Relationship between morphological development and sex hormone receptor expression of mammary glands with age in male rats

The aim of this study is to investigate the changes with age on morphology and sex hormone receptor expression in the mammary glands of male Sprague-Dawley rats, focusing on male-specific cells, “oxyphilic cells”, observed after sexual maturity. The mammary glands of male rats at 14, 21, 35, 50, 75 and 100 days old were examined by gross observation, microscopic observation using whole mount specimens, histological and immunohistochemical sections. Grossly, mammary glands showed brown color at 50–100 days old. In whole mount specimens, terminal end buds (TEBs) were observed at 14–50 days old and the number of TEBs was highest at 35 days old. Histologically, the male mammary glands contained small epithelial cells with scanty cytoplasm at 14–35 days old while ductal and lobular epithelial cells were changed into oxyphilic cells with abundant cytoplasm at 50–100 days old. Immunohistochemicaly, androgen receptor (AR), estrogen receptor (ER) and progesterone receptor (PgR) expressions were found in both mammary glands found at a young age and oxyphilic cells. In oxyphilic cells, AR expression was dominant compared to ER and PgR expressions and increased with age. From these results, the development at 50–100 days old might be strongly related to AR. Ultrastructural observation of oxyphilic cells confirmed a number of lipid droplets, deformed and/or enlarged mitochondria, lysosomes and peroxisomes in their cytoplasm.