Mitosis in milk secreting epithelial cells of mammary gland: an ultrastructural study

Abstract

In all stages of lactation mitotic configurations were observed in mammary gland epithelial cells of rats. An electron microscopic study is presented which shows that ultrastructure of such mitotic stages is normal and that mitotic cells contain typical products of milk secreting cells such as casein micelle-containing vesicles and milk fat droplets. Such secretory products can even be observed in the immediate vicinity of the chromosomes and microtubules of the spindle apparatus. The endoplasmic reticulum of mitotic cells appeared altered in that it did not show typical cisternal stacks characteristic of interphase cells. While the numbers of such mitotic cells were very low, especially from the second week of lactation on (always less than 0.1% of the milk secreting epithelial cells encountered), the observations clearly demonstrate that differentiation for milk secretory activity and cells division are not mutually exclusive. We conclude that postpartum growth of mammary gland epithelium and replacement of epithelial cells lost during desquamation into the milk liquids can occur by division of existing differentiated milk secreting cells and does not require mitotic activity of non-lactating 'stem cells' which are not observed in lactating alveoli.