Lactogenesis in the rat: an ultrastructural study of the initiation of the secretory process

Three specimens were taken from mammary glands of rats killed on the 18th and 21st days of pregnancy and on the 1st day of lactation. Ultrastructural features of the tissue were compared among rats within and between the two stages of development. The similarity among specimens from the same rats made feasible a comparison of serial biopsies obtained every 4 hr, starting on the afternoon of the 21st day of pregnancy. From the 18th to the 21st days of pregnancy, a marked increase in the amount of rough endoplasmic reticulum occurred. The alveolar cells of rats killed on both days and in biopsies obtained at 17 and 13 hr before partuirtion contained abundant small lipid droplets and vacuoles containing many protein granules with little clear fluid (stasis vacuoles). Alveolar lumina were distended with secretion by 17 hr before parturition. Between 8 and 12 hr. before parturition, the accumulated protein and lipid were rapidly extruded from the alveolar cells despite evidence of continued biosynthesis. It is suggested that active transport processes are initiated independently of milk synthesis before parturition.