Do men have mammary glands?
Male lactation
In zoology, male lactation is the production of milk from a male mammal's mammary glands. Such production is known as lactation. It is well-documented in the Dayak fruit bat. The term male lactation is not used in human medicine. It has been used in popular literature, such as Louise Erdrich's The Antelope Wife, to describe the phenomenon of male galactorrhea, which is a well-documented condition in humans, unrelated to childbirth or nursing. Newborn babies of both sexes can occasionally produce milk; this is called neonatal milk (also known as "Witch's milk") and not considered male lactation.
Mammary ridge
A mammary ridge, or crest, usually stops growing at eight weeks and its length is regressed starting at the caudal end and extending cranially, so that what remains is a round, ectodermic placode where the axilla develops. When shortening of the mammary crest is complete, the structure remains prominent in the areas where the mammary glands eventually form. The mammary lines begin to shorten and ectodermal cells begin to divide and grow into the mesenchymal cell layer. A basement membrane separating the expanding ectodermal crest structure and the underlying mesoderm usually remains. The mammary ridge then becomes recognizable in the thoracic region in the human embryo. Nipples develop on the milk lines of mammals.
In humans, milk lines form as thickenings of the epidermis of the mammary ridge, along the front surface of both sexes of mammals. Milk lines appear in the seventh week of embryonic development before human sexual differentiation, which explains why male humans have nipples. After initial development of the milk lines they go into remission. Most humans have two nipples, but in some cases more than two will develop. These additional nipples usually grow along the milk line.