Astrocyte Fibrous

FIGURE 1-14 Transverse sections of a myelinated axon (left) and the process of a fibrous astrocyte (right) in dog spinal cord. The axon contains scattered neurotubules and loosely packed neurofilaments interconnected by side-arm material. The astrocytic process contains a bundle of closely packed filaments with no cross-bridges, flanked by several microtubules. Sometimes, a lumen can be seen within a filament. X60,000. 

Figure 14.4 Different types of glial cells. Astrocytes connect capillaries and neurones. Fibrous astrocytes, with less branching and more filamentous processes, occur mainly in white matter while protoplasmic astrocytes are located principally in the grey matter. Oligodendrocytes form the myelin sheath by wrapping themselves around axons. The connection between the myelin sheath and the oligodendrocyte is permanent and provides material for the myelin sheath. Microgliocytes (microglia) are the phagocytes of the nervous system. The ciliated ependymal cells line the cavities of the central nervous system.

At the inner border of ONH, the ILM becomes continuous with the basement membrane of fibrous astrocytes lining the internal surface of the ONH [21]. However, the lateral borders between the ONH and the adjacent choroid and retina are not well defined. Furthermore, it was reported [49] that micro vessels in the prelaminar region of the ONH lack classical blood-brain barrier characteristics and display nonspecific permeability, possibly mediated by vesicular transport. Thus, there is a theoretical possibility that topically applied drugs can penetrate indirectly through the retrobulbar space and then, through the ONH, reach the posterior choroid and retina. It was reported that following retrobulbar administration of fluorescein, the dye rapidly accumulated in the ONH and penetrated later to the vitreous [50],... 

The protoplasmic ash ocytes, on the other hand, have more complex morphology. They contain highly branched processes that form membranous sheets surrounding the neuronal processes, cell bodies and end-feet on capillaries. In conh ast to fibrous astrocytes, these cells have fewer IFs and a greater density of organelles. 

Macroglial cells are of ectodermal origin and include the astrocytes, ependymal cells, oligodendrocytes, and Schwann cells. Astrocytes appear to be primarily supportive in function and under microscopic examination can be seen to envelope axons and dendrites and buttress against the adventitial layer of blood vessels. In the gray matter of brain, which is composed primarily of perikarya, they have a clear protoplasmic appearance, whereas in the white matter, which is composed primarily of neuronal processes, they appear fibrous. Ependymal cells line the ventricles of the brain, are ciliated. 

Astrocytes, oligodendrocytes, ependyma, and microglial cells are four major support cells in the mammalian CNS. Astrocytes provide a structural scaffold for other elements of the CNS and regulate the exchange of fluid. Fibrous and protoplasmic astrocytes are two types of astrocytes. Oligodendrocytes coat the axons in the CNS with myelin. The primary function of the ependyma is to interact with astrocytes to form a barrier separating the ventricles of the brain and cerebrospinal fluid (CSF) from the CNS... 

---