Studies of Mouse Mammary Glands. II. Cytomorphology of Mammary Transplants in Inguinal Fat Pads, Nipple-Excised Host Glands, and Whole Mammary-Gland Trans-

SUMMARY-The cytology of mammary transplants in inguinal glandfree fat pads, nipple-excised host glands / and whole mammary-gland trans-plants of 136 C57BLlCrgi mice was investigated by light and electron microscopy. Among 31 successful whole-gland transplants, 6 were suckled by youngj the fine structure of these suckled, whole-gland transplants and the glands in situ was similar. The unsuckled, whole-gland transplants, the nipple-excised host glands, and the inguinal glandfree transplants responded normally to the endogenous hormones of the hosts and were indistinguishable from the suckled glands up to the 1 st day after parturition. Within the next 24 hours / however, all nippleless glands started deviating from the controls and exhibited degenerative changes reminiscent of normal postlactational involution. The events of cytolysis at the ultrastructural level were identical in all regressing glands, irrespective of the circumstances that led to regression. The process of involution proceeded at a demon-strable rate for about 1 week, after which it became imperceptibly slow