Again, the specific hormonal requirments for lactogenesis in vitro varies between species. For, example, while the above combination of insulin, glucocorticoid and prolactin are required in many species, PRL alone can stimulate synthesis of casein and -lactalbumin in rabbit mammary tissue explants. However, addition of insulin, glucocorticoid and PRL significantly enhances milk protein synthesis over PRL alone, in the rabbit. In contrast, apparently the wallaby (a kangaroo, specifically the Macropus eugenii) requires only PRL to induce milk protein synthesis during lactogenesis, because addition of insulin and glucocorticoid in vitro does not further enhance the response to PRL (see Nicholas 1988 Ch. 6 In: The Developing Marsupial, Models fror Biomedical Research, Springer-Verlag, Berlin, pp. 68-85). Interestingly, progesterone does not seem to be inhibitory to lactogenesis in the wallaby, although it is necessary for mammary growth and development during pregnancy. Remember that the joey is born very immature after a short gestation. The joey attaches to a nipple and continues to nurse that same nipple. The structural and functional development of the mammary gland during lactation in the wallaby is closely linked to the development of the joey.
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Glucocorticoids are required in vitro for full initiation of milk secretion. There may be several roles of glucocorticoids in lactogenesis. They seem to be involved in development of the RER and other ultrastructural changes required for massive protein synthesis. They also may be directly involved in transcription of the casein and -lactalbumin genes.
De novo fatty acid synthesis ..
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Glucocorticoid concentrations in blood are fairly low during most of pregnancy, but increase markedly during the last few days prepartum. Another consideration when evaluating the effective concentration of glucocorticoids in blood is the concentration of corticoid-binding globulin (CBG), a blood protein that binds to corticoids and prevents them from having their actions on cells. Concentrations of CBG decrease in the prepartum period, thereby increasing available free hormone. Increased uptake of glucocorticoid by the mammary tissue coincides with lactogenesis, although a precise association with the first or second stage of lactogenesis has not been established. Glucocorticoid receptors in the mammary cells increase in numbers in late pregnancy. Both cortisol and PRL are required to maintain glucocorticoid receptor numbers.