Stereological analysis of the duct system of the rabbit parotid gland.

The duct system of the rabbit parotid gland constitutes about 5% of gland tissue volume and is divisible into intercalated and striated ducts in a volumetric ratio of about 3:2. The intercalated duct consists of low cuboidal epithelial cells (375 micrometer 2) surrounded by a myoepithlium, and the cells contain a few small secretion granules, particularly at the proximal end of the duct. The cells of the striated duct are larger (531 micrometer 3), columnal in shape, and show a mitochondrial compartment three times that of intercalated duct cells (16.5% of cell volume). These mitochondria are concentrated in a basal and perinuclear position, but they are largely absent from the apical cytoplasm, which is permeated with microfilaments and contains numerous small smooth membraned vesicles, but no clearly recognisable secretion granules. The lateral plasmalemma of these cells is complexly folded, and basal processes interdigitate with those of adjacent cells. This results in an increase in the ratio of apical to lateral/basal plasmalemma from 1:5 in intercalated cells to 1:24 in striated cells. Some slight changes in cell morphology were detected following isoprenaline-induced secretion of the gland in vivo. These included a small increase in the volume fraction of nuclei and mitochondria in intercalated duct cells, and depletion of their secretion granules. Change in striated cells was confirmed to a small increase in the volume of smooth membraned cytoplasmic vesicles. The structure of the duct and changes wrought by isoprenaline are discussed in the context of the role of the duct in the production of saliva.