Cell Biology International, 1996, Vol. 20, No. 11, 763–767 INVOLUTION OF MOUSE MAMMARY GLANDS DURING WHOLE ORGAN CULTURE OCCURS VIA APOPTOSIS OF EPITHELIAL TISSUE T. M. CASEY 1 , H. CHEN 2 , K. PLAUT* 1 and J. F. CHIU 2 1 Department of Animal and Food Sciences and 2 Department of Biochemistry, University of Vermont Burlington, VT 05405, U.S.A. Accepted 23 August 1996 Apoptosis was measured in mammary glands during whole organ culture, to determine whether regression resulting from hormone withdrawal results in epithelial cell death as in vivo involution. Glands were evaluated for morphology and DNA degradation prior to whole organ culture, after lobulo-alveolar development and 2, 4, or 6 days after hormone withdrawal. The data indicated that mammary regression during whole organ culture mimics in vivo involution and results in part from apoptosis of epithelial tissue.  1996 Academic Press Limited K: apoptosis; epithelial tissue; mouse; mammary glands INTRODUCTION Mammary gland differentiation is regulated and maintained by a complex interaction between hor- monal stimulation and the extracellular matrix. Involution is a dynamic transitional period of mammary development resulting in vivo from the cessation of the suckling stimulus. Involution is marked by a decrease in galactopoietic hormone levels and an increased expression of tissue remod- elling enzymes and programmed cell death, apop- tosis (Walker et al., 1989; Strange et al., 1992). The molecular mechanisms regulating this transitional period have yet to be elucidated. Mouse mammary whole organ culture (WOC) is a defined in vitro system that has been used to investigate cyclic mammary gland development (Ichinose and Nandi, 1966). WOC maintains the mammary cells in their natural environment while addition and removal of factors can be readily manipulated. During WOC, glands are hormonally stimulated to undergo lobulo-alveolar differentia- tion and express -casein mRNA (Ganguly et al., 1981). Withdrawal of all hormones except insulin causes mammary glands to regress in a manner similar to in vivo involution (Ganguly et al., 1981). Atwood et al. (1995) determined that apoptosis occurred in mammary tissue differentiated and involuted in WOC. Lactogenic hormones have been shown to have both positive and negative effects on the growth of other types of tissues. In particular, lympho- cytes and lymph tissue are very sensitive to glucocorticoid-induced apoptosis (Witorsch et al., 1993; Evans-Storms and Cidlowski, 1995). How- ever, Witorsch et al. (1995) demonstrated that the lactogenic hormone prolactin can inhibit gluco- corticoid induced apoptosis and induce mitosis in rat Nb2 lymphoma cells when cultured in prolactin alone. The objective of this study was to determine the type of mammary tissue apoptosis occurred in during involution of glands in WOC, and if the presence of mammary lymph nodes affected mam- mary involution in a positive or negative way. In this report, it is demonstrated that apoptosis occurs in the epithelium during involution of mammary glands in WOC with and without mammary lymph nodes, and the presence or absence of lymph nodes does not appear to effect the involution of glandular epithelium. MATERIALS AND METHODS Whole organ culture Animal experimentation was conducted under the regulations of Interdisciplinary Principles and Guidelines for the Use of Animals in Research, *To whom correspondence should be addressed. 1065–6995/96/110763+05 $25.00/0  1996 Academic Press Limited