Breasts begin developing in the embryo about 7 to 8 weeks after conception. They are unrecognizable at this stage consisting only of a thickening or ridge of tissue. From weeks 12 to 16, the various sub-components become more defined. Tiny groupings of cells begin to branch out laying the foundation for future ducts and milk producing glands. Other tissues develop into muscle cells which will form the nipple (the protruding point of the breast) and areola (the darkened tissue surrounding the nipple). In the later stages of pregnancy, the mother's hormones, which cross the placenta into the fetus, cause breast cells to organize into branching tube-like structures thus forming the milk ducts. In the final 8 weeks, lobules, (milk producing glands), mature and actually begin to secrete a liquid substance called colostrum. In both female and male newborns, swellings underneath the nipples and areolae can easily be felt and a clear liquid discharge, colostrum or "witch's milk", can be seen. These represent the effect of the mother's hormones and subside in the first few weeks of life.

From infancy to just before puberty, there is no difference between the female and male breasts. With the beginning of female puberty, however, the release of estrogen, at first alone, and then in combination with progesterone when the ovaries functionally mature, cause the breasts to undergo dramatic changes which culminate in the fully mature form. This process on average takes 3 to 4 years and is usually complete by age 16. Further maturation of the breast tissues occurs with lactation and is felt to be mildly protective against breast cancer.

The mature female breast is composed of essentially four structures: lobules or glands; milk ducts; fat and connective tissue (see diagram). The lobules group together into larger units called lobes. On average there are 15-20 lobes in each breast arranged roughly in a wheel spoke pattern emanating from the nipple/areolar area. The distribution of the lobes is not even, however. There is a preponderance of glandular tissue in the upper outer portion of the breast. This is responsible for the tenderness in this region that many women experience prior to their menstrual cycle. It is also the site of half of all breast cancers. The lobes empty into the milk ducts which course through the breast towards the nipple/areolar area. There, they converge into 6-10 larger ducts called collecting ducts which enter the base of the nipple and connect with the outside. During lactation (breast feeding), the breast milk follows this course on its way to the feeding infant.

Diagram 1:  Coronal Section through breast 

Diagram 2:  Saggital (cross section) through a breast showing glands and ducts

The consistency of breast lobes vary from woman to woman and may even vary in an individual from one side to the other. However, in general, the glandular portion of the breast has a firm, slightly nodular feel to it. Surrounding the lobes is breast fat. Unlike the lobes, the fat is almost always soft. The discrepancy in textures between these two components allows one to outline the lobes by carefully palpating (feeling) the breast. Interestingly, the difference in density between glandular breast tissue and breast fat is also the basis for mammography. In contrast, the ducts of the breast are usually not palpable unless they are engorged with milk, inflamed or contain a tumour.

The breasts of younger women are primarily composed of glandular tissue with only a small percentage being fat. Thus they are firmer than in older counterparts. As women age, especially with the loss of estrogen at menopause, the lobes involute (shrivel) and are replaced by fat. The breasts become softer and lose their support. Physical examination and mammography are easier to interpret and may well be more accurate.

Whereas all components of the breast are influenced by female hormones, the glandular tissue is most sensitive. Very dramatic and totally normal changes can occur in the consistency of the breasts during the menstrual cycle. These changes are most evident just prior to menstruation when levels of estrogen and progesterone are peaking. Right after menstruation, hormone levels are at their lowest and the breast becomes softer and less tender. This is the recommended time to perform breast self-examination, BSE, and to have a mammogram.

In post menopausal women, who are not taking estrogen supplementation, weight becomes a significant factor in the size and appearance of the breasts. Being mostly composed of fat at this point, small changes in body weight can produce significant changes in breast size.

There are several, well described congenital (present from birth) abnormalities of the breast. The most common of these are accessory nipples and/or breast tissue. This occurs in 2-6% of the population and often goes unrecognized. Accessory nipples are seen anywhere along the milk line (a ridge of tissue, present only in the fetus, extending from the underarm to the groin from which breasts develop). They are frequently multiple. Accessory breast tissue can also be found in this distribution but most often occurs in the underarm area. There is no special clinical importance to these other than being aware of their presence and including them in physical examinations.