Rat Mammary Gland Composition During Pregnancy and Lactation

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Abstract
Water, fat, and lactose con-centration were studied at 5-day intervals during pregnancy, parturition, and lactation in primiparous rats. A striking finding was the unchanging composition of the fat-free mammary tissues, as indicated by a constant proportion of dry weight for all of the periods studied. Thus, water content remained the same (80%) during preganncy and lactation. The percent fat of the gland declined steadily from 67% in the non-pregnant animal to 15% during lactation. Analysis of rat milk revealed fat and solids-not-fat concentrations to be about 13 and 17%, respectively. Lactose was found to be present in low amounts in the pregnant nonlactating gland, and to rise sharply at parturition. Both tissue lactose and milk lactose concentration rose significantly as lactation progressed, tissue values going from 596 [mu]g/100 mg on day 1 to 939 [mu]g/100 mg on day 20, and milk lactose from 12.3 mg/ml on the 1st day to 18.1 mg/ml on day 15. Fresh gland weight, when corrected for the estimated amount of milk retained, was found to be essentially constant during midlactation. A micro milking apparatus is described.