Functionally, the mammary glands produce milk; structurally, they are modified sweat glands. Mammary glands, which are located in the breast overlying the pectoralis major muscles, are present in both sexes, but usually are functional only in the female.
Externally, each breast has a raised nipple, which is surrounded by a circular pigmented area called the areola. The nipples are sensitive to touch, due to the fact that they contain smooth muscle that contracts and causes them to become erect in response to stimulation.
Internally, the adult female breast contains 15 to 20 lobes of glandular tissue that radiate around the nipple. The lobes are separated by connective tissue and adipose. The connective tissue helps support the breast. Some bands of connective tissue, called suspensory (Cooper’s) ligaments, extend through the breast from the skin to the underlying muscles. The amount and distribution of the adipose tissue determines the size and shape of the breast. Each lobe consists of lobules that contain the glandular units. A lactiferous duct collects the milk from the lobules within each lobe and carries it to the nipple. Just before the nipple, the lactiferous duct enlarges to form a lactiferous sinus (ampulla), which serves as a reservoir for milk. After the sinus, the duct again narrows and each duct opens independently on the surface of the nipple.
Mammary gland function is regulated by hormones. At puberty, increasing levels of estrogen stimulate the development of glandular tissue in the female breast. Estrogen also causes the breast to increase in size through the accumulation of adipose tissue. Progesterone stimulates the development of the duct system. During pregnancy, these hormones enhance further development of the mammary glands. Prolactin from the anterior pituitary stimulates the production of milk within the glandular tissue, and oxytocin causes the ejection of milk from the glands.
The breast is located on the anterior thoracic wall. It extends horizontally from the lateral border of the sternum to the mid-axillary line. Vertically, it spans between the 2nd and 6th intercostal cartilages. It lies superficially to the pectoralis major and serratus anterior muscles.
The breast can be considered to be composed of two regions:
- Circular body – largest and most prominent part of the breast.
- Axillary tail – smaller part, runs along the inferior lateral edge of the pectoralis major towards the axillary fossa.
At the centre of the breast is the nipple, composed mostly of smooth muscle fibres. Surrounding the nipple is a pigmented area of skin termed the areolae. There are numerous sebaceous glands within the areolae – these enlarge during pregnancy, secreting an oily substance that acts as a protective lubricant for the nipple.
The breast is composed of mammary glands surrounded by a connective tissue stroma.
The mammary glands are modified sweat glands. They consist of a series of ducts and secretory lobules (15-20).
Each lobule consists of many alveoli drained by a single lactiferous duct. These ducts converge at the nipple like spokes of a wheel.
Connective Tissue Stroma
The connective tissue stroma is a supporting structure which surrounds the mammary glands. It has a fibrous and a fatty component.
The fibrous stroma condenses to form suspensory ligaments (of Cooper). These ligaments have two main functions:
- Attach and secure the breast to the dermis and underlying pectoral fascia.
- 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.
There is a layer of loose connective tissue between the breast and pectoral fascia – known as the retromammary space. This is a potential space, often used in reconstructive plastic surgery.
Arterial supply to the medial aspect of the breast is via the internal thoracic artery (also known as internal mammary artery) – a branch of the subclavian artery.
The lateral part of the breast receives blood from four vessels:
- Lateral thoracic and thoracoacromial branches – originate from the axillary artery.
- Lateral mammary branches – originate from the posterior intercostal arteries (derived from the aorta). They supply the lateral aspect of the breast in the 2nd 3rd and 4th intercostal spaces.
- Mammary branch – originates from the anterior intercostal artery.
The veins of the breast correspond with the arteries, draining into the axillary and internal thoracic veins.
The lymphatic drainage of the breast is of great clinical importance due to its role in the metastasis of breast cancer cells.
There are three groups of lymph nodes that receive lymph from breast tissue – the axillary nodes (75%), parasternal nodes (20%) and posterior intercostal nodes (5%).
The skin of the breast also receives lymphatic drainage:
- Skin – drains to the axillary, inferior deep cervical and infraclavicular nodes.
- Nipple and areola – drains to the subareolar lymphatic plexus.