Mammary glands from progestin-treated mice. MPA (40 mg), Pg (40 mg), or control blank silastic pellets were implanted subcutaneously in 2-month-old (a) BALB/c or (b) C57BL/6 female mice. After 2 months, the mice were killed and the mammary glands processed for histological evaluation (H&E) or for WM (as described in the Materials and methods section). A representative image observed in at least four different mice of each group is presented. The numbers of ducts (black arrow), lobules (white arrow), and lateral branching (dotted arrow) were (c) quantified in each H&E stained slide and expressed as the number of structures per 400× field. MPA treatment induced an increase in the number of ducts and in lateral branching only in BALB/c mice, whereas Pg induced an increase in the number of lobules mainly in BALB/c. In control groups, animals were euthanized at diestrus. ***P < 0.001, treated versus control; a
P < 0.05 between controls of both strains. Magnification: 100×, bar: 100 μm. H&E, hematoxylin and eosin; MPA, medroxyprogesterone acetate; Pg, progesterone; WM, whole mount.