Synthetic microspheres transferred to the rat oviduct on day 1 of pregnancy mimic the transport of native ova

Microspheres of various materials and diameters were transferred microsurgically to the rat oviduct on Day 1 of pregnancy and autopsies were done at various times thereafter up to Day 10 to assess the recovery and segmental distribution of microspheres and eggs in the genital tract or the viability of embryos. The number and distribution of eggs in the treated and control sides after unilateral transfers were not different on Day 4 and 5 and the number of embryos implanted on Day 10 was not significantly affected after bilateral transfers. The segmental distribution of eggs and starch microspheres within the oviduct on Days 2, 3 or 4 showed that both are transported partly intermingled from ampulla to uterus. When microspheres of poly(DL-lactide-co-glycolide), starch, dextran or dextran blue were transferred, their distribution in the genital tract in the morning of Day 5 showed that poly(DL-lactide-co-glycolide) and dextran microspheres stayed longer in the oviduct while starch and dextran blue microspheres were transported to the uterus at the same time as the eggs. Transfer of starch microspheres of 40-60 microns to one oviduct and 180-200 microns in diameter to the opposite oviduct showed that distribution on one side was nearly identical to that of the other side from Days 2 to 5. We conclude that the behaviour of synthetic surrogate ova in rats differs from that in rabbits. The rat oviduct does not change the rate of transport of native eggs following transfer of synthetic surrogate ova.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prostaglandin E2-specific binding to the equine oviduct.

Prostaglandin E2 (PGE2) bound specifically (P less than 0.001) to ampullary and isthmic tissue on Day 2 and Day 5 after ovulation. No significant differences (P greater than 0.8) were detected between Day 2 and Day 5 in the specific binding of ampullary or isthmic tissue. Significantly more (P less than 0.05) PGE2 bound specifically to ampullary versus isthmic tissue on both days. Detection of PGE2-specific binding in the oviductal isthmus on Day 2 and Day 5 indicates that the oviduct is responsive to PGE2 when it is capable of transporting equine embryos.     

Egg transfer in the Mongolian gerbil (Meriones unguiculatus) during lactational delay of implantation

Lactating pregnant gerbils in which one oviduct was ligated during early pregnancy were used on Day 6-14 p.c. as recipients for eggs (morulae and blastocysts) recovered on Day 6-12 p.c. from lactating donors. Eggs were transferred to the ligated (experimental) horn and the litter was removed. About half of the recipients had implantations in the non-ligated (control) horn 15 days after litter removal. Of these 40 females, 37 had implantations in the experimental horn and 32 had some normal fetuses present. Of 179 eggs transferred into the ligated uteri, 61% implanted and 41% developed into normal fetuses. The time after copulation of recipient and donor did not appear to influence the number of implantations or post-implantation development of the transferred eggs.     

Embryos of different ages transferred to the rat oviduct enter the uterus at different times

Indirect evidence of embryo signalling to the oviduct was sought in rats by examining the transport of embryos of different ages. One-cell or four-cell embryos were transferred to the oviducts of recipient rats on Day 1 of pregnancy, and the number, condition, and location of native and transferred embryos was assessed on Day 4. To control for the effect of the presence of foreign embryos and excess number of eggs and the transfer procedure upon the fate of native embryos, other groups of rats were sham-operated or left undisturbed. Recipients had a mean number of ova significantly higher than controls. In controls and recipients of 1-cell embryos, the majority of eggs reached the morula stage and all of them were located in the oviducts. In those animals receiving 4-cell embryos, half of the eggs had reached the blastocyst stage and 28% were in the uteri (p less than 0.005). These results support the idea that advanced embryos can influence the timing of their entrance to the uterus in rats.     

Effect of embryonic treatment with oestradiol benzoate on reproductive morphology, ovulation and oviposition and plasma LH concentrations in female quail (Coturnix coturnix japonica)

Quail eggs were injected on Day 10 of incubation with 0, 5, 10, 20 or 40 micrograms oestradiol benzoate. Females hatching from these eggs were reared on a 16L: 8D photoperiod and egg laying was recorded. Blood samples were taken at 37, 40, 43, 46, 49, 52, 55, 58 or 61 days of age and LH concentrations were measured by a double-antibody radioimmunoassay. Birds were killed at 61 days of age; ovaries and oviducts were weighed and examined. Egg laying was greatly reduced by oestradiol benzoate treatment, but for birds that did lay, age at first oviposition was normal. LH levels were not affected by oestradiol benzoate treatment, and were highest at 40 and 49 days of age. Oestradiol benzoate had no effect on ovarian weight, number of follicles with diameter greater than 1 cm, or number of post-ovulatory follicles. Oestradiol benzoate had a dose-related effect on the likelihood that females would have two oviducts, and for those females that had retained the right oviduct, the left oviduct was smaller than normal. Oestradiol benzoate-treated females were more likely to have ovulated yolks in the body cavity. Embryonic treatment with oestradiol benzoate therefore appears to inhibit egg laying by causing oviduct abnormalities, rather than by (as happens in mammals) inhibiting ovulation.     

Blood flow in the ovary and oviduct of rats after sympathetic denervation

The effect of sympathetic denervation on blood flow in the ovary and oviduct was studied in rats undergoing oestrous cycles or at Day 14 of pregnancy. The ovary and oviduct on one side were denervated by briefly freezing the ovarian vascular pedicle and the ovarian suspensory ligament. Blood flow was measured using 15 microns 57Co-labelled microspheres while the rats were under barbiturate anaesthesia. In cyclic rats denervation raised blood flow to the oviduct by 90% the next day (P less than 0.01) and 39% at 4-10 days (0.05 less than P less than 0.1). Blood flow to the ovary was not affected. Denervation on Day 13 of pregnancy raised blood flow in the oviduct 5-fold at Day 14 (P less than 0.01) and denervation on Day 7 raised blood flow 3-fold on Day 14 (P less than 0.05). Blood flow to the luteal and non-luteal components of the ovary was not affected. Sham-operation did not affect blood flow in the oviduct or ovary. It is concluded that sympathetic nerves exert tonic vasoconstrictor control on the vasculature of the oviduct but not on that of the ovary, and that these nerves do not regulate the major changes in blood flow that occur in ovaries in various physiological states.     

In vitro and in vivo association of an oviduct estrus-associated protein with bovine zona pellucida

The interaction between the bovine egg zona pellucida and a 97 kDa estrus-associated protein produced by the oviduct was examined in vitro and in vivo. In vitro matured bovine eggs were incubated with oviduct fluid recovered throughout the estrous cycle from separate indwelling cannulae placed in the ampulla and isthmus of the same oviduct. Immunofluorescence techniques and a polyclonal antiserum against the 97 kDa protein were used to localize this protein on washed eggs previously incubated with oviduct fluid. Intensity and distribution of immunofluorescence varied with stage of cycle and to a lesser degree with region of oviduct from which the oviduct fluid was obtained. The most intense fluorescence was observed on the zonae pellucidae of eggs incubated with oviduct fluid pooled from days near estrus and ovulation compared to fluid pooled from luteal stage days. The immunofluorescence of isthmus-derived oviduct fluid was more intense than was ampulla-derived oviduct fluid collected near estrus. The zonae pellucidae of 7-day-old embryos flushed from the uterus displayed immunofluorescence comparable to that observed on the zonae pellucidae of eggs incubated in vitro with peri-estrus oviduct fluid. No immunofluorescence was observed associated with the perivitelline space, egg cytoplasm, or blastomeres. The apparent uptake of a 97 kDa estrus-associated protein by the zonae pellucidae of eggs in vitro and embryos in vivo may indicate that this protein functions in fertilization and/or early embryo development.     

Conceptus-mediated integrity of endometrial epithelial cells and maintenance of relaxin synthesis in pregnant rabbits: effects of unilateral oviduct ligation

A previous study indicated rabbit endometrial relaxin synthesis is stimulated by blastocyst (Lee VH, Fields PA, Biol Reprod 1990; 40:737-745). To evaluate this hypothesis, unilateral oviduct ligations were placed (A) at the oviduct isthmus on Day 1 post-copulation and (B), in a separate group of rabbits, at the infundibulum before copulation. Blastocysts migrate into and implant in the uterine horn contralateral to the ligated oviduct only (conceptus-bearing uterus). The uterine horn ipsilateral to the ligated oviduct will be referred to as the non-conceptus-bearing uterus. Uteri and ovaries were removed on Days 4-28 of pregnancy and were evaluated for relaxin using guinea pig anti-porcine relaxin serum and avidin-biotin light microscopy immunohistochemistry. Results were identical for both models. Blastocysts first attach to the antimesometrial uterine surface by Day 7 post-copulation. Implantation on the mesometrial surface occurs on Days 8-11. Relaxin was observed in antimesometrial endometrial glands of both conceptus and non-conceptus-bearing uteri on Days 4-7 of pregnancy. Beyond Day 7, relaxin was observed in antimesometrial and mesometrial endometrial glandular and luminal epithelial cells at implantation sites of the conceptus-bearing uterus only. Relaxin was not found between implantation sites. Endometrial epithelial cells of the non-conceptus-bearing uterus were regressing by Day 9. These data indicate a conceptus-mediated maintenance of endometrial epithelial cells. Furthermore, the data suggest a paracrine maintenance of epithelial cell integrity and relaxin synthesis since these parameters are preserved only in the conceptus-bearing uterus. Cell-cell communication between conceptus and endometrium appears to be specific since endometrium between implantation sites does not contain relaxin. Uterine tissue from pseudopregnant rabbits (Days 1-16) was evaluated. Relaxin was observed in the antimesometrial glands on Day 7 only. Like the endometrium in the ligation model, endometrial epithelial cells of the pseudopregnant rabbit uterus were regressing by Day 9. These results indicate that pregnancy is not required for, but may enhance, relaxin synthesis. In addition, endometrial epithelial cells regress in the absence of pregnancy. Regression of endometrial epithelial cells on Day 9 suggests that maternal recognition of pregnancy occurs during the preimplantation period (Days 4-8).     

Culture of in vitro produced bovine zygotes in vitro vs in vivo: implications for early embryo development and quality

The objectives of this study were to examine the effect of culture system on bovine blastocyst formation rates and quality. Presumptive IVM/IVF bovine zygotes were cultured either in vitro in synthetic oviduct fluid (SOF, 25 embryos/25 microL in 5% CO2, 5% O2, 90% N2 at 39 degrees C) or in vivo in the ewe oviduct (approximately 100 embryos per oviduct). The recovery rate after in vivo culture was 53% (813/1,530). The blastocyst rate on Day 7 was significantly higher for the in vitro system (28%, 362/1,278 vs 17%, 37/813; P< 0.0001). However, after culture in vitro for a further 24 h, there was no difference in Day 8 yields (36%, 457/1,278 vs 32%, 258/813, for in vitro and in vivo culture, respectively). There was no difference in blastocyst cell number between treatments (Day 7: 96 vs 103; Day 8: 78 vs 85 for in vitro and in vivo culture, respectively). Irrespective of culture system, Day 7 blastocysts had a significantly higher cell number than those appearing on Day 8. There was no difference in pregnancy rate at Day 35 after fresh transfer of a single Day 7 blastocyst (37.5%, 21/56 vs 45.3%/, 24/53 for in vitro and in vivo culture, respectively). After cryopreservation by freezing in 10% glycerol, VS3a vitrification or solid surface vitrification, the survival of in vitro cultured embryos was significantly lower than survival of embryos cultured in the ewe oviduct or those produced by superovulation of donors. In conclusion, these findings demonstrate that while bovine zygotes cultured in vitro are capable of rates of development similar to those of their in vivo cultured counterparts (in terms of Day 8 blastocyst yield, cell number and early pregnancy rate), there are significant differences in embryo cryosurvival. This suggests that current in vitro culture systems need to be improved to optimize embryo quality and pregnancy rates.     

Effects of delayed transfer and treatment with oestrogen on the transport of microspheres by the rat oviduct

Starch or dextran blue microspheres were transferred microsurgically to the infundibulum of the oviduct on Days 1, 2, or 3 of pregnancy of control and oestradiol-treated rats. The animals were killed a few hours to several days after transfer to assess the number and distribution of ova and microspheres in the tract. After transfer on Day 1 of pregnancy, microspheres and eggs crossed the ampullary-isthmic junction (AIJ) 18 h after ovulation. After transfer on Day 2 of pregnancy, more than 50% of microspheres were retained in the ampulla, indicating that the AIJ changes again 34 h after ovulation. Treatment with oestradiol did not advance the passage of eggs or microspheres across the AIJ but caused accelerated transport through the isthmus as soon as the eggs or microspheres reached this segment. Dextran blue microspheres were seen to move back and forth in the isthmus of control anaesthetized rats at a frequency of 5-6 times/min. Between 7 and 20 h after treatment with oestradiol the frequency of these movements was significantly augmented, indicating that increased frequency of contractions of the smooth muscle of the isthmus precedes and accompanies accelerated transport of ova through this segment.     

Salmonella enterica Serovar Enteritidis Genes Induced during Oviduct Colonization and Egg Contamination in Laying Hens

Salmonella enterica serovar Enteritidis is the predominant serovar associated with salmonellosis worldwide, which is in part due to its ability to contaminate the internal contents of the hen's egg. It has been shown that S. enterica serovar Enteritidis has an unusual tropism for the avian reproductive tract and an ability to persist in the oviduct and ovary. Factors allowing S. enterica serovar Enteritidis strains to contaminate eggs could be a specific interaction with the oviduct tissue, leading to persisting oviduct colonization. In vivo expression technology, a promoter-trap strategy, was used to identify genes expressed during oviduct colonization and egg contamination with S. enterica serovar Enteritidis. A total of 25 clones with in vivo-induced promoters were isolated from the oviduct tissue and from laid eggs. Among the 25 clones, 7 were isolated from both the oviducts and the eggs. DNA sequencing of the cloned promoters revealed that genes involved in amino acid and nucleic acid metabolism, motility, cell wall integrity, and stress responses were highly expressed in the reproductive tract tissues of laying hens.     

Oviduct-specific expression of tissue plasminogen activator in laying hens

Egg-laying hens are important candidate bioreactors for pharmaceutical protein production because of the amenability of their eggs for protein expression. In this study, we constructed an oviduct-specific vector containing tissue plasminogen activator (tPA) protein and green fluorescent protein (pL-2.8OVtPAGFP) and assessed its expression in vitro and in vivo. Oviduct epithelial and 3T3 cells were cultured and transfected with pL-2.8OVtPAGFP and pEGP-N1 (control vector), respectively. The pL-2.8OVtPAGFP vector was administered to laying hens via a wing vein and their eggs and tissues were examined for tPA expression. The oviduct-specific vector pL-2.8OVtPAGFP was expressed only in oviduct epithelial cells whereas pEGP-N1 was detected in oviduct epithelial and 3T3 cells. Western blotting detected a 89 kDa band corresponding to tPA in egg white and oviduct epithelial cells, thus confirming expression of the protein. The amount of tPAGFP in eggs ranged 9 to 41 ng/mL on the third day after vector injection. The tPA expressed in egg white and oviduct epithelial cells showed fibrinolytic activity, indicating that the protein was expressed in active form. GFP was observed only in oviducts, with no detection in heart, muscle, liver and intestine. This is the first study to report the expression of tPA in egg white and oviduct epithelial cells using an oviduct-specific vector.     

Hastened transport of equine embryos through the oviduct of the mare

The purposes of this experiment were 1) to test the hypothesis that placing rabbit embryos into the mare's uterus would hasten oviduct transport and 2) to determine if placing fluid into the uterus of bred mares on Day 4 and/or Day 5 would subsequently disrupt the mare's pregnancy. The hypothesis that placing rabbit embryos into the mare's uterus would hasten oviduct transport was not supported, since the uterine recovery rate of equine embryos on Day 5 was not significantly higher (P>0.05) for mares receiving rabbit embryos on Day 4 than for mares receiving no uterine infusion on Day 4 (1 10 vs 0 10 , respectively). However, placing fluid into the mare's uterus on Day 4 was apparently responsible for hastened oviduct transport, since mares with media infused into the uterus on Day 4 had a significantly higher (P<0.05) recovery rate of equine embryos on Day 5 than did mares receiving either rabbit embryos or no uterine infusion on Day 4 post ovulation (5 10 vs 1 10 or 0 10 , respectively). The Day-14 pregnancy rate was significantly higher (P<0.05) for mares receiving no uterine infusion on Day 4 or Day 5 than for mares receiving uterine infusion on Day 5 or uterine infusion on both Days 4 and 5 (9 10 vs 4 10 , 2 10 and 0 10 , respectively).     

Calcium-regulated fusion of yolk granules during early embryogenesis of Periplaneta americana

This work reported membrane fusion of yolk granules (YGs) during early embryogenesis of the insect Periplaneta americana (P. americana). We showed that eggs from Day 5 of embryogenesis possess a greater amount of enlarged YGs in comparison with Day 1. Day 5 is also the period when the largest amount of free calcium is found (approximately 17 mM) within the oothecae from early embryogenesis. Treatment of Day 1-YGs fraction with 17 mM Ca2+ resulted in a YG size pattern very similar to the one observed in Day 5 eggs, where enlarged YGs were formed. YG membrane fusion was observed by fluorescent membrane dye transfer from previously labeled small YGs to larger ones and was also visualized by electron microscopy. We also showed that the small "in fusion" YGs seemed to be acidic, suggesting that acidification is correlated with YG membrane fusion. Hence, it was shown that YGs are capable of membrane fusion in a calcium-dependent manner and this process probably occurs in vivo during early embryogenesis of P. americana.     

Ovulation and early embryogenesis in swine

Thirty gilts were used to examine if the sequence in which oocytes were released at ovulation contributed to differences in embryonic development and uterine secretions by Day 12 (Day 0 = onset of estrus). Oocytes of follicles destined to ovulate last were recovered 42 h after injecting proestrous gilts with hCG, incubated with a fluorescent stain, and returned to the donor's oviduct. These later-maturing oocytes subsequently became the lesser-developed (p less than 0.01) embryos on Day 4. In a second experiment, lesser- vs. more-developed Day 4 embryos from additional gilts were transferred to ligated uterine horns of nonpregnant gilts. Subsequently, the lesser-developed Day 4 embryos became the smaller (p less than 0.01) blastocysts within a litter on Day 12. Uterine flushings associated with lesser-developed embryos on Day 12 contained less estradiol (p less than 0.01), less total protein (p less than 0.10), and less acid phosphatase activity (p less than 0.05), but total content of calcium was not different compared to flushings that contained more-developed embryos. Analysis of uterine flushings with two-dimensional PAGE procedures indicated advanced uteroferrin-associated glycoprotein secretion from the horn that contained more-developed embryos. Results of these experiments suggested that oocytes of later-ovulating follicles were progenitors of smaller embryos, which probably stimulated uterine secretion later than more advanced littermates on Day 12.     

Studies of the Levamisole inhibitory effect on rat stromal-cell commitment to mineralization

The ability of Levamisole to decrease mineralization in skeletal tissue is usually related to its effect on alkaline phosphatase (ALP). However, Levamisole is also suspected to diminish mineralization by an additional mechanism which is unrelated to the ALP control of apatite crystal growth. To delineate the time in differentiation during which Levamisole inhibits mineralization, a tissue culture model system of bone marrow stromal cells was used. Secondary cultures of stromal cells were propagated in osteoprogenitor cell (OPC) induction medium for three weeks, followed by measurement of calcium precipitation. In situ ALP assays at pH 7.6 were also performed. When cells were cultured with 0.2 mM Levamisole for three weeks, Day 20 values of calcium precipitates were lower than in controls, but Day 20 ALP values were paradoxically higher. The correlation between calcium and ALP within each group was low. The correlation slightly improved, in uninhibited cultures, when Day 21 calcium values were matched with earlier Day 12 ALP values. This suggested the existence of a Levamisole-sensitive mechanism for mineralization inhibition effective prior to the culture's mineralization stage. To focus on this early effect on mineralization Levamisole was added to stromal cultures on different days and removed on Day 12. Levamisole decreased Day 21 mineralization when added on Days 0, 3, 5, and 7, but not when added on Day 9. The Levamisole-induced inhibition of mineralization was accompanied by an increase in Day 12 ALP specific activity, compared to controls, when added from Day 5 and thereafter. The results indicate that part of the ability of stromal cells to mineralize is determined during the first week of culture. The early inhibitory effect of Levamisole on mineralization was associated with increased Day 12 ALP activity.     

Ion concentrations in oviduct and uterine fluid and blood serum during the estrous cycle in the bovine

In the bovine up to 40% of embryos die before implantation but despite the importance of ions in oviduct and uterine fluid formation and in gamete, zygote and early embryo development there is very little published information on the ion concentrations of oviduct or uterine fluid. The free anions chloride, phosphate and sulphate and the free cations sodium, calcium, magnesium and potassium were measured in oviduct fluid on days 0, 2, 4 and 6 and in uterine fluid on days 6, 8 and 14 and in corresponding blood samples. Oviduct and uterine fluids were collected in situ. Sodium was 25-fold higher than potassium and 80-fold higher than the other ions and chloride was 10-fold higher than potassium and 40-fold higher than the other ions in oviduct and uterine fluid. Phosphate, sulphate, magnesium, potassium and calcium were at lower concentrations in all fluids. Oviduct calcium and sodium were higher on day 0 than other days. The most striking uterine differences were the higher potassium and lower chloride, sodium and magnesium on day 14 than other days. There were significant positive associations between oviduct and blood chloride, sulphate, magnesium and calcium while only uterine sulphate was positively related to its blood concentration. There was no relationship between fluid secretion rate and no association between the concentrations of systemic progesterone or oestradiol and any ion in oviduct or uterine fluid. The different concentrations and associations between ions in the oviduct, uterus and blood suggest a differential regulation of ion secretion by the oviduct and uterine epithelia.     

The sperm chemoattractant "allurin" is expressed and secreted from the Xenopus oviduct in a hormone-regulated manner

Recently, we cloned and sequenced the cDNA of allurin, a sperm chemoattractant isolated from the jelly of Xenopus laevis eggs [Proc. Natl. Acad. Sci. U.S.A. 78 (2001) 11205]. In this report, we demonstrate that allurin mRNA is expressed almost exclusively in the oviduct and that its expression is increased 2.5-fold by human chorionic gonadotropin over a 12-h period. Both dot blots and immunocytochemistry show that allurin is secreted from the upper two thirds of the oviduct that includes the pars recta and the proximal pars convoluta. Allurin appears to be deposited on the ciliated surfaces of luminal epithelial cells that come in direct contact with eggs as they move through the oviduct. Immune staining also demonstrates the presence of allurin in the serosal capsule of the oviduct. In contrast, allurin is not found within the tubular jelly-secreting glands or ducts that constitute a major portion of the oviduct wall. Therefore, we hypothesize that allurin is synthesized by nonciliated secretory cells in the luminal epithelium of the oviduct, is displayed on the ciliary layer and then mechanically mixed with jelly, and applied to eggs as they progress down the oviduct. This hypothesis is consistent with the fact that eggs progressing down the oviduct initially show evidence of allurin being incorporated into the J1 layer. Subsequently, allurin within J1 diffuses outward to J3 and eggs stored in the uterus now demonstrate a J3 localization of this chemoattractant.     

Progesterone abbreviates the nuclear retention of estrogen receptor in the rat oviduct and counteracts estrogen action on egg transport

Progesterone has synergistic or antagonistic effects on several estrogenic actions. The effects of progesterone on estrogen-induced accelerated ovum transport and on the dynamics of estrogen receptors in the rat oviduct were examined. The involvement of the progesterone receptors in these phenomena was assessed. On Day 1 of pregnancy, rats were treated with estradiol, estradiol plus progesterone, or either one plus the progesterone receptor-blocking agent RU486. Control animals received the oil vehicle alone. The number of eggs remaining in the oviduct was assessed 24 h after treatment. Cytoplasmic and nuclear estrogen receptor levels in the oviduct, as well as plasma concentrations of estradiol and progesterone, were measured at various intervals--up to 11 h and 24 h after treatment, respectively. Accelerated oviductal egg transport induced by estrogen was blocked by the concomitant administration of progesterone. This effect of progesterone was not associated with changes in estrogen circulating levels and was preceded by a reduction in the total amount of estrogen receptors and by a shortened retention of estrogen receptors in the nucleus. The effects of progesterone on egg transport and on the levels of estrogen receptors were reversed by blocking the progesterone receptor with RU486, suggesting that both effects were receptor-mediated. These findings demonstrate that progesterone antagonizes the effect of estrogen on oviductal egg transport in the rat, and suggest that this antagonism is mediated by a reduction both in the amount of estrogen receptors and in their retention time in the nucleus.     

Autotransfer of Day 4 embryos from oviduct to oviduct versus oviduct to uterus in the mare

Embryo autotransfer is defined as the collection of an embryo from and the transfer of this embryo into the same animal. The objectives of this study were to: 1) test the hypothesis that oviduct transport of the equine embryo from the oviduct into the uterus is not dependent on a unilateral embryo-corpus luteum interaction, 2) develop an embryo autotransfer technique for the mare and 3) compare the success rates of Day 4 embryos surgically autotransferred from the oviduct ipsilateral to ovulation to either the oviduct (n=10 mares) or the uterine horn (n=10 mares) contralateral to ovulation. Seventy percent (7 10 ) of the Day 4 embryos which were autotransferred to the oviduct contralateral to ovulation were transported through the oviduct and subsequently developed into embryonic vesicles detectable by ultrasonography between 10 and 21 days postovulation. This finding supported the hypothesis that oviductal embryo transport is not dependent upon the ipsilateral corpus luteum. Overall, sixty percent (12 20 ) of the autotransfers were successful. The success rate of uterine-transferred embryos was not significantly less (P>0.3) than that of oviductal-transferred embryos (5 10 vs 7 10 , respectively). Therefore, the Day 4 equine embryos were apparently mature enough to survive in the mare's uterus.