Page 888 - Textbook of Pathology, 6th Edition

872 tumours originating from it are termed gliomas, and reactive system, aqueduct, central canal of the spinal cord and cover
proliferation of the astrocytes being called gliosis. the choroid plexus. They are cuboidal to columnar cells and
i) Astrocytes. The astrocytes are stellate cells with have ciliated luminal surface, just beneath which are present
numerous fine branching processes. In routine haematoxylin small bodies termed blepharoplasts.
and eosin stains, an astrocyte has round or oval vesicular The ependymal cells influence the formation and
nucleus, but unlike neuron, lacks a prominent nucleolus. The composition of the cerebrospinal fluid (CSF) by processes of
cytoplasm is generally scanty. The processes radiate from active secretion, diffusion, absorption and exchange. The
the cell body. Depending upon the type of processes, two function of cilia is not very clear but probably they play a
types of astrocytes are distinguished: role in the circulation of CSF. The ependymal cells respond
to injury by cell loss and the space left is filled by proliferation
Protoplasmic astrocytes have branched processes and are of underlying glial fibres.
found mostly in the grey matter.
3. MICROGLIA. Microglia is the nervous system counter-
Fibrous astrocytes have long, thin processes and are present
mainly in the white matter. part of the monocyte-macrophage system. Although the term
Some astrocytic processes are directed towards neurons ‘microglia’ is commonly used but it is inappropriate since
and their processes, which others surround capillaries by these cells, unlike neuroglia, are not of neuroectodermal
terminal expansions called foot processes. The astrocytic origin. Microglial cells (or Hortega cells) are not fixed but
processes may not be visible by routine stains but can be are mobile cells. These cells are found throughout the brain
demonstrated by phosphotungstic acid haematoxylin and are often present close to the blood vessels. Normally,
(PTAH) stain. Ultrastructurally, these processes are microglial cells appear as small inconspicuous cells with
composed of abundant intermediate filaments, mostly bean-shaped vesicular nuclei, scanty cytoplasm and long
vimentin. cytoplasmic processes (Fig. 30.1,C). In response to injury or
The main functions of astrocytes in health are physio- damage, however, these cells have capability to enlarge in
logical and biochemical support to the neurons and size, proliferate and develop elongated nuclei, so called rod
interactions with capillary endothelial cells to establish blood cells. Microglial cells may actually assume the shape and
brain barrier. In case of damage to the brain, astrocytes act phagocytic function of macrophages and form gitter cells. The
like fibroblasts of other tissues. The astrocytes in respond to foci of necrosis and areas of selective hypoxic damage to the
injury undergo hyperplasia and hypertrophy termed ‘gliosis’ neurons are surrounded by microglial cells which perform
which is an equivalent of scar elsewhere in the body. phagocytosis of damaged and necrosed cells; this is known
SECTION III
as neuronophagia.
Gemistocytic astrocytes are early reactive astrocytes having
prominent pink cytoplasm. Long-standing progressive 4. DURA MATER. The dura mater is a tough fibrous
gliosis results in the development of Rosenthal fibres which covering of the brain which is closely attached to the skull
are eosinophilic, elongated or globular bodies present on the on its inner layer of endocranial periosteum. In the region of
astrocytic processes. spinal canal, it encloses a potential space, the epidural space,
Corpora amylacea are basophilic, rounded, sometimes between the bone and the dura. The dura is composed of
laminated bodies, present in elderly people in the white dense collagen, fused with periosteum of the skull.
matter and result from accumulation of starch-like material 5. PIA-ARACHNOID (LEPTOMENINGES). The lepto-
in the degenerating astrocytes. meninges (lepto=thin, slender) consisting of the pia and
ii) Oligodendrocytes. Oligodendrocytes are so named arachnoid mater form the delicate vascular membranous
Systemic Pathology
because of their short and fewer processes when examined covering of the central nervous system. The pia mater is
by light microscopy with special stains (oligo=short). In closely applied to the brain and its convolutions, while the
haematoxylin-eosin stained sections, these cells appear as arachnoid mater lies between the pia mater and the dura
small cells with a darkly-staining nucleus resembling that of mater without dipping into sulci. Thus, a space is left between
small lymphocyte. The cytoplasm appears as a clear halo the two layers of leptomeninges, known as subarachnoid space,
around the nucleus. Oligodendrocytes are present which contains the CSF. The major arteries and veins run in
throughout the brain in grey as well as white matter and are the subarachnoid space and small nutrient arteries pass into
most numerous of all other cells in the CNS. In grey matter, the cortex. Extension of the subarachnoid space between the
they are clustered around the neurons and are called satellite wall of blood vessels entering the brain and their pial sheaths
cells. In white matter, they are present along the myelinated form a circumvascular space called Virchow-Robin space.
nerve fibres and are termed interfascicular oligodendroglia. Another important potential space is enclosed between the
The major function of oligodendrocytes is formation and dura and the arachnoid membrane known as subdural space.
maintenance of myelin. Thus, in this respect they are
counterparts of Schwann cells of the peripheral nervous DEVELOPMENTAL ANOMALIES
system. Diseases of oligodendrocytes are, therefore, disorders These malformations are the result of various inherited and
of myelin and myelinisation such as inherited leucodys- acquired factors. The acquired conditions include viral
trophies and acquired demyelinating diseases. infections of the mother and foetus (e.g. rubella), intake of
iii) Ependymal cells. The ependymal cells are epithelium- drugs (e.g. thalidomide), exposure to ionising radiation and
like and form a single layer of cells lining the ventricular foetal anoxia. There are a large number of developmental