Phagocytosis in brain is caused by
Microglia is the nervous system counter- pa of the monocyte-macrophage system. Although the term 'microglia' is commonly used but it is inappropriate since these cells, unlike neuroglia, are not of neuroectodermal origin. Microglial cells (or Hoega cells) are not fixed but are mobile cells. These cells are found throughout the brain and are often present close to the blood vessels. Normally, microglial cells appear as small inconspicuous cells with bean-shaped vesicular nuclei, scanty cytoplasm and long cytoplasmic processes (Fig. 30.1,C). In response to injury or damage, however, these cells have capability to enlarge in size, proliferate and develop elongated nuclei, so called rod cells. Microglial cells may actually assume the shape and phagocytic function of macrophages and form gitter cells. The foci of necrosis and areas of selective hypoxic damage to the neurons are surrounded by microglial cells which perform phagocytosis of damaged and necrosed cells; this is known as neuronophagia. NEUROGLIA. The neuroglia provides suppoive matrix and maintenance to the neurons. It includes 3 types of cells: astrocytes, oligodendrocytes and ependymal cells Neuroglia is generally referred to as glia; thetumours originating from it are termed gliomas, and reactive proliferation of the astrocytes being called gliosis. i) Astrocytes. The astrocytes are stellate cells with numerous fine branching processes. In routine haematoxylin and eosin stains, an astrocyte has round or oval vesicular nucleus, but unlike neuron, lacks a prominent nucleolus. The cytoplasm is generally scanty. The processes radiate from the cell body. Depending upon the type of processes, two types of astrocytes are distinguished: Protoplasmic astrocytes have branched processes and are found mostly in the grey matter. Fibrous astrocytes have long, thin processes and are present mainly in the white matter. Some astrocytic processes are directed towards neurons and their processes, which others surround capillaries by terminal expansions called foot processes. The astrocytic processes may not be visible by routine stains but can be demonstrated by phosphotungstic acid haematoxylin (PTAH) stain. Ultrastructurally, these processes are composed of abundant intermediate filaments, mostly vimentin. The main functions of astrocytes in health are physio- logical and biochemical suppo to the neurons and interactions with capillary endothelial cells to establish blood brain barrier. In case of damage to the brain, astrocytes act like fibroblasts of other tissues. The astrocytes in respond to injury undergo hyperplasia and hyperophy termed 'gliosis' which is an equivalent of scar elsewhere in the body. Gemistocytic astrocytes are early reactive astrocytes having prominent pink cytoplasm. Long-standing progressive gliosis results in the development of Rosenthal fibres which are eosinophilic, elongated or globular bodies present on the astrocytic processes. Corpora amylacea are basophilic, rounded, sometimes laminated bodies, present in elderly people in the white matter and result from accumulation of starch-like material in the degenerating astrocytes. ii) Oligodendrocytes. Oligodendrocytes are so named because of their sho and fewer processes when examined by light microscopy with special stains (oligo=sho). In haematoxylin-eosin stained sections, these cells appear as small cells with a darkly-staining nucleus resembling that of small lymphocyte. The cytoplasm appears as a clear halo around the nucleus. Oligodendrocytes are present throughout the brain in grey as well as white matter and are most numerous of all other cells in the CNS. In grey matter, they are clustered around the neurons and are called satellite cells. In white matter, they are present along the myelinated nerve fibres and are termed interfascicular oligodendroglia. The major function of oligodendrocytes is formation and maintenance of myelin. Thus, in this respect they are counterpas of Schwann cells of the peripheral nervous system. Diseases of oligodendrocytes are, therefore, disorders of myelin and myelinisation such as inherited leucodys- trophies and acquired demyelinating diseases. iii) Ependymal cells. The ependymal cells are epithelium- like and form a single layer of cells lining the ventricular system, aqueduct, central canal of the spinal cord and cover the choroid plexus. They are cuboidal to columnar cells and have ciliated luminal surface, just beneath which are present small bodies termed blepharoplasts. The ependymal cells influence the formation and composition of the cerebrospinal fluid (CSF) by processes of active secretion, diffusion, absorption and exchange. The function of cilia is not very clear but probably they play a role in the circulation of CSF. The ependymal cells respond to injury by cell loss and the space left is filled by proliferation of underlying glial fibres. Ref:TEXTBOOK OF PATHOLOGY 6th EDITION - HARSH MOHAN PAGE NO:871-872 MICROGLIA.
Astrocytes
Microglia
oligodendrocytes
Ependymal cells
Nervous system