Continued removal of milk during lactation is key to maintaining lactation. The milking stimulus elicits a neuroendocrine reflex that causes release of oxytocin from the posterior pituitary, contraction of myoepithelial cells surrounding alveoli, and ejection of milk from the alveoli into the ductal system (milk letdown). In the absence of milk removal, intra-alveolar pressure builds, blood flow to the gland declines, and there is an accumulation of factors that inhibit lactation. The outcome is inhibition of milk secretion and stimulation of pathways for programmed death of epithelial cells. In rodents, mammary regression is rapid and extensive.[9,10]
For dairy cows, a nonlactating period between lactations (dry period) is important for maximizing milk production in the successive lactation. During a typical dry period there is extensive turnover of secretory cells, rather than extensive involution. This is likely because cows are in the final months of gestation when milking is terminated, and mitogenic effects of the hormones of pregnancy counterbalance the death-inducing effects of milk stasis. The replacement of secretory cells during the dry period may be necessary for maximizing milk production in the next lactation. By contrast, goats do not appear to require a dry period between successive lactations. The underlying basis for this is unclear, but may be associated with an ability of goats to replace mammary secretory cells during lactation. This may relate to the endocrine release of growth hormone during milking in goats, but not in cows, as growth hormone appears to enhance mammary cell turnover.
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