Lactation

Hi, in this post we’ll be discussing the process of lactation. I’ll describe the basic anatomy of the mammary glands and the anatomical differences between species. We’ll then take a look at the physiological events and hormones involved in mammogenesis, lactogenesis, galactopoeisis and involution. I’ll explain the neural and hormonal events associated with the milk ejection reflex as well as the influence of lactation on the oestrus cycle. I’ll finish by discussing a few of the common metabolic disorders associated with lactation.

Anatomy

Mammary glands are modified sweat glands and are composed of alveoli, lobuli and lobes. The alveoli are small vesicular cavities lined with secretory epithelium and surrounded by contractile myoepithelial cells. They are the functional units of the mammary glands. Lobuli are groups of 150-200 alveoli supported by connective tissue. Lobes are groups of lobuli surrounded by connective tissue.

Ducts drain from these structures into a gland cistern which is where milk is stored. The gland cistern opens to the teat cistern while the teat canal connects this with the surface of the skin. The teat canal is sealed by folds of epithelium and smooth muscle which contracts during the milk ejection reflex.

The mammary glands vary in teat number, the number of glands connected to each teat, and the amount of milk stored in the alveoli and excretory ducts between species. For example, cows have four teats that each have one duct while dogs have ten to twelve teats which have seven to twenty ducts each.

Mammogenesis

Mammogenesis refers to the development of the mammary glands. During puberty the rate of mammary development is disproportionately faster than normal body growth (this is known as allometric growth). During repeated oestrus cycles a duct framework is constructed within the mammary gland. 

 During puberty mammogenesis has two stages:

• Oestrogenic Phase: the ducts begin to lengthen, widen and branch and the presence of growth hormone enhances the development of the ducts. 
• Progesterone Phase: The terminal portions of each branch begin to form the initial portion of the alveoli.

However, the repeated exposure to oestrogen and progesterone during oestrus cycles only allows mammogenesis to progress so far and no alveolar development occurs until pregnancy.

Complete mammogenesis can only occur in the presence of oestrogen and progesterone which only occurs during pregnancy. The growth of the ducts and lobules continues in the first half of pregnancy while complete alveolar development takes place during the last trimester.

Progesterone and oestrogen act synergistically to cause the development of lobules and alveoli while prolactin and lactogen stimulate the growth and differentiation of epithelial cells.

Involution

The production of milk increases early during the period of time following the birth of the offspring; it then peaks and slowly declines. The production of milk corresponds to demand and if increased quantities of milk remain in the mammary gland after emptying involution occurs and milk production drops. This is why dairy cattle are milked twice a day – this prevents the build-up of pressure in the mammary glands and thus involution is avoided.

If the mammary glands aren’t emptied and the pressure builds up, pressure atrophy occurs. This causes a reduction in blood flow and an increase in the concentration of Feedback Inhibitor of Lactation (FIL). This substance causes a decrease in milk synthesis which is followed by involution after one or two days by apoptosis. Macrophages then phagocytose secretory cells and small ducts which leads to decreased milk production.

Lactogenesis

Lactogenesis refers to the initiation of milk secretion. The main hormone involved here is prolactin which causes the production of the milk proteins (α-lactalbumin and casein) which are necessary for colostrum. In addition, the α-lactalbumin protein interacts with enzymes in the golgi apparatus and this increases the synthesis of lactose. Lactose, the primary carbohydrate and energy source in milk, draws water into the alveoli through osmosis and this increases milk production.

Progesterone binds to prolactin receptors which inhibits the actions of prolactin and prevents milk secretion. Meanwhile, oestrogen promotes prolactin secretion and up regulates the formation of its receptors. Cortisol is also important in milk production because it increases the differentiation of the endoplasmic reticulum and Golgi apparatus. The suckle or milking reflex also maintains the secretion of prolactin.

Galactopoiesis

Galactopoiesis refers to the maintenance of lactation once it has been established and relies on both hormone concentrations and local mammary factors. The hormones involved are prolactin, growth hormone, cortisol and T3 (Triiodothyronine).

Prolactin increases the synthesis of  α-lactalbumin which maintains the synthesis of lactose. Growth hormone makes sure that the correct substrates are supplied to the mammary gland in order to support fat, protein and lactose inclusion to milk. Cortisol and T3 are required to maintain the secretory activity of the epithelial cells.

The removal of milk from the mammary glands causes the release of prolactin, which ensures that the milk is replaced.Intra-mammary pressure is also relieved, this prevents pressure atrophy and involution from occurring. Importantly, FIL is removed from the alveoli which allows milk synthesis to occur.

If milk is not removed from the mammary glands, there is no stimulation for the release of prolactin and intra-mammary pressure builds up. In addition, sympathetic nerves are activated and blood flow decreases which leads to the decreased availability of hormones and nutrients. This causes milk production to decline.

 

The Milk Ejection Reflex

The milk ejection reflex is a neurohormonal reflex that induces the emptying of the mammary gland. The skin of the teats or udder contains sensory nerve endings and when these are stimulated they fire afferent impulses to the hypothalamus. The hypothalamus causes the release of oxytocin from the posterior pituitary and prolactin from the anterior pituitary via dopamine. Oxytocin binds to receptors on the membrane of the myoepithelial cells which surround the alveoli and ducts. The contraction of the myoepithelial cells causes the pressure within the alveoli to increase, the resistance in the small excretory ducts to decrease, and the relaxation of the sphincter in the teat which reduces the resistance to flow. A few minutes separates stimulation and let-down in this reflex.

As explained earlier, prolactin causes milk to be produced by the mammary glands.

 

Lactation and Oestrus

 

In pigs, dogs and cats lactation inhibits the secretion of GnRH (gonadotropin releasing hormone) and thus the concentrations of FSH (follicle stimulating hormone) and LH (Luteinising Hormone) remain low. This means that folliculogenesis doesn’t happen and oestrogen and progesterone levels remain low.

 

However, in humans and dairy cows the reproductive cycle restarts while milk production is still high. Although progesterone levels are high because of this, they are not high enough to inhibit milk synthesis.

 

Metabolic Disorders

Because milk precursors are drawn from the blood, the same place that all tissues derive their nutrients from, diet is critical in animals that are lactating, especially in the 4-6 weeks post-partum. This is also the reason why some metabolic disorders occur during or after lactation.

 

Milk Fever (hypocalcaemia) is a result of the high calcium demand in the 72 hours following parturition and causes muscle weakness and death. Treated hypocalcaemia may have several consequences including:

·         Mastitis: there is a greater chance that bacteria can enter the teat canal because the muscle tone of the sphincter is decreased.

·         Uterine prolapse: this is due to poor tone of the muscles in the uterine wall.

·         Retained placenta and metritis: this is because of a reduction in uterine muscle contractile activity which may lead to infection.

·         Decreased productivity

Grass tetany (hypomagnesaemia) occurs in cows which are lactating heavily and aren’t being supplemented in their diet and are grazing on lush pastures. This is because rapidly growing lush pastures are often deficient in magnesium. Grass tetany leads to neuromuscular hyperirritability, muscle tremors, staggering, seizures and death.

That's all for this post, if you have any questions please feel free to ask :)