The rat submaxillary salivary gland has five distinct parenchymal zones.
- 1Acini consist of secretory and myoepithelial cells. An extensive network of canaliculi connect the many cells within an acinus to the main lumen. The fine structure of acinar secretory cells suggests that they are capable of great synthetic capacity; each cell having a large amount of ergastoplasm, many Golgi zones, and a great amount of secretory material. It is proposed that these cells are of the continually secreting type.
- 2Intercalated ducts consist of cuboidal cells and myoepithelium. This segment connects the acini to the main conduit system of the gland. The fine structure of the cuboidal cells indicates that they are essentially nonsecretory.
- 3The granular duct consists of three types of columnar cells; (a) dark narrow cells which contain many free ribosomes but no ergastoplasm or granules, (b) light granular cells which have varying amounts of ergastoplasm and granules, (c) dark granular cells which are full of granules while the other cell constituents including the nucleus, occupy a basal position. It is proposed that these three cells represent different secretory stages of the same cell type. This supports the interpretation that secretion in these cells is not continuous, but cyclic in nature.
- 4The striated duct forms a small portion of the total gland parenchyma and consists of tall columnar cells with extensive infolding of the basal plasma membrane, relatively little ergastoplasm and very few granules. It seems likely that ion and water metabolism is a specialized function of this segment.
- 5The excretory duct consists of three cell types: (a) tall columnar light cells, (b) dark columnar vesiculated cells and (c) small basal cells. The basal infoldings of these cells and the arrangement of many capillaries around these ducts suggests that this segment is primarily concerned with water transport.