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The breast is the upper ventral region of the torso of a primate, in left and right sides, which in a female contains the mammary gland that secretes milk used to feed infants.Both men and women develop breasts from the same embryological tissues. However, at puberty, female sex hormones, mainly estrogen, promote breast development, which does not occur in men, due to the higher amount of testosterone. As a result, women’s breasts become far more prominent than those of men.

During pregnancy, the breast is responsive to a complex interplay of hormones that cause tissue development and enlargement in order to produce milk. Three such hormones are estrogen, progesterone and prolactin, which cause glandular tissue in the breast and the uterus to change during the menstrual cycle.[2]

Each breast contains 15–20 lobes. The subcutaneous adipose tissue covering the lobes gives the breast its size and shape. Each lobe is composed of many lobules, at the end of which are sacs where milk is produced in response to hormonal signals.[2]


The English word breast derives from the Old English word brēost (breast, bosom) from Proto-Germanic breustam (breast), from the Proto-Indo-European base bhreus– (to swell, to sprout).[3] The breast spelling conforms to the Scottish and North English dialectal pronunciations.[4]



The human breast has two aspects: the functional aspect and the anatomic aspect.

The functional breast

The breast is an apocrine gland that produces milk to feed an infant child; for which the nipple of the breast is centred in (surrounded by) an areola (nipple-areola complex, NAC), the skin color of which varies from pink to dark brown, and has many sebaceous glands. The basic units of the breast are the terminal duct lobular units (TDLUs), which produce the fatty breast milk. They give the breast its offspring-feeding functions as a mammary gland. They are distributed throughout the body of the breast; approximately two-thirds of the lactiferous tissue is within 30 mm of the base of the nipple. The terminal lactiferous ducts drain the milk from TDLUs into 4–18 lactiferous ducts, which drain to the nipple; the milk-glands-to-fat ratio is 2:1 in a lactating woman, and 1:1 in a non-lactating woman. In addition to the milk glands, the breast also is composed of connective tissues (collagen, elastin), white fat, and the suspensory Cooper’s ligaments. Sensation in the breast is provided by the peripheral nervous system innervation, by means of the front (anterior) and side (lateral) cutaneous branches of the fourth-, the fifth-, and the sixth intercostal nerves, while the T-4 nerve (Thoracic spinal nerve 4), which innervates the dermatomic area, supplies sensation to the nipple-areola complex.[5][6]

The anatomic breast

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). At the sides of the chest, the breast tissue can extend into the axilla (armpit), and can reach as far to the back as the latissimus dorsi muscle, extending from the lower back to the humerus bone (the longest bone of the upper arm). As a mammary gland, the breast is an inhomogeneous anatomic structure composed of layers of different types of tissue, among which predominate two types, adipose tissue and glandular tissue, which effects the lactation functions of the breasts.

Morphologically, the breast is a cone with the base at the chest wall, and the apex at the nipple, the center of the NAC (nipple-areola complex). The superficial tissue layer (superficial fascia) is separated from the skin by 0.5–2.5 cm of subcutaneous fat (adipose tissue). The suspensory Cooper’s ligaments are fibrous-tissue prolongations that radiate from the superficial fascia to the skin envelope. The adult breast contains 14–18 irregular lactiferous lobes that converge to the nipple, to ducts 2.0–4.5 mm in diameter; the milk ducts (lactiferous ducts) are immediately surrounded with dense connective tissue that functions as a support framework. The glandular tissue of the breast is biochemically supported with estrogen; thus, when a woman reaches menopause (cessation of menstruation) and her body estrogen levels decrease, the milk gland tissue then atrophies, withers, and disappears, resulting in a breast composed of adipose tissue, superficial fascia, suspensory ligaments, and the skin envelope.

The dimensions and the weight of the breast vary among women, ranging approximately 500–1,000 grams (1.1–2.2 pounds) each; thus, a small-to-medium-sized breast weighs 500 grams (1.1 pounds) or less; and a large breast weighs approximately 750–1,000 grams (1.7–2.2 pounds.) The tissue composition ratios of the breast likewise vary among women; some breasts have greater proportions of glandular tissue than of adipose or connective tissues, and vice versa; therefore the fat-to-connective-tissue ratio determines the density (firmness) of the breast. In the course of a woman’s life, her breasts will change size, shape, and weight, because of the hormonal bodily changes occurred in thelarche (pubertal breast development), menstruation (fertility), pregnancy (reproduction), the breast-feeding of an infant child, and the climacterium (the end of fertility).[7][8][9]

Lymphatic drainage

Approximately 75% of the lymph from the breast travels to the ipsilateral (same-side) axillary lymph nodes, whilst 25% of the lymph travels to the parasternal nodes (beside the sternum bone), to the other breast, and to the abdominal lymph nodes. The axillary lymph nodes include the pectoral (chest), subscapular (under the scapula), and humeral (humerus-bone area) lymph-node groups, which drain to the central axillary lymph nodes and to the apical axillary lymph nodes. The lymphatic drainage of the breasts is especially relevant to oncology, because breast cancer is a cancer common to the mammary gland, and cancer cells can metastasize (break away) from a tumour and be dispersed to other parts of the woman’s body by means of the lymphatic system.

Shape and support

The morphologic variations in the size, shape, volume, tissue density, pectoral locale, and spacing of the breasts determine their natural shape, appearance, and configuration upon the chest of a woman; yet such features do not indicate its mammary-gland composition (fat-to-milk-gland ratio), nor the potential for nursing an infant child.[10][11] The size and the shape of the breasts are influenced by normal-life hormonal changes (thelarche, menstruation, pregnancy, menopause) and medical conditions (e.g. virginal breast hypertrophy).[12] The shape of the breasts is naturally determined by the support of the suspensory Cooper’s ligaments, the underlying muscle and bone structures of the chest, and the skin envelope. The supensory ligaments sustain the breast from the clavicle (collarbone) and the clavico-pectoral fascia (collarbone and chest), by traversing and encompassing the fat and milk-gland tissues, the breast is positioned, affixed to, and supported upon the chest wall, while its shape is established and maintained by the skin envelope.

The base of each breast is attached to the chest by the deep fascia over the pectoralis major muscles. The space between the breast and the pectoralis major muscle is called retromammary space and gives mobility to the breast. Some breasts are mounted high upon the chest wall, are of rounded shape, and project almost horizontally from the chest, which features are common to girls and women in the early stages of thelarchic development, the sprouting of the breasts. In the high-breast configuration, the dome-shaped and the cone-shaped breast is affixed to the chest at the base, and the weight is evenly distributed over the base area. In the low-breast configuration, a proportion of the breast weight is supported by the chest, against which rests the lower surface of the breast, thus is formed the inframammary fold (IMF). Because the base is deeply affixed to the chest, the weight of the breast is distributed over a greater area, and so reduces the weight-bearing strain upon the chest, shoulder, and back muscles that bear the weight of the bust.

The chest (thoracic cavity) progressively slopes outwards from the thoracic inlet (atop the breastbone) and above to the lowest ribs that support the breasts. The inframammary fold, where the lower portion of the breast meets the chest, is an anatomic feature created by the adherence of the breast skin and the underlying connective tissues of the chest; the IMF is the lower-most extent of the anatomic breast. In the course of thelarche, some girls develop breasts the lower skin-envelope of which touches the chest below the IMF, and some girls do not; both breast anatomies are statistically normal morphologic variations of the size and shape of women’s breasts.[13]


The basic morphological structure of the human breast – female and male – is determined during the prenatal development stage. 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 usually rest on her chest; these development stages of secondary sex characteristics (breasts, pubic hair, etc.) are illustrated in the five-stage Tanner Scale.[14] During thelarche, the developing breasts sometimes are of unequal size, and usually the left breast is slightly larger; said condition of asymmetry is transitory and statistically normal to female physical and sexual development.[15] Moreover, breast development sometimes is abnormal, manifested either as overdevelopment (e.g. virginal breast hypertrophy) or as underdevelopment (e.g. tuberous breast deformity) in girls and women; and manifested in boys and men as gynecomastia (woman’s breasts), the consequence of a biochemical imbalance between the normal levels of the estrogen and testosterone hormones of the male body.[16]


Approximately two years after the onset of puberty (a girl’s first menstrual cycle), the hormone estrogen stimulates the development and growth of the glandular, fat, and suspensory tissues that compose the breast. This continues for approximately four years[clarification needed] until establishing the final shape of the breast (size, volume, density) when she is a woman of approximately 21 years of age.[11] About 90% of women’s breasts are asymmetrical to some degree,[11] either in size, volume, or relative position upon the chest. Asymmetry can be manifested in the size of the breast, the position of the nipple-areola complex (NAC), the angle of the breast, and the position of the inframammary fold, where the breast meets the chest.

For about 5%[11] to 10%[17] women, their breasts are severely different, with the left breast being larger in 62% of cases.[17] This is due to the breast proximity to the heart and a greater number of arteries and veins, along with a protective layer of fat surrounding the heart located beneath it.[18] Up to 25% of women experience notable breast asymmetry of at least one cup size difference.[10][11][19][20][21][22]

If a woman is uncomfortable with her breasts’ asymmetry, she can minimize the difference with a corrective bra[18] or use gel bra inserts.[18] Alternatively, she can seek a surgical solution. Options include a minimally invasive procedure known as platelet injection fat transfer, which transfers fat cells from a woman’s thighs to her smaller breast.[11] More invasive procedures include corrective mammoplasty, such as mastopexy, breast reduction, or breast augmentation, depending on the nature of the asymmetry.[11][17] Most surgeons will only perform an augmentation procedure to treat asymmetry if the woman’s breasts differ by at least one cup size.[11]

Hormonal change

Because the breasts are principally composed of adipose tissue, which surrounds the milk glands, their sizes and volumes fluctuate according to the hormonal changes particular to thelarche (sprouting of breasts), menstruation (egg production), pregnancy (reproduction), lactation (feeding of offspring), and menopause (end of menstruation). For example, during the menstrual cycle, the breasts are enlarged by premenstrual water retention; during pregnancy the breasts become enlarged and denser (firmer) because of the prolactin-caused organ hypertrophy, which begins the production of breast milk, increases the size of the nipples, and darkens the skin color of the nipple-areola complex; these changes continue during the lactation and the breastfeeding periods. Afterwards, the breasts generally revert to their pre-pregnancy size, shape, and volume, yet might present stretch marks and breast ptosis. At menopause, the breasts can decrease in size when the levels of circulating estrogen decline, followed by the withering of the adipose tissue and the milk glands. Additional to such natural biochemical stimuli, the breasts can become enlarged consequent to an adverse side effect of combined oral contraceptive pills; and the size of the breasts can also increase and decrease in response to the body weight fluctuations of the woman. Moreover, the physical changes occurred to the breasts usually are recorded in the stretch marks of the skin envelope; they are historical indicators of the increments and the decrements of the size and the volume of a woman’s breasts throughout the course of her life.

Breast ptosis

Ptosis is a normal consequence of aging[23] where the breast tissue sags lower on the chest and the nipple points downward.[24] The rate at which a woman develops ptosis depends on many factors including genetics, smoking, body mass index, number of pregnancies, the size of breasts before pregnancy, and age.[25]

Plastic surgeons categorize ptosis by evaluating the position of the nipple relative to the inframammary crease (where the underside of the breast meets the chest wall). This is determined by measuring from the center of the nipple to the sternal notch (at the top of the breast bone) to gauge how far the nipple has fallen. The standard anthropometric measurement for young women is 21 centimetres (8.3 in). This measurement is used to assess both breast ptosis and breast symmetry. The surgeon will assess the breast’s angle of projection. The apex of the breast, which includes the nipple, can have a flat angle of projection (180 degrees) or acute angle of projection (greater than 180 degrees). The apex rarely has an angle greater than 60 degrees. The angle of the breast apex is partly determined by the tautness of the suspensory Cooper’s ligaments. For example, when a woman lies on her back, the angle of the breast apex becomes a flat, obtuse angle (less than 180 degrees) while the base-to-length ratio of the breast ranges from 0.5 to 1.0.[23]

Functions and health


The primary function of the breasts – as mammary glands – is the feeding and the nourishing of an infant child with breast milk during the maternal lactation period. The round shape of the breast helps to limit the loss of maternal body heat, because milk production depends upon a higher-temperature environment for the proper, milk-production function of the mammary gland tissues, the lactiferous ducts. Regarding the shape of the breast, the study The Evolution of the Human Beast (2001) proposed that the rounded shape of a woman’s breast evolved to prevent the sucking infant offspring from suffocating while feeding at the teat; that is, because of the human infant’s small jaw, which did not project from the face to reach the nipple, he or she might block the nostrils against the mother’s breast if it were of a flatter form (cf. chimpanzee); theoretically, as the human jaw receded into the face, the woman’s body compensated with round breasts.[26]

In a woman, the condition of lactation unrelated to pregnancy can occur as galactorrhea (spontaneous milk flow), and because of the adverse effects of drugs (e.g. antipsychotic medications), of extreme physical stress, and of endocrine disorders. In a newborn infant, the capability of lactation is consequence of the mother’s circulating hormones (prolactin, oxytocin, etc.) in his or her blood stream, which were introduced by the shared circulatory system of the placenta; neonatal milk, the milk from a lactating infant, is also known as witch’s milk. In men, the mammary glands are also present in the body, but normally remain undeveloped because of the hormone testosterone, however, when male lactation occurs, it is considered a pathological symptom of a disorder of the pituitary gland.


In considering the human animal, zoologists proposed that the human female is the only primate that possesses permanent, full-form breasts when not pregnant. Other mammal females develop full breasts only when pregnant. The zoologist Desmond Morris proposed that the rounded shape of a woman’s breasts evolved as frontal, secondary sex characteristic that is a sexual-attraction counterpart to the buttocks, and so encouraged frontal copulation, the reason being that while other primates mate by means of the rear-entry position, the upright, bipedal human being was likelier to successfully copulate face to face in the missionary position.

As an ethologist, Morris further proposed that breasts, a secondary sex characteristic located on the woman’s chest, encouraged face-to-face sexual intercourse that led to the establishment of an emotional bond between man and woman; social progress from an essentially procreational function of human biology.[27] Furthermore, the symmetry of the breasts, and the general symmetry of the human body, influence what men and women consider physical attractiveness in a mate with whom to reproduce. Bodily beauty, evolutionary psychology proposes that a symmetrical body signals genetical health to a potential mate and so is the product of a morphologically stable line of people who physically developed without interference by disease. Therefore, because the breasts are especially sensitive to developmental interference (genetic and environmental), breast symmetry indicates a woman of good health and thus of good breeding stock, who shall successfully bear more (surviving) children than will a woman with asymmetrical breasts.[28]

Mammary diseases

The breast is susceptible to numerous benign and malignant conditions. The most frequent benign conditions are puerperal mastitis, fibrocystic breast changes and mastalgia. Breast cancer is one of the leading causes of death among women.


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