The breasts, also referred to as mammary glands (Latin: mammae) are the most prominent superficial structures in the anterior thoracic wall, especially in women. It is an organ found in both males and females, but its mammary glands, which are accessory to reproduction in the adult females, are rudimentary and functionless in men and consist of only a few small ducts or epithelial cords. The mammary glands are in the subcutaneous tissue overlying the pectoralis major and minor muscles. The amount of fat surrounding the glandular tissue determines the size of non-lactating breasts. Usually, the fat present in the male breast is not different from that of subcutaneous tissue elsewhere in the body, and the glandular system does not normally develop. At the greatest prominence of the breast is the nipple, surrounded by a circular pigmented area of skin called the areola (Latin: small area).
The breast is a modified apocrine sweat gland and begins to develop as early as the 4th week as a downgrowth from a thickened mammary ridge (milk line) of ectoderm along a line from the axilla to the inguinal region. Supernumerary nipples or even glands proper may form at lower levels on this line.
Gross Features of the Breast
Despite individual variations in size, the extent of the base of the breast is fairly constant: from nearly the midline to near the midaxillary line, and from the 2nd to the 6th ribs. It overlies pectoralis major muscle, overlapping on to serratus anterior muscle and to a small part of rectus sheath and external oblique muscle.
The roughly circular body of the breast (female breast) rests on a bed that extends transversely from the lateral border of the sternum to the midaxillary line, and from the 2nd to 6th ribs, and thus roughly spans the area on which the base of the breast sits. Two thirds of the bed of the breast are formed by the pectoral fascia overlying the pectoralis major muscle; the other third, by the fascia covering the serratus anterior muscle. Between this pectoral fascia and the breast is a loose connective tissue plane or potential space called the retromammary space (bursa). This plane, containing a small amount of fat, allows the breast some degree of movement on the pectoral fascia. A small part of the upper outer quadrant of the mammary gland may extend along the inferolateral edge of the pectoralis major toward the axillary fossa (armpit), forming an axillary tail or process (of Spence).
This axillary tail is not always present, but when present, lies in the medial wall of the axilla and may be a discrete mass poorly connected with the duct system. Some women discover the axillary process (especially when it may enlarge during a menstrual cycle) and become concerned that it may be a lump (tumor) or enlarged lymph nodes. The mammary gland is firmly attached to the dermis of the overlying skin, especially by substantial skin ligaments called the suspensory ligaments (of Cooper). These condensations of fibrous connective tissue, particularly well developed in the superior part of the gland, help support the mammary gland lobules. During puberty (ages 8-15 years in both sexes), the breasts normally enlarge, owing in part to glandular development but primarily from increased fat deposition. The areolae and nipples also enlarge. Breast size and shape are determined by genetic, ethnic, and dietary factors. The lactiferous ducts give rise to buds that form 15-20 lobules of glandular tissue, which constitute the parenchyma of the mammary gland. Each lobule is drained by a lactiferous duct, which usually opens independently on the nipple. The lactiferous ducts branch off in a radial direction towards the nipple like the spokes of bicycle wheel.
Deep to the areola, each duct has a dilated portion, the lactiferous sinus, in which a small droplet of milk accumulates or remains in the nursing mother. As the infant begins to suckle, compression of the areola (and the lactiferous sinus beneath it) expresses the accumulated droplets and encourages the infant to continue nursing as the hormonally mediated let-down reflex ensues and the mother’s milk is secreted into – not sucked from the gland by – the baby’s mouth.
The areola contain numerous sebaceous glands, which enlarge during pregnancy and secrete an oily substance that provide a protective lubricant for the areola and nipple, which are particularly subject to chaffing and irritation as mother and baby begin the nursing experience. Some large sebaceous or areola glands may form small elevations called tubercles of Montgomery.
The nipples are conical or cylindrical prominences in the centre of the areola. The nipples have no fat, hair or sweat glands. The tips of the nipples are fissured with the lactiferous ducts opening into them. The nipples are composed mostly of circularly arranged smooth muscles fibres that compress the lactiferous duct during lactation and erect the nipples in response to stimulation as when a baby begins to suckle. In most women, the breast enlarge slightly during the menstrual period from increased release of the gonadotropic hormones – follicle stimulating hormone (FSH) and luteinizing hormone (LH) – on the glandular tissue. For the anatomical location and description of tumors and cysts, the surface of the breast is divided into four quadrants.
The breast is supplied by three main arteries which form anastomosing network. The arteries are the lateral thoracic artery, internal thoracic artery and the thoracoacromial artery.
The lateral thoracic sends branches that curl around the border of pectoralis major and other branches that pierce the muscle. The internal thoracic artery also sends branches through the intercostal spaces beside the sternum; those of the 2nd and 3rd spaces are the largest. Similar but small perforating branches arise from the intercostal arteries. Pectoral branches of the thoracoacromial artery supply the upper part of the breast.
The venous drainage of the breast is mainly by deep veins that run with the main arteries to internal thoracic artery and by the axillary veins. Some drainage to posterior intercostal veins provides an important link to vertebral veins and hence a pathway to metastatic spread to the vertebral bones. Superficial veins may anastomose across the midline (though not all Authorities subscribe to this suggestion).
The lymphatic system of the breast is very important in relation with the spread of malignant disease. There are numerous lymphatic capillaries in the breast. Lymph passes from the nipple, areola and lobules of the gland to a lymphatic plexus called the subareolar lymphatic plexus and from this plexus, lymph (over 75%) from the lateral part of the breast (upper and lower outer quadrants) drains into the axillary and infraclavicular lymph nodes. Lymph from the rest part of the breast, that is, from the medial part (upper and lower inner quadrants) drains through the intercostal spaces into the internal thoracic (parasternal) lymph nodes.
The overlying skin of the breast is supplied by the cutaneous branches of the 4th-6th intercostal nerves (T4-T6). Sympathetic fibres supply the blood vessels and glands, but the control of lactation is hormonal.