The myoepithelial cells of the virgin mouse were round or ovoid and devoid of processes; a few scattered bundles of short myofibrils were observed. The threshold for oxytocin was above 1000 µU/ml. The cells enlarged and processes developed during pregnancy; the myofibrils were more abundant, packed and were arranged parallel to the long axis of the cells. At the eighteenth day of pregnancy the processes were very long. Myofibrils occupied the greater part of the cells and many micropinocytotic vesicles were present. The threshold for oxytocin dropped to 25-100 µU/ml. No significant ultrastructural changes were observed during lactation; the threshold dropped again to 10-25 µU/ml. The myofibrils appear to be composed of globular subunits; no changes were found in their ultrastructure through pregnancy and lactation. The marked myoepithelial growth is complete by the end of pregnancy. The simultaneous evolution of the myoepithelial growth and of response to oxytocin suggests that the former is the anatomical basis of the latter.

Estradiol benzoate induced in oophorectomized mice changes in myoepithelial ultrastructure and function, resembling those occurring during pregnancy. The administration of progesterone did not modify the ultrastructure or the response to oxytocin of nonpregnant animals. These results suggest that estradiol is an important factor in the regulation of myoepithelial ultrastructure and function, whereas progesterone appears unnecessary.