Electron Microscopy of Centrioles of Normal and Reactive Astrocytes

This paper described the fine structure of centrioles in the normal astrocytes of the optic nerve of frogs and in reactive astrocytes during Wallerian degeneration following the removal of the eye ball. The materials were fixed in 1% osmium tetroxide, embedded in Araldite or styrene-methacrylate, sectioned with glass knives and stained with potassium permanganate or sodium hydroplumbite. The results obtained were as follows: Two centrioles are present in the cytoplasm adjacent to the nucleus. Each of them is a hollow cylinder about 170 mμin diameter and 380 mμ in length. The wall of a centriole usually consists of a set of 9 outer fibers; each of the outer fibers is composed of two or three tubular subfibers which appear as doublets or triplets. The subfiber is approximately 15 mμ in diameter and is composed of an electron-dense, thin wall and a less-dense lucid core. The adjacent subfibers of an outer fiber are quite intimately associated with each other on one side. They are all inclined in the same direction at an angle of about 30° to 45° to the circumference of the centriole, thus having a spiral or pinwheel appearance in transverse section. Connecting lines between subfibers in adjacent triplets are seen. Usually the lumen of the centriole appears lucid. It, however, often contains such elaborate central structures as a cart-wheel like structure or a central fiber. The central fiber is usually composed of two subfibers. During Wallerian degeneration, the centrioles of the reactive astrocytes show some changes in their fine structure. Some of them appear shorter in length. The appearance of a satellite, decrease in the number of the outer fiber, decrease of the subfibers in the central fiber and the accumulation of an electron-dense, amorphous substance around the centrioles are observed. In both normal and reactive astrocytes, there are often found rudimentary cilia.