Morphological analysis of single astrocytes of the adult cat central nervous system visualized by HRP microinjection

The morphology of individual astrocytes of the adult cat was analyzed by HRP microinjection and light microscopy. The astrocytes had generally two types of processes: (1) thread-like processes of relatively constant width with few ramifications and few lamellar appendages and (2) the sinuous processes with clusters of lamellar appendages. The former processes were morphologically characterized as follows: (1) Those of fibrous astrocytes were frequently remarkably long, ranging from 600 to 1500 microns. They were much longer than any astrocytic processes hitherto reported in the literature. In contrast, those of protoplasmic astrocytes were usually short (30-400 microns), and were sometimes decorated with lamellae. (2) The processes often terminated in endfeet on the subpial surface of the brain tissue or on blood vessel walls. The number of endfeet per cell varied from 1-11 with a tendency to split into two populations close to each extreme number. Another type of endfoot was also found, i.e. swellings along the trunk of the processes which made side-to-side contact with the vessel wall. The sinuous processes rich in lamellae were predominant in protoplasmic astrocytes, and clearly corresponded to the sheet- or veil-like processes of Golgi-impregnated astrocytes. They formed an ellipsoidal field (100 microns for the longer, and 50-60 microns for the shorter, diameter) around the nucleus.