The breasts are paired structures that lie in front of the anterior thoracic wall over the pectoralis major muscle.
In this article, we shall discuss the anatomy of the breast, its nerve supply, blood supply, lymphatic drainage along with various aspects of clinical significance.
The breasts contain the mammary glands which are an accessory gland of the female reproductive system. Their primary function is to produce milk to feed the young ones (lactation or breastfeeding).
Read more about Breastfeeding – Benefits, Problems, and Tips for Better Feeding
The breasts are present in both males and females. In males, however, breast is present in rudimentary form. In females under the effect of hormones, the breasts undergo rapid enlargement and development during puberty and pregnancy.
The breast is located on the anterior thoracic wall in the superficial fascia of the pectoral region.
Horizontally it extends from the lateral border of the sternum to the mid-axillary line.
Vertically, it lies between the second and sixth ribs.
The size and shape of the breast depends on genetic, racial and dietary factors
It is composed of two parts:
- Circular part: It is the largest and most prominent part of the breast which is present on the anterior thoracic wall.
- Axillary part: It is the smaller part that is present in the axillary fossa. It is also called the axillary tail of Spence.
At the center of the breast is a conical projection called the nipple. It is composed of circular and longitudinal smooth muscle fibers. The nipple lies at the level of the fourth intercostal space. However, in multiparous women, its position is variable.
The skin surrounding the base of the nipple is pigmented. It forms a circular area called areola. The areola is rich in sebaceous glands, particularly in its outer margin. These enlarge during pregnancy and lactation to form raised tubercles (of Montgomery). These tubercles secrete an oily substance that provides lubrication to the nipple and prevents it from cracking during lactation. Areola also contains some sweat glands and accessory mammary glands. It has a rich nerve supply and has many sensory end organs at the terminations of nerve fibers. The skin of areola and nipple does not contain hair. Also, there is no fat adjacent to it.
The deep surface of the breast is related to the following structures in the following order:
- The base of the breast lies on the pectoral fascia which is a layer of connective tissue associated with the pectoralis major muscle.
- Further deep are present the following muscles: the pectoralis major, the serratus anterior, and parts of the external oblique muscle of the abdomen inferolaterally and the uppermost portion of the rectus abdominis muscle inferomedially.
- Between the breast and pectoral fascia is a potential space composed of loose connective tissue. This space is called the retromammary space. The presence of this loose connective tissue allows the normal breast to move over the pectoralis major muscle. The retromammary space is clinically significant as it is used in reconstructive plastic surgery.
Quadrants of the Breast
The breast can be divided into four quadrants. These are:
• Upper outer (superolateral) quadrant
• Upper inner (superomedial) quadrant
• Lower outer (inferolateral) quadrant
• Lower inner (inferomedial) quadrant
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The majority of breast cancers develop in the upper outer quadrant of the breast.
The breast is composed of mammary glands surrounded by a connective tissue stroma.
The mammary glands are modified sweat glands. They are made up of 15-20 lobulated masses or lobes which are radially arranged around the nipple.
These lobes consist of smaller units called lobules.
Each lobule consists of an intralobular terminal duct with multiple outpouchings called acini or ductules.
Structurally the lobule is composed of a tubuloalveolar unit with two-layered epithelium. The inner layer is made up of glandular epithelial cells that can produce milk. The outer layer is composed of myoepithelial cells which have the ability to contract and help to propel the milk towards the ducts.
The ducts from each lobule empty into a lactiferous duct that further empties onto the surface of the nipple. The function of the lactiferous ducts is to transport milk from the lobules to the nipple. Around the nipple, these ducts are surrounded by smooth muscle, contraction of which makes the nipple become erect.
The normal breast tissue of adult women is composed of three types of lobules (lobule type 1, 2, and 3). At birth, breast tissue consists of only a few ducts. The formation of type 1 lobules starts during puberty. Changes in the levels of estrogen and progesterone which occur during each menstrual cycle stimulate the type 1 lobules to produce new alveolar buds and evolve to more mature structures, known as type 2 and type 3 lobules. After puberty, further enlargement and development of breasts occur during pregnancy and lactation.
Connective Tissue Stroma
Surrounding the mammary glands is a supporting structure known as the connective tissue stroma. It is composed of fibrous and fatty tissue.
The fibrous stroma forms septa known as the suspensory ligaments (of Cooper). These ligaments anchor the skin and mammary gland to the pectoral fascia and also separate the various lobules of the breast. With age these ligaments become lax, resulting in drooping of the breasts.
The fatty tissue forms the main bulk of the gland. Fat is present all over the breasts, except beneath the areola and nipple.
The ratio of connective tissue to glandular tissue varies among individuals. During pregnancy and lactation, the breast is composed of mainly glandular tissue and very little stroma. During menopause, the relative amount of fatty stromal tissue is much more in comparison with the glandular tissue.
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The breast is a highly vascular structure.
The medial aspect of the breast receives its arterial supply from the internal thoracic artery (a branch of the subclavian artery).
The lateral part of the breast receives its arterial supply from the following blood vessels:
- The lateral thoracic, superior thoracic and thoracoacromial branches (branches of the axillary artery).
- Lateral mammary branches (branches of the posterior intercostal arteries derived from the aorta).
- Mammary branch(branch of the anterior intercostal artery).
The veins of the breast correspond with the arteries. The main venous drainage is to the axillary vein followed by the internal thoracic veins.
The rich blood supply of the breast enables various reduction techniques to be performed since the viability of the skin flaps after surgery is ensured.
About 75% of the lymph (particularly from the lateral quadrant of the breast) drains to the axillary lymph nodes on the same side of the body, The axillary lymph nodes include the pectoral, lateral, posterior, central and apical lymph-node groups.
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Approximately 20% of the lymph (mainly from the medial quadrant) drains to the parasternal nodes.
Out of the remaining lymph, some drains to posterior intercostal nodes while a small amount (mainly from the lower quadrant) drains to the inferior phrenic (abdominal) nodes or to the other breast.
The skin of the breast, except the nipple and areola, drain into the axillary, inferior deep cervical, and inferior clavicular nodes. The nipple and areola drain to the subareolar lymphatic plexus.
From the axillary nodes, lymph travels to infraclavicular and supraclavicular nodes and from them to the subclavian lymphatic trunk. Lymph from parasternal nodes travels to the bronchomediastinal trunk. Both these trunks independently join the junction of the internal jugular and subclavian veins to form the brachiocephalic veins.
The lymphatic drainage of the breasts is important because it plays a major role in the spread of breast cancer to other parts of the body (cancer metastasis).
Read more about Metastasis or Metastatic Disease
The breast is mainly supplied by the anterior and lateral cutaneous branches of the 4th to 6th thoracic intercostals nerves. The nipple is supplied by the lateral cutaneous branch of the 4th thoracic nerve. Supraclavicular nerves from the lower fibers of the cervical plexus also supply the upper and lateral portions of the breast.
These nerves contain both sensory and autonomic nerve fibers. The sensory fibers convey messages from the skin, and autonomic fibers relay messages to smooth muscle and blood vessels. The nerves do not control the secretion of milk.
The secretion of the milk is not under the control of nerves but is regulated by various hormones including prolactin and oxytocin.
Read more about Breast Milk Production and Regulation
Read more about Diet for Breastfeeding Mothers – What to Eat and What to Avoid?
Diseases of the breasts
A large number of diseases can affect the breast. This includes both benign and malignant disorders. The most common benign conditions include fibrocystic breast disease, mastitis, mastalgia, fibroadenoma, etc.
It is the most common cause of cancer death among women. It can occur in both men and women but is much more common in women. Increasing age, family history of breast cancer, obesity postmenopausal hormonal therapy are some of the factors which increase the risk of breast cancer. Breastfeeding, on the other hand, has a protective role in breast cancer.
Knowledge of the lymphatic drainage of the breasts has great practical importance in predicting the metastasis of breast cancer. Since the axillary lymph nodes drain the majority of the lymph from the breast tissue, they are the most common site of breast cancer metastasis.
Obstruction in the lymphatic drainage of the breast by cancer cells may cause the skin to become thick and leather-like with prominent pores. This gives it an orange peel appearance (peau d’orange).
Read more about Breast Cancer While Breastfeeding
Axillary part of the breast
It is important to remember that breast tissue is also present in the axilla and all the diseases of the breast can occur within this axillary breast tissue. Enlargement and painful axillary lumps during menstruation are suggestive of fibrocystic disease of breast involving the axillary region. Also, breast cancer can occur within the axilla and so examination of the axilla must be included while self-examining the breast for early detection of breast cancer.
Both females’ and males’ breasts develop from the same embryological tissues. In males, breasts are present in a rudimentary form. This is because the lower levels of estrogens and higher levels of testosterone, which suppress the effects of estrogens don’t allow the development of breast tissue in males.
Enlargement of the breast in males is called gynecomastia. It is commonly seen at puberty. It may also be due to liver disease or due to the intake of drugs like diethylstilbestrol for prostate cancer.
It can be performed to increase or reduce the size of breasts, or to reconstruct the breast after mastectomy in cases of breast cancer.
Breast augmentation (augmentation mammoplasty or boob job) and breast lift (mastopexy) procedures are done only for cosmetic reasons. A woman can very much breastfeed her babies in future even after undergoing these procedures.
Breast reduction (reduction mammoplasty) may be done to treat asymmetrical breasts or breast hypertrophy (macromastia and gigantomastia). It can lead to decreased sensation in the nipple-areola complex, and result in low milk supply in women in future.