Analyses of the rat mammary gland show that the increase in the milk-protein mRNAs during the development of lactation and the rapid disappearance of these sequences during involution are not accompanied by similar changes in the poly(A) content. During the development of lactation the casein mRNA is initially in great excess to the whey-protein mRNA and this differential expression of the genes for the two types of milk proteins is again observed during early involution. Since the amounts of poly(A) and of both milk-protein mRNAs are also similar to the amounts found in the gland during late pregnancy, these results indicate that during early involution the mammary gland has reverted to the pattern of mRNA metabolism that occurs during late pregnancy.

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