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J. Anim. Sci. 2003. 81:1265-1273
© 2003 American Society of Animal Science

Relationships between uterine and fetal traits in rabbits selected on uterine capacity1

M. J. Argente*,2, M. A. Santacreu{dagger}, A. Climent{dagger} and A. Blasco{dagger}

* Departamento de Tecnologia Agroalimentaria, División de Producción Animal, Universidad Miguel Hernández 03312 Orihuela, Spain and and {dagger} Universidad Politécnica de Valencia, Departamento de Ciencia Animal 46071 Valencia, Spain

2 Correspondence:
Carretera de Beniel Km 3,2 (phone: +34 96 6749708; fax: +34 96 6749677; E-mail:
mj.argente{at}umh.es).


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
The aim of this study was to investigate whether uterine capacity (UC) in rabbits was related to uterine horn length and weight and whether these uterine traits and vascular supply were related to fetal development and survival. Data from 48 unilaterally ovariectomized (ULO) does of the High and 52 ULO does of the Low UC lines of a divergent selection experiment on UC were used. Does were slaughtered on d 25 of fifth gestation. The High line showed higher ovarian weight (0.08 g, P < 0.05) linked to a higher ovulation rate (1 ovum, P < 0.05) and greater length of the empty uterine horn. There were no differences between lines in the remaining doe traits. The number of implanted embryos and live fetuses, fetal survival, and uterine weight and length were positively associated and explained most of the observed variation. Average weights of the live fetuses and their fetal and maternal placentae were not related to uterine weight and length. The linear regression coefficient of full uterine horn length on the number of live fetuses was 2.43 ± 0.21. The weight of the full uterine horn showed a small quadratic relationship (P < 0.05) with the number of live fetuses. Full uterine horn length, after adjusting for the number of embryos, was negatively associated (P < 0.001) with the number of dead fetuses. The linear regression coefficient of average fetal placental weight of the live fetuses on number of implanted embryos was higher (P < 0.10) in the Low line (-0.23 ± 0.04 vs. -0.12 ± 0.04). The linear regression coefficient of average weight of the live fetuses on the average weight of their fetal placentae was higher (P < 0.10) in the High line (2.56 ± 0.47 vs. 1.27 ± 0.57). The High line was more efficient, most likely because an increase in intrauterine crowding has a lesser effect on the development of fetal placentae and fetuses. The fetal position within the uterus did not affect the proportion of dead embryos. Fetuses with placentae receiving a single blood vessel had a higher probability of death (P < 0.001) and the lowest weight. There was no difference between lines for individual weight of the live fetuses, but the High line showed higher individual weights of fetal (P < 0.01) and maternal placentae (P < 0.10). Live fetuses in the mid-portion of the uterus were lighter in weight (P < 0.05) than in the oviductal and cervical regions (20.3 vs. 21.6 and 21.7g). Increasing uterine capacity increases uterine length and decreases weights of fetus and fetal placenta in rabbits.

Key Words: Blood Vessels • Fetal Development • Fetus • Placenta • Rabbits • Uterus


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
Prenatal losses are a limiting factor of litter size in rabbits, pigs, and mice. Approximately 30 to 40% of ova shed do not result in fetuses at term (Blasco et al., 1993). It has been suggested that losses resulting from intrauterine crowding are due to limitations in uterine capacity (Ford et al., 2002; Vallet et al., 2002). Uterine capacity is defined as the maximal number of fetuses that the dam is able to support at birth when ovulation rate is not a limiting factor (Christenson et al., 1987). In rabbits and mice, unlike pigs, there is no embryonic migration between horns. Thus, unilateral ovariectomy (ULO) in these species doubles the ovulation rate in the remaining ovary and the adjacent uterine horn is crowded with embryos. In pigs, it is necessary to perform a unilateral hysterectomy-ovariectomy (UHO) to obtain intrauterine crowding similar to that in ULO does. The litter size in ULO (Clutter et al., 1990, mice; Blasco et al., 1994, rabbits) or UHO females (Christenson et al., 1987) has been used to estimate uterine capacity. Fetal development and survival seem to be related to the vascular blood supply reaching the placenta (Hafez, 1965; Ducan, 1969) and to placenta functionality (Ford et al., 2002). In mice, the number of blood vessels arriving at each implantation site was used to estimate the vascular supply to each fetus (Wirth-Dzieciolowska, 1987). The surface attachment area between the placenta and endometrium has been suggested as a limiting factor of uterine capacity in pigs due to its type of noninvasive placentation (Ford et al., 2002). Wilson et al. (1999) suggested that uterine capacity in pigs should be defined more correctly as the total amount of placental mass or surface area that a dam can support to term. The aim of this study is to investigate whether uterine capacity in rabbits is related to length and weight of the uterus and whether these uterine traits and vascular supply might be related to fetal development and survival.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 Implications
 Literature Cited
 
Animals
Rabbits came from the second and third generation of a divergent selection experiment on uterine capacity in ULO does. Uterine capacity was estimated as litter size in ULO does. The left ovary was removed in all does before puberty via midventral incision at 14 to 16 wk of age (Blasco et al., 1994). The females were first mated at 18 wk of age and at 10 d after parturition thereafter. More details on this divergent selection experiment can be found in Argente et al. (1997). Records were available from 33 ULO does of the High-uterine capacity line and 28 ULO does of the Low-uterine capacity line in the second generation, and from 15 ULO does of the High-uterine capacity line and 24 ULO does of the Low-uterine capacity line in the third generation. Does were weighed on d 25 of their fifth gestation, and then slaughtered by stunning and exsanguination. The right ovary and the uterine tract were collected. The number of corpora lutea was recorded. An external examination of the uterine horn was made to locate the implantation site of each fetus. The number of blood vessels arriving at each implantation site was counted (Figure 1Go). As proposed by Wirth-Dzieciolowska (1987) in mice, the number of blood vessels was used to estimate the vascular supply to each fetus. Afterward, the mesometrium was trimmed from the right and left uterine horns. The right ovary and right uterine horn with its fetuses were weighed. The length of the right uterine horn was measured, and then it was opened lengthwise and the position and status of each fetus were recorded by starting on the ovarian end. There were three uterine positions: oviduct (the first fetus nearest the ovarian end), middle (fetuses in middle of the uterine horn), and cervix (the last fetus in the uterine horn from the ovarian end). The fetuses were classified according to status as live fetus, dead fetus, atrophic fetal and maternal placenta, and atrophic maternal placenta or decidual reaction. Each fetus with its fetal and maternal placentae was removed from the uterine horn. The fetus and its maternal and fetal placentae were separated and weighed (Figure 2Go). In total, there were 363 and 400 live fetuses in the High and Low lines and 119 and 128 dead fetuses in the High and Low lines, respectively. Finally, the dissected empty uterine horn was weighed and the length of the empty uterine horn measured.



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  		Figure 1. Right ovary and uterine tract of a unilaterally ovariectomized (ULO) doe. There were nine implantation sites within this uterus.

 


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  		Figure 2. Fetus (A) with fetal (B) and maternal placentae (C) at d 25 of gestation.

 
Traits
Traits Measured on the Does.
Variables measured were ovulation rate (estimated as the number of corpora lutea), the number of implanted embryos (estimated as the number of implantation sites), the number of live fetuses, the number of dead fetuses (implanted embryos minus live fetuses), embryonic survival (implanted embryos/ovulation rate), fetal survival (live fetuses/implanted embryos), prenatal survival (live fetuses/ovulation rate), ovarian weight, length of the full and empty right uterine horn, and weight of the full and empty right uterine horn.

Traits Measured on the Fetuses.
Variables measured were individual weight of the fetus, individual weight of its fetal and maternal placenta, and individual placental efficiency (measured as individual weight of the fetus divided by individual weight of its fetal placenta, according to Wilson et al., 1999). The average of weights of the live fetuses and weights of their fetal and maternal placentae were calculated for each female. Average placental efficiency was also calculated.

Statistical Analysis
Differences Between Lines for Doe Traits.
The traits were analyzed using the following model:

where µ was the general mean, Gi was the generation effect (second and third), Lj was the uterine capacity line effect (High and Low), Gi x Lj was the interaction generation per line, and eijk was the random residual term. The model for ovarian weight included ovulation rate as a covariate. For uterine length and weight, number of implanted embryos was added to the model as a covariate. The GLM procedure of SAS (SAS Inst., Inc., Cary, NC) was used for these analyses.

Differences Between Lines for Individual Traits of the Fetuses.
The individual traits of the fetuses were analyzed with a mixed model that included a random effect of mother of the fetuses. The model was:

where Sk was the status of the fetus, Lj x Sk was the interaction line per status, mijl was the random effect of mother of fetuses, and eijklm was the random residual term. The MIXED procedure of SAS was used for these analyses.

Relationships Between the Traits of the Doe and the Average Traits of the Live Fetuses.
A principal component analysis was performed using STATGRAPHICS (Manugistic, Inc., Rockville, MD) to examine the relationships between the traits of the doe and the average traits of the live fetuses. Moreover, in order to assess the relationships of fetal traits on number of implanted embryos, the following model was used:

where xijk was the covariate, b was the overall regression coefficient, and (b x L)j was the interaction between the regression coefficient and the line. Within line j, b + (b*L)j was the regression coefficient. If the interaction (b x L) was significantly different from zero, the regression coefficients of the High- and Low-uterine capacity lines were different. A quadratic term was included in the former model to study the quadratic relationships between traits. The determination coefficient R2 and the R2/R2max were calculated to examine the accuracy of the regressions. The R2max is the R2 before fitting any model (Draper and Smith, 1998). R2max is less than unity because there are repeated observations within each class of the covariate. The SAS GLM procedure was used for these analyses. The same model was used to study the relationships between average weights of fetuses and their placentae, although R2max cannot be calculated in this case because there are not repeated observations for each placental weight.

Uterine length and weight depended on number of implanted embryos. In order to analyze the relationships between length and weight of the full uterine horn and the number of dead fetuses free of the effect of the number of implanted embryos, a regression of the residual values of uterine length and weight on the number of dead fetuses was performed. The residual values were estimated using the following model:


Uterine Position, Blood Supply and Fetal Development.
The distribution of the percentage of live and dead fetuses per uterine position and the number of blood vessels was analyzed using a contingency {chi}2 test. Moreover, the weights of live fetus and their placentae were analyzed with a mixed model that included a random effect of mother of the fetuses. The model was:

where Pk was the fixed effect of the position of fetus in the uterine horn, Vl was the fixed effect of the number of blood vessels reaching the implantation site of each fetus (one, two, three, and four or more vessels), mijm was the random effect of mother of fetuses, and eijklmn was the random residual term. The SAS MIXED procedure was used for these analyses.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
Implications
 Literature Cited
 
Differences Between Lines
Differences in doe traits between the High- and Low-uterine capacity lines are shown in Table 1Go. The High line showed a greater ovulation rate (14.7 vs. 13.7 ova, P < 0.05), which agrees with the results from the whole set of data from generations 2 and 3 that considered up to the first four parities (Argente et al., 1997). The High line had a higher ovarian weight than the Low line (0.97 vs. 0.88 g, P < 0.05), but this difference disappeared when ovulation rate was added to the model as a linear covariate, so it should be due to the higher ovulation rate in the High line. No differences were found for number of implanted embryos and number of live fetuses at 25 days. However, Argente et al. (1997) reported a large difference between the High and Low lines for number of implanted embryos (1 and 1.7 embryos) and uterine capacity (0.8 and 1 kitten) with the whole data set from generations 2 and 3.


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  		Table 1. Least squares means for doe traits in the High- and Low-uterine capacity lines
 
The length of the empty uterus was higher in the High line (approximately 10%; see Table 1Go). No differences were found for the remaining doe traits. The difference in length of the empty uterus between the lines may be due to a higher litter size in previous parities or to a correlated response to selection for uterine capacity. Uterine length has been suggested as an important component of uterine capacity in the pig (reviewed by Ford et al., 2002). Development of placentae in pigs is limited by the availability of uterine space (Vallet and Christenson, 1993), and a reduction in prenatal survival and litter size was associated with a restriction in uterine space per fetus (Webel and Dziuk 1974; Wu et al., 1987; Chen and Dziuk, 1993). However, Galvin et al. (1993) found that Meishan sows had a higher litter size and a smaller uterine length than European breeds, and Gama and Johnson (1993) did not report any correlated increase in length and weight of the uterus in a line selected for increased litter size. Moreover, selection for uterine capacity in pigs had no effect on uterine length either, although empty uterine weight was increased 8% (Christenson and Leymaster, 2000). Thus, it seems that no relationships appear between uterine length and uterine capacity in pigs, and the same may hold true for rabbits.

Table 2Go shows the differences between the High- and Low-uterine capacity lines for individual weight of the fetus, its fetal placenta, placental efficiency, and maternal placenta. The individual weights of maternal and fetal placentae were lighter in live fetuses of the Low line. However, a lower development of fetal and maternal placenta in the Low line was not associated with a lower weight of live fetuses in this line. Moreover, placental efficiency, measured as the ratio of fetus weight:fetal placenta weight, was higher in the Low line, which shows that the definition of "placental efficiency" proposed by Wilson et al. (1999) in pigs might not work in rabbits. There are no studies on rabbits regarding the relationship between placental efficiency and litter size or uterine capacity. The greater litter size in Meishan pigs has been related to both smaller placentae and lighter fetuses (Christenson, 1993; Biensen et al., 1998). This result suggests that selection for placental efficiency in pigs would decrease the placental size, and therefore more conceptuses could be accommodated in the same amount of uterine space (Biensen et al., 1998; Wilson et al., 1998). However, the results in pigs have been contradictory. Wilson et al. (1999), with a reduced sample size, found that selection for placental efficiency resulted in a correlated increase in litter size. Nevertheless, selection for uterine capacity over 11 generations raised uterine capacity by approximately one pig per uterine horn (Leymaster and Christenson, 2000), but this response was not accompanied by an improvement in placental efficiency (Vallet et al., 2001).


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  		Table 2. Least squares means for individual weight of the fetus (IWF), individual weight of its fetal placenta (IWFP), individual placental efficiency (IPE), and individual weight of the maternal placenta (IWMP) by status of fetus (s1= live fetus, s2 = dead fetus, s3 = atrophic fetal and maternal placentae, s4 = atrophic maternal placenta or decidual reaction) in does of the High- and Low-uterine capacity lines
 
Regarding the traits of the dead fetuses, those of the Low line weighed approximately one-half that of the dead fetuses of the High line. The lower weight of the dead fetuses in the Low line was accompanied by an approximately 20% lighter fetal and maternal placentae weight (see Table 2Go). It seems reasonable to suggest that the lower weight of the dead fetuses found in the Low line is due to the fact that the fetuses died sooner in this line, given that the live fetuses reaching 25 d of gestation weigh the same in both lines. Alternatively, if there were reabsorption of fetuses, both lines would have the same speed of reabsorption. Hence a lighter weight in dead fetuses of the Low line would also indicate that the fetuses die sooner in this line.

Relationships Between the Traits of the Doe and the Average Traits of the Live Fetuses
The results of the principal component analysis are shown in Figure 3Go. The first two principal components explain 62% of the total variation. Principal component 1 shows that the number of implanted embryos and live fetuses, fetal survival, and uterine traits are positively related and explain most of the observed variations. Component 2 shows that weight of the fetus and its fetal and maternal placentae form a second positively associated group. Placental efficiency is negatively correlated with the development of fetal and maternal placenta, which again shows that the concept of placental efficiency may not be applicable to rabbits. The number of implanted embryos and live fetuses showed negative relationships with fetal and placental weight, as also was reported in pigs (Vallet et al., 2002). Contrary to what might be expected, fetal survival and uterine length and weight were not associated with average fetal and placental weight.



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  		Figure 3. Projection of the variables in the plane defined by the first and second principal components. The first and second principal components accounted for 42 and 20% of the variation, respectively. IE = number of implanted embryos, LF = number of live fetuses, FS = fetal survival, LFU = length of the full right uterine horn, LEU = length of the empty right uterine horn, WFU = weight of the full right uterine horn, WEU = weight of the empty right uterine horn, WFs1 = average weight of the live fetuses, WFPs1 = average weight of fetal placenta of the live fetuses, WMPs1 = average weight of maternal placenta of the live fetuses, PEs1 = average placental efficiency of the live fetuses.

 
Figures 4 and 5GoGo show the relationships between the number of live fetuses and either length or weight of the full uterine horn. Length of the full uterine horn was linearly related to the number of live fetuses. The number of live fetuses was highly correlated with the length of the uterine horn (r = 0.79). Each additional fetus was associated with an increase of 2.43 cm in the length of the uterine horn, or 7% of total uterine length. In the pig, Wu et al. (1987) also found a high correlation between the length of the uterine horn and the number of fetuses (r = 0.81) in the last week of gestation; the results of Wu et al. (1987) showed that the uterine horn was 5% of the total uterine length longer with each additional fetus. There were no differences between the High and Low lines on the linear regression of length of the full uterine horn on the number of live fetuses. The weight of the full uterine horn showed a small quadratic relationship with the number of live fetuses (Figure 5Go). In rabbits, the vascular supply to each implantation site is reduced when the number of fetuses rises (Duncan, 1969). This limited blood supply may produce smaller fetuses and a higher fetal mortality rate. No differences were found between lines for quadratic relationships of weight of the full uterine horn on number of live fetuses.



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  		Figure 4. Relationship between number of live fetuses and length of the full right uterine horn (LFU). LFU = (15.6 ± 1.9) + (2.4 ± 0.2) LF; R2 = 0.63; R2/R2max = 0.80.

 


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  		Figure 5. Relationship between number of live fetuses and weight of the full right uterine horn (WFU). WFU = (7.5 ± 41.4) + (49.3 ± 1.1) LF - (1.5 ± 0.7) LF2; R2 = 0.75; R2/R2max = 0.89.

 
Fetal survival showed a moderate correlation with length and weight of the full uterine horn (r = 0.56 and r = 0.59). Each additional centimeter or gram of uterine horn was associated with 1.2 and 0.1% increases in fetal survival, respectively. A reduction in the length of the full uterine horn, even after adjusting for number of implanted embryos, was associated with an increase in number of dead fetuses (Table 3Go). Each additional dead fetus decreased the length and weight of the uterine horn by 2.09 cm and 22.14 g, respectively. There were no differences between lines for the linear regressions of length and weight of the uterine horn on the number of dead fetuses. Length and weight of the full uterine horn did not show any quadratic relationship with the number of dead fetuses.


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  		Table 3. Coefficients of linear regression (b) for doe traits and average traits of fetal variables
 
Live fetuses were highly correlated with the number of implanted embryos; the regression coefficient of the number of live fetuses on the number of implanted embryos was 0.76 ± 0.07, and the correlation was high (r = 0.73, R2/R2max = 0.82). No differences were found between the linear regressions for the High and Low lines of number of live fetuses on number of implanted embryos. These results show that the functional uterine horn of ULO does was very efficient, although this uterine horn was overcrowded with an average of twice as many embryos as is found under normal conditions.

An increase of one implanted embryo was associated with a decrease in the average weights of maternal placentae, fetal placentae, and fetuses of 0.03, 0.18, and 0.29 g, respectively. This represents 2, 4, and 1% of the weights of maternal placentae, fetal placentae, and fetuses (Table 3Go), respectively. An increase in the number of implanted embryos had a lesser effect on the average weight of fetal placenta in the High line. The linear regression coefficients were -0.12 ± 0.04 (r = -0.36, R2/R2max = 0.28) in the High line vs. -0.23 ± 0.04 (r = -0.68, R2/R2max = 0.69) in the Low line. This reduction in the development of the fetal placenta and fetus with each additional implanted embryo is due to a reduction in available uterine space per fetus, as reported by Vallet and Christenson (1993) in pigs, and a decrease in blood flow reaching each fetus in rabbits, as found by Duncan (1969). The type of implantation in rabbits is hemochorial (Ramsey, 1975) and more invasive than in pigs; therefore, it is possible that the development of the placental surface area is not as strongly associated with fetal growth as has been proposed in pig (reviewed by Ford et al., 2002).

Placental efficiency was not related to number of implanted embryos in the High line; the linear regression coefficient of placental efficiency on number of implanted embryos was not different from zero in this line (0.06 ± 0.05, r = 0.22, R2/R2max = 0.11) and 0.21 ± 0.05 (r = 0.51, R2/R2max = 0.59) in the Low line. This shows the lack of utility of the concept of placental efficiency in rabbits. As indicated previously, Wilson et al. (1999) and Vallet et al. (2001) found contradictory results for the utility of placental efficiency in pigs when defined as the ratio of fetal weight:placental weight. Therefore, the study of other traits that can better measure placental efficiency has been proposed, such as placental vascular density (Biensen et al., 1998; Vonnahme et al., 2002), placental vascular endothelial growth factor, which is related to the vascular permeability of the placenta/endothelium (Vonnahme et al., 2001), and the efficiency of fetal erythropoiesis (Vallet et al., 2002).

Development of fetal placenta was correlated with development of maternal placenta, and fetal development was related to development of its fetal placenta. However, the regression coefficients of the High and Low line may be different (P < 0.10). The linear regression coefficients of average weight of fetal placenta on average weight of maternal placenta were 0.75 ± 0.30 (r = 0.35, R2/R2max = 0.14) in the High line vs. 1.41 ± 0.27 (r = 0.58, R2/R2max = 0.36) in the Low line. Moreover, fetal development was more correlated with development of fetal placenta in the High line: The linear regression coefficient was 2.56 ± 0.47 (r = 0.63, R2/R2max = 0.42) in the High line vs. 1.27 ± 0.57 (r = 0.37, R2/R2max = 0.15) in the Low line.

Uterine Position, Blood Supply, and Fetal Development
Table 4Go shows that there were no differences in the percentages of dead fetuses among the different positions within uterine horn (oviduct, middle, or cervix). The lightest live fetuses were located in the middle of the uterine horn, weighing 7% less than those of the oviduct and cervix. Also, fetal and maternal placentae presented lower weights in this uterine position (20 and 10%, respectively, see Table 5Go). The fetuses in the middle of the uterine horn had a lower availability of uterine space than those near oviduct or cervix because their littermates flanked them on both sides. This smaller available uterine space could limit development of the placenta, and in turn, fetal development. The heaviest maternal and fetal placentae were located near the oviduct, probably due to the greater uterine space per fetus and greater blood flow in this region, as reported by Duncan (1969). In rabbits, Lebas (1982) and Poigner et al. (2000) also found that the heaviest fetuses were located in the position nearest the oviduct, and the lightest fetuses developed in the intermediate uterine positions, which concurs with our results. Also, at the end of gestation in pigs, Waldorf et al. (1957), Perry and Rowell (1969), and Wise et al. (1997) found that the heaviest fetuses were located on the ovarian ends, and there was a decline in fetal weight from the ovarian end toward the middle. McLaren (1965) observed in mice that the fetuses at either end of the uterus had a greater weight than fetuses in the middle, and that the fetuses at the ovarian end tended to have a greater weight than fetuses at the cervical end.


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  		Table 4. Percentage of live and dead fetuses per position of the fetus in the uterine horn (oviduct, middle, and cervix) and number of blood vessels reaching each implantation site
 

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  		Table 5. Least squares means for individual weight of the live fetuses (IWF), individual weight of fetal placenta of the live fetuses (IWFP), individual placental efficiency of the live fetuses (IPE) and individual weight of maternal placenta of the live fetuses (IWMP) per position of the fetus in the uterine horn (oviduct, middle, and cervix), and number of blood vessels reaching each implantation site
 
Table 4Go shows that fetuses with a poor blood supply seemed to have a higher probability of death. Table 5Go shows that an increase in the weights of fetuses and their fetal and maternal placentae were associated with an increase in the number of blood vessels at each implantation site. The implantation sites with a single blood vessel had a lower fetal weight (8%) and lighter placentae (17 and 7% fetal and maternal placenta, respectively) than implantation sites with four or more blood vessels. This agrees with the findings of Wirth-Dzieciolowska (1987) in mice that embryos implanted in places with a single blood vessel have a higher likelihood of mortality and lower weight than the embryos receiving a higher number of blood vessels. Therefore, uterine vascular supply reaching each implantation site seems to have an important effect on development of fetuses and their placentae, as well as on fetal survival.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
Literature Cited
 
The length of the overcrowded uterine horn of unilaterally ovariectomized does increases linearly with the number of live fetuses in rabbits. Shorter uterine horns have a higher number of dead fetuses at the same number of implanted embryos. Length and weight of the uterine horn are not related to development of fetuses and their placentae. Nonetheless, fetal development and survival are correlated with blood supply. Uterine capacity is related to both blood supply and length of the uterine horn in rabbits.


   Footnotes
 
1 This study was supported by CICYT (AGF98-0382-C02-01). We acknowledge the detailed comments of M. A. Mirando and one referee. English text version revised by N. Macowan. Back

Received for publication August 2, 2002. Accepted for publication December 19, 2002.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 


Argente, M. J., M. A. Santacreu, A. Climent, G. Bolet, and A. Blasco. 1997. Divergent Selection for uterine capacity in Rabbits. J. Anim. Sci. 75:2350–2354.[Abstract/Free Full Text]

Biensen, N. J., M. E. Wilson, and S. P. Ford. 1998. The impact of either a Meishan or Yorkshire uterus on Meishan or Yorkshire fetal and placental development to days 70, 90, and 110 of gestation. J. Anim. Sci. 76:2169–2176.[Abstract/Free Full Text]

Blasco, A., M. J. Argente, C. S. Haley, and M. A. Santacreu. 1994. Relationships between components of litter size in unilaterally ovariecomized and intact rabbit does. J. Anim. Sci. 72:3066–3072.[Abstract]

Blasco, A., J. P. Bidanel, G. Bolet, C. S. Haley, and M. A. Santacreu. 1993. The genetics of prenatal survival of pigs and rabbits: a review. Livest. Prod. Sci. 37:1–21.

Chen, Z. Y., and P. J. Dziuk. 1993. Influence of initial length of uterus per embryo and gestation stage on prenatal survival, development, and sex ratio. J. Anim. Sci. 71:1895–1901.[Abstract]

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Christenson, R. K., K. A. Leymaster, and L. D. Young. 1987. Justification of unilateral hysterectomy-ovariectomy as a model to evaluate uterine capcity in swine. J. Anim. Sci. 67:738–744.

Christenson, R. K., and K. A. Leymanster. 2000. Effects of selection for ovulation rate or uterine capacity on gravid uterine, farrowing, and weaning traits in swine. J. Anim. Sci. 78(Suppl. 1): 202. (Abstr.)

Clutter, A. C., M. K. Nielsen, and R. K. Johnson. 1990. Alternative Methods of selection for litter size in mice: I. Characterization of base population and development of methods. J. Anim. Sci. 68:3536–3542.[Abstract]

Draper, P. J., and H. Smith. 1998. Applied Regression Analysis. 3rd ed. John Wiley & Sons, New York.

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M. L. Moce, M. A. Santacreu, A. Climent, and A. Blasco
The effect of divergent selection for uterine capacity on fetal and placental development at term in rabbits: Maternal and embryonic genetic effects
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