Basal Alar Roof And Floor Plates

As a result of continuous addition of neuroblasts to the mantle layer, each side of the neural tube shows a ventral and a dorsal thickening. The ventral thickenings, the basal plates, which contain ventral motor horn cells, form the motor areas of the spinal cord; the dorsal thickenings, the alar plates, form the sensory areas (Fig. 19.8A). A longitudinal groove, the sulcus limitans, marks the boundary between the two. The dorsal and ventral midline portions of the neural tube, known as the roof and floor plates, respectively, do not contain neuroblasts; they serve primarily as pathways for nerve fibers crossing from one side to the other.

Roof plate Alar plate

Sulcus limitans

Basal plate a

Sulcus limitans

Marginal Plate And Alar Plate
Floor plate

Mantle layer

Marginal layer

Central canal

Intermediate horn

Neuroepithelial B layer

Dorsal median septum

Dorsal sensory horn

Dorsal median septum

Dorsal sensory horn

Intermediate Horn
Ventral fissure

White matter

Ventral motor horn

Basal plate a

Intermediate horn

Neuroepithelial B layer

Ventral motor horn

Electron Micrograph Spinal Cord

Figure 19.8 A and B. Two successive stages in the development of the spinal cord. Note formation of ventral motor and dorsal sensory horns and the intermediate column. C. Scanning electron micrograph of a section through the spinal cord of a mouse embryo showing a stage similar to that in A. SG, spinal ganglion.

Figure 19.8 A and B. Two successive stages in the development of the spinal cord. Note formation of ventral motor and dorsal sensory horns and the intermediate column. C. Scanning electron micrograph of a section through the spinal cord of a mouse embryo showing a stage similar to that in A. SG, spinal ganglion.

In addition to the ventral motor horn and the dorsal sensory horn, a group of neurons accumulates between the two areas and forms a small intermediate horn (Fig. 19.85). This horn, containing neurons of the sympathetic portion of the autonomic nervous system, is present only at thoracic (T1-T12) and upper lumbar levels (L2 or L3) of the spinal cord.

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