The submandibular gland is composed of several different types of cells. Mucous cells are primarily full of secretory droplets of mucin (a ropy viscous fluid), pushing the flattened nuclei to the basal portions of the cells. Serous cells (80 percent of human submandibular gland cells) are shaped like pyramids with more rounded nuclei and are also pushed to the base of the cell by secretory droplets of serum-albumin (thinner, more watery). The mixed alveoli (20 percent of human submandibular gland cells) of the submandibular gland are composed of serous cells topping mucous cells and in humans, purely mucous alveoli are not common. Lined by columnar cells with apical nuclei, the striated ducts are long and very conspicuous in gland sections under the microscope. They play a role in the secretion and absorption of salts, which modify the composition of the saliva. The striations are infoldings of the basal plasma membrane with rows of elongated mitochondria in the pockets.