ELECTRON MICROSCOPY OF BASOPHILIC COMPONENTS OF CYTOPLASM

Abstract

Electron microscopy of thinly spread cells grown in vitro has defined, as a component of the cytoplasm, a complex reticulum of strands and vesicles which has come to be called the endoplasmic reticulum. The component is limited by a membrane similar in thickness (ca. 80 Å) to the plasma membrane of the cell. It separates the content of the strands and vesicles from the general matrix of the cytoplasm and gives to the whole component the character of a finely divided vacuolar system. The elements of the system are frequently large enough to be resolved by light microscopy and can be shown to have an affinity for basic dyes. The system has therefore been identified as a basophilic component of the cytoplasm (25). With the techniques now available for thin sectioning the same system of vesicles and interconnecting canaliculi can be demonstrated in cells fixed in situ and studies of such material have defined the system as a universally occurring component of the cytoplasm. In its structure it varies greatly from one cell type to another and even within a single kind of cell under different physiological states. In one form it is common for it to be made up of extraordinarily thin, flattened vesicles referred to as cisternae. In certain types of cells, particularly acinar cells of exocrine glands, these vesicles are organized in parallel arrays and such organizations have been shown to coincide with the distribution of the basophilic component.. Thus in thin sections as in cultured cells the reticulum is identified with the basophilic material. In very recent electron microscopy of thin sections, Palade (16) has observed that a dense granular component of the cytoplasmic matrix is usually adjacent to the membrane surfaces of the reticulum. In rapidly growing cells where the basophilia is diffuse and where elements of the reticulum may not be prominent, granules of this latter character (100-300 Å diameter) are abundant and scattered throughout the cytoplasmic matrix. This suggests therefore that in the other instances where the intensely basophilic component and the endoplasmic reticulum coincide, the latter structure owes its basophile properties to the quantities of the granular material which associate preferentially with its membrane surfaces. This conclusion finds support in studies of cell fractions (1, 23) which have defined small cytoplasmic particles that are extraordinarily rich in ribonucleic acid. It is also supported by the observation that parts of the nucleolus consist of closely packed granules of similar size and density. It appears then that the basophilic component when it exists as a distinct and localized entity of the cytoplasm, consists of a concentration of vesicular and canalicular elements of the endoplasmic reticulum with small basophilic granules associated. Where the basophilia is diffuse it seems to be an expression of the diffuse distribution of the small granules.