MILK SYNTHESIS AND SECRETION AND ITS COMPOSITION

22.15.1 Secretion and its Constituents

Milk secretion is the process in which the milk constituents synthesized by epithelial cells are released into the alveoli’s lumen. Most of these constituents originate from the alveo­lus itself.

The secretion of milk constituents, including fat and protein, involves distinct processes within the epithelial cells. Milk fat forms as small droplets within these cells, which then merge as they move towards the cell’s apical part. These fat droplets are subsequently released from the epithelial cell through a pinching-off mechanism (Figure 13). During this process, the fine apical membrane of the epithelial cell encases the milk fat globule’s outer layer.

FIGURE 22.6 Diagram showing involution of the mammary gland in bovine and rodents.

Casein and lactose molecules are released as secretory vesicles surrounded by a membrane. These vesicles migrate towards the apical part of the cell before being discharged into the alveolar lumen. Before release, the secretory vesi­cle’s wall fuses with the cell membrane, and its contents are discharged into the alveolar cavity, repairing the membrane loss caused by the release of fat droplets. This secretion process is continuous, with each epithelial cell contributing to the overall secretion.

The molecules of proteins, milk sugar, vitamins, and minerals are evenly distributed within the milk.

22.15.2 Rate of Secretion of Milk

Epithelial cells display a heightened milk synthesis rate within the first 10 hours post-milking, which gradu­ally diminishes afterward. When a cow or buffalo goes unmilked, milk synthesis and secretion typically cease within a specific timeframe following the last milking ses­sion. The cessation of milk secretion is due to increased pressure within the alveolus, which constricts blood ves­sels, hindering the flow of blood and the supply of milk precursors to epithelial cells. Low-producing cows are less affected by extended milk intervals due to having less secretory tissue in the udder. Consequently, younger and low-producing cows experience a greater increase in udder pressure per unit milk, underscoring the importance of short milking intervals for such animals.

Leaving a significant amount of milk (2kg or more) in the udder after incomplete milking can lead to a perma­nent reduction in milk production. Milk secretion is influ­enced by various factors including parity, lactation stage, milking interval, frequency of milking, residual milk, and milking management practices. The rate of milk secretion plays a crucial role in determining the additional milk yield achievable with more frequent milking. For instance, milk­ing high-producing cows (>12-15kg∕day) three times daily instead of twice daily can result in a 20% increase in milk yield. Further increases in milk yield (5 to 10%) can be achieved by milking four times per day.

22.15.3 Role of Nerves in Milk

Secretion Regulation

The regulation of milk secretion by the nervous system has not been elucidated extensively. Many studies indicate that frequent milking of dairy animals stimulates milk secre­tion. Mammary glands respond to the following neural fac­tors for milk synthesis.

It has been observed that the production capacity of the mammary gland in rats can be modulated by the nutritional demand of the pups or litter. In ruminants, increased milk­ing frequency (IMF) of 2-3 times or intense suckling by the calf results in increased milk synthesis. Thus, IMF can be utilized to augment milk production and production efficiency.

In dairy animals, such as cows, milking three times a day results in 15-20% more milk production. Similarly, milking four times is preferable to three times, leading to an additional increase in milk production. This indicates that the mammary gland is especially sensitive to the demand of offspring in early lactation, influencing the shape of the lactation curve and lactation persistency.

Increased milking frequency (IMF) in ruminants and rodents leads to:

1.An increase in mammary cell number

2. An increase in mammary cell activity

3. Additionally, the effect of suckling intensity on mammary gland development has also been reported.

In rodents, mammary development with increased suckling is accompanied by:

1.An increase in DNA

2. An increase in RNA

3. An increase in the RNA:DNA ratio

22.15.4 Regarding the Impact of IMF and

Suckling Intensity on Hormones

In rodents, the act of suckling prompts the release of both prolactin and oxytocin, wherein prolactin serves to stimu­late mammary cell differentiation. An increased litter size is associated with elevated prolactin levels within the pitu­itary gland. This elevation in prolactin levels is linked to increased mammary cell activity. When oxytocin is injected into rats, it prolongs lactation by facilitating the complete evacuation of the mammary gland, although it does not impact milk yield. During late lactation in rodents, cortisol is recognized as the limiting factor; however, administering cortisol does not consistently result in a significant increase in milk yield. This suggests that local factors play a cru­cial role in regulating the mammary response to systemic hormones.

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