Anatomy of the Breast

The human breast is formed early in fetal life from an invagination of the ectoderm. During the fourth week of pregnancy, a raised area can be seen in the developing fetus. Near term, 15-25 ducts form the fetal breast. Male and female breast tissue develop in the same fashion. The withdrawal of maternal hormones can cause breast engorgement in the newborn. Shortly after birth, a neonate's breasts may produce a small amount of milk called "witches' milk." Study neonatal milk shows that it greatly resembles the components of mother's milk (Pittard 294-298). Production in the neonatal breast quickly subsides and the glands become the mammary disk of childhood. "The human mammary gland is the only organ that is not fully developed at birth (Lawrence and Lawrence 35)." The mammary glands will remain inactive until puberty.

Organogenesis begins just before the onset of puberty in the female (age 10 or 12). The internal breast structures begin to expand rapidly under the influence of estrogen. Typically a girl's first period will begin about a year or two after her breasts begin to grow (Love and Lindsey 11) With every menstrual cycle a new phase of growth occurs which includes extensive branching of the ductal system and organization of the internal structures. Fat deposits in the breast give it a more adult, rounded appearance. The greatest changes occur by age 20 but the breast continues to develop until age 35 (Riordanand Auerbach 94). The breast is not considered fully mature until a woman gives birth and begins to produce milk (Love and Lindsey 15).

The breast tissue follows a teardrop shape. The top of the tear is located in axillary region and is called the "tail of Spence." The main body of the breast, the corpus mammae, is the bottom portion of the drop. Breast tissue and/or extra nipples may occur anywhere along the "milk line," a line extending downward from the inner arm toward the inner thigh. However, ectopic breast and/or nipple tissue can occur anywhere on the body. Hyperthelia or supernumerary nipples often resemble simple moles. Hyperadenia, (breast tissue without a nipple) or polymastia (breast tissue with a nipple) is difficult to detect except during pregnancy and lactation. The most commonly reported site for hyperadenia is in the axillary fold. Occasionally the extra tissue may also have an ill-defined nipple that mother assumes is a mole. In all the cases I have seen the condition is bilateral. Extra breast tissue in the axillary region is separate from the "tail of Spence." During pregnancy areas of hyperadenia and/or hyperthelia may become sensitive. Extra glandular tissue can be expected to experience growth during pregnancy. If a pregnant woman complains of a tender "mole" it should be examined to see if it is extra breast tissue. It may be appropriate to remove excess tissue if it causes pain, embarrassment, engorgement or mastitis as these areas are not fully functional breasts. (Lawrence and Lawrence 40-41 ) Breast tissue high in the axilla cannot be seen on mammography and is difficult to palpate on breast examination. Breast surgeons recommend removal of the extra tissue not only as a comfort measure but that the area is a potential site where breast cancer may hide.

The adult female breast is made up of glandular tissue, supportive and connective tissue, and protective fatty tissue. The stroma or supportive tissue of the breast contains connective tissue, fat, blood vessels, nerves and lymphatics. The breast is suspended by Cooper's ligaments. Breast sagging is not a result of breastfeeding, but the result of pregnancy hormones and gravity loosening the Cooper's ligaments. The stroma appears to keep the lobes from encroaching upon each other, maintaining an orderly structure within the breast.

The glandular tissue is composed of the lobi, lobuli, and alveoli and resembles a bunch of grapes. There between 15 and 25 lobes, arranged in a wheel spoke pattern in each breast. The lobuli are clusters of alveoli. The alveoli are the milk producing units of the breast. Individual alveoli empty into small lactiferous ducts that converge in each lobulus with several lobuli forming a lobe. The larger ducts of the lobe converge under the areola ** and finally end at the nipple. There are multiple openings in the nipple corresponding to the internal lobes. [Tables: A,B,C] Occasionally, one or more of the ducts may end at the areola and may leak milk during pregnancy and lactation. This is a normal variation.

 

Table A: Traditional View	Table B: Ultrasound Findings**	Table C: Microscopic Anatomy
**Ultrasound imaging of the lactating breast by Ramsay, et al, tossed our image of the lactating breast out the window. In the study, published in the Journal of Anatomy 2005, researchers were unable to see any structures resembling a milk "sinus" in the breast. Our ideas of internal breast anatomy may date back to a book from 1840 called "The Anatomy of the Breast," by Sir Astley Paston Cooper. As a part of his "research," hot, coloured wax was injected into the duct system of the breasts from the bodies of 7 lactating women to help identify the network. The wax itself may have enlarged the ducts closest to the areola, where multiple ducts converge, leading to a misconception of sinuses. Obviously imaging techniques have vastly improved since 1840.

Breast Size

The size of the breast varies greatly depending on the amount of adipose tissue present within the breast. Breast size bears no relationship to the amount of milk produced. The rare exception is a condition called Insufficient Glandular Development of the Breast (Neifert, Seacat, Jobe, Lact Failure 823-828). In these cases some or all of the lobes in the breast have not fully developed. The breast with Insufficient Glandular Development can appear long, thin and tube like, almost pointed at the areola and nipple or it may appear pubescent: flat with little or no development of the nipple and areola. [Many women with these types of Insufficient Glandular Development may have breast augmentation surgery to give the breast or breasts an adult, rounded appearance.] Occasionally the breast may appear normal. Upon palpation, the breast feels empty in the areas  where insufficient development has occurred. Little or no firm glandular tissue can be felt beneath the skin in affected areas. The condition can affect one or both breasts or it may only affect a portion of the breast(s). Definitive diagnosis can be made with ultra sound or mammography.

External Anatomy

External structures of the breast are the nipple, areola and Montgomery glands [Table D]. The nipple functions as a nozzle for delivery of the milk. The nipple is the most sensitive to tactile stimulation and pain. The darker portion behind the nipple is called the areola and can vary widely in size and color. The milk sinuses lie directly below the areola. The compression of the milk sinuses beneath the areola delivers milk to the nipple. The darkening of the areola during pregnancy may serve to act as a visual target for the newborn. Secondary areolar darkening or patchy pigmentation behind the areolar rim can also occur in pregnancy. Surrounding the areola, are areas that elevate during pregnancy called Montgomery glands. It is widely believed that these sebaceous glands produce a waxy substance that both lubricates and protects the nipple and areola with an antibacterial action but no evidence of this function exists (Riordan and Auerbach 96).

Table D: External Anatomy

 

Breast Growth in Pregnancy

Pregnancy brings increased growth within the breast. By the time the baby is born, the glandular tissue in the breast has completely replaced the fatty tissue (Eiger and Olds 41). Before pregnancy most of the glandular tissue in the breast looks like a fruit tree in winter (merely branches and twigs). The first trimester of pregnancy causes the internal structures to branch and sprout. Under the influence of 10 to 20 fold increase in placental lactogen, colostrum appears near the end of the second trimester. The breast will produce colostrum if the fetus is born at 16 weeks. However, "the division and differentiation of mammary epithelial cells and presecretory alveolar cells into secretory milk-releasing alveolar cells (Lawrence and Lawrence 55-56)," occur in the third trimester. Third trimester changes may account in part for the difficulty reported by mothers of premature infants (under 32 weeks) regarding maintaining a good milk supply when they are exclusively pumping long term, especially in prima-paras.

Involution of the Lactating Breast

Following lactation, the breasts involute. If milk is not removed from the breasts the glands become distended. This distention interferes with the blood supply to the breasts and milk production ceases. There is also evidence that an enzyme produced by the unremoved milk decreases production. Milk remaining in the alveoli is gradually reabsorbed and the alveoli collapse or rupture. Initially, after weaning, the breasts may appear smaller then pre-pregnancy size. This is due to the lack of adipose or fatty tissue within the breast. The amount of adipose tissue gradually increases and the breast returns to its resting state. Some residual growth of the glandular tissue remains after lactation. Women can often express a drop or two of milk for up to twelve months following weaning.