Other multicellular glands typically form tubular invaginations from the surface. The end pieces of the gland contain the secretory cells; the portion of the gland connecting the secretory cells to the surface serves as a duct. If the duct is unbranched, the gland is called simple; if the duct is branched, it is called compound. If the secretory portion is shaped like a tube, the gland is tubular; if it is shaped like a flask, the gland is alveolar or acinar; if the tube ends in a saclike dilation, the gland is tubuloalveolar. Tubular secretory portions may be straight, branched, or coiled; alveolar portions may be single or branched. Various combinations of duct and secretory portion shapes are found in the body. Classification and description of exocrine glands may be found in Table 5.6.
Mucus and serous glands are so named because of the type of secretion produced.
The secretory cells of exocrine glands associated with the various body tubes (e.g., the alimentary canal, respiratory passages, and urogenital system) are often described as being mucus, serous, or both.
Mucus secretions are viscous and slimy, whereas serous secretions are watery. Goblet cells, secretory cells of the sublingual salivary glands, and surface cells of the stomach are examples of mucus-secreting cells. The mucus nature of the secretion results from extensive glycosylation of the constituent proteins with anionic oligosaccharides. The mucinogen granules, the secretory product within the cell, are therefore PAS positive (see Fig. 5.26a). However, they are water soluble and lost during routine tissue preparation. For this reason, the cytoplasm of mucus cells appears to be empty in H&E–stained paraffin sections. Another characteristic feature of a mucus cell is that its nucleus is usually flattened against the base of the cell by accumulated secretory product (Fig. 5.40).
In contrast to mucus-secreting cells, serous cells produce poorly glycosylated or nonglycosylated protein secretions. The nucleus is typically round or oval (Fig. 5.41). The apical cytoplasm is often intensely stained with eosin if its secretory granules are well preserved. The perinuclear cytoplasm often appears basophilic because of an extensive rough endoplasmic reticulum, a characteristic of protein-synthesizing cells.
Serous cell–containing acini (sing., acinus) are found in the parotid gland and pancreas. Acini of some glands, such as the submandibular gland, contain both mucus and serous cells. In routine tissue preparation, the serous cells are more
FIGURE 5.40 • Mucus-secreting compound gland. Photomicrograph showing two small lobes of a mucus-secreting gland associated with the larynx. Each displays the beginning of a duct (D) into which mucin is secreted (arrows). The individual secretory cells that form the acinus (A) are difficult to define. Their nuclei (arrowheads) are flattened and located in the very basal portion of the cell, a feature typical of mucus-secreting glands. The cytoplasm is filled with mucin that has been retained during preparation of the tissue and appears stained. 350.