The male mammary gland: a target for the xenoestrogen bisphenol A

Abstract

Males of some strains of mice retain their mammary epithelium even in the absence of nipples. Here, we have characterized the mammary gland in male CD-1 mice both in whole mounts and histological sections. We also examined the effects of bisphenol A (BPA), an estrogen mimic that alters development of the female mouse mammary gland. BPA was administered at a range of environmentally relevant doses (0.25-250μg/kg/day) to pregnant and lactating mice and then the mammary glands of male offspring were examined at several periods in adulthood. We observed age- and dose-specific effects on mammary gland morphology, indicating that perinatal BPA exposures alter the male mammary gland in adulthood. These results may provide insight into gynecomastia, the most common male breast disease in humans, where proliferation of the mammary epithelium leads to breast enlargement.

Figure 1. Morphology of the female and male mammary glands throughout life

Whole-mount mammary glands were collected from control male and female mice at E18, PND1, PND5, PND10, 3-4 months of age, 7-9 months of age and 12-15 months of age. Images were collected at 0.8x magnification for all glands to allow visual comparisons of the sexual dimorphism of the gland. At all ages examined, the male gland (middle panel) was significantly smaller than glands collected from female siblings (left panel). High magnification images of the male mammary gland (right panel) illustrate the changes in mammary gland morphology observed over time. In all panels, the scale bar represents 500μm for E18, PND1 and PND5 and 2mm for PND10, 3-4 months of age, 7-9 months of age, and 12-15 months of age.

Figure 2. Histological and immunohistological analyses of the 7-9 month old male mammary epithelium reveal many similarities to the female gland

H&E staining of excised male mammary epithelial ducts reveal a simple cuboidal epithelium with an obvious lumen in both the females (A) and the males (B). The extracellular matrix is highly eosinophilic and the ducts are surrounded by lipid-filled adipocytes. B) Immunohistochemical detection of smooth muscle actin, a marker of myoepithelial cells, reveals the presence of these cells below the ductal epithelium in both females (C) and males (D). E) Immunohistochemical analysis reveals ERα expression throughout the epithelial compartment in the males. In total, approximately 15% of epithelial cells were strongly ERα-positive. F) Immunohistochemistry for Ki67, a marker of proliferation, indicates a low level of expression in male ductal epithelium. Approximately 2-3% of cells were strongly Ki67-positive. G) A TUNEL reaction marking apoptotic cells in the male mammary epithelium. Approximately 60% of cells were marked by TUNEL, indicating a high rate of apoptosis. In panels C-G, arrowheads indicate positive cells; hematoxylin was used as a counter stain. Scale bar = 50μm.

Figure 3. Analyses of control and BPA-exposed male mammary glands reveals age- and dose-specific effects on two morphological parameters, number of branching points and ductal area

A-B) Comparisons of morphological parameters in control and BPA-exposed animals at 3-4 months of age. C-D) Comparisons of morphological parameters in control and BPA-exposed animals at 7-9 months of age. E-F) Comparisons of morphological parameters in control and BPA-exposed animals at 12-15 months of age. For all panels, ANOVA p<0.05. Bonferroni posthoc analysis revealed differences between treated groups and controls, indicated by red bars (p<0.05). Of note, at 3-4 months of age, the lowest doses were most disruptive of mammary gland morphology. At 7-9 months of age, the moderate doses were most disruptive. And finally at 12-15 months of age, the highest doses of BPA tested had the greatest effects on mammary gland morphology. On all graphs, numbers indicate the sample size analyzed.

Figure 4. Whole mount mammary glands from control and BPA-exposed males

At the three ages examined, mammary glands from a control and two BPA treatments groups are shown. The middle panel always includes a treatment that had significant differences from controls, and the right panel includes a treatment that was not statistically distinguishable from controls. In samples where the epithelium was small, it is indicated by a black box. All samples from the same age were taken at the same magnification. Scale bar = 1mm in all panels.

Figure 5. Perinatal BPA exposure alters proliferation in adult male mammary glands

Immunohistological analyses were conducted in sections of 7-9 month old male mammary glands to quantify the expression of Ki67 (A), TUNEL-positive cells (B), and the ratio between these two factors, expressed as the apoptosis: proliferation ratio (C). Additionally, the percentage of ERα-positive cells was quantified (D). For all measures, ANOVA values are indicated on graphs. When p<0.05, Bonferroni posthoc tests were performed, *p<0.05 compared to controls. For all groups, n=4-5 were analyzed.