Studies on the particulate components of rat mammary gland. 4. The distribution of ribonucleic acid

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Abstract
The distribution of total protein and ribonucleic acid phosphorus in 5 fractions of the rat mammary gland have been determined at 3 stages of the lactation cycle: on the 20th day of pregnancy and on the 3d and 18th days of lactation. At the end of pregnancy, ribonucleic acid phosphorus is largely localized in the "soluble" fraction of the mammary gland. Lactation produces a redistribution of total ribonucleic acid phosphorus such that the majority of it is found in the smaller particulate fractions of the tissue suspension. Lactation produces a relative increase in the amount of protein associated with the smaller particulate fraction at the expense of the "large-particle" fraction. All these fractions, however, show an increase in total amount from late pregnancy to late lactation. The ratio of ribonucleic acid phosphorus to total protein is low in all fractions in late pregnancy but has risen to a high value in the "small-particle" fraction by the end of lactation.