1967 Volume 38 Issue 8 Pages 328-333
It is clearly established that the milk yield can be affected by the quantity and quality of the parenchyma of mammary gland. It is also known that the desoxyribonucleic acid (DNA) content of the mammary gland can indicate the quantity of the parenchyma of mammary gland and that the intensity of protein formation can be expressed by the ribonucleic acid (RNA) content which may correlate with the quality of the parenchyma of mammary gland.
The present experiment was undertaken to study the changes of the nucleic acid content in the mammary gland at the peak-period of lactating mice in order to make advance in future selection experiments.
All the mice used were CFW strain. Fourty one female mice were divided into 4 groups at random and sacrificed on 10, 12, 14 and 16 days postpartum, respectively. Using dry powder of the abdominal-inguinal mammary glands, the fraction of nucleic acid was extracted by the method of SCHNEIDER. DNA and RNA contents were determined following a YAGI's modification of DIscaa and MEJBAUM, respectively. Litter size was adjusted to 8 pups per mother at birth and to 6 pups on 5 days postpartum. The body weight changes of mother and litter were recorded. Weight of fresh and dried fat free tissue (D.F.F.T.) of mammary gland were measured and total DNA-P, total RNA-P contents and the ratio of RNA-P/DNA-P were estimated. The results obtained were summarized as follows:
1) Mean and standard error of the weight of mother, weights of fresh and D.F.F.T. of mammary gland, total DNA-P, DNA-P/mg D.F.F.T., total RNA-P, RNA-P/mg D.F.F.T., RNA-P/DNA-P ratio and pup weight are shown in table I. There were no significant differences among the days of lactation in those characters since the variances were fairly large.
2) The deviation of those characters at 12 days postpartum was smaller than these of other days of lactation.
3) The coefficient of correlation between total DNA-P and D.F.F.T. was not significant. 4) The curve of RNA-P/DNA-P seemed to be parallel with the pup weight gain (Fig. III).
5) The total RNA-P remained higher during 10 to 12 days than during 14 to 16 days, but the total DNA-P indicated a tendency to decrease gradually from days 10 to 16 of lactation (Fig. I, II.).
6) Highly significant coefficients of correlation were found between RNA-P/DNA-P and litter weight gain (r=0.45). When. the effect. of days of lactation was held constant, its partial correlation coefficient was also significant (r=0.41), (Table II).
7) On improving milking ability, high DNA, RNA and RNA/DNA ratio are necessary and from the evidences obtained in the present experiment of small variation in measures at 12 days postpartum, it is considered that the selection of mice on milking ability with the ratio of RNA/DNA on 12th day of lactation might be, useful.
