Histology of the central nervous system

The intent of this article is to assist pathologists inexperienced in examining central nervous system (CNS) sections to recognize normal and abnormal cell types as well as some common artifacts. Dark neurons are the most common histologic artifact but, with experience, can readily be distinguished from degenerating (eosinophilic) neurons. Neuron degeneration stains can be useful in lowering the threshold for detecting neuron degeneration as well as for revealing degeneration within populations of neurons that are too small to show the associated eosinophilic cytoplasmic alteration within H&E-stained sections. Neuron degeneration may also be identified by the presence of associated macroglial and microglial reactions. Knowledge of the distribution of astrocyte cytoplasmic processes is helpful in determining that certain patterns of treatment-related neuropil vacuolation (as well as some artifacts) represent swelling of these processes. On the other hand, vacuoles with different distribution patterns may represent alterations of the myelin sheath. Because brains are typically undersampled for microscopic evaluation, many pathologists are unfamiliar with the circumventricuar organs (CVOs) that represent normal brain structures but are often mistaken for lesions. Therefore, the six CVOs found in the brain are also illustrated in this article.