Prenatal growth in the albino rat: Effects of number, intrauterine position and resorptions

Abstract

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...

Selective Ligation of Uterine Artery Branches Accelerates Fetal Growth in the Rat

“…The mean birth weight of the term pups of our A litters not only significantly exceeded that of controls but was also considerably greater than that reported for fetuses of naturally occumng litters (3,13,14). The enhancing effect of A on fetal growth is also confirmed by the observation that the mean weight of pups in litters orignally 12 to 14 reduced to six and seven significantly exceeded that of control pups with naturally occurring litters of eight.…”

“…A roughly significant negative correlation exists between body weight and naturally occumng litter number in the rat when litters ranging from two to 17 fetuses are considered (3). When the upper and lower extremes of litter size are excluded, this relation is no longer demonstrable (3,13).…”

“…Less is known about the effects of physical factors such as litter number and uteroplacental blood flow, and few animal models are available to assess the effects of these variables on intrauterine growth. In normal litters of polytococcous animals, the number of fetuses in a litter is roughly inversely related to birth weight (3,4). Whereas reducing litter number by such means as aspiration (5) or electrocautery (6) may enhance the growth of the remaining fetuses, these techniques have not been assessed in a systematic manner.…”

Relationships between fetal body weight of Wistar rats at term and the extent of skeletal ossification

“…Data showing that rat fetal weight at term is not influenced by whether the fetus comes from the left or the right uterine horn were also reported by Barr et al (8). Conflicting (9), for instance, reported a position effect in mice according to a U-shaped pattern, i.e., the heaviest fetuses were located toward the ends of the uterine horn and the lightest ones in the middle.…”

“…In rabbits, a weight gradient from the ovarian to the cervical end of the uterine horns was found by Rosahn and Green (12). A few studies performed on rats using different methods to define the position of the fetuses have apparently come to a common conclusion that fetuses located in the middle horn position are heavier than those at both ends of the uterine horn (8,13,14). Barr and Brent (15) also found that, in the rat, such an association between fetal body weight and intrauterine position may not be dependent on hemodynamic factors such as the loopartery configuration that provides blood supply to the uterine horn.…”