This growth occurs along thickened strips of ectoderm called the mammary ridges , which extend from the armpit to the groin. In humans, unlike in dogs and other mammals, the mammary glands only persist in the pectoral region. The original solid growths canalize to form ducts under the influence of estrogen produced in the placenta. At birth the mammary glands of males and females are identical, composed of about 15 to 20 rudimentary lactiferous ducts . In females, the breasts develop further after puberty under the influence of estrogen secreted by the developing follicle and corpus luteum each month.
Enlargement occurs due to deposition of fat and connective tissue.
The nipple becomes enlarged and pigmented. Further growth and branching of the lactiferous ducts also occurs. Pregnancy Even more intense growth and branching of the lactiferous ducts occurs during pregnancy, under the influence of high serum estrogen levels from the corpus luteum and later the placenta. Growth hormone, glucocorticoids, prolactin, and insulin also play a role in growth. Final development of the mammary glands occurs under the influence of progesterone, which in concert with the hormones mentioned above causes alveoli to bud from the ends of the lactiferous ducts.