Galactopoiesis

Galactopoiesis can be defined as the maintenance of established lactation. The term may often be used to describe the enhancement of established lactation. In cattle, the milk production follows a dynamic curve with a rapid accelerating phase and reached peak around 6 weeks then decline till the end of lactation.

Due to high energy requirement and nutrient partitioning, the animal exhibits negative energy balance (NEB) during early and peak lactation and the mobilization of body reserves leads to decreasing body weight during this critical period. During NEB state, the health and fertility of the animals may be compromised.

The ability of the animals to sustain the peak yield with a lesser decrease is termed as production persistency. An ani­mal with higher production persistency has increased overall gain per lactation alternatively high persistency may also have a negative impact on the animal’s health and fertility.

Galactopoiesis depends upon milk synthesis and milk removal. Mammary gland has to ensure a sufficient number of secretory cell population by increasing cell proliferation and decreasing cell loss.

The key components that contribute to galactopoiesis and lactation persistency are discussed below.

25.5.1 Activity of Milk Secretory Cells

During the initiation of lactation, the numberof secretory cells are highest with less activity and milk production per cell was lowest. During peak lactation, the secretory cell differentia­tion leads to an increase in average yield per cell which is sustained throughout the lactation period. Decline in milk yield after peak lactation is therefore due to loss in secretory cell number due to apoptosis rather than decreased secretory activity.

25.5.2 Milk Secretion and Milking Frequency

After being secreted from secretory epithelium, the milk is distributed between two main components. Some portion of milk remains in the lumen of mammary alveoli and small ducts (alveolar milk) and some portion descend to cistern (cisternal milk). The cisternal milk can be obtained without milk ejection whereas the alveolar milk can only be obtained after milk ejection reflex. Huge species differences exist with respect to the storage of milk in different compartments. In goat and sheep, cisternal milk filling initiates immediately and 75% milk can be stored following a milking interval of 12-14 h. In contrast, there is no milk in the cistern up to 2 h post milking and only 20% cisternal storage of milk is achieved between normal milking interval. To obtain maxi­mum persistency milk has to be removed from the mammary gland. Incomplete milking hastens lactation persistency. Absence of milk removal leads to increased intramammary pressure and decreased blood flow to mammary gland. Some local autocrine inhibitors of milk secretion such as feedback inhibitor of lactation (FIL) result in the accumulation of milk in the alveolar ducts and cause partial inhibition of milk synthesis and secretion. If long-term stasis of milk occurs, it can lead to complete cessation of lactation and mammary involution begins.

Local factors: Feedback inhibitor of lactation (FIL): The rate of milk secretion in response to the frequency of milking is regulated by a local inhibitory glycoprotein termed as a Feedback inhibitor of lactation (FIL).

Beside FIL, there are several other local factors thought to be involved in autocrine regulation of milk secretion (see mammary gland involution). One such inhibitory factor in the form of proteolytic casein fragment has been identified in the udder of lactating goat. The enzyme carbonic anhydrase was identified in the capillary endothelium which is related to mammary metabolism, milk composition, and milk flow rate in goat.

25.5.3 Hormonal Control of Galactopoiesis

25.5.3.1 Somatotropin

Growth hormone or somatotropin is involved in tissue metab­olism and nutrient partitioning during lactation. The main roles of somatotropin are to promote lipolysis in adipocytes and gluconeogenesis in the liver during the state of negative energy balance. Exogenous administration of growth hor­mone for 10-12-week treatment period resulted in 40% increase in milk yield of dairy cows and genetic engineering explored the potential of using recombinantly derived bovine somatotropin (rbST) in the biology of lactation and its com­mercial use in augmentation of lactation in bovines. The review of Bauman and Vernon (1993) on the roles of bovine somatotropin in different body tissues is summarized below in Table 25.24.

The direct role of bST on the mammary tissue is a matter of debate as it was reported that growth hormone did not bind to receptors in the bovine mammary gland. Rather an alter­nate hypothesis was proposed on the indirect effects of ST in association with the IGF complex which binds to its receptor on mammary epithelial cells and mediates the action of growth hormone. The nutritional status of the animals plays a crucial role in regulating IGF axis in cattle.

Table 25.24 The roles of somatotropin in different body tissues

increase in circulating IGF-I levels but this effect of bST was abolished in undernourished animals.

25.5.3.2 Prolactin

The role of prolactin in monogastric animals on galactopoiesis is well-established. In rat, suppression of prolactin resulted in decreased milk secretion and administration of prolactin increases milk secretion during the early phase of lactation in rabbits.

25.5.3.3 Thyroid Hormones

Thyroid hormones are galactopoietic in nature. Administra­tion of T₃ and T₄ or feeding of thyroprotein increases milk production for 2-4 months along with increased butterfat percentage. Long-term supplementation of thyroid hormones or thyroproteins increased milk production during early lac­tation but there was a rapid decline in later lactation. Supple­ment thyroid hormones during the negative energy balance state of early lactation has little effect on galactopoiesis as the galactopoietic effect of thyroid hormones are mediated through increased metabolism. Thyroid hormones potentiate the action of other galactopoietic hormone such as prolactin for lactose and casein synthesis.

25.5.3.4 Glucocorticoids

Evidence on glactopoietic effect of glucocorticoids in ruminants are controversial. Studies reported that adrenalec­tomy reduces milk yield in cows. In contrast, the administra­tion of dexamethasone inhibits milk production in dairy cows. This negative effect of glucocorticoids on galactopoiesis is thought to be induced by the inhibition of prolactin release or the reduction of mammary responsiveness to prolactin.

25.5.3.5 Insulin

Insulin concentration is negatively correlated with milk pro­duction. But administration of insulin together with supple­mentation of extra glucose stimulates lactation. Insulin has no role in mammary uptake of acetate, β-hydroxybutyrate, triglycerides, amino acids, and glucose.

25.5.3.6 Ovarian Steroids

Exogenous administration of 17β-estradiol reported to decrease milk production in dairy cows. Ovariectomy in nonpregnant dairy cows improves persistency of lactation. Oral contraceptives containing estrogen also reduces milk production in woman. The mechanism by which estrogen affects milk production is yet to be discovered. But the inhibition of lactogenic effect of prolaction and GH by estra­diol could be the reason for decreased milk production upon estrogen administration.

Progesterone has a negative effect on lactogenesis but during galactopoiesis or established lactation progesterone has no effect on milk yield. It may be due to the fact that mammary tissue lacks progesterone receptor during lactation and progesterone has a greater affinity for milk fat rather its own intracellular receptor.

25.5.4 Factors Affecting Milk Yield and Composition

In mammals, milk is primarily produced to nourish the young. But the cow possesses most advanced type of mammary gland from the evolutionary point of view which can able to produce far more milk than the calf can consume. Milk production can be augmented further through genetic selection, nutritional manipulation or supplementation, better managemental strategies, and advanced milking technologies. The quantity and quality of milk are dependent upon the availability of secretory tissues in the mammary gland and efficiency of these tissues to synthesize milk components together with the availability of suitable nutrients as the precursors of milk components. The selection of cows for increased production may compromise their fertility and disease resistance. Production stress also compromises the host immunity and makes the animals more prone to metabolic and systemic diseases which in turn affect milk production and composition.

There are several genetic and non-genetic factors that affect milk yield and compositions

25.5.4.1 Breed

Milk yield varies considerably among different breeds of cattle. Generally, heavier breeds produce more milk com­pared to lighter breeds. Zebu cattle have lower production potential compared to exotic cows. Table 25.25 depicts the comparison of production performances of zebu, exotic, and cross-bred cattle along with different buffalo breeds.

In zebu cattle, the milk composition varied significantly among different breeds except for lactose. The fat percentage was higher in Jerseys and Guernseys, and more compared to Holstein Friesians and Ayrshires. The highest intrabreed variability was seen in fat percentage followed by solids­not-fat (SNF), protein, and lactose. The composition of milk constituents among different breeds of cattle and buffalo has been presented in Table 25.26.

25.5.4.2 Genetics

Economical traits such as milk yield were dependent on genetics. Milk yield has heritabilities around 0.25 whereas percentage milk fat, protein, and lactose have heritabilities around 0.5 (Table 25.27). The heritability of lactation persis­tence, peak yield, and milking rate is moderate while that of mastitis resistance is low.

The repeatability of milk yield and different milk produc­tion traits are presented in Table 25.28. Milk yield is highly repeatable (repeatability 0.95). The percentage of protein and SNF are also highly repeatable (0.85 and 0.90) but the repeatability of fat percentage is low (0.60). The characters with moderate to high repeatability can be improved through genetic selection.

The correlation coefficients between milk yield and vari­ous milk production traits are presented in Table 25.29. The fat, protein, solid not fat, and total solids are negatively correlated with milk yield.

Table 25.25 Milk yield of different cattle and buffalo breeds

Table 25.26 Milk composition of different cattle and buffalo breeds

Source: Wilcox (1992)

Table 25.29 The correlation coefficients between milk yield and various milk constituents

Source: Wilcox (1992)

25.5.4.3 Environment

Lactating cows are more cold-resistant due to the production of metabolic heat as a result of increased feed intake. The dairy cows may sustain their production until the temperature fall below —5 °C. Bos indicus of tropics are more heat tolerant compared to Bos Taurus of European origin due to low BMR and lower feed intake. But simultaneously they also have lower milk production.

Lactating cows are more susceptible to heat stress which decreases their feed intake. A significant decline in milk production was found at a temperature humidity index (THI) of 77. The critical values for minimum, mean, and maximum THI for milk production were 64, 72, and 76, respectively for dairy cows. There was 0.32 kg decrease in milk yield with the increase per unit THI. Milk composi­tion traits except milk protein are highest in hot humid season compared to other seasons. The milk of cows calved during winter exhibits more milk fat and SNF compared to the cows calved during summer.

25.5.4.4 Nutrition

The mammary secretory epithelium requires a constant sup­ply of nutrient precursors to produce milk. Therefore, both the milk yield and composition are affected by dietary manip­ulation. Through dietary manipulation, fat and protein con­tent have been altered up to a range of 0.3-0.6%, respectively but the lactose was reported to be unchanged with respect to dietary manipulation. Acetate and butyrate are the main precursors of milk fat. Lower intake of roughage to carbohy­drate decreases milk fat by decreasing the production of acetate and butyrate. The concentration of propionate is neg­atively correlated with milk fat but it promotes milk protein synthesis by increasing the availability of glutamate. On a dry matter basis, the minimum forage-to-concentrate ratio of 40: 60 is required to ensure milk fat percentage above 3.6. In contrast, finely chopped forage increases the amount of pro­pionate hence the milk protein percentage. Starch is essen­tially required for optimum microbial protein synthesis and its inclusion in the diet positively influences milk yield and the percentage of protein in milk. Milk fat percentage can also be increased by adding a minimum of 28% neutral detergent fiber. Supplementation of saturated fatty acids in the diet was reported to increase milk fat percentage, in contrast, unsaturated fatty acids decrease fat percentage. But the rumen microbes are unable to utilize lipids as an energy source thus supplementation of fatty acids in the diet may decrease milk protein percentage. Therefore, it is recommended that fatty acid supplemented diets should be enriched with amino acids to maintain optimum milk protein percentage. There are certain feed additives that promote milk fat synthesis such as sodium bicarbonate, magnesium oxide, and methionine hydroxy analog with high-energy diets. They facilitate the transfer of blood lipids to mammary gland.

25.5.4.5 Parity and Stage of Lactation

It was reported that the total milk production of a cow reaches a peak around fifth lactation when the cow is 7-8 years old with maximum skeletal size which is around 30% more compared to first lactation. The recurring increments of milk production are 13% from first to second, 9% from second to third, 5% from third to fourth, and 3% from fourth to fifth lactation. There is a plateau in milk production after fifth lactation after which milk production declines beyond 12 years of age. Mammary glands also increase in size on subsequent lactations till fifth which is due to skeletal matu­ration and increase in body weight to accommodate a larger udder.

Milk constituents are also influenced by the lactation stage. The fat and SNF content are decreased by 0.05% and 0.1%, respectively in each successive lactation. Parity has no significant effect on fat and SNF content in some Zebu (Tharparkar, Red Sindhi) and crossbred (Karan swiss cow).

The transition from colostrum to milk occurs within the first few days postpartum with abrupt changes in the compo­sition. Colostrum contains more protein and minerals but less lactose than milk. Colostrum also has 25% total solid which is much higher than milk. Colostrum is also having more calcium, magnesium, sodium, phosphorus, and chloride but less potassium compared to milk. However, these changes are still continuing up to 6-8 weeks postpartum at a slower rate. An increase in the protein content decreases lactose and vice versa and the SNF content remains fairly constant till the 8th month of lactation and then increases under the influence of hormones of pregnancy but declines gradually in unbred cows over the remaining lactation period.

25.5.4.6 Milking Management

Milking at unequal intervals results in less milk production compared to those milked at regular intervals. This reduction in milk production is more pronounced in high-yielding cows compared to low yielders. Milking twice a day yields 40% more milk production than once a day which can be further increased to 5-20% thrice a day and 5-10% when milked four times a day. Increasing the frequency of milking to three times a day can increase milk yield by up to 20%. The probable reasons for increased milk yield in response to milking frequency are less intramammary pressure with fre­quent milking, increased stimulation for oxytocin release, and less negative feedback on secretory cell results due to accumulation of milk. Around 10-20% of total milk is left in the udder after milking termed as residual milk. Poor milking procedure decreases the milk yield by increasing the fractions of residual milk.

25.5.4.7 Pregnancy

Milk production of cow is compromised during gestation especially after 4 or 5 months due to nutrient partitioning for the growth and maintenance of the fetus. The proportion of milk constituents is increased with the advancement of gestation after fourth month of pregnancy. Milk SNF, pro­tein, and lactose contents are altered during pregnancy but fat and mineral content are not affected in HF crossbred. The alterations in the milk composition during gestation can be explained by increased estrogen levels in the maternal circulation.

25.5.4.8 Dry Period

A dry period of 42-60 days is common practice that facilitates the replacement of old and damaged mammary epithelial cells and increases milk production in the next lactation. The milk production was reported to be decreased by 4.5% for a short dry period (4-5 weeks) and by 19.1% for no dry period. But after conventional dry period the cow exhibits negative energy balance during early lactation due to reduced feed intake and high milk yield which may con­tinue for several months associated with metabolic disorders and impaired fertility. Therefore, shortening of dry period is recommended to improve the energy balance in early lacta­tion through decreased milk yield after calving. The milk protein, lactose, and SNF percentage were reported to be increased by a shorter dry period but milk fat content was not affected.

25.5.4.9 Body Condition Score (BCS)

The state of NEB during the transition period and early lactation mobilizes body adipose for milk production. There­fore, BCS is negatively correlated with milk yield. Superior milk-producing cows genetically have lower BCS throughout lactation compared to those of lower genetic merit. Data have suggested that 20% of the increase in milk production is due to increased body weight. BCS is slightly negatively correlated to milk fat, lactose, and SNF content and positively correlated with protein.

25.5.4.10Photoperiod

Increased photoperiod has a positive effect on milk produc­tion in many species including cattle. Increased milk produc­tion at a tune of 25 kg/cow has been reported with increasing light exposure from 12 to 16-18 h. The exposure of light in the retina stimulates photoperiod which transmits inhibitory signal to pineal gland through retino-hypothalamic tract. In pineal gland, this inhibitory signal decreases melatonin syn­thesis by inhibiting the enzyme N-acetyltransferase. Decreased melatonin stimulates prolactin release which mediates galactopoietic effect. But prolactin is only galactopoietic in rodents and it had a limited role in milk yield in an established lactation in cattle. Therefore, this hypothesis did not hold true for cows, instead this photoperiodic induction of galactopoiesis may be mediated through increased IGF-I secretion. Increasing day length exposure did not affect milk composition but a minor decrease in milk fat percentage has been reported. Photoperiodic induction of milk yield can also be mediated through increased dry matter intake in cattle.

25.5.5 Milk Ejection Reflex

It is a neuroendocrine reflex leading to passive withdrawal of milk from alveoli, cistern, and ducts of mammary glands. As discussed in previous chapter, after secretion from secretory alveoli, milk is stored in the udder in the form of cisternal and alveolar fractions. The cisternal milk fractions can be removed after loosening the teat sphincter barrier. The alveo­lar milk fraction that locates in the alveoli and small ducts are fixed by capillary force. The removal of alveolar milk requires its forceful expulsion into the cistern. Milk ejection reflex facilitates this forceful milk expulsion.

25.5.5.1 Pathways of Milk Ejection Reflex

Neuroendocrine reflex, as the name implies, consists of neu­ral (afferent) and endocrine (efferent) pathways.

Neural (afferent) pathway: The neural component starts after the stimulation of pressure-sensitive nerve receptors located at the tip of teats. The nerve impulse travels through the spinothalamic nerve tract to the brain, espe­cially to the supraoptic nuclei (SON) and paraventricular (PVN) nuclei of the hypothalamus. These hypothalamic nuclei secrete oxytocin stored in the secretory terminals of the neurohypophysis. The suckling-induced oxytocin is released into bloodstream which initiates the endocrine (efferent pathway).

Endocrine (efferent) pathways: Once oxytocin is released, it reaches the mammary gland through systemic circulation. Oxytocin binds with its specific receptors in the mammary gland and contracts the myoepithelial cells. These cells are located between the basement membrane and epithe­lial cells of alveoli. They are also termed as basket cells and are having long cytoplasmic processes which cover epithelial cells. Due to the contraction of myoepithelial cells, the interalveolar pressure increases leading to the expulsion of milk from the alveoli into the cisternal system.

Mechanism of action of oxytocin on myoepithelial cell con­traction: The receptors of oxytocin belong to rhodopsin- type (Class 1) of the G-protein coupled receptor superfam­ily. Ligand binding triggers intracellular signaling pathways that activates Phospholipase-C (PLC) which converts phosphatidyl inositol-bis-phosphate (PI2) into diacylglycerol (DAG) and phosphatidyl inositol-tri-phos- phate (PI3). PI3 mediates the release of calcium from sarcoplasmic reticulum. Calcium after binding with cal­modulin (CaM) activates myosin light chain kinase (MLCK) and phosphorylation of myosin. Phosphorylation of myosin initiates the contraction of myoepithelial cells.

25.5.5.2 Inhibition of Milk Ejection Reflex

Inhibition of milk ejection is brought about by either central (brain) or peripheral (mammary gland) level. In central inhi­bition, the release of oxytocin from the neurohypophysis is inhibited whereas in peripheral inhibition the secretion of oxytocin is normal but oxytocin is unable to exert its effect on mammary gland.

25.5.5.2.1 Central Inhibition

A number of stimuli are associated with the central inhibitory pathway of the milk ejection reflex. But the possible mecha­nism responsible for this inhibition was not clearly under­stood in cows. Two possible mechanisms of central inhibition are proposed.

Opioid control: The endogenous opioid system (EOP) is thought to be the main regulator of central inhibition of oxytocin release. The EOP system inhibits oxytocin release at three different levels, viz., neuronal terminals in the neurohypophysis, supraoptic, and paraventricular nuclei of the hypothalamus and inputs to oxytocin neurons. Pro-opiomelanocortin is the common precursor of both β-endorphin and cortisol. Therefore, it seems that cortisol may be a mediator of central inhibition. But the administration of cortisol does not have any role in milk ejection in cattle. Therefore, the involvement of cortisol in this inhibitory control may be ruled out.

Noradrenergic control: The noradrenergic cells (A2 cell group) of nucleus tractus solitarii in medullary structures and dopaminergic cells of the posterior and periventricular hypothalamus also involve in the oxytocin release. A2 cells secret noradrenalin which mediates excitatory con­trol over oxytocin release via α 1-adrenergic receptors in PVN and SON whereas adrenaline released from adrenal medulla inhibits oxytocin release via β-adrenergic receptors. However, the role of catecholamines in the central inhibition of milk ejection reflex is questionable as peripheral catecholamine concentration was unaltered during central inhibition in cows and adrenergic blocking agents failed to abolish central inhibition of milk ejection reflex in cows.

25.5.5.2.2 Peripheral Inhibition

The failure of oxytocin to stimulate milk ejection at the mammary gland level under normal oxytocin secretion is known as peripheral inhibition of milk ejection. It is brought about either through the blocking of oxytocin receptor in the mammary gland or by the ability of oxyto­cin to reach mammary gland through the systemic circula­tion. Catecholamines are important mediators of peripheral inhibition of milk ejection. There is a close association between oxytocin-containing neuron and sympathetic ner­vous system in brain and the effect of oxytocin sympathetic nervous system in the mammary gland. The arteries, arterioles, milk duct system, and smooth muscles are hav­ing sympathetic innervations. α-adrenoceptor agonists stim­ulate the contraction of these muscle whereas β-adrenoceptor agonists and α-adrenergic antagonists cause the relaxation of these muscles. Earlier it was believed that sympathetic stimulation inhibits blood flow in the udder by vasoconstriction and oxytocin hardly reaches myoepithelial cells. Administration of an α-adrenergic agonist increased the teat length without reducing the milk flow in the cistern indicating that the contraction of the vascular smooth muscles of the udder are not responsible for the inhibition of milk flow. Another hypothesis suggested that the inhibition of milk ejection by α-adrenergic agonists is possibly due to constriction of teat wall and mammary ducts and enhanced milk flow by β-adrenergic agonists is possibly due to relaxation of the teat sphincter, teat wall, and large mammary ducts.

25.5.5.3 Factors Affecting Milk Ejection Reflex

Suckling stimulus: Both suckling stimulus and machine milking have a positive effect on the milk ejection reflex; however, suckling stimulus is stronger compared to machine milking. The suckling stimulus has a potent effect on milk ejection in cows with previous suckling experience.

Presence of calf: The release of oxytocin is more when the milking is performed in presence of calf. But immediate removal of calf after postpartum before initiation of milking did not influence oxytocin release. The separation of own calf and suckling by an alien calf also decreases oxytocin secretion.

Relocation: Change in the milking surroundings causes inhi­bition of milk ejection reflex. Relocation within familiar surroundings also inhibits milk secretion and oxytocin release. The relocation causes emotional stress to the animals and releases β-endorphin which in turn inhibits oxytocin release by the central inhibitory pathway.

Methods of milking-. Hand milking induces more oxytocin release compared to machine milking. However, mechan­ical stimulation is sufficient enough to induce normal oxytocin secretion and milk ejection under normal conditions.

Stress: Stress in the animals by any means results in inhibi­tion of milk ejection. Stress-induced inhibition of milk ejection is thought to be mediated by adrenaline. Some authors suggested that inhibition of milk ejection upon stress is mediated through the release of ACTH and corti­sol release in rat but machine and hand milking or even suckling stimulate the release of cortisol in cows. Thus, its inhibitory role in milk ejection is not well established.

Learning Outcomes

• Mammary gland: Mammary glands are the exo­crine glands modified from sweat (sudoriferous) gland. From the evolutionary point of view, the mammary gland of bovines is the most advanced form which can produce far more milk than required for a calf. Mature mammary gland consists of udder, teat or nipple, associated ducts, alveoli composed of epithelial secretory cells and supporting tissues. The primary secretory units of the mammary gland are alveoli.

• Mammogenesis: The growth of the mammary gland is termed as mammogenesis and occurs through a series of structural and functional devel­opment, differentiation, and involution regulated by hormones and growth factors. Mammary secretory tissues are developed from the ectoderm; however, the blood and lymph vessels, connective tissue, fat pad, and smooth muscles are derived from the meso­derm. Mammogenesis in female can be broadly classified into five stages namely prenatal, prepuber­tal, postpubertal, pregnancy, and early lactation. Majority of mammary growth occurs around gesta­tion and lactation. Hormones like estrogen, proges­terone, placental lactogen, glucocorticoids, oxytocin, and insulin like growth factors play piv­otal role in mammogenesis.

• Lactogenesis: Lactogenesis is the biological pro­cess of onset of milk secretion which includes the enzymatic and cytological differentiation of mam­mary alveolar cells in early pregnancy to full lacta­tion after parturition. Lactogenesis comprises of two stage process; appearance of pre-colostrum (stage-I) and onset of copious milk secretion at parturition (stage-II). The precursors for milk synthesis are either derived directly from blood or synthesized de novo. Prolactin, growth hormone, insulin, estro­gen, progesterone, glucocorticoids, thyroid hormones, and prostaglandins regulate galactopoiesis. It is a neuroendocrine reflex leading to passive withdrawal of milk from alveoli, cistern, and ducts of mammary glands called milk ejection reflex.

• Galactopoiesis: Galactopoiesis can be defined as the maintenance of established lactation. In cattle, milk production follows a dynamic curve with a rapid accelerating phase and reached peak around 6 weeks then decline till the end of lactation. The typical lactation curve of bovines can be split into early (day 1 to 100 days), mid (101-200 days), and late (201-300 days) followed by a dry period of 65 days. Milk yield and composition vary with species, breed, stages of lactation, and nutritional status of the animals.

• Mammary gland involution: Involution is a biological process by which the lactating mammary gland undergoes a series of tissue remodeling pro­cesses after cessation of milk secretion to restore into a virgin like state mediated through a series of events involving apoptosis and tissue remodeling.

Exercises

Objective Questions

Q1. The main supporting system of the udder in cow is

Q2. Gland cistern of a cow can store mL of milk.

Q3. What is the primary function of Furstenberg's rosette?

Q4. What is the common name of subcutaneous abdominal vein?

Q5. Synthesis and processing of dimeric of IgA in mono­gastric animals is facilitated by which cellular compo­nent of mammary gland?

Q6. The prenatal development of mammary gland of bovines starts around days of

embryonic life.

Q7. _____________________ hormone helps in mammary

ductal growth and______________________ hormone

helps in mammary lobulo-alveolar growth.

Q8. What facilitates the glucose sparing action in the mam­mary gland of cattle?

Q9. Specialized cell fragments involved in triglyceride syn­thesis in goat mammary gland is known as

Q10. A2 milk is found in_____________________ cattle.

Subjective Questions

Q1. “Fear and excitement inhibit milk ejection reflex” jus­tify the statement.

Q2. What is the difference between allometric and isomet­ric growth of the mammary gland?

Q3. What is feedback inhibition of lactation?

Q4. How does milk SCC help to indicate intramammary infections?

Q5. How does the exposure of light facilitate galactopoietic effect?

Q6. Briefly describe the mechanism of milk fat secretion.

Q7. Why milk ejection reflex is called neuroendocrine reflex?

Q8. What is the role of myoepithelial cells?

Q9. What is lactogenic hormone complex?

Q10. Briefly describe the partitioning nutrients around lactogenesis?

Answer to Objective Questions

A1. Median suspensory ligament

A2. 100-400 mL

A3. To provide local defense against pathogen by recruiting leukocytes especially lymphocytes and plasma cells

A4. Milk vein

A5. Mammary epithelial cells

A6. 32 days

A7. Estrogen, progesterone

A8. Absence of citrate cleavage enzymes essential for gen­eration of cytoplasmic acetyl CoA from glucose

A9. Christiesomes

A10. Zebu cattle

Keywords for Subjective Questions

A1. Catecholamines, peripheral inhibition

A2. Body growth, mammary growth, relative proportion

A3. Local inhibitory glycoprotein, milk secretion

A4. Infection, leukocyte recruitment, higher SCC

A5. Photoperiod, IGF, mammary epithelial growth

A6. Cytoplasmic lipid droplets, microlipid droplets, apical vesicle route, secretory vesicle route

A7. Neural stimulation, oxytocin release, milk secretion

A8. Modified smooth muscle, contraction, squeezing of mammary epithelium

A9. Growth hormone, prolactin, insulin

A10. Metabolic adaptation, negative energy balance, glucose-sparing action

Further Reading

Books

Akers RM (2001) Lactation and the mammary gland. Iowa State Press, Ames, IA, p 278

Alais C, Alais C, Godina AL (1985) Ciencia de la leche: principios de tecnica lechera. Editorial Reverte, Barcelona

Banerjee GC (1998) Lactation. In: Textbook of animal husbandry, 8th ed. IBH Publishing Co. Pvt. Ltd., New Delhi, pp 340-380

Bauman DE, Davis CL (1974) Biosynthesis of milk fat. In: Larson BL, Smith VR (eds) Lactation: a comprehensive treatise, vol 2. Aca­demic, New York, pp 31-75

Cowie AT (1984) Lactation. In: Austin CR, Short RV (eds) Reproduc­tion in mammals, vol 3, 2nd edn. Cambridge University Press, Cambridge

Cowie AT, Tindal JS (1972) The physiology of lactation, Monographs of the Physiological Society No. 22. Edward Arnold Ltd., London

Dang AK, Singh M (2002) Somatic cell counts of milk. National Dairy Research Institute, Karnal, Haryana

Folley SJ (1956) The physiology and biochemistry of lactation. Oliver and Boyd, Edinburg

Guthrie AH (1989) Introductory nutrition. Times Mirror/Mosby College Publishing, St. Louis, MO

Mehta BM (2015) Chemical composition of milk and milk products. In: Cheung PCK, Mehta BM (eds) Handbook of food chemistry. Springer-Verlag, Berlin

Mukherjee J, Das PK (2019) Immune responses of mammary gland. In: Sar TK (ed) Mastitis: symptoms, triggers and treatment, 1st edn. Nova Science Publishers, Inc., New York, p 89

Nickerson SC, Akers RM (2011) Mammary gland I anatomy. In: Fuquay JW, Fox PF, McSweeney PLH (eds) Encyclopedia of dairy sciences, vol 3, 2nd edn. Academic Press, San Diego, pp 328-337

SchmidtGH (1971) Biology of lactation. W. H. Freeman, SanFrancisco

Thiagarajan R (2014) Textbook of animal breeding. Satish Serial Pub­lishing House, New Delhi

Turner CW (1952) The mammary gland, 1st edn. Missouri, CO, Lucas Brothers Publishers, p 383

Wilcox CJ (1992) Genetics: basic concept. In: Van Horn HH, Wilcox CJ (eds) Large dairy herd management. ADSA, Champaign, IL, pp 1-8

Research Articles

Alhussien MN, Dang AK (2018) Milk somatic cells, factors influencing their release, future prospects, and practical utility in dairy animals: an overview. Vet World 11(5):562-577

Bauman DE, Vernon RG (1993) Effects of exogenous bovine somato­tropin on lactation. Annu Rev Nutr 13:437-461

Belitz HD, Grosch W, Schieberle P (2009) Food chemistry, 4th revised and extended edn. Springer, Berlin, pp 498-545

De K, Mukherjee J, Prasad S, Dang AK (2011) Effect of different physiological stages and managemental practices on milk somatic cell counts of Murrah buffaloes. Buffalo Bull 30(1):72-75

Department of Animal Husbandry and Dairying. Govt. of India, New Delhi, India.

Linzell JL, Peaker M (1971) Mechanism of milk secretion. Physiol Rev 51(3):564-592

Masson PL, Heremans JF (1971) Lactoferrin in milk from different species. Comp Biochem Physiol B 39(1):119-129

Misra SS, Sharma A, Bhattacharya TK, Kumar P, Saha RS (2008) Association of breed and polymorphism of α-s1and α-s2casein genes with milk quality and daily milk and constituent yield traits of buffaloes (Bubalus bubalis). Buffalo Bull 27:294-301

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