The function of the breasts in females is to supply milk to an infant in a process called lactation. Breast milk is produced by the mammary glands (Pic. 1), which are modified sweat glands. The milk itself exits the breast through the nipple via 15 to 20 lactiferous ducts that open on the surface of the nipple. These lactiferous ducts each extend to a lactiferous sinus that connects to a glandular lobe within the breast itself that contains groups of milk-secreting cells in clusters called alveoli. The clusters can change in size depending on the amount of milk in the alveolar lumen. Once milk is made in the alveoli, stimulated myoepithelial cells that surround the alveoli contract to push the milk to the lactiferous sinuses. From here, the baby can draw milk through the lactiferous ducts by suckling (Pic. 2).
The morphological structure of the human breast is identical in males and females until puberty. For a girl in puberty, during thelarche (the breast-development stage), the female sex hormones (principally estrogens) promote the sprouting, growth, and development of the breasts, in the course of which, as mammary glands, they grow in size and volume, and rest on her chest; these development stages of secondary sex characteristics (breasts, pubic hair, etc.) are illustrated in the five-stage Tanner Scale (The scale defines physical measurements of development based on external primary and secondary sex characteristics, such as the size of the breasts, genitals, testicular volume and development of pubic hair.) (Pic. 3).
Approximately two years after the onset of puberty (a girl's first menstrual cycle), the hormone estrogen, in conjunction with growth hormone, stimulates the development and growth of the glandular, fat, and suspensory tissues that compose the breast. This continues for approximately four years until establishing the final shape of the breast (size, volume, density) when she is a woman of approximately 21 years of age.
Morphologically, the breast is a cone with the base at the chest wall, and the apex is at the nipple, the center of the nipple-areola complex (Pic. 4). In women, the breasts overlay the pectoralis major muscles and usually extend from the level of the second rib to the level of the sixth rib in the front of the human rib cage; thus, the breasts cover much of the chest area and the chest walls. At the front of the chest, the breast tissue can extend from the clavicle (collarbone) to the middle of the sternum (breastbone).
The dimensions and weight of the breast vary widely among women, ranging from approximately 500 to 1,000 grams (1.1 to 2.2 pounds) each. A small-to-medium-sized breast weighs 500 grams (1.1 pounds) or less and a large breast can weighs approximately 750 to 1,000 grams (1.7 to 2.2 pounds) or more.
Structure of the mammary gland:
As a mammary gland, the breast is composed of layers of different types of tissue, among which predominate two types:
The breast also is composed of connective tissues (collagen, elastin), and the suspensory Cooper's ligaments.