On the ultrastructure of the canine mammary gland during pregnancy and lactation.

During pregnancy and lactation marked changes are observed in the fine structure of the secretory cells in the Beagle mammary gland: especially pronounced are differences in cellular height, shape and size of the nuclei and distribution of mitochondria. In later stages of pregnancy a proceeding development of those cellular organelles involved in synthesis and extrusion of secretory material (i.e. rough endoplasmic reticulum, Golgi apparatus) can be observed. Myoepithelial cells which can be first discerned from secretory cells by ultrastructural features from day 40 on show only minor variations of their ultrastructure during pregnancy and lactation.