dysraphism.

Test yourself

Think of the answer, then tap the question to see it.

Which plate of the cord gives rise to anterior horn motor neurons?

The basal plate, ventral to the sulcus limitans.

Which signal secreted by the notochord ventralizes the neural tube?

Sonic hedgehog (SHH), later also made by the floor plate.

At what vertebral level does the cord end at birth, and why does it rise?

Near L3; the vertebral column outgrows the cord, so the conus ends ever higher (L1–L2 in adults).

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: 13.1 The Embryologic Perspective - Anatomy and Physiology 2e (OpenStax, 2022); Tethered Spinal Cord Syndrome (National Institute of Neurological Disorders and Stroke, 2024); Langman's Medical Embryology, 13th ed., ch. 18: Central Nervous System (Wolters Kluwer, 2015).

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Representative model, for educational purposes only.