Abstract
Several Strains of inbred mice (DDD, DDK, DDM, DKI, DDO, DSD, DY, etc) have been established in Japan from the common ancestor, so called “dd” mice which are not genetically uniform and have been maintained by pen-breeding [6, 9], and these strains are widely used for medical and biological researches [9] .
Unlike other strains, DSD mice have small litter size and low weaning rate and most of the new born pups in this strain die shortly after birth. These defects are not ameliorated by the improvement of feeding conditions. On the other hand, foster-nursing of the new born pups of DSD mice to the lactators of the other strains prevents the pups from death and results in the rise of weaning rate. Therefore, the inbreeding depression is likely to exert serious influence on some characteristics related to lactational performnance of this strain. Mammary development during pregnancy is an important factor for subsequent lactation.
The present experiment was carried out to compare mammary development and mammotropin levels of the pituitary and the placenta during pregnancy between DDD mice with normal rearing ability and DSD mice, as one step to elucidate the causes of the low rearing ability of DSD mice.
Virgin mice of both strains were mated with males at about 70 days of age. The day when vaginal plug or sperm was found or the last estrous day was designated as day 1 of pregnancy.
The body weight, number of placentae and anterior pituitary weight were significantly higher in DDD mice than in DSD mice (P<0.01) (Table 1) . DDD mice were also significantly higher than DSD mice in mammary lobulo-alve-olar formation on days 14 and 19 of pregnancy and in mammary nucleic acid contents on day 19 employed as the indices of mammary development (P<0.01) (Table 2) . Furthermore, DSD mice were inferior to C3H/He and C57BL/6 mice in these measures. Although mammary lobuloalveoli existed in the gland of DSD mice on day 14 of pregnancy, the developmental degree was much higher in DDD mice than in DSD mice (Fig.1-A, B) . On day 19, the well developed lobulo-alveoli covered the gland in DDD mice, and several alveoli contained the milk-like substance in their lumina (Fig.1—C) . On the other hand, the degree of lobulo-alveolar formation was much lower and no secretion was found in the alveoli in DSD mice even on day 19 (Fig.1—D) . Pituitary prolactin and growth hormone contents determined by disc electrophoresis [10] were significantly higher in DDD mice than in DSD mice (P<0.01), although there were little differences between strains in concentration per mg pituitary (Table 3) . Placental mammotropin level estimated by local mammary lobulo-alveolar de-2 times as many as that of the latter.
All the results would strongly suggest that the low pituitary and placental mammotropin secretion is one of the causes of low mammary development during pregnancy of DSD mice, which, in turn, might mostly account for the low rearing ability of this strain.
velopment of young C3H/He virgin mice injected with water extract equivalent to total 6 placentae was similar in both strains (Table 3), indicating that total placental mammotropin level was much higher in DDD mice than in DSD mice, because the number of placentae of the former was about
[Correction] EXPERIMENTAL ANIMALS Vol.22 No.2 (1973) pp.e1a-e1a
