The fetal breast parenchyma first becomes transiently sensitive to hormonal stimulation at approximately 15 weeks of development.
At this time in the male fetus, the presence of testosterone affects the breast mesenchyme, which involutes around the epithelial stalk and causes the primitive breast bud to be isolated in the subdermal stroma, preventing development of the alveolar duct system.
In the female fetus, the absence of testosterone stimulation results in the default development of epithelium-lined milk ducts from the breast buds, which occurs from weeks 20 to 32.
Further development of the lobules and ductal elements occurs through the remainder of gestation, which is largely independent of hormonal stimulation.
Near term, the fetal breast again comes under the influence of hormonal stimulation. At this time, the stimulation is from maternal and placental sources, which cause secretion from the breast glandular alveoli. Removal of maternal hormonal stimulation after birth causes reversion to a simple, non-secreting ductular organization.
Further development occurs at puberty with the production of estrogens and progesterone, causing a recurrent growth in the ductal elements, which have been dormant since shortly after birth.
There is elongation of the mammary ducts, development of the ductular epithelium, and growth of the terminal ductule lobular units, with accompanying growth of connective tissue elements, including fat and fibrous tissue, to form the normal, mature female breast.
Changes in the breasts
Breast stage 1 (B1)
Breast: Prepubertal; no glandular tissue
Areola and papilla: Areola conforms to general chest line
Breast stage 2 (B2)
Breast: Breast bud; small amount of glandular tissue
Areola: Areola widens
Breast stage 3 (B3)
Breast: Larger and more elevation; extends beyond areolar parameter
Areola and papilla: Areola continues to enlarge but remains in contour with the breast
Breast stage 4 (B4)
Breast: Larger and more elevation
Areola and papilla: Areola and papilla form a mound projecting from the breast contour
Breast stage 5 (B5)
Breast: Adult (size variable)
Areola and papilla: Areola and breast in same plane, with papilla projecting above areola
Puberty and Maturity
In response to hormone stimulation, the breasts enlarge due to the growth of ductal and alveolar tissues and an increase in fat depostis.
The nipple and areola also enlarge and become more senstitive to touch. When the woman begins to menstruate, the breasts undergo a periodic premenstrual phase that varies with the individual but can include an increase in size, swelling and tenderness.
The symptoms subside within a few days of the onset of bleeding. During pregnancy, the breasts increase in size dramatically due to the influence of progesterone.
The nipple and areola become deeply pigmented and increase in size. Most of the fat is replaced by the necessary machinery to produce milk by late pregnancy.
After delivery the breasts begin to secrete milk.
The gland rapidly returns to the prepregnant state when nursing ceases.
The postmenopausal breast may retain its shape but the milk producing machinery is mostly replaced by fat.
In most females, the beginning of a breast bud is the first physical sign of puberty. While the traditional mean age of female sexual development was in the early 11s, over the past decade in developed countries, this age has been decreasing. For example, in the United States , the mean age of onset of breast development is 8.87 years for African-American girls and 9.96 years for white girls. The mean ages for the onset of pubic hair are 8.78 years and 10.51 years, respectively. Potential reasons for this decrease in age of onset, while unknown, might include improved nutrition, increasing obesity, hormonal exposures and other environmental/societal alterations. There may be important future consequences of earlier maturation with regards to teen behavior, sexual activity and pregnancy as well as future lifetime health consequences of early sexual maturation such as potential increase risk of breast cancer.
During puberty, the female's breasts develop and the ovaries, uterus, vagina, labia, and clitoris increase in size. The uterus and ovaries increase in size fivefold to sevenfold. Completion of puberty in females averages 4 years but can range from 1.5 years to 8 years. In the average adolescent female, the growth spurt starts about 1 year before breast development and this is followed by an average of 1.1 years until PHV and then followed in an average of 1 year by menarche. The typical sequence of pubertal events in females is seen below which demonstrates the usual early occurrence of peak height velocity at an average SMR of 2 and the late occurrence of menarche at an average SMR of 4. Menarche occurs in 19% of adolescents during PH3 and in 5 6% during PH4. There is little or no correlation between adult height and either age of onset of growth spurt, age of PHV, velocity at peak, or pubertal height gain. However, there is a correlation between adult height and the height at onset of growth spurt or height at PHV.
The breast undergo many changes during a woman's lifetime.
They don't start to mature until puberty when the ovaries start producing the hormone estrogen
The breasts are very sensitive to hormonal changes and during the first two weeks of the menstrual cycle, estrogen stimulation causes growth of the ductal elements.
During the later two weeks, progesterone, another hormone, causes the lobules to grow.
This can cause painful swelling in the breasts just before menstruation.
Pregnancy / Menopause
If pregnancy occurs, other hormonal changes prepare the breast for milk production. With menopause, estrogen levels diminish and the breast tissue shrinks. Much of the breast is then replaced with fatty tissue
The breasts become fully developed under the influence of estrogen, progesterone and prolactin during preganancy.
Prolactin causes the production of milk, and oxytocin release (via the suckling reflex) causes the contraction of smooth muscle cells in the ducts to eject the milk from the nipple.
The first secretion of the mammary gland after delivery is called colostrum.
It contains more protein and less fat than subsequent milk and contains antibodies that impart some passive immunity to the infant. Most of the time it takes 1-3 days after delivery for milk production to reach appreciable levels.
The drop in circulating estrogens and progesterone caused by the expulsion of the placenta at delivery initiates milk production.
Estrogen antagonizes the positive effect of prolactin on milk production.
The physical stimulation of suckling causes the release of oxytocin and stimulates prolactin secretion stimulating more milk production