Relationship between litter size adjusted surgically and gestation period and time of delivery inthe rat

Abstract

In an attempt to study the relationship between litter size and mammary gland growth and mammary secretory function around parturition, we observed the relationship between litter size adjusted surgically and the gestation period and the time of delivery.
Rats of the Wistar strain, which had been bred by random mating in a closed system, were used at 3 to 4 months of age. Animals were housed in an animal room where the temperature was maintained at approximately 25°C and illuminated for 14 hours per day (lights on 6 : 00 to 20:00). After mating, the vaginal smear was taken every morning between 9:00 and 10:00 hr and the day of finding sperm was designated as day 1 of pregnancy. Each pregnant rat was kept in an individual cage thereafter. They were allowed unrestricted access to a commercial diet and water.
On day 8 of pregnancy, pregnant rats were undergone laparotomy under light ether anesthesia and the number of conceptus was reduced to various sizes less than 10 by crushing embryos with a forceps. In some rats, all conceptuses were remained intact after counting the number.
On day 20 of pregnancy, the pupping cages were transferred to another room with the same environmental conditions for the convenience of checking the delivery and inspected as frequently as possible throughout the entire 24 hrs. The dim light by a small flashlight was carefully used to determine the time of parturition during the dark period. The time of parturition was defined as the time at which the 1st pup was delivered. In one group, the mother was killed immediately after the delivery of the 1st pup and counted the number of fetuses remained in the uterus, and the sum of them was considered as the litter size at birth. In another group, the mother was killed 3 or 8 hrs after the delivery of the 1st pup and confirmed the completion of delivery of all pups by inspecting the uterus so that the length of parturition as well as litter size were recorded.
Total of 85 pregnant rats gave birth on day 22 to day 24 of gestation and an inverse relation between the gestation period and the litter size was observed. The regression line of gestation period on litter size in mothers whose litter sizes were less than 10 is Y=23.46-0.17X, where Y is the gestaion period and X is the litter size, with a correlation coefficient of -602 (P <0.01) (n=71). In most rats whose conceptus was reduced surgically, the litter size at birth was 1 or 2 less than the number of conceptuses remained on day 8 of pregnancy.
In spite of our careful inspection, we failed to confirm the time of delivery of the 1st pup in 19 rats, but could confirm that of the 2nd pup in 8, of the 3rd in 3, of the 4th in 4, of the 5th in 2, of the 6th in 1, and of the 10th pup in 2 rats. On assumption from the regression equation mentioned below, it may be safe to say that 17 rats of those also delivered during the light period. Thus, regardless of the day of parturition, more than 90% of deliveries occurred during the light phase. The distribution of the time of parturition showed similar pattern each day, i.e. the high frequency between 11:00 and 15:00 hr. These results may suggest a possibility of photoperiodic regulation of parturition.
Although interval between each pup was not always regular, the overall relationship between 44 observations of length of paturition (Y, min) and litter size (X) was presented as a strait-line regression equation, Y=53.02+4.55X, with a correlation coefficient of 0.348 (P <0.05). The results were discussed in relation to the literature.