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storage diseases

Chromatolysis

Axonal damage

neurons from any cause, breakdown of myelin, or injury to nerve fibers are followed by an increase in the number of astrocytes and glial fibers; this process is called gliosis, which is composed entirely of cellular processes.

Oligodendrocytes are the myelin-forming cells of the CNS. In white matter they are frequently lined up along the myelinated fibers. In gray matter they tend to cluster around the neurons, where they are called satellite cells. Since they are responsible for synthesis and maintenance of myelin, they are affected in disorders that depend on myelin and myelination, mainly the leukodystrophies and demyelinating diseases (e.g., multiple sclerosis). The reaction to injury is limited: The cell turnover is very slow and the presence of axons is necessary for their proliferation.

Microglial cells, which are brain-intrinsic cells, have phagocytic properties and contribute to the CNS's macrophage response to injury. The conditions under which the microglial reaction is found cover a range from mild proliferation around chromatolytic neurons to the removal of degenerating terminals and the invasion of totally necrotic brain as in an infarct. Since they express major histocompatible antigens, micro-glial cells also act as antigen presenting cells.

3. Connective Tissue

Connective tissue encloses the meninges, including the dura, arachnoid, and the pia, covering the complete surface of the brain. The properties of separate compartments (subarachnoidal, subdural, and epidur-al) result in different effects of lesions, even in cases in which they are also the origin and target of disorders (e.g., infections and tumors). Extradural lesions tend to be localized, whereas subarachnoidal lesions (e.g., subarachnoidal bleeding) can expand widely over the surface of the whole brain. An important function of the mesenchyme is the resorption of the cerebrospinal fluid (CSF) into the venous sinus via arachnoid granulation. Interruption of resorption at these sites due to obliteration after inflammation or hemorrhage leads to an accumulation of CSF with the potential risk of increased intracranial pressure and hydrocephalus.

4. Blood Vessels

The blood vessels in the CNS are similar in structure and function to those elsewhere in the body, with the important exception of the capillaries. The capillaries within the CNS differ from most other capillaries because they are not fenestrated. Tight junctions are present between adjacent cells and the endothelial cell basement membrane is intimately surrounded by a network of astrocytic processes. These special structural features are important constituents of the blood-brain barrier. In general, an inflammation of the CNS (e.g., encephalitis) and/or the meninges (e.g., meningitis) is associated with a disturbance of the blood-brain barrier. Such malfunction of the blood-brain barrier enables the visualization of brain lesions by contrast enhanced functional imaging. Congenital or acquired malformations of the cerebral blood vessels (e.g., angioma, AV fistula, and cavernoma), inflammation (e.g., viral, bacterial, autoimmunological, radiogenic, granulomatous, sarcoidous, and angiitis), structural anomalies (congophilic angiopathy), venous or arterial obstruction (sinus thrombosis and thrombotic or embolic arterial occlusion), or disturbed autoregulation of the diameter of the small brain vessels (e.g., microangiopathy) can cause a variety of brain lesions, such as lacunar or territorial ischemia, hematoma, edema, or calcifications.

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