Walsh & Hoyt: Astrocytic Tumors

Astrocytes can be separated based on their histologic appearance into two main types: fibrillary and protoplasmic. Fibrillary astrocytes are present throughout the central nervous system (CNS) except in gray matter, where protoplasmic astrocytes predominate. Fibrillary astrocytes contain fine fibers in their cell bodies and processes, whereas protoplasmic astrocytes have only a few such fibers that are seen only with electron microscopy. The nuclei of fibrillary astrocytes are irregular in shape and contain numerous indentations, whereas the nuclei of protoplasmic astrocytes are round or ovoid and have a smooth outline. The processes of fibrillary astrocytes, some of which make contact with capillaries and others with pia mater, are smooth, long, and branch freely, and those of protoplasmic astrocytes are irregular and have fewer branches. These histologic differences help distinguish the two types of astrocytes, particularly when they are examined in sections stained with gold-mercury or silver impregnation.