Definition, Lactation Process, Lactation Hormones

Development Of Mammary Glands

At Birth

At the time of birth, the 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 the mammary gland between males and females.

At Puberty

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

Read And Learn More: Medical Physiology Notes

During Pregnancy

• During pregnancy, the mammary glands enlarge to a great extent accompanied by marked changes in structure.
• During the first half of pregnancy, the duct system develops further with the appearance of many new alveoli. No milk is secreted by the gland now.
• During the second half, there is the enormous growth of glandular tissues and the development is completed for the production of milk just before the end of the gestation period.

Role Of Hormones In the 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 the duct system; so the normal development of the 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 the 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 the growth of mammary glands during pregnancy.
• Normally, prolactin is inhibited by prolactin inhibiting hormone secreted from the hypothalamus. However, prolactin secretion starts increasing from the 5th month of pregnancy. At that time, it acts directly on the mammary glands and causes the proliferation of epithelial cells of the alveoli.

4. Placental Hormones

The estrogen and progesterone secreted from the placenta are essential for the further development of mammary glands during pregnancy. Both the hormones stimulate proliferation of ducts and glandular cells during pregnancy.

5. Other Hormones

Growth hormones, 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 the corpus lutein, mammary glands, and placenta and its major function is to facilitate dilatation of the cervix during labor.

Lactation

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

1. Milk secretion
2. Milk ejection.

Milk Secretion

The synthesis of milk by alveolar epithelium and its passage through the duct system is called milk secretion. This process 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 a small amount of milk secretion occurs in the 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

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

2. Maintenance of Milk Secretion or Galactopoiesis

Prolactin is important for the initiation of milk secretion rather than the maintenance of milk secretion. Galactopoiesis depends upon other hormones like growth hormone, thyroxine, and cortisol, which are essential for a continuous supply of glucose, amino acids, fatty acids, calcium and other substances necessary for milk production.

Rote of the hypothalamus in galactopoiesis

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

Milk Ejection

• Milk ejection is the discharge of milk from the mammary gland. It depends upon the suckling exerted by the baby and on the contractile mechanism in the breast, which expels milk from the alveoli into the ducts.
• Milk ejection is a reflex phenomenon. It is called the milk ejection reflex or milk letdown reflex. It is a neuroendocrine reflex.

Milk Ejection Reflex

It is explained.

Effect Of Lactation On Menstrual Cycle

• A woman who nurses her child regularly does not have a menstrual cycle for about 24-30 weeks after delivery It is because regular nursing the baby stimulates prolactin secretion continuously.
• Prolactin inhibits GnRH secretion resulting in the suppression of gonadotropin secretion. In the absence of gonadotropins, 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 a sufficient quantity of gonadotropins is secreted, the menstrual cycle starts.

Breast Milk

Breast or human milk is the milk secreted by human female. It 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 a sufficient quantity of all the substances necessary for infants like iron, vitamins, and minerals.
• Besides nourishment of infants, breast milk also provides several antibodies which help the infant resist infection by lethal bacteria. Even some neutrophils and macrophages are secreted in milk. 1 nese phagocytic cells protect the infant by destroying microbes in the infant’s body.

Disadvantages Of Animal Milk

1. It causes irritation of the Gl tract and anemia
2. Excess proteins and fats in animal milk are difficult to digest and absorb by the infants
3. The high content of casein is harder to digest resulting in Gl bleeding and anemia
4. High concentrations of sodium and potassium in animal milk cause overstraining of immature kidneys in infants
5. Low iron content in animal milk develops iron deficiency anemia
6. It has a low content of vitamins and essential fatty acids.