Figure 2.

Acute progesterone exposure elicits branching morphogenesis in the mammary gland of the ovariectomized mouse. A–C are high-magnification images of mammary gland whole mounts from untreated, progesterone-treated (for 76 h), or sesame oil-treated (for 76 h) ovariectomized wild-type mice, respectively. Note the overt dilation of the ductal network and the emergence of numerous ductal side branches (arrows) in response to 76 h progesterone exposure (compare B with A). Sesame oil treatment for 76 h fails to induce these morphological responses (compare C with B). D–F show representative H&E stained sections of the whole mounts shown in A–C, respectively. Note the significant expansion of the epithelial compartment (arrow) after progesterone treatment for 76 h (compare E with D and F). G and H show whole mounts of mammary glands from ovariectomized mice continuously treated with progesterone for 1 and 2 wk, respectively. In both panels, note the clear increase in ductal side branching and limited alveolar budding with progesterone exposure (arrows). I, Absence of these mammary morphological changes in the ovariectomized wild-type mouse after 2 wk sesame oil treatment. J–L, H&E stained sections of the whole mounts shown in G–I, respectively; a marked increase in epithelial content is observed with progesterone exposure for 1 and 2 wk (compare J and K with L). Scale bar in A applies to B, C, and G–I, respectively; scale bar in D applies to E, F, and J–L, respectively. Also see supplemental Figs. 1 and 2.