Introduction
The central nervous system contains two types of cells:
(1) The nerve cells proper;
(2) the connective tissue of the central nervous system (neuroglia), including the ependymal lining of the brain cavities.
In addition to these two types, there are the cells of the meninges whish cover:
(a) The central nervous system;
(b) The blood vessels which enter it;
(c) The nerves which arise from it.
The neuroglia consists of three types of cells. Two are derived from ectoderm in common with most of the nervous system- the astrocytes and the oligodendroglia. The third type forms the microglia- a phagocytic type of cell derived from the mesoderm as are the blood vessels.
Astrocytes are star-shaped cells with processes radiating from them. They pervade the central nervous system, forming a surface layer throughout it. At least one of the numerous processes passes to form an end-foot on an adjacent capillary, so that together they ensheathe the capillaries of the central nervous system. Because of this and because their biochemistry alters with that of the nerve cells adjacent to them, it is believed that they play an important part in nerve cell metabolism and transfer of substances from the blood to the nerve cells. Two main types are present with many forms.
(1) The nerve cells proper;
(2) the connective tissue of the central nervous system (neuroglia), including the ependymal lining of the brain cavities.
In addition to these two types, there are the cells of the meninges whish cover:
(a) The central nervous system;
(b) The blood vessels which enter it;
(c) The nerves which arise from it.
The neuroglia consists of three types of cells. Two are derived from ectoderm in common with most of the nervous system- the astrocytes and the oligodendroglia. The third type forms the microglia- a phagocytic type of cell derived from the mesoderm as are the blood vessels.
Astrocytes are star-shaped cells with processes radiating from them. They pervade the central nervous system, forming a surface layer throughout it. At least one of the numerous processes passes to form an end-foot on an adjacent capillary, so that together they ensheathe the capillaries of the central nervous system. Because of this and because their biochemistry alters with that of the nerve cells adjacent to them, it is believed that they play an important part in nerve cell metabolism and transfer of substances from the blood to the nerve cells. Two main types are present with many forms.
1. The potoplasmc astrcytes whose processes branch and rebranch to form a dense bush, and which are found in areas where nerve cells predominate (gray matter)
2. The fibrillary astrocytes are found predominantly among the bundles of nerve fibres (white matter). Their long, thin processes branch infrequently and appear smooth in stained preparations. Astrocytes are numeros in old injuries of the nervous system. They seem to react as fibrocytes do in other parts of body to form a scar, but there is no good evidence that they undergo mitotic division. However, their rapid accumulation at the site of an injury, and the numerous processes which they form there have been suggested as the reason for the failure of central nervous system axons to grow across wounds.
2. The fibrillary astrocytes are found predominantly among the bundles of nerve fibres (white matter). Their long, thin processes branch infrequently and appear smooth in stained preparations. Astrocytes are numeros in old injuries of the nervous system. They seem to react as fibrocytes do in other parts of body to form a scar, but there is no good evidence that they undergo mitotic division. However, their rapid accumulation at the site of an injury, and the numerous processes which they form there have been suggested as the reason for the failure of central nervous system axons to grow across wounds.