Breast Anatomy and Development During Lactation

Cross section biomedical illustration of changes in breast during pregnancy

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The breast development of an unborn child begins very early during your pregnancy. As early as six weeks, an embryo begins to develop.

Embryonic Breast Development - Conception to 12 Weeks

Breast development starts during the fourth week of gestation with the growth of a basic milk streak. Milk lines, or "ventral epidermal ridges," are seen by the sixth week of the embryo's "life." They arch down from the armpit to the groin on both sides. The breast develops along the ridge over the chest, and the remainder of the ridge, for the most part, disintegrates.

Embryonic Breast Development - 12 to 40 Weeks

Between 12 to 16 weeks gestation, specialized cells transform even further into the smooth muscle of the nipple and areola. At this point, mammary buds form and create milk ducts from cells that are close together. Sex hormones in the placenta come into the fetal circulation and control the development of the breast; this remains until 32 weeks gestation. Then, from 32 to 40 weeks, milk ducts, which contain colostrum, form within the nipple.

Embyronic Breast Development - 40 weeks to Birth

In the last weeks of pregnancy, the fetal mammary gland size grows four times more than its original size, and the nipple and areola grow even more and become darker. At birth, the only existing parts of breast anatomy are the milk ducts within the nipple—the alveoli, or milk-producing cells—have not yet formed and not a lot of changes will happen now until the onset of puberty. After birth, the newborn's mammary tissue might secrete colostrum.

Breast Changes During Puberty

Estrogen, the hormone that starts puberty in girls, triggers breast changes. Breast growth happens generally because of ample fat accumulation. The duct system also develops and spreads, and groups of tiny, dense cells form at the duct endings. These are future alveoli. The average development of breasts occurs between ages 10 to 11, however, the normal range is between 8 to 13 years of age.

Breast Changes During the Menstrual Cycle

During each menstrual cycle, maturation and intense growth of duct tissue takes place during the follicular and ovulatory phases, which peaks in the late luteal phase and then wanes. During each ovulatory cycle, high levels of ovarian steroids, mostly progesterone, promote additional mammary growth that never goes back to what it had been in the earlier cycle. Total development of mammary function exists only in pregnancy.

Breast Tissue Composition

The breast is made up of three types of tissue: glandular, fibrous (including suspensory ligaments) and adipose (or fatty) tissue. The relative proportion of glandular, fibrous, and adipose tissue changes with age, menstrual cycle, pregnancy, and nutritional situation.

Adult Breast Anatomy

Within mature breast tissue, significant structures are present. These include:

  • alveoli, or tiny sacs in which milk is stored and secreted
  • myoepithelial cells, which surround the alveoli and contract so that the milk is forced out into milk ducts
  • ducts and ductules, which filter out milk from the alveoli
  • lactiferous sinuses, or extensions of the ducts under the areola that store milk
  • lobes, or the branching network of all of the above-mentioned structures; each breast has approximately 20, ending in the nipple.

Other Significant Parts of the Breast

  • Cooper's ligaments
  • Axillary tail, or part of the mammary gland that extends into the armpit
  • Blood supply, which comes from particular mammary arteries
  • Musculature
  • Innervation, or nerve supply
  • Lymph supply and drainage: Lymphatic vessels transport this moving fluid that originates from blood and body fluid. Lymph is removed from the mammary gland and the tissues around it by two clusters of lymphatic vessels—one is for the skin; the other is for the actual breast, the nipple, and the areola.

Breast Appearance

Breasts vary from woman to woman. The parts of the breast recognized by the naked eye are symmetry; nipple size and shape; areolar color, size and shape, and the Montgomery Glands (tiny little bumps on the areola which secrete natural oils to lubricate the area and help to prevent bacteria from breeding.)

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