The process of lactation is commonly observed in all mammals and refers to the process of milk production by the mammary glands.
Lactation: An overview
Lactation, also referred to as “nursing”, is the process of milk production by the mammary glands in all sexually mature mammalian females. It is usually triggered during pregnancy to ensure that the young ones of the organism receive the required nutrients. It is a natural process that ensures that all mammals can provide the required nutrient for their offspring. Some anomalous incidences in the context of lactation are as follows-
- Some newborns (of both sexes) can produce milk from their mammary glands as a result of the hormonal influences of the hormones produced by the mother during the gestation period. This is considered to be physiologically normal, even though it is rare, and does not require any testing.
- Monotremes, also known as egg-laying mammals, do not have mammary ducts through which milk is transported. Milk is instead transported via ducts in the abdomen.
- Milk production is a normal phenomenon in males only among the Dayak fruit bats found in Southeast Asia.
It is important to note that lactation not only refers to the process of milk production but also to the period during which mammalian mothers feed their offspring.
In humans, certain hormones influence and regulate the process of lactation. These hormones are usually secreted during the second and third trimesters of pregnancy and are primarily responsible for the induction of milk production and growth of the milk ducts. Some of the most important hormones that regulate the process of lactation are as follows-
A male Dayak fruit bat
- Progesterone and estrogen are responsible for the growth and differentiation of the milk duct system. They also inhibit milk production during pregnancy to prevent lactation before birth.
- Prolactin is primarily responsible for alveolar growth and differentiation. The milk duct system is composed of tightly maintained structures that are controlled by the hormone prolactin. Milk production is also regulated via the maintenance of an osmotic balance within the milk ducts.
- Human Placental Lactogen or HPL is produced in copious amounts by the placenta during the second month of pregnancy and is mainly responsible for the growth and differentiation of the milk duct system.
- Glucocorticoids (Example: cortisol) and the Adrenocorticotropic hormone are responsible for the maintenance and regulation of tight junctions.
- The growth hormone is similar to prolactin and plays an important role in the process of galactopoiesis (regulation and sustained maintenance of milk production).
- Oxytocin is responsible for the contraction and relaxation of the smooth muscles around the alveoli which eventually lead to the movement of milk into the mammary ducts. It plays an important role in the ‘milk ejection reflex.’
- Other important hormones required in optimal amounts during the lactation period are Follicle Stimulating Hormone (FSH), Human Chorionic Gonadotrophin (hCG), and Luteinising Hormone (LH).
Milk ejection reflex
This mechanism, also known as the let-down mechanism, refers to the transportation of breast milk through the ducts from the alveoli to the nipples. This mechanism is controlled by a variety of stimuli and can be conditioned to occur in response to the sound of a baby’s cry.
The milk ejection reflex is characterized by the following steps-
- Suckling by the infant activates the paraventricular and supraoptic nuclei in the hypothalamus which in turn stimulates the production of oxytocin by the posterior pituitary gland.
- Oxytocin induces the contraction and relaxation of the smooth muscles and myoepithelial cells that surround the alveoli.
- This results in the movement or transportation of milk that is already present in the alveoli, through the milk ducts and outside the body, through the nipples of the lactating mother.
Lactation is a natural process observed in all mammals (except monotremes), which mainly involves the production of milk by the mammary glands to provide nutrients to offspring. The process of lactation is regulated and sustained by several hormones in humans. These hormones are produced both during gestation and after parturition. The brain of the mother eventually becomes conditioned to a set of stimuli that activate the “milk ejection reflex” leading to the secretion of milk.