Normal breast anatomy

The mammary, or breast, gland is a paired organ and is located on the anterior chest wall. The mammary gland is attached to the sternum with its base, covers the edge of the pectoral muscles and reaches the anterior axillary line. The shape of the mammary gland is determined by the woman's race, age and constitutional features. The size and shape of the mammary gland can vary widely.

The normal volume of the mammary gland is 200-300 cm3. The smaller it is, the more stable the shape of the gland. The place of attachment of the mammary gland to the chest can be located in height from the 2nd to the 6th rib (the diameter of the attachment site varies from 12 to 15 cm); in width - from the edge of the sternum to the anterior axillary (axillary) line. In profile, 2/3 of the height is occupied by a straight or slightly concave suprapapillary segment, the lower third - by a convex subpappillary segment. The skin fold formed between the lower segment of the gland and the anterior chest wall forms the lower border of the organ.

The central part of the anterior surface of the mammary gland is occupied by the nipple, which forms a cylindrical or conical protrusion. The nipple consists of epithelial and muscle tissue. In its central part are the terminal sections of the excretory milk ducts. The top of the nipple is crossed by grooves, in which there are 15 to 25 small openings of the milk ducts. The outer part of the nipple is formed mainly by skin structures, inside which there are both radial and circular muscle fibers. Contraction or relaxation of the muscle structures of the nipple and areola causes physiological changes in the state of the nipple and the terminal section of the milk ducts.

A small pigmented area of skin (4 to 5 cm in diameter) around the nipple is called the areola. On the areola there are numerous elevations, tubercles, which are sweat glands. Subcutaneous muscle fibers form the areolar muscle. Together with the nipple, the areola is directed slightly upward.

The skin of the mammary gland is thin and mobile. It easily slides over the gland and gathers into folds. In the area of the nipple and areola, the skin is especially thin. There is no subcutaneous tissue layer here.

The body of the mammary gland is a soft glandular organ consisting of adipose, glandular and connective tissue, and is located under the skin in a fascial sheath formed by split layers of the superficial fascia.

The fatty tissue intimately embraces the body of the mammary gland, forming the anterior and posterior layers. The anterior (subcutaneous or pre-glandular layer) is interrupted in the retroareolar region, where the terminal sections of the milk ducts pass. The pre-glandular fatty tissue is located in the form of separate accumulations, which, under the influence of involution processes, are formed into fat lobules.

The connective tissue structures of the mammary gland are represented by its own capsule (anterior and posterior layers of the split fascia), coarse collagen fibers extending from it in the form of Cooper's ligaments, delicate fibrillar tissue located between the glandular elements and structures that are part of the wall of the milk ducts.

In the anterior sections, Cooper's ligaments connect the body of the mammary gland to the deep layers of the dermis, and in the posterior sections, they attach to the fascial sheath of the pectoral muscles. Cooper's ligaments, passing deep into the gland, envelop the fatty tissue like a capsule, forming a fatty lobe. The place where Cooper's ligaments attach to the glandular structures is called Durette's ridge.

The function of the mammary gland is to produce and secrete milk. The functioning fibroglandular tissue of the mammary gland is called parenchyma.

The parenchyma of the mammary gland is represented by complex alveolar-tubular glands collected in small lobules, from which the lobes are formed. In accordance with the overall dimensions of the mammary gland, the sizes of the glandular lobes vary from 1-2 cm in length and 1.5-2.0 cm in width (small glands), to 5-6 cm in length and 3-4 cm in width (large glands). The number of lobes (as well as their sizes) depends on the size of the mammary glands and varies from 6-8 (small glands) to 20-24 (large glands). The lobes are located radially in relation to the nipple and can overlap one another. The glandular lobule and glandular lobe do not have an external capsule and are not so much an anatomical as a functional unit of the mammary gland. An excretory milk duct extends from each glandular lobule. During pregnancy, numerous microscopic bubbles - acini - are formed inside the glandular lobules, at the ends of the milk ducts. The acinus produces milk during lactation and atrophies or disappears after breastfeeding ends. The bulk of the glandular tissue is located in the upper outer quadrant and in the posterior parts of the mammary gland. Often, glandular tissue is located in the axillary region, forming the axillary lobe. Between the glandular structures of the parenchyma is loose and delicate connective tissue. The complex of glandular and connective tissue is united by the term fibroglandular tissue.

A complex network of milk ducts connects the acini (during pregnancy and lactation), glandular lobules and lobes of the mammary gland, forming galactophores of the 1st to 3rd order. The sizes of the terminal milk ducts extending from each lobule (galactophores of the 1st order) and the lobar ducts (galactophores of the 2nd order) in a hormonally calm mammary gland do not exceed 2 mm in diameter. Behind the areola are the main, largest ducts with a diameter of up to 3 mm (galactophores of the 3rd order). These main ducts make a bend before exiting onto the surface of the nipple in the form of openings, forming the milk sinus. The milk sinus acts as a reservoir during lactation. The outer part of the milk duct is formed by connective tissue structures. The inner part of the duct consists of a single-layer cuboidal epithelium located on the basal membrane. As a result of the cyclic action of sex hormones, epithelial cells produce and then reabsorb milk duct secretion in the non-lactating mammary gland. During lactation, milk is present in the lumen of the ducts.

The mammary gland is supplied with blood mainly by branches of the external thoracic and subclavian arteries and to a lesser extent by the intercostal arteries. The arteries form a wide network of anastomoses behind the areola. Deep veins accompany the arteries of the same name. Venous outflow is carried out through the superficial and deep network. Venous anastomoses can form a bizarre pattern around the base of the areola.

The lymphatic system of the mammary gland consists of intraorgan lymphatic capillaries, extraorgan drainage vessels and regional lymph nodes. A large number of lymphatic vessels are located under the skin in the form of a superficial lymphatic network. Lymphatic vessels in the interlobular space form a deep network of lymphatic vessels in the form of lacunae and plexuses between glandular lobules and milk ducts. There are no lymphatic vessels in the glandular lobules themselves. Intraorgan and extraorgan lymphatic vessels and groups of lymph nodes can connect with each other in two ways. In the first case, the connection is carried out directly between the lymphatic vessels and regional lymph nodes, and in the second, this connection occurs with the obligatory involvement of the subareolar lymphatic network. Such a complex system of lymph drainage of the mammary gland is one of the causes of specific damage to various regional lymph nodes.

Depending on the quadrant of the breast tissue lesion, different groups of regional lymph nodes are involved in the tumor process. When the tumor is localized in the upper outer quadrant, the anterior and central axillary regional lymph nodes are most often affected. When the malignant tumor is localized in the upper inner quadrant, metastases may appear quite early in the axillary lymph nodes on the opposite side, as well as in the lymph nodes of the anterior mediastinum.

The innervation of the mammary gland is carried out by nerve trunks located along the skin and inside the glandular tissue. Superficial sensitivity is carried out by the thoracic, brachial and intercostal nerve trunks.

Physiology of the mammary gland

From birth to old age, the mammary glands undergo complex physiological changes. Until the end of early puberty at 7-8 years (stage 1), no special structural changes are observed. Stage 2 of puberty (8-9 years) is characterized by a unilateral enlargement of the girl's mammary gland in the retro-nipple region. This is the stage of physiological asymmetric hypertrophy of the mammary glands. By the age of 10-11, the sizes of both glands become equal. In stage 3 of puberty, there is a further increase in the size of the mammary gland not only behind the nipple, but also along its periphery (12-13 years). In stage 4, the mammary gland acquires a conical shape. At 15 years, the formation of the mammary gland is completed (stage 5). During this period, the mammary gland acquires a more rounded shape.

The onset of mammary gland formation (thelarche) occurs long before the first menstruation (menarche). With the onset of menstruation (from 12-14 years), cyclic morphofunctional changes begin to occur in the mammary glands under the influence of estrogens. Days 1-10 - tubuloacinous involution, days 11-16 - galactophoric proliferation, hypervascularization of connective tissue, days 17-28 - acinous proliferation, glandular hyperplasia and gradual limitation of vascularization of connective tissue. Superficial blood flow predominates, sometimes painful tension of the gland is noted. By the end of the cycle, there is a gradual increase in the volume of the mammary glands by 20%.

Involution processes begin in the mammary gland after the end of the first pregnancy. Involution of the mammary glands is especially rapid in the premenopausal period and during menopause.

Periods of age-related restructuring of the mammary gland include:

• 1. period - gradual loss of glandular structures of the mammary gland (35-40 years);
• 2. period - thickening of the cylindrical epithelium of the ducts, associated with a decrease in the number of myoepithelial cells, irregular thickening of the basement membrane and especially with fibrous restructuring of connective tissue (40-45 years);
• 3. period - dilation, and sometimes cystic expansion of some milk ducts, compressed by fibrous tissue (45-50 years);
• 4. period - slow obliteration of milk ducts, as well as small-caliber vessels (after 50 years); during this period, in parallel with connective tissue sclerosis, excessive formation of adipose tissue occurs. Involutional processes of the mammary glands are not simultaneous. It is always possible to note the predominance of one or another process.

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