Which hormone is primarily responsible for stimulating the growth and development of mammary glands?

Prolactin is the hormone primarily responsible for stimulating the growth and development of mammary glands. It plays a crucial role in lactation by promoting the differentiation of mammary epithelial cells, which are essential for milk production. Prolactin levels rise during pregnancy, preparing the mammary glands for breastfeeding after the birth of the offspring. This hormone not only initiates lactation but also maintains it during nursing by helping to regulate the production of milk in response to the infant's needs.

While oxytocin is important for the milk ejection reflex, allowing for the release of milk during breastfeeding, it does not directly stimulate the growth of mammary glands. Oestrogen contributes to the overall development of the female reproductive system and the growth of mammary tissues, but its primary function is not to promote milk production. Relaxin is mainly involved in preparing the body for childbirth by relaxing the ligaments and does not play a role in mammary gland development. Therefore, prolactin is specifically identified for its direct involvement in the growth and functional development of mammary glands.