Spinal Cord:
Development (Page 1 of 2)

Click for a Micrograph of a 8 week spinal cord

Click for Page 2, the functional aspects of alar and basal plates

As the spinal cord develops neuroblasts of the neural tube’s mantle layer proliferate in 2 zones. In cross section the mantle layer develops a characteristic “butterfly”-shape of gray matter. The lateral walls of the tube thicken but leave a shallow, longitudinal groove called the sulcus limitans which separates the developing gray matter into a dorsal alar plate and a ventral basal plate. These plates signal the future locations of sensory and motor functions, respectively. The sulcus limitans extends the length of the spinal cord and through the mesencephalon.

Alar plate - Neuronal cell bodies here form nuclei which constitute the uninterrupted dorsal gray matter (or columns) that receive and relay input from afferent (sensory, somatic and visceral) neurons. It extends the length of the cord. These neurons receive sensory information from axons in the dorsal roots of spinal nerves.

Basal plate - Cell bodies here form the uninterrupted ventral gray matter (or columns) of efferent neurons that extend the length of the cord. Axons of these neurons project motor fibers to skeletal muscle and form the ventral roots of the spinal nerves.

Proliferation of alar and basal plates causes accompanying changes. Bulges of the plates result in the formation of the longitudinally running
dorsal and ventral median septi. The lumen of the neural tube becomes reduced to a small central canal.

The marginal layer increases in mass due to the addition of longitudinally running intersegmental axons, long ascending axons from the gray matter, long descending axons from supraspinal levels and incoming dorsal root sensory fibers. The mass of fibers in the marginal layer subsequently becomes myelinated (beginning in the 4th month) and is called white matter. The dorsal gray and ventral gray columns partially segregate the white matter into
posterior, ventral and lateral white funiculi.

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