Short Essay on Breast (Mammary Gland)

 

Short Essay on Breast (Mammary Gland)

The breast, or mammary gland, is the most important structure present in the pectoral region. Its anatomy is of great practical importance and has to be studied in detail.

The breast is found in both sexes, but is rudimentary in the male. It is well developed in the female after puberty. The breast is a modified sweat gland. It forms an important accessory organ of the female reproductive system, and provides nutrition to the newborn in the form of milk.

Situation:

The breast lies in the superficial fascia of the pectoral region. It is divided into four quadrants, i.e. upper medial, upper lateral, lower medial and lower lateral.

A small extension of the upper lateral quadrant called the axillary tail of Spence, passes through an opening in the deep fascia and lies in the axilla. The opening is called foramen of Langer.

Extent:

(i) Vertically, it extends from the second to the sixth rib.

(ii) Horizontally, it extends from the lateral border of the sternum to the midaxillary line.

Deep Relations:

The deep surface of the breast is related to the following structures in that order.

1. The breast lies on the deep fascia (pectoral fascia) covering the pectoralis major.

2. Still deeper there are the parts of three muscles, namely the pectoralis major, the serratus anterior, and the external oblique muscle of the abdomen.

3. The breast is separated from the pectoral fascia by loose areolar tissue, called the retro mammary space. Because of the presence of this loose tissue, the normal breast can be moved freely over the pectoralis major.

Structure of the Breast:

The structure of the breast may be conveniently studied by dividing it into the skin, the parenchyma, and the stroma.

A. The skin:

It covers the gland and presents the following features.

1. A conical projection, called the nipple, is present just below the centre of the breast at the level of the fourth intercostal space. The nipple is pierced by 15 to 20 lactiferous ducts.

It contains circular and longitudinal smooth muscle fibres which can make the nipple stiff or flatten it, respectively. It has a few modified sweat and sebaceous glands. It is rich in its nerve supply and has many sensory end organs at the termination of nerve fibres.

2. The skin surrounding the base of the nipple is pigmented and forms a circular area called the areola. This region is rich in modified sebaceous glands, particularly at its outer margin. These become enlarged during pregnancy and lactation to form raised tubercles of Montgomery.

Oily secretions of these glands lubricate the nipple and areola, and prevent them from cracking during lactation. Apart from sebaceous glands, the areola also contains some sweat glands, and accessory mammary glands. The skin of the areola and nipple is devoid of hair, and there is no fat subjacent to it.

B. The parenchyma :

It is made up of glandular tissue which secretes milk. The gland consists of 15 to 20 lobes. Each lobe is a cluster of alveoli, and is drained by a lactiferous duct. The lactiferous ducts converge towards the nipple and open on it. Near its termination each duct has a dilatation called a lactiferous sinus.

Alveolar epithelium is cuboidal in the resting phase and columnar during lactation. In distended alveoli, the cells may appear cuboidal due to stretching, but they are much larger than those in the resting phase. The smaller ducts are lined by columnar epithelium, the larger ducts by two or more layers of cells, and the terminal parts of the lactiferous ducts by stratified squamous keratinised epithelium.

The passage of the milk from the alveoli into and along the ducts is facilitated by contraction of myoepitheliocytes, which are found both around the alveoli and around the ducts, lying between the epithelium and the basement membrane.

C. The stroma:

It forms the supporting framework of the gland. It is partly fibrous and partly fatty.

The fibrous stroma forms septa, known as the suspensory ligaments (of Cooper) which anchor the skin and gland to the pectoral fascia.

The fatty stroma forms the main bulk of the gland. It is distributed all over the breast, except beneath the areola and nipple.

Blood Supply :

The mammary gland is extremely vascular. It is supplied by branches of the following arteries.

Internal thoracic artery, a branch of the subclavian artery, through its perforating branches.

The lateral thoracic, superior thoracic and acromiothoracic (thoracoacromial) branches of the axillary artery.

Lateral branches of the posterior intercostal arteries.

The arteries converge on the breast and are distributed from the anterior surface. The posterior surface is relatively avascular.

The veins follow the arteries. They first converge towards the base of the nipple where they form an anastomotic venous circle, from where veins run in superficial and deep sets.

1. The superficial veins drain into the internal thoracic vein and into the superficial veins of the lower part of the neck.

2. The deep veins drain into the internal thoracic, axillary and posterior intercostal veins.

Nerve Supply :

The breast is supplied by the anterior and lateral cutaneous branches of the 4th to 6th intercostal nerves. The nerves convey sensory fibres to the skin, and autonomic fibres to smooth muscle and to blood vessels. The nerves do not control the secretion of milk. Secretion is controlled by the hormone prolactin, secreted by the pars anterior of the hypophysis cerebri.

Lymphatic Drainage :

Lymphatic drainage of the breast assumes great importance to the surgeon because carcinoma of the breast spreads mostly along lymphatics to the regional lymph nodes. The subject can be described under two heads, the lymph nodes, and the lymphatics.

Lymph Nodes :

Lymph from the breast drains into the following lymph nodes.

1. The axillaiy lymph nodes, chiefly the anterior (or pectoral) group. The posterior, lateral, central and apical groups of nodes also receive lymph from the breast either directly or indirectly.

2. The internal mammary (parasternal) nodes which lie along the internal thoracic vessels.

3. Some lymph from the breast also reaches the supraclavicular nodes, the cephalic (delto- pectoral) node, the posterior intercostal nodes (lying in front of the heads of the ribs), the subdiaphragmatic and subperitoneal lymph plexuses.

Lymphatic Vessels :

A. The superficial lymphatics drain the skin over the breast except for the nipple and areola. The
lymphatics pass radially to the surrounding lymph nodes (axillary, internal mammary, supraclavicular and cephalic).

B. The deep lymphatics drain the parenchyma of the breast. They also drain the nipple and areola.

Some further points of interest about the lymphatic drainage are as follows.

1. About 75% of the lymph from the breast drains into the axillary nodes; 20% into the internal mammary nodes; and 5% into the posterior intercostal nodes.

Among the axillary nodes, the lymphatics end mostly in the anterior group (closely related to the axillary tail) and partly in the posterior and apical groups. Lymph from the anterior and posterior groups passes to the central and lateral groups and through them to the apical group. Finally it reaches the supraclavicular nodes.

2. The internal mammary nodes drain the lymph not only from the inner half of the breast, but from the outer half as well.

3. A plexus of lymph vessels is present deep to the areola. This is the subareolar plexus (of Sappy). Subareolar plexus and most of lymph from the breast drain into the anterior or pectoral group of lymph nodes.

4. The lymphatics from the deep surface of the breast pass through the pectoralis major muscle and the clavipectoral fascia to reach the apical nodes, and also to the internal mammary nodes.

5. Lymphatics from the lower and inner quadrants of the breast may communicate with the subdiaphragmatic and subperitoneal lymph plexuses after crossing the costal margin and then piercing the anterior abdominal wall through the upper part of the linea alba.

Development of the Breast :

1. The breast develops from an ectodermal thickening, called the mammary ridge, milk line, or line of Schultz. This ridge extends from the axilla to the groin.

It appears during the fourth week of intrauterine life, but in human beings, it disappears over most of its extent persisting only in the pectoral region. The gland is ectodermal, and the stroma mesodermal in origin.

2. The persisting part of the mammary ridge is converted into a mammary pit. Secondary buds (15-20) grow down from the floor of the pit. These buds divide and subdivide to form the lobes of the gland. The entire system is first solid, but is later canalised. At birth or later, the nipple is averted at the site of the original pit.

3. Growth of the mammary glands, at puberty, is caused by oestrogens. Apart from oestrogens, development of secretory alveoli is stimulated by progesterone and by the prolactin hormone of the hypophysis cerebri.

4. Developmental anomalies of the breast are:

(a) Amastia (absence of the breast),

(b) Athelia (absence of nipple),

(c) Polymastia (supernumerary breasts),

(d) Polythelia (supernumerary nipples),

(e) Gynaecomastia (development of breasts in a male) which occurs in Klinefelter’s syndrome.

Human Milk :

Human milk is composed of about 88% water, 7% lactose, 4% fat, and 1% protein (caseins and lactalbumin). It also contains various ions (calcium, phosphate, sodium, potassium and chloride), vitamins and antibodies of IgA variety.

Milk secreted in the later part of pregnancy, and for a few days after parturition is known as colostrum. It is rich in fat and poor in nutrients, the fat being contained in colostral corpuscles. It is rich in immunoglobulins.

Under the influence of maternal oestrogens, the infant’s breast may secrete milk during the first one or two weeks after birth, a fat-free fluid called ‘witch’s milk’.

Lactation in mothers is active for about 5-6 months after parturition and then diminishes progressively, so that the infant is weaned by about 9 months of age.