The Structure and Composition of Rat Reticulocytes : I. The Ultrastructure of Reticulocytes

1. Fixed, sectioned rat reticulocytes, embedded in araldite, have a regular outer limiting margin like that of erythrocytes; however, unlike erythrocytes they contain cytoplasmic organelles similar to those in erythroblasts and other mammalian cells. These include mitochondria, ribosomes and a sparse endoplasmic reticulum consisting of rounded vesicles and occasional small cisternae. The endoplasmic reticulum appears to consist wholly of "rough-surfaced" elements.

2. Many mitochondria have disorientated cristae or ruptured outer membranes. The possible origin of these alterations is discussed.

3. Ribosomes appear as electron-opaque granules 10-20 mµ in diameter in sections of whole cells. Most of them occur free in the cytoplasm while a few appear to be attached to the surface of vesicles and cisternae of the endoplasmic reticulum. They can be isolated from ruptured cells by high speed centrifugation and have a similar chemical composition to ribosomes from other mammalian tissues.