ABSTRACT
The CNS is made up of the spinal cord and the brain.
Spinal Cord The fluid-filled lumen that runs through the neural tube (structure that gives rise to the CNS) is called the neural canal or neurocoele. This relatively simple structure gives rise to the spinal canal of the spinal cord and its contained fluid becomes the cerebrospinal fluid. The walls of the neural tube are quite thin, originally organized into epithelium that is the thickness of one cell layer. This neuroepithelium gives rise to three kinds of cells:
e Ependymal cells, which make up the inside lining of the neural tube
e Neuroblasts, which give rise to the various kinds of neurons (nerve cells) found in the brain and spinal cord (or CNS)
e Glioblasts, from which two types of CNS glial cells arise-astrocytes and oligodendrocytes (Fig. 13-1)
As the spinal cord develops, it becomes much thicker in comparison to the surrounded fluid than the neural tube was. The cells of the wall of the spinal cord are arranged into three layers (from the spinal canal outward): • Ventricular layer, which contains the ependy-
mal cells • Mantle (intermediate) layer, which contains
the cell bodies of the neuroblasts, which in turn contain the nuclei of the nerve cells
• Marginal layer, which is made up of the processes (neurites: axons and dendrites) of the developing neurons
The mantle layer has a gray appearance and is therefore called gray matter; the marginal layer is similarly referred to as white matter, because of its appearance. From the center outward, we have spinal canal, ependyma, gray matter, and white matter (Fig. 13-2).
