ANATOMY & PHYSIOLOGY OF THE BREAST

Anatomy & Physiology of the Breast

The breast is an organ whose structure reflects its special function: the production of milk for lactation (breast feeding). The epithelial component of the tissue consists of lobules, where milk is made, which connect to ducts that lead out to the nipple. Most cancers of the breast arise from the cells which form the lobules and terminal ducts. These lobules and ducts are spread throughout the background fibrous tissue and adipose tissue (fat) that make up the majority of the breast. The male breast structure is nearly identical to the female breast, except that the male breast tissue lacks the specialized lobules, since there is no physiologic need for milk production by males.

Anatomically, the adult breast sits atop the pectoralis muscle (the "pec" chest muscle), which is atop the ribcage. The breast tissue extends horizontally (side-to-side) from the edge of the sternum (the firm flat bone in the middle of the chest) out to the midaxillary line (the center of the axilla, or underarm). A tail of breast tissue called the "axillary tail of Spence” extend into the underarm area. This is important because a breast cancer can develop in this axillary tail, even though it might not seem to be located within the actual breast.

The breast tissue is encircled by a thin layer of connective tissue called fascia. The deep layer of this fascia sits immediately atop the pectoralis muscle, and the superficial layer sits just under the skin. The skin covering the breast is similar to skin elsewhere on the body and has similar sweat glands, hair follicles, and other features. A clinician will examine the skin in addition to the breast tissue itself when performing a breast exam.

Vasculature of the breast

Arteries carry oxygen-rich blood from the heart to the chest wall and the breasts, while veins take de-oxygenated blood back to the heart.

The arterial supply of the breast is from:

1.   Internal thoracic artery

2.    Lateral thoracic and thoracoacromial arteries

3.    Posterior intercostal arteries

Venous drainage of the breast is mainly through the axillary vein, and some drainage occurs through the internal thoracic veins.

Drainage of lymph (a clear fluid that travels through the body’s arteries, circulates through tissues to help clean them, and then drains away through the lymphatic system) through the breast is very important, especially in metastases involving breast cancer cells. Lymph runs from the nipple, areola, and lobules into a special network, which then drains to the axillary lymph noses, parasternal and subclavicular nodes.

The nerve supply of the breast is derived from the intercostal nerves. Nerves convey sensory fibres to the skin of the breast, so there is sensation of the breast, and nerve fibres also run to the blood vessels and smooth muscles within the breast.

Hormonal regulation effect on breasts

During puberty, growth, and development of the breast occurs due to oestrogens and progesterones. After this development, changes in breast morphology occur during the ovarian cycle due to hormonal fluctuations.

During pregnancy, estrogen, and progesterone produced by the corpus luteum stimulate proliferation and development of the breast. Prolactin (produced by the adenohypophysis), somatomammotrophin from the placenta, and adrenal corticoids also play a role during pregnancy.

After delivery, levels of circulating hormones decrease with degeneration of the corpus luteum and the placenta. Milk secretion is stimulated by increased prolactin (as a neurohormonal response to suckling) and adrenal cortical steroids. Oxytocin is also produced, which stimulates the contraction of myoepithelial cells in the mammary glands to eject milk.

With the absence of ovarian hormones after menopause, breast glands atrophy, and secretory cells of the alveoli degenerate. Connective tissue also shows degenerative changes with a decrease in stromal cells and collagen.

References

Ross MH, Gordon GI, Pawlina W. Histology : A Text & Atlas, USA: Lippincott Williams & Wilkins; 2003.

Moore KL, Dalley AF. Clinically Orientated Anatomy. Canada: Lippincott Williams & Wilkins; 1999.

Summary

The mammary gland is classified as a branched tubuloalveolar structure with hormone-responsive lobules surrounded by a loose connective tissue stroma. The glands making up the breast are embedded in adipose tissue separated by bands of connective tissue. The breast is unique because it completes the majority of its development after birth, undergoing hormonally regulated changes during puberty. It varies moderately during each menstrual cycle, develops additionally during pregnancy, and differentiates following parturition during the process of lactation. The breast regresses after lactation to a much less differentiated state through the process of involution, which occurs following each cycle of pregnancy, parturition, and lactation—following reduction of estrogen and progesterone at menopause, the breast involutes, reverting to a near prepubertal structure. These complex developmental processes are controlled by a combination of hormonal stimulation, growth factors, and other physical elements constituting the mammary gland's microenvironment.