Physiology of milk secretion

presenter:

Karma Choki

Introduction

• The defining characteristic of the class Mammalian is the ability to produce milk, an externally secreted fluid designed specifically to nourish the young

• The provision of milk frees the mother from the necessity of providing a specialized environment for rearing of the young

Cont……

• It allows birth to occur at a relatively early stage of development and provides a time of intense maternal interaction with the newborn during early behavioral development

• In addition the nutritional reserves of the mother may be able to sustain the suckling through a period of famine

Biochemistry of human milk

• Human milk is a unique, species-specific,

complex nutritive fluid with immunologic and growth promoting properties

• It evolves to meet the changing needs of the baby during growth and maturation

• Breast milk is the most appropriate source of nutrition up to the age of 6 months

LACTATION• The cyclical process of milk synthesis and secretion is termed

lactation

• Lactation occurs with the help of 2 hormones, prolactin and

oxytocin

• During the second stage of lactogenesis, the breast becomes

capable of milk production but for the ongoing synthesis and

secretion of human milk, the mammary gland must receive

hormonal signals

• These signals are produced by direct response to stimulation

of the nipple and areola (mammae), are then relayed to the

central nervous system

The stages of lactation

1. Mammogenesis: Mammary (breast) growth occurs. The size and weight of the breast increases

2. Lactogenesis

Stage 1 (late pregnancy): Alveolar cells are differentiated from secretary cells

Stage 2 (day 2 or 3 to day 8 after birth): The tight junction in the alveolar cell closes. Copious milk secretion begins. Breasts are full and warm. Endocrine control switches to autocrine (supply-demand) control

3. Galactopoiesis (later than 9 days after birth to beginning of involution): Established secretion is maintained. Autocrine system control continues

4. Involution (average 40 days after last breastfeeding): Regular supplementation is added. Milk secretion decreases from the buildup of inhibiting peptides

Prolactin• It is a polypeptide hormone synthesized by

lactotrophic cells in the anterior pituitary

• Prolactin is necessary for the secretion of milk by the cells of the alveoli and its level of prolactin in the blood increases markedly during pregnancy

• It stimulates the growth and development of the mammary tissue, in preparation for the production of milk

• However, milk is not secreted then, because progesterone and oestrogen, the hormones of pregnancy, block this action of prolactin

• After delivery, levels of progesterone and oestrogen fall rapidly, prolactin is no longer blocked, and milk secretion begins

Prolactin reflex

• More prolactin is produced at night, so breastfeeding at night is especially helpful for keeping up the milk supply

• Prolactin seems to make a mother feel relaxed and sleepy, so she usually rests well even if she breastfeeds at night

• Suckling affects the release of other pituitary hormones, including gonadotrophin releasing hormone(GnRH), follicle stimulating hormone, and luteinisinghormone, which results in suppression of ovulation and menstruation

• Frequent breastfeeding can help to delay a new pregnancy hence, breastfeeding at night is important to ensure this effect

Oxytocin

Hormone involved in the milk ejection or letdown reflex

The suckling infant stimulates the touch receptors that are densely located around the nipple and areola

This creates afferent impulses that stimulate the posterior pituitary gland and it releases oxytocin

Oxytocin causes the contraction of the myoepithelial cells that line the ducts of the breast which then expel milk from alveoli into ducts and subareolar sinuses that empty through a nipple pore

Oxytocin reflex

• The oxytocin reflex is also sometimes called the “letdown reflex” or the “milk ejection reflex”

• Oxytocin is produced more quickly than prolactin

• It makes the milk that is already in the breast flow for the current feed, and helps the baby to get the milk easily

• Oxytocin starts working when a mother expects a feed as well as when the baby is suckling

• The reflex becomes conditioned to the mother's sensations and feelings, such as touching, smelling or seeing her baby, or hearing her baby cry, or thinking lovingly about him or her

Cont…..• If a mother is in severe pain or emotionally upset, the

oxytocin reflex may become inhibited, and her milk may suddenly stop flowing well

• If she receives support, is helped to feel comfortable and lets the baby continue to breastfeed, the milk will flow again

• It is important to understand the oxytocin reflex, because it explains why the mother and baby should be kept together and why they should have skin-to-skin contact

• Oxytocin makes a mother's uterus contract after delivery and helps to reduce bleeding

• The contractions can cause severe uterine pain when a baby suckles during the first few days

COLOSTRUM

Definition:

• It is the first milk secretion of the breast during pregnancy and the first 2-3 days following delivery which contains a great quantity of proteins and calories in addition to antibodies and lymphocytes

• It is a serous fluid, yellow in color and alkaline in nature with higher specific gravity

• Foremilk: is the milk (typically lower in fat) available at the beginning of a feeding

• Hindmilk: is milk at the end of a feeding, which has a higher fat content than the foremilk at that feeding

Colostrum is composed of:

• High protein

• Low carbohydrate

• High minerals esp. Na, Cl

• Antibody- Secretory IgA (High)

Importance of Colostrum:

• It is more nutritious

• It provides protection against infection due to presence of antibodies ( IgA)

• It has got laxative action due to the presence of large fat globules

Ingredients that make up breast

milk:• Protein: Protein is responsible for the body's

metabolism and the growth and development of body cells and tissues

• Carbohydrates: Carbohydrates are responsible for energy and calories and is important for brain growth and development. The main carbohydrate in human milk is lactose (having about twice as much as cow's milk).

• Fats: Fats (or lipids) are essential for energy and calories, development of the brain, nerves and blood vessels

• Minerals: Minerals present in breast milk include calcium, phosphorus and magnesium. These are important for bone growth and muscle functioning.

• Vitamins: Water soluble vitamins B and C are concentrated more in the fore milk (or the milk at the beginning of a feed) and fat soluble vitamins such as A, D E and K are more concentrated in the hind milk (or towards the end of the feed).

• Enzymes: There are 3 enzymes present in breast milk. They include lysozyme (an ant-bacterial enzyme), lipase (which aids fat digestion) and amylase (which digests carbohydrates).

• Hormones: The hormones present in breast milk include prolactin, prostaglandins, insulin, thyroid stimulating hormone, thyroxine and growth hormone. These aid the baby's growth and development and are not present in formula milk

• Anti-infective agents: There are many anti-infective agents present in breast milk including leucocytes that are capable of surrounding and destroying bacteria and immunoglobulin's from the mother's immunity (called IgA, IgG, IgM and IgD).

• Anti-allergenic agents: Babies who are exclusively breastfed for the first 6 months of their life have less chance of developing food allergies, skin disorders (such as eczema) and allergic breathing disorders, (such as asthma).

References

• PEAKER, M. (2001). The aqueous phase of

milk: ion and water transport. Symposia of

the Zoological Society of London 41, 113-

134.

• SEELIG, L. L.,JR. & BEER, A. E. (2003).

Transepithelial migration of leukocytes in

the mammary gland of lactating rats.

Biol.Reprod. 22, 1157-1163.