Properties of nervous system

E. Embryonic Development of Nerve Tissue: All neurons and supporting cells derive from embryonic ectoderm.

F. Aging and Repair: Mature neurons are incapable of mitosis and are often used as examples of terminally differentiated cells. Neurons of the elderly may contain abundant lipofuscin pigment. The inability of neurons to divide makes repair of injured nerve tissue more difficult than for most other tissues. Neuron cell bodies lost through injury or surgery cannot be replaced, but if an axon is severed or crushed and the cell body remains intact, regeneration of the injured axon is possible. Supporting cells, unlike neurons, can divide if stimulated by injury.


G. Meninges: The brain and spinal cord are separated from the bony compartments that house them (skull and vertebral canal) by 3 connective tissue layers termed the meninges. The outer layer, or dura mater, is dense connective tissue bound tightly to the periosteum of the surround ing bone. The middle layer, or arachnoid, has 2 components: (1) a layer of loose connective tissue in contact with the dura mater, and (2) many connective tissue trabeculae (strands) that attach the arachnoid to the underlying pia mater. The spaces between the arachnoid trabeculae contain cerebrospinal fluid. Projections of the arachnoid into sinuses in the dura are called arachnoid villi. The innermost layer, or pia mater, is a thin, richly vascularized layer of loose connective tissue that is firmly attached to the surface of the brain or spinal cord but separated from the neurons by neuroglial cells processes. Ramified, cuboidal epithelium-covered projections of the pia matter into the ventricles of the brain are collectively termed the choroid plexus; they produce the cerebrospinal fluid by selective ultrafiltration of the blood plasma.