Lactation is the process by which milk is produced from the female mammary glands (breasts) to feed the young ones. Breast milk production and regulation of its secretion are controlled by a complex interplay of physical, emotional and hormonal factors.
Lactation or breastfeeding is a hallmark feature of all female mammals.
Breast milk provides ideal nutrition and immunity to the infant. Besides providing a large number of health benefits to the child, breastfeeding is also beneficial to the mother. It encourages the uterus to return to its pre-pregnancy size, helps to lose maternal weight and protects the mother against various diseases including cancers.
Read more about the Advantages of breastfeeding: Breastfeeding – Benefits, Problems, and Tips for Better Feeding
Structure of the Female Breast
Breast or mammary gland is a paired structure that lies in front of the thoracic wall over the pectoralis major muscle.
The adult female breast is made up of 15-20 lobulated masses or lobes of glandular tissue which are radially arranged around the nipple. These lobes consist of smaller units called lobules.
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Each lobule consists of an intralobular terminal duct with multiple outpouchings called acini or ductules. Structurally it is composed of a tubuloalveolar unit with two-layered epithelium.
The inner layer is made up of glandular epithelial cells that can produce milk. The outer layer is composed of myoepithelial cells which have the ability to contract and help to propel the milk towards the ducts.
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The normal breast tissue of adult women is composed of three types of lobules (lobule type 1, 2, and 3). These lobules drain into a network of ducts or connecting channels called lactiferous ducts which transport milk from the lobules to the nipple. In between the glandular structures and ducts, the breast is composed of fibrous tissue and fatty tissue.
At birth only a few ducts are present. Formation of type 1 lobules starts during puberty. Changes in the levels of estrogen and progesterone which occur during each menstrual cycle stimulate the type 1 lobules to produce new alveolar buds and evolve to more mature structures, known as type 2 and type 3 lobules. Once puberty is completed, the female breast does not undergo further changes until pregnancy.
Physiology of Breast Milk Production and Regulation
Preparation of breasts (Mammogenesis)
Under the effect of various hormones, both the ducts and alveoli (glandular tissue) of breast start growing during pregnancy to prepare for breastfeeding.
Development of lobules occurs with the formation of alveoli. The newly formed lobules have increased size and number of epithelial cells. This is known as stage II mammogenesis.
During the period of pregnancy and lactation, the breast is composed of mainly glandular tissue and very little stroma.
The marked proliferation of secretory tissue occurs mainly under the effect of the hormone progesterone and human chorionic gonadotropin.
Synthesis and secretion of milk (Lactogenesis)
Lactogenesis is the process of developing the ability to synthesize and secrete milk. It occurs in 2 stages.
Stage I lactogenesis (secretory initiation)
Synthesis of milk begins during the late stages of pregnancy. The lumen of the alveoli starts filling with secretory yellowish fluid or colostrum. However, it is not secreted during pregnancy because the high levels of estrogen and progesterone present in the body prevent its secretion.
Stage II lactogenesis (secretory activation)
After delivery, when estrogen and progesterone levels fall, prolactin begins its milk secretory activity. Stimulation of the breast causes the levels of prolactin to rise which in turn stimulates the breast tissue to synthesize milk.
Prolactin levels rise and peak in about 45 minutes after stimulation and return to the pre-breastfeeding state after about three hours.
The secretory activity is also influenced by other hormones like growth hormone, glucocorticoids, insulin, and thyroxine.
Ejection of milk (Galactokinesis)
Suction exerted by the baby during suckling and the contractile mechanism of the breast tissue leads to the discharge of milk from the breast alveoli into the ducts.
Milk ejection or milk let out reflex: When a baby suckles, a reflex is set up. Impulses from the nipple and areola cause the release of prolactin from the anterior pituitary gland and oxytocin from the posterior pituitary gland. For continuous synthesis and secretion of milk, the mammary tissue must receive hormonal signals.
Prolactin
Prolactin promotes the growth and proliferation of mammary glandular epithelium and stimulates it to produce milk.
It is a polypeptide hormone which is synthesized by lactotrophic cells in the anterior pituitary. Structurally it is similar to growth hormone and placental lactogen.
Its secretion is both positively and negatively controlled. It is mainly regulated by hypothalamic inhibitory factors such as dopamine which act on dopamine receptors present in lactotrophs. Suckling of the baby and emptying of the breast are the most important factors which stimulate the production of prolactin.
Oxytocin
Oxytocin causes the milk present in the alveoli to move into the ducts from where it can be expressed by the mother or sucked by the baby.
The stimulation provided by suckling sends signals to the hypothalamus which causes the release of oxytocin from the posterior pituitary. This causes the myoepithelial cells to contract, forcing milk from the alveoli into the ducts and out through the nipple. Besides this, oxytocin also helps to reduce maternal stress and enhances the bonding between the mother and the child.
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Maintenance of lactation (Galactopoiesis)
Milk secretion is a continuous process. Once lactation is established, milk production is maintained and regulated by a complex interplay of physical, emotional and hormonal factors.
Regular suckling and removal of milk from the breast provide the physical stimuli for production of hormones needed for further milk production.
Emotional factors such as touch, smell, cry or even thought of the baby provide stimulus to the mother’s body to release milk-producing hormones.
If milk is not removed from the breasts, the increased pressure within the breasts and negative feedback will reduce milk production. If milk is removed from the breasts, the negative feedback is removed and milk secretion is resumed. Thus the amount of milk produced is regulated by the needs of the baby.
If the baby is not able to feed and empty the breast, it is very important to empty the breast either manually or using a breast pump, so as to provide positive feedback for milk production, else milk production can reduce or cease altogether.
Read more about Breast Pump – Types, Working, Uses and Selection Tips
With the cessation of lactation, the reduction in feedback signals results in suppression of hormone production and no more milk is produced. This results in the involution of mammary glands. However full involution does not occur and a permanent increase in the size and number of lobules occurs following pregnancy.
References
- Lawrence RA, Lawrence RM. Breastfeeding: a guide for the medical profession. 6th Edition. London: Mosby; 2005.
- Riordan J. Breastfeeding and human lactation. Boston, USA: Jones and Bartlett; 2004. The biological specificity of breast milk.
- Butte N, Lopez-Alarcon MG, Garza C. Nutrient adequacy of exclusive breastfeeding for the term infant during the first six months of life. Geneva: World Health Organization; 2002.
- WHO. Infant feeding: the physiological basis. Bulletin of the World Health Organization. 1989;67(Suppl.):1–107. [PMC free article] [PubMed]
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