Data from 103 litters of albino rats were analyzed for the effects of the number of fetuses in the litter and in each uterine horn, resorptions, and intrauterine position on variations in fetal weight. The right uterine horn tended to contain more fetuses than the left. There appeared to be systemic control over both the number of fetuses in the litter and their distribution between the horns. The number of fetuses per litter and the number per horn had separate and different effects on fetal weight. The number of fetuses in the horn was a more important determinant of fetal weight than was the number in the litter. Fetal weight was higher and the effect of the number of fetuses in the horn was stronger in horns with early resorptions than in horns without resorptions. Resorptions had no effect on the weight of adjacent fetuses or on fetal weight variation in the horn which suggests that resorptions influence fetal growth systemically rather than locally. Fetal mortality was highest in the lower one-third of the uterine horn and lowest in the upper one-third. Fetal weight variation increased as the number o f fetuses per horn increased and as mean fetal weight decreased. Intrauterine position was a significant determinant of fetal weight. The heaviest fetuses occupied the middle of the uterine horn while the lightest fetuses were at either end. The importance of these findings in prenatal growth and teratologic studies is discussed.There has been a great deal of interest in fetal growth and the association between intrauterine growth retardation and congenital malformation (Warkany et al., '61; Yerushalmy et al., '65; Brent and Jensh, '67). Many factors, both genetic and environmental, have been reported to influence the rate and extent of fetal growth. It has been suggested that variation in prenatal growth is largely determined by the intrauterine environment (McLaren and Michie, '60). One means of studying growth and the action of teratogens is through the use of laboratory models. Perhaps the most frequently used models are the rodentia: mice, rats, and guinea pigs.Several workers have published data from mice and guinea pigs in regard to the control of prenatal growth by intrauterine factors (Eckstein and McKeown, '55; Eckstein et al., '55; Hashima '56; Healy et al., '60; McLaren and Michie, '59, '60; McLaren, '63, '65). It has been assumed that the principles set forth for mice hold true also for rats. Both mice and rats are used for teratological studies in this laboratory. We have noticed significant AM. J. ANAT., 128: 413-428. differences between the two species. The purposes of this paper are to report fetal growth data in rats and compare them with known growth data in mice.
MATERIALS AND METHODSThe litters of 103 primigravid Wistar albino rats were used. These animals had served as "controls" for various experiments over the past five years (Jensh and Brent, '67; Jensh et al., '70). Each female rat was caged with a male overnight (5 : 00 PM to 9:00 A M ) . The zygotes were considered to be...
Fetal weight data from 28 litters of Wistar rats were analyzed for the effect of position in the uterine horn on fetal weight. The deviation of each fetus from the mean weight of all fetuses in a uterine horn was divided by the standard deviation of the mean. The standard scores thus obtained were studied for position effect. A pattern of fetal weights was found in which the heaviest fetus occupied the middle position with a progressive decrease in weights toward the ovarian and cervical ends of the horn. The lightest fetus in each horn was most likely to be found occupying either one of these extreme positions. These results are discussed with respect to a hemodynamic theory of fetal growth control that has been postulated for the guinea pig and mouse, and it is concluded that this theory is not applicable to the normal rat pregnancy.
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