Both men and women have mammary glands, but these milk-producing glands are undeveloped and typically nonfunctional in males. The release of estrogen after puberty prompts the development of the glandular tissue in women.
Although both men and women have mammary glands, this tissue seldom develops in men. The endocrine system regulates the development of mammary glands, and they begin working in response to the hormonal changes brought on by childbirth. These glands begin producing milk only after labor.
Overdevelopment of the male breast, or gynecomastia, is a response to too much estrogen or too little testosterone. The glandular tissue present in the male breast increases in size to form an enlarged breast. This condition usually occurs in babies, teenagers and older men. In babies and teenagers, the condition tends to go away without medical intervention according to WebMD.