A. Supporting Cells of the CNS: There are about 10 neuroglial cells per neuron in the CNS. Glial cells are generally smaller than neurons. Their processes, although abundant and exten sive, are indistinguishable without special stains. Identification is usually based on nuclear morphology. The major supporting cells in the CNS are the macroglia, including astrocytes and oligodendrocytes, the microglia, and the ependymal cells.
1. Astrocytes are the largest glial cells. Their nuclei, also the largest, are irregular, spheric, and pale-staining with a prominent nucleolus. Their branching cytoplasmic processes often have, at their tips, expanded pediclcs, or vascular end-feet. These surround capillaries of the pia mater and are important components of the blood-brain barrier . Proteplasmic astro cytes (messy cells) are more common in gray matter. They have ample granular cytoplasm and short, thick, highly branched processes. Fibrens astrecytes are more common in white matter. Silver stains show their cytoplasm to be full of fibrous material. Their long, thin processes are less branched than those of protoplasmic astrocytes. 2. Oligedendroglia or oligodendrocytes, the most numerous glial cells, are found in both gray and white matter. Their spheric nuclei fall between those of astrocytes and microglia in terms of size and staining intensity. Like the Schwann cells of the PNS, oligodendrocytes form myelin and occur in long rows as required to myelinate entire axons. Unlike a Schwann cell, each may have several cell processes and may provide myelin for segments of several axons. Unmyelinated axons of the CNS are not sheathed. 3. Microglia, the smallest and rarest of the glia, are found in both gray and white matter. Their nuclei are small and elongate (often bean-shaped), and their chromatin is so condensed that they often appear black in H&E-stained sections. Their processes are shorter than those of astrocytes and are covered with thorny branches. Microglial cells may derive from mes enchyme, or they may be glioblasts (immature oligodendrocytes) of neuroepithelial origin. Some microglia may be components of the mononuclear phagocyte system and have phago cytic capabilities. When neural injury is unaccompanied by vascular injury, phagocytic cells in the lesioned area appear to derive from macroglia. 4. Ependymal cells derive from ciliated neuroepithelial cells of the internal lining of the neural tube. In adults, they retain their epithelial nature and some cilia, and they line the remnants of the neural tube (ventricles and aqueducts of the brain and the central canal of the spinal cord). The lining resembles a simple columnar epithelium, but epcndymal cells have basal cell processes that extend deep into the gray matter. The ependymal lining is con tinuous with the cuboidal epithelium of the choroid plexus.
SUPPORTING CELLS
GENERAL FEATURES OF NERVE TISSUE & THE NERVOUS SYSTEM
ØA. Two Classes of Cells: Nerve tissue consists of the neurons that transmit impulses and the supporting cells that surround them. It contains little extracellular material.
Ø1. Neurons. These cells are highly specialized to carry out nerve tissue functions. Neurons receive, integrate, and transmit electrochemical messages. Each has a cell body, also called the soma ("body") or perikaryon ("around the nucleus"), comprising the nucleus and the surrounding cytoplasm and plasma membrane. Each neuron has a variable number of dendrites, cytoplasmic processes that collect incoming messages and carry them toward the soma, and a single axon, a cytoplasmic process that transmits messages to the target cell. Axons of most neurons have a myelin sheath formed by supporting cells and interrupted by gaps called nodes of Ranvier. Myelinated axon segments between the gaps are called internodes. 2. Supporting cells. These cells are called neuroglia ("nerve glue") or glial cells. Their functions include structural and nutritional support of neurons, electrical insulation, and enhancement of impulse conduction velocity along axons.
