DEVELOPMENT OF MAMMARY GLANDS

DEVELOPMENT OF MAMMARY GLANDS

Physio Guideline

AT BIRTH

At the time of birth, mammary gland is rudimentary and consists of only a tiny nipple and few radiating ducts from it.

AT CHILDHOOD

Till puberty, there is no difference in the structure of mammary gland between male and female.

AT PUBERTY

At the time of puberty and afterwards there is a vast change in the structure of female mammary gland due to hormonal influence. The beginning of changes in mammary gland is called thelarche. It occurs at the time of puberty, just before menarche. At puberty, there is growth of duct system and formation

of glandular tissue. During every sexual cycle, at the time of menstruation there is slight regression and

in between the phases of menstruation, proliferative changes occur. On the whole, progressive enlargement occurs, which is also due to the deposition of fat.

DURING PREGNANCY

During pregnancy, the mammary glands enlarge to a great extent accompanied by marked changes

in structure. During first half of pregnancy, the duct system develops further with appearance of many new alveoli. No milk is secreted by the gland now. During the second half, there is enormous growth of glandular tissues and the development is completed for the production of milk just before the end of gestation period.

ROLE OF HORMONES IN GROWTH

OF MAMMARY GLANDS

Various hormones are involved in the development and growth of breasts at different stages:

1. Estrogen

2. Progesterone

3. Prolactin

4. Placental hormones

5. Other hormones.

1. ESTROGEN

Growth of Ductile System

Estrogen causes growth and branching of duct system; so the normal development of duct system in breasts at puberty depends upon estrogen. Estrogen is also responsible for the accumulation of fat in breasts.

2. PROGESTERONE

Growth of Glandular Tissue

The development of stroma of the mammary glands depends upon progesterone activity. Progesterone also stimulates the development of glandular tissues.

3. PROLACTIN

Prolactin is necessary for milk secretion. However, it also plays an important role in growth of mammary

glands during pregnancy. Normally, prolactin is inhibited by prolactin-inhibiting hormone secreted from hypothalamus. However, prolactin secretion starts increasing from 5th month of pregnancy. At that time, it acts directly on the mammary glands and causes proliferation of epithelial cells of alveoli.

4. PLACENTAL HORMONES

Estrogen and progesterone secreted from placenta are essential for further development of mammary

glands during pregnancy. Both the hormones stimulate the proliferation of ducts and glandular cells during pregnancy.

5. OTHER HORMONES

Growth hormone, thyroxine and cortisol enhance the overall growth and development of mammary glands

in all stages. Relaxin also facilitates the development of mammary glands. It is secreted by corpus luteum, mammary glands and placenta. Its major function is to facilitate dilatation of cervix during labor.

LACTATION

Lactation means synthesis, secretion and ejection of milk. Lactation involves two processes:

A. Milk secretion

B. Milk ejection.

MILK SECRETION

Synthesis of milk by alveolar epithelium and its passage through the duct system is called milk secretion. Milk secretion occurs in two phases:

1. Initiation of milk secretion or lactogenesis

2. Maintenance of milk secretion or galactopoiesis.

1. Initiation of Milk Secretion or Lactogenesis

Although small amount of milk secretion occurs at later months of pregnancy, a free flow of milk occurs only after the delivery of the child. The milk, which is secreted initially before parturition is called colostrum. Colostrum is lemon yellow in color and it is rich in protein (particularly globulins) and salts. But its sugar content is low. It contains almost all the components of milk except fat.

Role of hormones in lactogenesis

Prolactin is responsible for lactogenesis. During pregnancy, particularly in later months, large quantity of prolactin is secreted. But the activity of this hormone is suppressed by estrogen and progesterone secreted by placenta. Because of this, lactation is prevented during pregnancy. Immediately after the delivery of the baby and expulsion of placenta, there is sudden loss of estrogen and progesterone. Now, the prolactin is free to exert its action on breasts and to promote lactogenesis.

2. Maintenance of Milk Secretion or Galactopoiesis

Galactopoiesis depends upon the hormones like growth hormone, thyroxine and cortisol, which are essential for continuous supply of glucose, amino acids, fatty acids, calcium and other substances necessary for the milk production.

Role of hypothalamus in galactopoiesis

Galactopoiesis occurs till 7 to 9 months after delivery of child provided feeding the baby with mother’s milk is continued till then. In fact, the milk production is continued only if feeding the baby is continued. Suckling of nipple by the baby is responsible for continuous milk production. When the baby suckles, the impulses from touch receptors around the nipple stimulate hypothalamus. It is suggested that hypothalamus releases some prolactin-releasing factors, which cause the prolactin secretion from anterior pituitary. Prolactin acts on glandular tissues and maintains the functional activity of breast for subsequent nursing.

MILK EJECTION

Milk ejection is the discharge of milk from mammary gland. It depends upon suckling exerted by the baby

and on contractile mechanism in breast, which expels milk from alveoli into the ducts. Milk ejection is a reflex phenomenon. It is called milk ejection reflex or milk let-down reflex. It is a neuroendocrine reflex.

EFFECT OF LACTATION ON

MENSTRUAL CYCLE

Woman who nurses her child regularly does not have menstrual cycle for about 24 to 30 weeks after delivery It is because, regular nursing the baby stimulates prolactin secretion continuously. Prolactin inhibits GnRH secretion resulting in suppression of gonadotropin secretion. In the absence of gonadotropin, the ovaries become inactive and ovulation does not occur. When the frequency of nursing the baby decreases (after about 24 weeks) the secretion of GnRH and gonadotropins starts slowly. When sufficient quantity of gonadotropins is secreted, the menstrual cycle starts.

BREAST MILK

Breast or human milk forms the primary source of nutrition for infants.

COMPOSITION

Breast milk contains about 88.5% of water and 11.5% of solids. Important solids are lactose, lactalbumin, iron, vitamins A and D and minerals.

ADVANTAGES OF BREAST MILK

Breast milk is always considered superior to animal milk (cow milk or goat milk) because it consists of

sufficient quantity of all the substances necessary for infants like iron, vitamins and minerals.

Besides nourishment of infant, the breast milk also provides several antibodies, which help the infant

resist the infection by lethal bacteria. Even some neutrophils and macrophages are secreted in milk.

These phagocytic cells protect the infant by destroying microbes in the infant’s body.

DISADVANTAGES OF ANIMAL MILK

1. It causes irritation of GI tract and anemia

2. Excess proteins and fats in animal milk are difficult to digest and absorb by the infants

3. High content of casein is harder to digest resulting in GI bleeding and anemia

4. High concentrations of sodium and potassium in animal milk causes overstraining of immature

kidneys in infants

5. Low iron content in animal milk develops iron deficiency anemia

6. It has low content of vitamins and essential fatty acids.

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