The cells of this variety of epithelium are approximately of the same diameter in all directions. They may be almost strictly cubical or spherical, but are usually polyhedral as the result of mutual compression, their contiguous surfaces being flattened. They are usually disposed in a single layer upon a surface furnished by the underlying tissues, as, for example, in tubular or racemose glands, but they may be aggregated to form a solid mass of cells filling a sac, as in the sebaceous glands of the skin, or in strands or columns, variously disposed, as in the liver and suprarenal bodies.
It is this form of epithelium that is chiefly concerned in performing the functions of secretion, and, for this reason, it is frequently designated as "glandular epithelium".
The appearance of the individual cells varies considerably according to the functions that they perform and the stage of functional activity which obtained at the time cellular changes were arrested when the particular specimen was prepared for study. It will suffice for present purposes of description to call attention to the fact that the cytoplasm is usually highly granular, partly because of its own structure, partly because many of the substances elaborated and stored within the cells as the result of their functions appear in the form of granules (metaplasm). The nature of these granules varies. They may be albuminoid, zymogenic granules, or minute drops of fatty substances, which may coalesce to form distinct oily globules, or they may consist of carbohydrates, e. g., glycogen. The granular condition of the cytoplasm may be so marked as to render the detection of the nucleus difficult in unstained specimens.
In this form of epithelium the presence of two nuclei in a single cell is more frequent than in the other varieties.