Spot a neuroglial cell of mesodermal origin
1)Ependymal cell 2)Microglia
3)Astrocyte 4)Oligodendrocyte
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Explanation:
non-nervous, interstitial component made up of stellate or spindle-shaped cells, morphologically distinct from neurons, which he named neuroglia, or “nerve glue.” It was not until the early part of the twentieth century that this interstitial element was classified as consisting of distinct cell types [3,4]. Today, we recognize three broad groups of glial cells: (i) true glial cells or macroglia, such as astrocytes and oligodendrocytes, of ectodermal origin, the stem cell of which is the spongioblast; (ii) microglia, of mesodermal origin; and (iii) ependymal cells, also of ectodermal origin and sharing the same stem cell as true glia. Microglia invade the CNS at the time of vascularization via the pia mater, the walls of blood vessels and the tela choroidea. Glial cells differ from neurons in that they possess no synaptic contacts and retain the ability to divide throughout life, particularly in response to injury. The rough schema represented in Figure 1-3 demonstrates the interrelationships between the macroglia and other CNS components.
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Virtually nothing can enter or leave the central nervous system parenchyma without passing through an astrocytic interphase
The complex packing achieved by the processes and cell bodies of astrocytes underscores their involvement in brain metabolism. Although astrocytes traditionally have been subdivided into protoplasmic and fibrous astrocytes [4], these two forms probably represent the opposite ends of a spectrum of the same cell type. However, Raff et al. [22] have suggested that the two groups might derive from different progenitors and that the progenitor of the fibrous astrocyte is the same as that of the oligodendrocyte. The structural components of fibrous and protoplasmic astrocytes are identical; the differences are quantitative. In the early days of EM, differences between the two variants were more apparent owing to imprecise techniques, but with the development of better procedures, the differences became less apparent.
Protoplasmic astrocytes range in size from 10 to 40 μm, frequently are located in gray matter in relation to capillaries and have a clearer cytoplasm than do fibrous astrocytes (Fig. 1-12). Within the perikaryon of both types of astrocyte are scattered 9-nm filaments and 24-nm microtubules (Fig. 1-13); glycogen granules; lysosomes and lipofuscin-like bodies; isolated cisternae of the rough ER; a small Golgi apparatus opposite one pole of the nucleus; and small, elongated mitochondria, often extending together with loose bundles of filaments along cell processes. A centriole is not uncommon. Characteristically, the nucleus is ovoid and the nucleochromatin homogeneous, except for a narrow, continuous rim of dense chromatin and one or two poorly defined nucleoli. The fibrous astrocyte occurs in white matter (Fig. 1-13). Its processes are twig-like, being composed of large numbers of 9-nm glial filaments arranged in tight bundles. The filaments within these cell processes can be distinguished from neurofilaments by their close packing and the absence of side-arms (Figs. 1-13 and 1-14). Desmosomes and gap junctions occur between adjacent astrocytic processes.
Figure 1-12. A protoplasmic astrocyte abuts a blood vessel (lumen at L) in rat cerebral cortex.
Figure 1-12
A protoplasmic astrocyte abuts a blood vessel (lumen at L) in rat cerebral cortex. The nucleus shows a rim of denser chromatin, and the cytoplasm contains many organelles, including Golgi and rough endoplasmic reticulum. ×10,000. Inset: Detail (more...)
Figure 1-13. A section of myelinating white matter from a kitten contains a fibrous astrocyte (A) and an oligodendrocyte (O).
Figure 1-13
A section of myelinating white matter from a kitten contains a fibrous astrocyte (A) and an oligodendrocyte (O). The nucleus of the astrocyte (A) has homogeneous chromatin with a denser rim and a central nucleolus. That of the oligodendrocyte (O) is denser (more...)
In addition to protoplasmic and fibrous forms, regional specialization occurs among astrocytes. The outer membranes of astrocytes located in subpial zones and adjacent to blood vessels possess a specialized thickening. Desmosomes and gap junctions are very common in these regions between astrocytic processes. In the cerebellar cortex, protoplasmic astrocytes can be segregated into three classes, each ultrastructurally distinct: the Golgi epithelial cell, the lamellar or velate astrocyte and the smooth astrocyte [1].
Astrocyte functions have long been debated. Their major role is related to a connective tissue or skeletal function since they invest, possibly sustain and provide a packing for other CNS components. In the case of astrocytic ensheathment around synaptic complexes and the bodies of some neurons, such as Purkinje cells, it has been speculated that the astrocyte isolates these structures.