II. CYTOPATHOLOGY - PATHOLOGICAL CHANGES IN CELL COMPONENTS
Neurons are more sensitive to injury than other cell types in
the CNS. There may be selective vulnerability of groups of neurons to specific
types of processes. The following sections illustrate types of neuronal
reactions occurring in various disorders.
- 1. Central chromatolysis (Axonal reaction): When the axon of
a neuron is cut or damaged, the axon and its myelin sheath undergo degeneration
distal to the lesion (Wallerian degeneration). The sequence of events that
takes place in the cell body is known as central chromatolysis or axonal
- a) The cell body swells.
b) The Nissl bodies disperse and move peripherally.
c) The nucleus is displaced peripherally in the cell.
These changes begin within 24-48 hours and reach a peak at about 2 weeks.
This sequence of events is an attempt by the neuron to increase protein
synthesis and restore the integrity of its axon after injury; in the PNS,
axonal regeneration occurs in most cases. Central chromatolysis is most
dramatically seen in anterior horn cells after damage to a peripheral nerve.
- 2. Wallerian Degeneration: degeneration of axons and myelin
sheaths distal to a lesion.
- 3. Ischemic cell change: Neurons are quickly injured by ischemia
(regional absence of blood supply). After 6-12 hours morphological changes
include acute shrinkage, angularity, and homogeneous eosinophilia of the
cytoplasm. The nucleus becomes shriveled, pyknotic and hyperchromatic. These
changes are part of the process of cell death. Affected cells are called
ischemic neurons or red neurons or eosinophilic neurons.
- 4. Chronic cell change (simple atrophy): Chronic cell change
is generally seen in degenerative conditions of unknown etiology. In chronic
cell change the cell body shrinks and becomes more angular; the nucleus
becomes pyknotic and hyperchromatic, but the cytoplasm is not eosinophilic
as in the ischemic neuron.
- 5. Inclusions: Intracellular inclusions are mainly found in
two types of conditions: viral infections and degenerative diseases.
- a) In many cases of viral encephalitis, characteristic inclusion
bodies occur in the cytoplasm or nucleus of infected cells. The inclusion
bodies are not pathognomonic except in rabies, where Negri bodies occur.
- b) In some degenerative diseases, specific types of intraneuronal
inclusions characteristically occur. For example, in Parkinson's disease,
large spherical intracytoplasmic inclusions called Lewy bodies are
found. In Alzheimer's disease, cytoplasmic accumulation of abnormal neurofilaments,
called neurofibrillary tangles occurs.
- 6. Storage in neurons: Neurons may be the site of storage of
uncatabolized substances, such as lipids, in a number of inborn errors of
metabolism resulting from deficiencies of lysosomal enzymes.