Ultrastructural study of rat mammary gland during pregnancy

Abstract

Abstract Virgin female Sprague‐Dawley rats were mated, and after the appearance of vaginal plugs were placed in individual cages. The animals were sacrificed at different intervals from the second day of gestation until delivery. Breast tissue was studied with the light and electron microscope. The study suggests that breast tissue undergoes changes as early as four days after fertilization and continues throughout pregnancy. The changes start with accumulation of many ribosomes and polysomes within the epithelial cell cytoplasm. Lipid droplets appear early and increase in number as pregnancy advances. Two types of proteinaceous particles appear within the cytoplasm on the seventh day of gestation, and are totally secreted into the lumina before parturition. The particles are believed to play a role in colustrum composition. The Golgi apparatus plays no role in the formation of the proteinaceous particles. The particles are formed in relation to rough surfaced endoplasmic reticulum. The changes seen are limited to the ductular epithelium and are not seen in the large ducts. In animals treated with colchicine, mitoses are seen in the ductular epithelium very early in pregnancy and continue throughout the gestation period.