Answer:
At six weeks, the 9-mm human embyro shows a linear ectodermal ridge (mammary ridge) extending from nipple to groin bilaterally. By the fourth month, the gland has developed with thickening and growth of the ectoderm into the underlying mesoderm, with the formation of 20 secondary outgrowths which form the lactiferous ducts. The areola is created by the ectoderm and is recognizable by the fifth month. By the seventh month the small duct acini have formed. The main ducts are concentrated together and open into a central pit. This area develops smooth muscle fibers and becomes the nipple, which becomes elevated soon after birth.
Changes occur during the menstrual cycle. The proliferative phase occurs on days 3 to 7, characterized by increased levels of estrogen, which causes epithelial proliferation. The stroma is dense and has increased numbers of fibroblasts and plasma cells. During the second week (the follicular phase) there is increased production of progesterone. Three types of epithelial cells are present: myoepithelial, B cells, and A cells. The secretory phase occurs during days 21 - 27 and is under the influence of estrogen, progesterone and prolactin. Breast volume increases 15 to 30 ml. Apocrine secretion and dilatation of the acinar lumen occurs. The final phase is associated with withdrawal of estrogen and progesterone. During menopause, there is breast involution and gradual atrophy of the glandular tissue. There is replacement of the epithelium and stroma by fat.
For innervation, see Breast Augmentation, question 1.
Aesthetic measurements:
sternal notch or midclavicle to nipple: 19-21 cm
nipple to midline: 9-11 cm
nipple to inframammary crease: 7-8 cm
breast base: 11-14 cm
areolar diameter: 35-45 mm
nipple diameter: 5-8 mm
nipple projection: 4-6 mm