During pregnancy and lactation, subcutaneous white adipocytes in the mouse mammary gland
transdifferentiate reversibly to milk-secreting epithelial cells. In this study, we demonstrate by
transmission electron microscopy that in the post-lactating mammary gland interscapular
multilocular adipocytes found close to the mammary alveoli contain milk protein granules. Use
of the Cre-loxP recombination system allowed showing that the involuting mammary gland of
whey acidic protein-Cre/R26R mice, whose secretory alveolar cells express the lacZ gene during
pregnancy, contains some X-Gal-stained and uncoupling protein 1-positive interscapular
multilocular adipocytes. These data suggest that during mammary gland involution some milk-
secreting epithelial cells in the anterior subcutaneous depot may transdifferentiate to brown
adipocytes, highlighting a hitherto unappreciated feature of mouse adipose organ plasticity