J. Anim. Sci. 2003. 81:2575-2582
© 2003 American Society of Animal Science
Evaluation of the uterine environment and embryos of prepubertal gilts1,2
H. J. Henning and
D. L. Davis3
Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201
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Abstract
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A series of three experiments was conducted to test the functional status of the uterus and embryos in prepubertal gilts. In Exp. 1, gilts were induced to ovulate by treating with gonadotropins followed by hCG 72 or 96 h later, and were artificially inseminated 24 h after hCG. Five of the 10 gilts treated at 120 d of age, but none of the gilts treated at 100 of age, maintained pregnancies. We next tested the function of the uterine environment by transferring embryos from postpubertal females into gilts of various ages that had been induced to ovulate but not inseminated (Exp. 2). Pregnancy rate at d 50 of gestation was 44% (4/9) for 100-d-old recipients, 67% (2/3) for 140-d-old recipients, and 60% (3/5) for postpubertal recipients (P > 0.20). Therefore, uteri of 100-d-old gilts are able to maintain pregnancies with conceptuses from postpubertal gilts. In Exp. 3, embryos from 100-d-old and postpubertal gilts were transferred into postpubertal recipients. Uterine horns of recipients were surgically separated before transfer, and embryos from 100-d-old and postpubertal females were transferred to opposite horns of some recipients (experimental). Other recipients received embryos from postpubertal females in both uterine horns (control). When examined on d 50 to 60 of gestation, three of five control gilts were pregnant and three of seven experimental gilts were pregnant (P > 0.50). In experimental recipients, the survival of embryos from 100-d-old gilts was 38% (8/21) compared to 57% (15/26) for embryos from postpubertal gilts (P > 0.30). Because all uterine horns of pregnant recipients contained fetuses, these results support the hypothesis that embryos from 100-d-old gilts are able to initiate and maintain pregnancies in the uteri of postpubertal gilts. Therefore, the uterine environment of 100-d-old gilts provides an environment that supports development of embryos produced by postpubertal gilts, and the embryos produced by 100-d-old gilts can survive and develop in the uteri of postpubertal gilts. It was only the combination of embryos and uteri of 100-d-old gilts that did not permit pregnancy to be maintained.
Key Words: Embryo Transfer • Pigs • Pregnancy • Prepubertal Females • Uterus
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Introduction
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Conception is first possible at puberty and age at spontaneous puberty has been established by natural and artificial selection. Attempts to alter age at first pregnancy in pigs have utilized exogenous gonadotropins to induce premature ovulations. Those studies reveal that several maturations must occur. The hypothalamic-pituitary-ovarian axis and the tubular reproductive tract must respond appropriately to result in ovulation, fertilization, and embryonic development (Ellicott et al., 1973
; Christenson et al., 1985).
It is established that sequential treatment with PMSG and hCG induces ovulation as early as 100 d of age in the gilt (Dial et al., 1984). The oocytes released at these precocious ovulations can be fertilized; however, the pregnancies are not maintained (Dziuk and Gelbach, 1966; Ellicott et al., 1973; Segal and Baker, 1973). There are indications that the corpus luteum (CL) in such gilts are more sensitive to PGF2
than are the CL of postpubertal gilts (Rampacek and Kraeling, 1979), and prepubertal gilts respond differently to exogenous estrogen than postpubertal gilts (Rampacek and Kraeling, 1978), but the causes of pregnancy loss have not been established. In particular, the competence of the uterus and conceptuses has not been tested. The latter information is required if the deficiencies preventing pregnancy are to be identified.
This report presents studies of the functional status of the reproductive system of the prepubertal gilt. The hypotheses that the uterus and/or embryos of prepubertal gilts are not able to maintan pregnancies were evaluated. The age at which gilts in the research herd could first maintain pregnancy was determined. Next, the ability of the uteri and embryos of pregnancy-incompetent gilts to participate in pregnancy were evaluated by transferring embryos between pre- and postpubertal females.
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Materials and Methods
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Experiment 1
Age at Ovulation Induction.
Thirty-four gilts were treated with gonadotropins (PG600; 400 IU of PMSG and 200 IU of hCG; Intervet America, Millsboro, DE) at approximately 100 (99 to 102), 120 (117 to 123), and 170 (167 to 171) d of age. The numbers of gilts treated for each age were 10, 12 and 12, respectively. Equal numbers of gilts in each age group received hCG (Intervet; 500 IU) at either 72 or 96 h after PG600. Gilts were inseminated artificially one time, 24 h post hCG. Estrus detection was performed each day by observing the standing response in the presence of a boar. Inseminations were with 6 x 109 motile sperm extended to 100 mL (75 mL in 100-d-old gilts) in Beltsville Thaw Solution (Purcel and Johnson, 1975). Extended semen was stored up to 48 h before insemination.
Experiment 2
Embryo donors were sows and postpubertal gilts (n = 9) and recipients were postpubertal (n = 5, approximately 180-d-old), noncyclic peripubertal (n = 3, approximately 140-d-old) or prepubertal (n = 9, approximately 100-d-old) gilts. Sows providing embryos were injected i.m. the day their litters were weaned with PG600, and 84 h later with 500 IU of hCG (Peters et al., 2000). They were inseminated 24 and 36 h after hCG injection. To control time of ovulation in postpubertal gilts, they were checked for estrus daily and inseminated artificially on the day of standing estrus and the following day. Between 21 and 40 d of gestation gilts were injected i.m. with PGF2, (Lutalyse, Pharmacia & Upjohn, Kalamazoo, MI) at 0800 and 1600 (15 and 10 mg, respectively) (d -4) to regress the CL and allow gonadotropins to be administered precisely relative to luteolysis. Gonadotropins (PG600) were injected the following day at 0800 (d -3), and hCG (500 IU) was injected 84 h later (d 0). Gilts were inseminated artificially 24 and 36 h post hCG, and embryos were recovered and transferred on d 4 or 5 (hCG = d 0) of gestation. A total of 383 embryos and unfertilized oocytes was recovered from 20 donors (nine fourth-parity sows and 11 postpubertal gilts). Cleaving embryos that showed no signs of fragmentation or degeneration were selected for transfer. Most (n = 254) transferred embryos were at the four-cell to morula stages and four were two-cell. Embryos were transferred into 17 recipients. Embryos were transferred between 1 and 4 h after removal from the donor, except on two occasions when for logistical reasons, 11 embryos from three donor animals were cultured overnight and transferred into four recipients the following day. Two prepubertal gilts, a peripubertal gilt, and a cyclic gilt each received two to four embryos that had been cultured overnight, plus 12 to 14 embryos collected within 4 h of transfer. Two of these recipients were pregnant at slaughter.
Experiment 3
Donors were postpubertal (n = 17) and prepubertal gilts (n = 21) treated with PG600 and hCG to control the time of ovulation as described for Exp. 2. Before embryo transfer, the body of the uterus was isolated and the mesometrium pierced at a relatively nonvascular site near one uterine horn and within 10 cm of the uterine bifurcation. The uterine horn was severed using a gastrointestinal automatic metal stapling device (AutoSuture, United States Surgical Corp., Norwalk, CT). Uterine horn isolation allowed the transfer of embryos from 100-d-old and postpubertal gilts into opposite uterine horns of the same recipient. Two groups of postpubertal gilts were used as recipients. Five gilts received embryos from postpubertal gilts in both uterine horns (controls) to verify the procedures for separating uterine horns permitted the maintenance of pregnancy. Another seven gilts received embryos from postpubertal gilts in one uterine horn and embryos from 100-d-old gilts in the other uterine horn (experimental). Transfer of embryos from 100-d gilts was alternately into the severed or contralateral horn in succeeding recipients.
Embryos (n = 197) were transferred into 12 recipients on d 4 or 5 of gestation. Seven embryos had two or three cells at transfer, and the remaining 190 embryos had four or more cells. Embryos were transferred between 2 and 8 h after removal from the donors.
General Procedures
Animals and Surgical Procedures.
The Institutional Animal Care and Use Committee approved all animal procedures. Sows (line C22) and crossbred gilts (lines C22 x 326) from PIC USA (Franklin, KY) were used for these experiments. Anesthesia was induced with 15 to 30 mL of pentothal i.v. (5%; Abbott Laboratories, North Chicago, IL) and 5 mL of atropine sulfate i.m. (0.054%; Phoenix Pharmaceutical, St. Joseph, MO), administered to inhibit salivation. Halothane (Halocarbon Laboratories, River Edge, NJ) was applied by inhalation to maintain anesthesia. For embryo recovery, the donor uteri, oviducts, and ovaries were exposed through a midventral incision. A total of 60 mL of HEPES-buffered Tyrodes medium was used to flush the oviducts and uterus (Peters et al., 2000). Recovered oocytes and embryos were evaluated for developmental stage and normalcy using an inverted microscope (400x). Embryos were transferred to recipients that had four or more CL and not more than two follicular cysts (defined as follicles >15 mm in diameter). Larger numbers of cysts are associated with infertility (Davis et al., 1979). Embryo transfer was accomplished by loading embryos in a catheter (3.5 Fr. Tom Cat, Sherwood Medical, St. Louis, MO). The catheter was inserted into the uterus by cannulating the salpinx through a small puncture wound approximately 1.5 cm from the uterine horn, and the embryos expelled into the uterine lumen.
Evaluation of Pregnancy Status.
Blood (10 mL) was collected from the anterior vena cava to monitor concentrations of progesterone in serum. In Exp. 1, blood was collected on d 10 and one time between d 18 and 22. In Exp. 2 and 3, blood was collected on d 0 (before hCG injection), either d 4 or 5 (before embryo transfer), d 12, once between d 18 and 22, and on d 35 or 36. Blood was stored for 24 h (5°C), and then centrifuged (1,000 x g) for 20 min to harvest serum that was frozen (-15°C) until assayed for progesterone.
Concentrations of progesterone were quantified with RIA according to Davis et al. (1985). Intra- and interassay CV were 2.4 and 9.2%, respectively. Progesterone concentrations in serum on d 18 to 22 were used only as an indication of extension of the luteal phase indicative of pregnancy signaling by conceptuses. Progesterone concentrations on other days were evaluated as a quantitative indication of luteal function. Gilts with progesterone levels of 
4 ng/mL were slaughtered at midgestation (approximately d 66, 57, and 58 for Exp. 1, 2, and 3, respectively) and their uteri and ovaries collected. In Exp. 1, serum from some gilts had 2 to 4 ng/mL of progesterone on d 18 to 22. These gilts were examined at laparotomy. Otherwise samples used for pregnancy confirmation contained either < 2 or 4 ng/mL of progesterone.
Weights and Lengths.
Individual fetuses and placentas were weighed. The crown-to-rump length of each fetus was measured. Fetuses were considered viable if they had a uniform color and consistency and were anatomically intact. Ratios of fetal weight to placental weight and number of fetuses to number of CL were calculated.
Statistical Analyses
Quantitative data were analyzed using procedures GLM and LSMEANS of SAS (SAS Inst., Inc., Cary, NC). Models included type of recipient (Exp. 2) or donor (Exp. 3) as independent variables. Models evaluating fetal and placental size included the number of conceptuses per uterus (Exp. 1 and 2) or uterine horn (Exp. 3) as covariates. Categorical data were evaluated by 
2 analysis.
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Results
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Experiment 1
Initial analyses revealed no (P > 0.10) differences attributable to the 72 vs. 96 h intervals from PG600 to hCG. Therefore, this effect was not considered in subsequent analyses. At PG600 treatment, gilts differed in BW (P < 0.0001) with 100-, 120-, and 170-d-old gilts weighing 50, 73, and 116 ± 2.4 kg, respectively. Estrus was observed for 11 of 12 170-d-old gilts, three of 12 120-d-old gilts, and two of 10 100-d-old gilts. The 170-d-old gilts were expected to be peripubertal. To verify the induced ovulation was the first for 170-d-old gilts, progesterone concentrations were determined 3 d before PG600 injection and progesterone <1 ng/mL was detected in all 170-d-old gilts. Concentrations of progesterone in serum >4 ng/mL were taken as indicative of ovulation, and there was a tendency (P < 0.15) for fewer 100-d-old gilts to have elevated progesterone concentrations on d 10 than for older gilts (Table 1). However, concentrations of progesterone did not (P > 0.10) differ among age groups at either d 10 or d 18 to 22 (Figure 1). Both of the 100-d-old gilts with elevated progesterone on d 18 to 22 had exhibited estrus.
For gilts that ovulated, early pregnancy failure is indicated by low progesterone concentrations at d 18 to 22. Early pregnancy loss occurred for 60% of the 100-d-old group and 42 and 25% for the 120- and 170-d-old gilts, respectively. Fewer (P = 0.09) 100-d-old gilts than 120- and 170-d-old gilts had elevated progesterone at d 18 to 22, and no 100-d-old gilts maintained pregnancies to midgestation (Table 1
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Two of the 100-d-old gilts and one each of the 120- and 170-d-old gilts had progesterone concentrations of 2 to 4 ng/mL at d 18 to 22. These gilts were examined by laparotomy within the following week and were not pregnant (Table 1). Among these gilts, the 170-d-old gilt had CL present in both ovaries and her uterus appeared postpubertal in size. Of the two 100-d-old gilts, one had a small, immature uterus and the other had a uterus of postpubertal size, but neither had CL in their ovaries. The 120-d-old gilt had a small uterus and no CL.
Gilts with extended luteal function were slaughtered at d 65 after hCG at which time gilts treated at 100-, 120-, and 170-d of age differed (P < 0.05) in weight at 108, 142, and 160 ± 5.1 kg. No gilts given PG600 at 100 d of age had viable fetuses at d 65. In comparison, five 120-d-old gilts and seven 170-d-old gilts had viable fetuses (Figure 2). The number of fetuses (P < 0.05) increased with age. Fetal and placental weights did not differ (P > 0.3) between 120- and 170-d-old gilts. However, uterine weight was greater (P < 0.0002) for 170-d-old vs. 120-d-old gilts (Figure 2).
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Figure 2. Fetal, placental and uterine characteristics and number of corpora lutea (CL) in Exp. 1. aThe number of fetuses was less (P < 0.05) for 120 d-old gilts than for 170-d-old gilts. bThe uterine weight of 120-d-old gilts was less (P < 0.0002) than the uterine weight of 170-d-old gilts.
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Experiment 2
Concentrations of progesterone in serum on the day of embryo recovery were less (P < 0.1) for donor sows (4.1 ± 1.6 ng/mL) than for donor gilts (7.8 ± 1.0 ng/mL). All but one recipient had progesterone less than 1 ng/mL on d 0. The exceptional postpubertal gilt had 8.5 ng/mL of progesterone on d 0 and did not remain pregnant. Based on elevated progesterone, she was not proestrual at hCG injection and her data are excluded.
Concentrations of progesterone on d 0 did not differ (P > 0.10) among the three recipient groups (Figure 3). However, at embryo transfer (d 4 or 5), postpubertal recipients tended (P 
0.07) to have greater progesterone than other groups, a trend that continued (P < 0.05) on d 8 and d 35 or 36. Postpubertal recipients also had more (P < 0.02) CL (31.3 ± 3.3) than the peripubertal (6.5 ± 4.8) and prepubertal (14.3 ± 3.4) gilts.
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Figure 3. Concentrations of progesterone in serum of recipient gilts in Exp. 2. Bars represent, from left to right, concentrations at d 0, d 4 to 5, d 8, and d 35 to 36. aTends (P < 0.07) to be greater for postpubertal than for prepubertal and peripubertal groups. bGreater (P < 0.02) for postpubertal than prepubertal gilts. cGreater (P < 0.05) for postpubertal than for pre- and peripubertal.
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When slaughtered, three of five postpubertal, two of three peripubertal, and four of nine prepubertal gilts were pregnant (P > 0.20). Interval from hCG injection to slaughter was 53 to 62 d and similar (P > 0.10) among groups. The mean number of embryos transferred was 15 ± 0.4 and did not differ between recipient groups. Survival of embryos in pregnant gilts was 57, 50, and 33% for pre-, peri-, and postpubertal gilts, respectively, and did not differ (P > 0.35) between groups. However, postpubertal recipient gilts had fetuses with greater crown-to-rump lengths (P 0.001) and weights (P 0.05) (Figure 4). Placental weights (P < 0.02) were greater for postpubertal recipient gilts than for other groups. The ratio of fetal weight to placental weight was greater (P < 0.05) for 100-d-old prepubertal gilts than for postpubertal gilts.
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Figure 4. Fetal (A) and placental (B) characteristics in Exp. 2. aPostpubertal greater (P < 0.05) than pre- and peripubertal gilts. bPrepubertal greater (P 0.05) than peri- and postpubertal gilts.
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Experiment 3
Pregnancies were maintained in both control and experimental recipients (Table 2). Pregnancy rates, embryonic survival, and fetal and placental characteristics did not differ between the two groups. Effect of side of transfer on embryonic survival was evaluated in control transfers and did not (P > 0.40) differ (36.6 ± 7.3 vs. 54.7 ± 18.6 % for severed and intact horns, respectively).
Survival of Embryos from Prepubertal Gilts.
Three of five (60%) gilts receiving only embryos from postpubertal gilts were pregnant at an average of 56 d gestation. Three of seven (43%) recipients of embryos from both pre- and postpubertal gilts were pregnant at an average of 59 d gestation (Table 2). Fetuses were found in both uterine horns of all pregnant recipient gilts at slaughter. Examination verified that the severed uterine horns ended blindly preventing embryo migration between the horns.
The number of embryos transferred to each uterine horn ranged from five to nine (Table 3). Surviving fetuses in pregnant gilts were similar in number (P > 0.50) between control and experimental recipient groups. Both recipient groups had conceptuses of similar size (P 0.50), as evidenced by fetal and placental weights and fetal crown-to-rump lengths. Fetal:placental weight ratios were also similar (P 0.60) between recipient groups. Slaughter weights (P > 0.80) and gestational age at slaughter (P > 0.50) were similar between recipient groups. Among pregnant experimental recipients, survival of embryos from 100-d-old gilts (8/21, 38%) and postpubertal gilts (15/26, 57%) was not different (P > 0.30) (Table 3). The crown-to-rump length and fetal weight tended (P < 0.10) to be greater and placental weight was numerically, but not significantly (P > 0.10), greater for conceptuses from postpubertal gilts compared with those from the 100-d-old gilts.
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Discussion
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The complete failure of pregnancies in inseminated 100-d-old gilts (Exp. 1) is consistent with earlier reports (Dziuk and Gelbach, 1966; Ellicott et al., 1973; Neville et al., 1971) and provides a model to investigate pregnancy failures in prepubertal gilts. Previous reports show that pregnancy loss in prepubertal gilts occurs before d 30 of gestation (Rampacek and Kraeling, 1978). In Exp. 1, no 100-d-old gilts maintained pregnancies to midgestation; however, some had extended luteal function, as indicated by elevated progesterone on d 18 to 22. Similarly Rampacek et al. (1976) reported that progestin concentrations were elevated on d 14 for some prepubertal gilts that did not maintain pregnancies. Pregnancy recognition in pigs occurs in two phases, and elevated estrogen is required on d 11 and d 14 to 16 (Geisert et al., 1987). Therefore, it may be that the first stage of pregnancy recognition is accomplished in some 100-d-old gilts but that the second phase of pregnancy recognition fails. The interpretation that lack of luteal support is at least part of the problem is supported by experiments showing maintenance of pregnancy in some prepubertal gilts that were treated with progestins and estrogens (Ellicott et al., 1973).
In Exp. 2, pregnancy rates and embryo survival on d 50 of gestation were similar in prepubertal and postpubertal recipients of embryos from postpubertal donors. Therefore, the uterine environment of 100-d-old gilts is adequate to support embryonic and fetal development.
In Exp. 3, embryos produced by 100-d-old gilts developed to midgestation when gestated in uterine horns of postpubertal gilts and adjacent to uterine horns containing postpubertal conceptuses. It is clear that unilateral pregnancies do not survive in pigs (Anderson, 1966). Even a short segment of nonpregnant uterine horn causes regression of adjacent CL (du Mesnil du Buisson, 1961), but based on gross appearance, all pregnant recipients had bilateral CL maintenance. Therefore, the pregnancies observed in experimental recipients required pregnancy recognition signals to be sent by conceptuses provided by prepubertal and postpubertal gilts each in their separate uterine horn. It is concluded that the embryos from 100-d-old gilts are developmentally competent and able to send the appropriate signals to establish and maintain pregnancy.
From the combined results of Exp. 1, 2, and 3, it is concluded that it is the combination of prepubertal embryos and a prepubertal uterus that leads to pregnancy loss. Before conducting these experiments, it was anticipated that the embryos and/or the uteri of prepubertal gilts were unable to produce pregnancies. Peters et al. (2000) found that embryos from 100-d-old gilts were less developmentally competent in vitro than embryos from postpubertal donors. Murray and Griffo (1976) and Groothuis et al. (1997) reported that uteri of prepubertal gilts secrete less of several components in the histotroph in response to exogenous steroids, compared to peri- or postpubertal gilts although the major constituents appear but in reduced quantities by 90-d of age. Therefore, it appears that both the embryos and uteri of 100-d-old gilts are marginally competent for pregnancy establishment and maintenance.
Because both the uterus and conceptuses participate in establishing pregnancy in pigs (Bazer et al., 1984), and because the numbers of embryos produced by 100-d-old gilts are low and variable (present experiments and Peters et al., 2000), it may be that pregnancy signals generally fail to reach the required threshold for pregnancy recognition in 100-d-old gilts. In postpubertal pigs, the minimal number of conceptuses required to maintain pregnancy is four (Polge et al., 1966). However in 100-d-old gilts, the required number of conceptuses could be considerably higher. Alternatively, the combination of the uterus and conceptuses of prepubertal gilts may not be capable of pregnancy maintenance.
Regarding the results of Exp. 2 and 3, it may be that the postpubertal uterus is able to compensate for a marginal prepubertal embryo transferred into it and that a relatively robust postpubertal embryo is able to overcome deficiencies in a prepubertal uterine environment when transferred into the prepubertal gilt. A physiological explanation for such compensating effects could be the redundancy in source of some products of pregnancy. For example, both the uterus and conceptus produce prostaglandins (Rosenkrans et al., 1992; Davis and Blair, 1993) and retinol binding protein (Trout et al., 1991; 1992). For these products a uterine source might be able to compensate for inadequate embryonic production and vice versa.
Such compensatory abilities of the uterus and conceptus have not been evaluated. However, compensation might also function to rescue marginal embryos or overcome marginal uterine environments in postpubertal females. If compensation is a valid concept, it may be that improving either the uterus or the embryos of postpubertal females will be useful for enhancing embryonic survival in postpubertal females. It follows from this reasoning that the constituents produced by both the conceptus and uterus have potential to provide compensation.
In Exp. 3, the prepubertal donor gilts had fewer CL and provided fewer embryos. As a result, fewer embryos were available for transfer. Low responsiveness of prepubertal gilts to PG600 was anticipated based on earlier work (Peters et al., 2000). To overcome the limited supply of prepubertal embryos, uterine horns of recipients were surgically separated and embryos from pre- and postpubertal gilts were transferred to separate but adjacent uterine horns within the same postpubertal gilt. Therefore, the embryos produced by prepubertal gilts may have benefited from the postpubertal embryos in the adjacent uterine horn within the same pregnant gilt. There are no reports of such a mechanism operating in pig pregnancies, but intercornual effects cannot be ruled out. Such effects could be tested by an experiment comparing pregnancy survival in recipients of 100-d embryos to both uterine horns with transfers of 100-d and postpubertal embryos to opposite horns, as was done in Exp. 3.
Estrogen is considered to be the signal for pregnancy recognition in pigs (Geisert et al., 1987), but an earlier report (Rampacek and Kraeling, 1978) showed that hysterectomized prepubertal gilts failed to maintain CL if estrogen was administered. The authors concluded that the hypothalamic-pituitary axis of prepubertal gilts is more sensitive to estrogen’s negative feedback effects and that the resulting decrease in LH secretion probably contributes to CL failure. In Exp. 2, embryos produced by postpubertal gilts and transferred to prepubertal gilt uteri initiated and maintained pregnancy. Therefore, an inappropriate response to estrogen is not an adequate explanation for lack of pregnancy recognition in prepubertal gilts. It may be that the exogenous estrogen administered by Rampacek and Kraeling (1978) provided a pharmacological stimulus that is not encountered in pregnancy. Growth of the conceptuses, as revealed by measurements at slaughter, provides additional information on the capabilities of prepubertal uteri and conceptuses. Embryos transferred from postpubertal gilts into 100-d-old gilts survived at least as well (57%) as when transferred to peripubertal and postpubertal recipients (50% and 33%, respectively). However, fetuses and placentas developing in 100-d-old and peripubertal gilts were smaller than those developing in postpubertal gilts (Figure 4). These results suggest inadequacies in the maternal environment provided by young gilts.
Regarding uterine and conceptus function, it is interesting that, in Exp. 1, 120-d-old gilts had approximately half the uterine weight and half the number of fetuses of 170-d-old gilts and their fetal and placental weights were similar. From our data we cannot determine whether 120-d-old gilts would maintain fetal and placental weights if they gestated more fetuses or whether their uterine weights would increase in proportion to increases in fetal number.
In Exp. 3, the fetuses developing from 100-d-old gilt embryos after transfer to postpubertal gilts tended to be smaller than fetuses developing from postpubertal gilt embryos. Retarded fetal growth also was observed in prepubertal gilts by Puglisi et al. (1978), resulting in neonatal pigs that were smaller than usual but otherwise normal.
Reduced conceptus growth in Exp. 2 could have resulted from the reduced secretion of constituents of the histotroph by the prepubertal gilt’s uterus, as reported by Murray and Griffo (1976) and Groothuis et al. (1997). The latter authors found that retinol binding protein and uteroferrin were reduced in prepubertal gilts treated with progesterone. These or other transport proteins, if insufficient, might limit placental and fetal growth. Other possible problems in prepubertal gilt pregnancies include aspects of signaling at the maternal-trophoblast interface. In this regard, Wegmann et al. (1993) proposed that intrauterine growth retardation might be due to failure of proper cytokine production and/or interaction at the maternal-placental interface. Activation of T-cells occurs in the endometrium (Wegmann, 1987), and the uterine immune system of the female pig responds to the presence of conceptuses (Engelhardt et al., 1997). Therefore, inadequacies of cytokine signaling also could contribute to the fetal growth reduction in prepubertal gilts.
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Implications
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Results of these experiments were not anticipated based on previous work, therefore they introduce new ideas about pregnancy establishment and conceptus development in the pig. Although intimate interactions between the conceptus and uterus have been appreciated for some time, the concept that either component can compensate for the other had heretofore not been suggested. If this concept is correct, it is possible that enhancing either the conceptus or the uterine component can increase embryo survival in postpubertal females. Regarding fetal and extra-embryonic development, the prepubertal gilt may provide a model for placental insufficiency as a contributor to decreased growth, and perhaps quality, of pigs and other livestock. It is also possible that pregnancy in prepubertal gilts could be a model for intrauterine growth retardation in humans.
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Footnotes
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1 Contribution No.01-410-J from the Kansas Agric. Exp. Stn. 
2 We appreciate the assistance of T. Rathbun and R. Mosteller in conducting this research and E. Specht in manuscript preparation.
3 Correspondence: 256 Weber Hall (phone: 785-532-1224; fax: 785-532-7059; E-mail ddavis{at}oznet.ksu.edu).
Received for publication April 22, 2003.
Accepted for publication July 11, 2003.
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Abstract
Introduction
