Embryonic mammals develop a mammary ridge along the ventral trunk. Different species of mammals develop breasts and nipples at different point along this ridge. In humans, additional breasts and nipples can develop along this ridge.
Women have been documented at having up to eight pairs of functional breasts (Jones, 1991).
Because oxytocin is not only the signal for milk letdown but also a signal used for other events such as orgasm, a woman who has expressed the hormones which promote milk development in the breast--even without pregnancy--may release milk during orgasm.
Although some have proposed that larger breast size may be linked to breast cancer, this has yet to be supported in a comprehensive study. Women with breast augmentation have lower levels of breast cancer, which may be due, at least in part, to a smaller breast size prior to the augmentation (Koch, 2004). Increase in breast size has been observed as a side effect of chronic antidepressant therapy, perhaps involving an increase in prolactin levels (Amsterdam, 1997).
MAMMARY GLANDS IN MALES
All infants have some degree of breast development caused
by the hormones they experienced in the placenta which were inducing
changes in their mmothers' breasts. The mammary glands of some infants,
male and female, can produce watery secretions shortly after birth.
The secretions of infant mammary glands was once referred to as "witch's
milk" and was thought to have healing powers (Jones, 1991).
Estrogens are produced in males, probably by Sertoli cells, and result in modest breast development in some pubescent boys (Jones, 1991). Although mammary glands are typically inactive in males, they are capable of function. Male mammary tissue has occasionally been shown to be capable of milk production in mammals ranging from humans to domesticated animals to wild bats (Diamond, 1995; Jones, 1991).
Although the presence of many drugs in breast milk can be accounted for by diffusion alone, other such as cimetidine, acyclovir, and nitrofurantoin seem to be actively pumped into milk using xenobiotic transporters. ABC Cassette proteins (such as the multidrug resistant protein), solute carrier superfamily (SLC) and pH-dependent proton/peptide cotransporters, PEPT1 and 2 (SLC15A1 and 2) are transporters expressed in the breast (Ito, 2003).