The difference between male and female nipples is simply one of development.
Male nipples are underdeveloped; although they contain exactly the same structures and enervation as female nipples, they don't receive the necessary dose of oestrogen to develop fully, so they remain in the prepubescent, immature state.
Female nipples DO receive a surge of oestrogen at puberty, which stimulates them to develop; they enlarge, and the milk ducts develop to accommodate the corresponding development of breast tissue.

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