Mammary alveolar epithelial cells convert to brown adipocytes in post-lactating mice

Abstract

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.

Keywords: Cre-loxP recombination system; adipose organ; electron microscopy; mammary gland; pregnancy.

Figure 1

The mouse anterior subcutaneous depot during lactation. Upper panels, pictures of the gross anatomy of the interscapular depot fitted on mouse templates. In non-pregnant female mice (A), the interscapular fat (dotted area) is easily recognized by its brownish color; after 10 days of lactation (B) it turns white and the depot acquires a white fat-like macroscopic appearance. In sections processed for immunohistochemistry (C), interscapular multilocular adipocytes containing large lipid droplets exhibit very weak UCP1 expression. Mammary gland alveoli (G) are found close to both brown (BAT) and white (WAT) adipose tissue lobules. At higher magnification (D), no clear boundaries are visible between the secretory epithelium and the adipose parenchyma; note the weak and homogenous UCP1 staining in multilocular adipocytes. D is the enlargement of the area framed in C. Bar: 3 cm in A, 250 μm in B, 40 μm in C.

Figure 2

Transmission electron microscopy images from specimens of peripheral anterior mammary gland in contact with interscapular BAT on day 1 of mammary gland involution. In A, the epithelial cells bordering the alveolar lumen (Lu) contain numerous mitochondria (m), stacked rough endoplasmic reticulum (RER), and large vacuoles containing milk protein secretory granules; the vacuole indicated by the arrow is enlarged in B. C: a multilocular adipocyte containing several “brown-like” mitochondria (M) packed with laminar cristae and two milk protein secretory vacuoles (enlarged in D). Cap: capillary, My: myoepithelial cells, N: nucleus, L: lipid droplet. Bar: 1 μm in A and C, 0,3 μm in B and D.

Figure 3

X-Gal staining of the peripheral anterior mammary gland in contact with interscapular BAT of a WAP-Cre/R26R mouse 10 days post-lactation. In A, not only mammary epithelial structures (Ep), but also some multilocular adipocytes (arrows) show the blue X-Gal precipitate. UCP1 immunohistochemistry and hematoxylin counterstaining, performed in the same section (B), show that the X-Gal-positive multilocular adipocytes also exhibit UCP1 immunoreactivity (arrows). bV: blood vessel. Bar: 25 μm.