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It is quite common for breasts to be asymmetrical, with about 88% of women having breasts that differ in size, shape, or position. This asymmetry can be influenced by various factors, including genetics, weight changes, hormonal changes, puberty, pregnancy, breastfeeding, and underlying medical or skeletal conditions. While breast asymmetry is generally not a cause for concern, significant differences in breast size or density may indicate an increased risk of breast cancer or other abnormalities, making regular mammograms and medical consultations important for early detection and diagnosis.
| Characteristics | Values |
|---|---|
| Number of breasts | Most people are born with two breasts |
| Breast composition | Glandular tissue, connective or fibrous tissue, and fatty tissue |
| Muscle composition | The pectoralis major muscle forms the base of the breast |
| Muscle function | Provides support and mobility to the breast |
| Lymphatic system | Lymph vessels and lymph nodes are present in the breast |
| Nerve supply | Intercostal nerves T3-T5 and lower cervical plexus |
| Hormonal influence | Estrogen and progesterone affect breast tissue and milk production |
| Size and shape | Varies among individuals, with one breast often larger or shaped differently than the other |
| Nipple characteristics | Contains muscle fibres that cause erection when contracted; usually symmetrical |
| Areola characteristics | Circular, pigmented area surrounding the nipple; may vary in size and colour |
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What You'll Learn
The pectoralis major muscle forms the base of the breast
The pectoralis major muscle is a thick, fan-shaped or triangular convergent muscle that forms the base of the breast. It is the largest and most superficial muscle in the chest area, making up the bulk of the chest muscles. It lies underneath the breast tissue and forms the anterior wall of the axilla. The pectoralis major arises from parts of the clavicle, sternum, costal cartilages of the true ribs, and the aponeurosis of the abdominal external oblique muscle. It has two heads, the clavicular and the sternocostal, which reference their area of origin. The muscle is active in deep or forced inspiration but not expiration.
The pectoralis major is responsible for several actions, primarily the movement of the shoulder joint. This includes flexion, adduction, and internal rotation of the humerus. The clavicular part contributes to flexion, horizontal adduction, and inward rotation of the humerus, while the sternocostal part contributes to downward and forward movement of the arm and inward rotation when accompanied by adduction. The pectoralis major also assists in elevating the thorax and supporting the weight of the body, as in crutch-walking or parallel-bar work.
The muscle receives most of its blood supply from branches of the internal mammary and thoracoacromial arteries. It receives dual motor innervation from the medial pectoral nerve and the lateral pectoral nerve.
In rare cases, the pectoralis major muscle may be absent due to Poland Syndrome, a congenital condition that can result in the absence of the muscle on one side of the body. This condition can also be associated with an underdeveloped breast.
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The breast is anchored to the pectoralis major by suspensory ligaments
The breasts are paired structures located on the anterior thoracic wall, in the pectoral region. They are present in both males and females, but are more prominent in females following puberty. The breasts lie over the pectoralis major muscle, which forms the base of the breast. The pectoralis major is a broad, fan-like muscle that originates from the lateral sternum and clavicle and inserts at the humeral head. The muscle receives most of its blood from branches of the internal mammary and thoracoacromial arteries.
The fibrous stroma condenses to form these suspensory ligaments, which have two main functions. Firstly, they attach and secure the breast to the dermis and underlying pectoral fascia. Secondly, they separate the secretory lobules of the breast. The base of the breast lies on the pectoral fascia, a flat sheet of connective tissue associated with the pectoralis major muscle. It acts as an attachment point for the suspensory ligaments.
The breast can be considered to have two regions: the circular body, which is the largest and most prominent part, and the axillary tail, which is smaller and runs along the inferior lateral edge of the pectoralis major towards the axillary fossa. The nipple is located in the centre of the breast and is surrounded by the areola, a pigmented area of skin. The nipple and areola together are referred to as the nipple-areola complex (NAC), an important anatomical classification in surgical procedures such as mastectomies.
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The nipple contains muscle fibres that contract to make it erect
The nipple is the area at the centre of the areola, the circular darker-coloured area of skin surrounding the nipple on the surface of the breast. The nipple contains muscle fibres. When these muscle fibres contract, the nipple becomes erect or pointed outward. This is caused by the release of oxytocin, which is linked to sexual arousal and pair bonding. Nipple erection during sexual arousal or breastfeeding is caused by the release of oxytocin. Oxytocin is also released during breastfeeding, promoting the production and release of prolactin. This creates maternal feelings and decreases a woman's anxiety while increasing feelings of bonding and trust.
The nipple contains hundreds of nerve endings. Generally, a female breast is more sensitive to stimulation due to hormones. However, males can also experience pleasure from the nerve endings in their nipples. Nipple stimulation may produce sexual excitement, and erect nipples can indicate an individual's sexual arousal. Both men and women report that breast stimulation may be used to initiate and enhance sexual arousal. A few women report experiencing orgasm from nipple stimulation. Nipple erection can significantly impact people's perceptions of and behaviour towards women. For example, a psychological study found that the presence of nipple erection can influence people's views of a woman's intelligence, morality, and sexuality. Another study found that men are more willing to help women with erect nipples, indicating that they view them as more sexually attractive and approachable.
The breast has many blood vessels and lymph vessels. Lymph vessels are thin tubes similar to blood vessels that transport lymph fluid away from the breast into small bean-shaped masses of lymphatic tissue (lymph nodes) around the breast. The breast covers a large area, from just below the collarbone to the armpit and across to the breastbone. It is made up of fat, connective tissue, glands, and ducts. The main function of a woman's breasts is to make, store, and release milk to feed a baby. After giving birth, hormones in a woman's body stimulate the glands in the lobules throughout the breast to make milk.
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The breast is influenced by the sex hormone oestrogen
The female breast is influenced by the sex hormone oestrogen in several ways. Oestrogen is responsible for the growth of milk ducts in the breasts. During puberty, when the ovaries start to make and release oestrogen, fat collects in the connective tissue, causing the breasts to enlarge. The duct system also grows during this time. Oestrogen controls the growth of the ducts, while the hormone progesterone controls the growth of the glandular buds.
Oestrogen levels fluctuate throughout the menstrual cycle. In the first half of the cycle, oestrogen levels increase, stimulating the growth of milk ducts. This leads to ovulation halfway through the cycle. In the second half of the cycle, progesterone takes over, stimulating the formation of milk glands. These hormonal changes cause cyclical changes in the breasts, such as swelling, pain, and soreness.
Oestrogen also plays a role in pregnancy. As in puberty, oestrogen stimulates the growth of ducts during pregnancy, while progesterone stimulates the growth of glandular buds. By the fifth or sixth month of pregnancy, the breasts are fully capable of making milk.
Additionally, oestrogen levels influence breast cancer risk. Around 70% of breast cancers are sensitive to oestrogen, and the growth of these cancers can be minimised by taking drugs that block oestrogen production or its ability to attach to breast cancer cells. After menopause, when oestrogen production from the ovaries stops, the fat cells in the breasts produce greater amounts of an enzyme called aromatase, which promotes oestrogen production. This increase in local oestrogen production can contribute to the initiation and promotion of breast cancer.
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The breast is affixed to and supported by the chest wall
The breasts of women lie over the pectoralis major muscles and extend from the second rib to the sixth rib at the front of the rib cage. The breasts cover a large area of the chest, extending from the collarbone to the armpit and across to the breastbone. The base of each breast is attached to the chest by the deep fascia over the pectoralis major muscles. The base of the breast is semi-circular, while the shape and position of the breast above the surface vary. The space between the breast and the pectoralis major muscle is called the retromammary space, which gives mobility to the breast. The chest progressively slopes outwards from the thoracic inlet to the lowest ribs that support the breasts. The inframammary fold (IMF), 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 lowest extent of the anatomic breast.
The breast is positioned, affixed to, and supported upon the chest wall, while its shape is established by the suspensory Cooper's ligaments, the underlying muscle and bone structures of the chest, and by the skin envelope. The suspensory ligaments sustain the breast from the clavicle (collarbone) and the clavico-pectoral fascia (collarbone and chest) by encompassing the fat and milk-gland tissues. The breast attaches to the chest wall along its entire perimeter, posterior wall, and horizontal septum, with the perimeter functioning as the primary anchor of the breast to the chest wall. The structure of the perimeter attachment is both periosteal and fascial and requires sharp dissection to remove it from the chest wall.
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Frequently asked questions
It is completely normal for breasts to be a little uneven. In most cases, having one breast bigger than the other is not a problem. Breast asymmetry can be caused by genetics, pregnancy, breastfeeding, menopause, and certain medical conditions.
If you're concerned or self-conscious about your breasts, you can visit a professional bra fitter to help you find a bra that fits both sides of your chest and hides any differences. You can also consider meeting with a mental health professional or joining an online support group.
The only time when changes in breast size should be concerning is when they happen suddenly for no apparent reason. While it may be due to a benign cause, it could also be a sign of breast cancer or lymphoma. Researchers are also finding that uneven breasts may be a risk factor for breast cancer.
You can talk to your healthcare provider about surgical procedures if your breasts are causing health problems or you cannot accept them. Some surgeries can help improve the symmetry of your breasts.
