Secretion by striated ducts of mammalian major salivary glands: Review from an ultrastructural, functional, and evolutionary perspective

In addition to their role in electrolyte homeostasis, striated ducts (SDs) in the major salivary glands of many mammalian species engage in secretion of organic products. This phenomenon usually is manifested as the presence of small serous-like secretory granules in the apical cytoplasm of SD cells . The composition of these granules is largely unknown, except in the case of the cat and rat submandibular gland, where the granules have unequivocal ly been shown to contain kallikrein. In some species, the apical cytoplasm of SD cells contains variable numbers of vesicles, both spherical and elong ated, that vary in appearance from 'empty' to moderately dense. In the rat parotid gland, lucent vesicles transport glycoproteins to the luminal surfa ce where they are incorporated into the apical plasmalemma and the glycocal yx. There is a strong possibility that in various species some of these ves icles are involved in transcytosis of antibodies to the saliva from their s ource (plasma cells) in the surrounding connective tissue. In addition, ves icles may engage in transfer of growth factors from the saliva to the inter stitium. In a few species, conventional SDs have been replaced by ducts tha t are wholly given over to secretion, i.e., they entirely lack basal striat ions; although such ducts occupy the histological position of conventional SDs, it is not clear whether they represent a new type of duct or merely ar e modifications of SDs. Broad-based comparisons of ultrastructural and othe r data about SDs offer some insight into evolutionary history of salivary g lands and their role in the adaptive radiation of mammals. Evolutionary pat terns emerged when we made interspecific comparisons across mammalian order s. Among the bats, there is a clear relationship between SD secretion and g eneral categories of diet. Anat Ree 264: 121-145, 2001. (C) 2001 Wiley-Liss , Inc.