The fine structure of secretory granules in submandibular glands of the rat during early postnatal development.

The secretory end-pieces of the submandibular gland of rats during the first week of postnatal development are studied with regard to the fine structure of the secretion granules in these end-pieces. The terminal ends of the secretory ducts during this period consist of two types of cells; one cell is an acinar-type and the other is a duct-type found in the gland of adult rats. The secretion granules of the acinar-type cells are similar in appearance to those of the acinar cells in the gland of adult rats, and the structure of these granules remains the same throughout the week. However, granules widely different in appearance are present in the duct-type cells, and their structure varies in different cells as well as within a single cell at different stages of development. These granules contain unusual substructures which are not found in the secretion granules of adult rats, suggesting that the granules are transitory. Granules containing short tubular profiles are predominant in the gland of one day-old rats. A large number of granules in three day-old rats contain elongated tubules. More granules of widely different substructures are present in the gland of seven day-old rats than in the gland of younger rats. The matrix of the granules in seven day-old rats is of higher density than that of the granules in younger rats. In the dense matrix of these granules, less dense tubules form fingerprint-like or somewhat more irregular patterns. Two types of secretory granules occur in the submandibular gland of the adult rat; one type is found in acinar cells and the other type in the granulated portion of secretory ducts. The granules of the acinar cells are of a mucous type and those of the ducts are of a serous type (Shackleford and Klapper, '62). In the electron microscope, the granules of acinar cells are characterized by the ill-preserved appearance of their membrane and electron translucent interior (Scott and Pease, '59; Kurtz, '64; Tamarin and Sreebny, '65; Tamarin, '67). The granules of ducts, on the other hand, are bound by a distinct membrane and appear dense with no obvious internal structure. These granules occur in the convoluted portions of secretory ducts and are of varying density (Scott and Pease, '59; Leeson and Jacoby, '59; Tamarin and Sreebny, '65). ANAT. REC., 168: 463-476. During the early postnatal period, submandibular glands of the rat are immature (Jacoby and Leeson, '59; Leeson and Jacoby, '59) ; microscopic observations of the submandibular glands of newborn rats have shown that neither typical acini nor granular ducts are present. In neonatal rats, secretory granules are located in the terminal end-piece of intralobular ducts, which has been described as the "terminal tubule" (Jacoby and Leeson, '59). The differentiation of acini and granulated ducts proceeds to completion during the first six weeks of the postnatal period. The chemical composition of the secretory granules in the terminal tubules appears to be different from that of the granules in the mature gland, as indicated by