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Biology of Reproduction 60, 1483-1487 (1999)
©Copyright 1999 Society for the Study of Reproduction, Inc.

Articles

Retinol Administration to Superovulated Ewes Improves In Vitro Embryonic Viability1

Dawn M. Eberhardta, Whitney A. Willa, and James D. Godkin2,a

a Department of Animal Science, The University of Tennessee, Knoxville, Tennessee 37901


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Retinol and its metabolites, all-trans retinoic acid and 9-cis retinoid acid, are regulators of cellular growth, differentiation, and development and have been implicated in reproductive processes including folliculogenesis and embryonic survival. Three experiments were conducted to identify effects of retinoid treatment of superovulated ewes upon subsequent in vitro embryonic development. Ewes were treated with all-trans retinol (ROH), all-trans retinoic acid (RA), 9-cis retinoic acid (CIS), or vehicle (Control) on the first and last day of FSH treatment. Embryos were recovered at the morula stage, cultured in vitro for 96 h, and observed for blastocyst formation. Embryos from ROH-treated animals had a higher (p < 0.01) incidence of blastocyst formation than RA-, CIS-, or vehicle-treated animals (72% vs. 27%, 33% and 32%, respectively). In experiment 2, ewes were given ROH or vehicle and treated as above. ROH treatment resulted in an increased percentage of embryos forming blastocysts (70% vs. 22%, p < 0.05). In experiment 3, ewes were treated with ROH or vehicle, and embryos were collected at the 1- to 4-cell stage and cultured for 7 days. ROH treatment resulted in increased blastocyst formation (79% vs. 5%, p < 0.05). The majority of embryos (60% vs. 6%; p < 0.01)) from vehicle-treated animals failed to develop beyond the 8-cell stage in comparison with those from ROH animals. ROH treatment of superovulated ewes increased embryonic viability and positively impacted embryonic development.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Retinol and its cellular metabolites, all-trans retinoic acid and 9-cis retinoic acid, are collectively known as retinoids. These compounds influence embryonic morphogenesis, cell growth, and differentiation in many cell types including embryonic stem cells and embryo carcinoma cells. Differentiation induced by retinoids in vitro is accompanied by specific changes in expression of homeobox genes, growth factors, and their receptors (reviewed in [1]).

Systemic and intercellular transport of retinol is accomplished via a specific protein, retinol-binding protein (RBP). Cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP) are involved in intracellular retinol homeostasis (reviewed in [2]). CRBP has three major functions: retinol accumulation; stimulation of retinol mobilization from retinol ester stores; and delivery of the retinol, via direct transfer to dehydrogenase, for conversion to retinal and eventually retinoic acid, which can then undergo conversion to 9-cis retinoid acid. The cellular transport/metabolism of retinoic acid is accomplished via the small cytoplasmic proteins cellular retinoic acid-binding proteins, CRABP and CRABP-2. The actions of retinoic acid are mediated through two subgroups of nuclear receptors, retinoic acid receptors and retinoid X receptors. Ligand-receptor complexes initiate gene activation or repression through association with specific response elements found in promoter regions of target genes [3].

Retinoids are essential to reproduction in both males and females [4]. Deficiencies in vitamin A lead to decreased ovarian size, decreased ovarian steroid concentrations, abortion, and eventually reproductive senescence. Several studies indicate a positive effect of retinol supplementation when diets are adequate in vitamin A. In litter-bearing species, administration of retinol or ß-carotene has been reported to increase embryo survival in mice [5], rabbits [6], and swine [7–9]. In cattle, retinol administration, in combination with superovulation, increased the number of transferable embryos but did not affect ovulation rate in comparison with that in superovulated control animals [10]. Three experiments were conducted to identify the effect of retinoid treatment in combination with superovulation and natural service upon subsequent ovine embryonic development. The retinoids utilized in this study were chosen because retinol exerts its effect via interaction between its biological active metabolites all-trans retinoic acid and 9-cis retinoic acid and nuclear receptors.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Materials

Lutalyse was purchased from the Upjohn Company (Kalamazoo, MI). Synchromate B was purchased from Rhone Merieux, Inc. (Athens, GA); 9-cis retinoic acid was kindly provided by Hoffmann La Roche (Nutley, NJ). Porcine FSH was purchased from Sioux Biochemical (Sioux City, IA). Fetal bovine serum (FBS) was purchased from Atlanta Biologicals (Norcross, GA). Synthetic oviductal fluid (SOF) was purchased from Specialty Media, Inc. (Lavallette, NJ). Falcon organ culture dishes were purchased from Fisher Scientific (Pittsburgh, PA). All-trans retinol and all-trans retinoic acid and all other reagents were purchased from Sigma Chemical Co. (St. Louis, MO).

Animals

Estrous cycles of sexually mature crossbred ewes were synchronized using progestin implants (Synchromate B) combined with prostaglandin F2{alpha} (Lutalyse) injections, and superovulation was induced by multiple FSH injections. Briefly, animals were administered one implant and 6 days later received two Lutalyse injections (15 mg i.m.) 12 h apart. Superovulation was induced using a total of 24 units of FSH administered twice daily in decreasing doses over 3 days (5.5, 4.4, 3.3 units per injection, respectively) beginning 9–11 days after implant administration. Retinoid treatments were administered on the first and last day of FSH injections. Implants were removed at the time of the fifth FSH injection, and animals were checked for estrus 24 h later. Ewes exhibiting behavioral estrus were hand-bred to intact rams every 12 h until signs of estrus were no longer detected. All animals were maintained on high-quality hay and fed ad libitum, with free-choice access to a sheep and goat mineral premix that contained 1 million IU vitamin A per pound.

In the first experiment, performed under decreasing day length (fall), 25 ewes were randomly assigned to one of the following treatments: 1) all-trans retinol (ROH; 500 000 IU, n = 6); 2) all-trans retinoic acid (RA; 15 mg, n = 6); 3) 9-cis retinoic acid (CIS; 15 mg, n = 7); or 4) vehicle (Control; n = 6), which was corn oil. Animals were surgically ovohysterectomized at 144 h post-implant removal, and uteri were gently flushed twice with culture medium (tissue culture medium 199 [TCM 199]) to collect morula-stage embryos. Two ewes from the RA group were dropped from the study: one because of a total lack of response to the FSH treatment, the other because of overstimulation resulting in more than 50 ovulations with none of the ova fertilizing. This left 4 animals in the RA group, 5 in each of the ROH and Control groups, and 7 in the CIS group.

The second experiment was a repetition of the first, with the exceptions that it was performed under increasing day length (winter) and only the ROH and Control treatments were administered (in combination with FSH) to 24 ewes (12 per treatment) not used in the previous experiment. RA and CIS treatments were not utilized because of failure to improve embryonic viability in experiment 1. One ewe from the Control group was dropped from the study for failure to respond to FSH treatment.

The third study involved two identical experiments performed sequentially in the fall and winter. Results were not different between seasons, and the data were combined. A total of 24 ewes, not used in the previous experiments, received either ROH (n = 12) or Control (n = 12) treatment in combination with FSH, followed by natural mating at estrus, as in experiment 2. At 84 h post-implant removal, ewes were salphincectomized and oviducts gently flushed with culture medium in order to recover 1- to 4-cell embryos.

Ovulation and Fertilization Rate

At the time of embryo recovery, ovulation rate was determined by counting corpora lutea (CL) on each ovary. Embryo/oocyte recovery rates were determined by dividing the embryo/oocyte number by CL number. Fertilization rate was determined by dividing the number of cleaved embryos by the total number of embryos/oocytes recovered from each ewe.

Embryo Grading

Embryos (morulae) collected in experiments 1 and 2 were categorized according to morphology, developmental stage, and quality based on a procedure developed for bovine embryos [11]. Quality grades ranged from 1 to 4, with 1 = excellent, 2 = good, 3 = poor, 4 = degenerate. One individual (D.M.E.), who was unaware of treatments at the time, performed all of the grading.

Embryo Culture

In the first two experiments, morula-stage embryos were cultured in TCM 199 with Earle's salt supplemented with 10% FBS and 1 mM glutamine [12]. The FBS had been twice stripped of low molecular weight molecules with charcoal, and retinol concentrations were below detection levels as determined by fluorescent analysis [13]. In the third experiment, 1- to 4-cell embryos were cultured in SOF supplemented with 3 mg/ml BSA and essential and nonessential amino acids [14]. Both media were prepared weekly, filtered through a 0.2 µm filter, and allowed to equilibrate for 2 h in a humidified atmosphere at 38.5°C containing 5% CO2 in air.

Morula-stage embryos (experiments 1 and 2) were washed a minimum of three times in the outer well of organ culture dishes and then transferred with a minimum amount of medium into the inner well, which contained 3 ml of TCM 199. Embryos from each ewe were cultured in one dish, and there were no significant differences in the average number of embryos per dish between treatments. Embryos were cultured for 96 h and observed daily for blastocyst formation and complete hatching from the zona pellucida. No further development was observed after 72 h in culture, and all data presented reflect that time period.

In experiment 3, embryos were treated as above except that culture medium was SOF (see above). Embryos were observed every 48 h until embryos hatched or failed to develop for two consecutive viewings (168-h maximum).

Statistical Analyses

Data were checked for normality and analyzed using the Statistical Analysis System (SAS Institute Inc., Cary, NC). ANOVA was used with mixed models procedure (PROC MIXED) to detect differences in ovulation rate, embryo recovery rate, fertilization rate, embryonic quality, in vitro embryonic development to blastocyst, and embryonic hatching due to retinoid treatment. Differences due to retinoid treatment were tested utilizing protected least-significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Ovulation rate was not affected by retinoid treatment within any experiment or between experiments, and ranged from 8 to 33, with an average of 19.33 CL per ewe excluding data from the animal that did not respond to FSH treatment and the one animal from which > 50 unfertilized oocytes were recovered. Embryo/oocyte recovery rates were not different between treatments (p < 0.50) or experiments (p < 0.32) and ranged from 82% to 93%. Fertilization rate was also not influenced by retinoid treatment (p < 0.12) and ranged from 83% to 93%. No differences were observed in in vivo developmental stage at time of collection or in speed of development (progression to the next developmental stage) in vitro. Results from experiments performed during decreasing day length (fall) were not different from those for experiments performed during increasing day length (winter).

Retinol in combination with superovulation significantly improved embryonic viability as measured by blastocyst formation in vitro. In the first experiment, embryos were collected from 21 ewes treated with ROH (n = 5), RA (n = 4), CIS (n = 7), or Control (n = 5), resulting in 96, 84, 97, and 93 embryos per group, respectively. Embryos from each treatment were graded immediately after collection, and the score was not different between treatments (1.9 ± 0.1, 2.8 ± 0.2, 2.4 ± 0.2, and 2.1 ± 0.1 for ROH, RA, CIS, and Control, respectively). Embryos from the ROH-treated animals had a greater than 2-fold increase in vitro blastocyst formation than those from RA, CIS, or Control animals (72% vs. 27%, 33%, and 32%; p < 0.05) (Fig. 1A). In addition, ROH treatment improved (p < 0.05) embryonic hatching rates in vitro in comparison with the rates for CIS and Control animals but was not different from that for RA-treated animals (73%, 38%, 36%, and 55%, respectively) (Fig. 1B).



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  		FIG. 1. Effect of retinoid treatment of superovulated donors on embryonic development in vitro. ROH increased embryonic development to blastocyst stage in comparison with RA, CIS, and Control treatments (A). ROH treatment improved embryonic hatching rates when compared with rates in CIS and Control animals but was not different from RA treatment (B). In the second experiment, ROH treatment improved embryonic development to the blastocyst stage (C) and rate of embryonic hatching (D) in comparison with Control values. Animal numbers were ROH = 5, RA = 4, CIS = 7, Control = 5, and ROH and Control = 12 for experiments 1 and 2, respectively (see Materials and Methods). Error bars represent SEM. Columns with different letter subscripts are significantly different (p < 0.05).

In the second experiment, treatment of donors with ROH resulted in a dramatic increase in the percentage of embryos that formed blastocysts compared with the control value (70% vs. 22%; n = 243 and 218, respectively) (Fig. 1C). ROH treatment resulted in an increase to nearly 3-fold in hatching rate in comparison with vehicle treatment (70% vs. 27%, p < 0.05) (Fig. 1D).

In the third experiment (Fig. 2), the effect of ROH treatment of the dam (24 ewes) on in vitro development of 1- to 4-cell embryos was investigated. ROH treatment significantly (p < 0.05) improved the number of embryos that progressed through the 8-cell in vitro block (94% vs. 40%). As in the first two experiments, ROH treatment resulted in a dramatic increase (p < 0.05) in blastocyst formation (79% vs. 5%; n = 230 and 202, respectively) and blastocyst hatching (71% vs. 0%, respectively).



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  		FIG. 2. Effect of ROH treatment of superovulated ewes on subsequent in vitro embryonic development (n = 12 ewes per treatment). Columns represent the terminal stage of development. ROH treatment significantly increased the number of embryos that progressed through the 8-cell in vitro block in comparison with control treatment. Columns within a developmental stage with * are significantly different (p < 0.05).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Experiments were performed over a period of 2 years under conditions of both decreasing (fall) and increasing (winter) day length and included over 70 ewes producing more than 1300 embryos. Results from every experiment demonstrated that ROH treatment, in combination with superovulation, dramatically improved the in vitro developmental competence of resultant embryos. In the first experiment, the incidence of blastocyst formation and hatching of embryos from animals treated with ROH, but not RA, was dramatically higher than for embryos from vehicle-treated animals. The failure of retinoic acid (either all-trans retinoic acid or 9-cis retinoic acid) to increase embryonic development or hatching may be explained by the fact that a specific systemic transport mechanism exists for retinol but not retinoic acid. Systemic and intercellular transport of retinol is accomplished by a specific protein, RBP [15]. RBP is produced predominantly by the liver [16], and it is synthesized in several reproductive tissues including the ovary [17], oviduct [18], endometrium [19, 20], and testes [21, 22]. The differential results of our retinoid treatments may be analogous to results from studies in which retinol, but not retinoic acid, was effective in restoring reproductive functions in vitamin A-deficient rats [23–26].

Previous studies by others have indicated that retinol administration prior to ovulation may positively impact reproductive performance. In swine, a single injection of retinol palmitate, given about 5 days prior to estrus, increased litter size [8, 9]. In cattle, a single injection of retinol palmitate, administered 5–7 days prior to induced estrus and in combination with FSH, resulted in an increased number of embryos evaluated to be of high quality when collected on Day 7. Significantly more blastocysts were collected from retinol-treated cows, but overall ovulation rates and numbers of embryos were not affected [10]. We too observed no effect of retinol treatment on either ovulation rates or fertilization rates. In contrast to results from the study in cattle, there was no influence of retinol treatment of ewes on the quality, as judged by embryo score, or quantity of morula collected. However, since we did observe an increase in embryonic viability in vitro, it is possible that our embryo scoring technique was not effective in discriminating qualitative differences between embryos. Treatment did not affect the rate (speed) of in vivo development. In vitro development of embryos from control and retinol-treated ewes was parallel, in terms of time, up until the time when controls failed to progress.

In experiments 1 and 2, embryos from Control exhibited a relatively low rate of development from the morula to blastocyst stage. We suggest that this was in part the result of culture conditions that included a) serum that had been twice stripped with charcoal and b) culture of embryos in a relatively large volume (3 ml) of medium. We chose to not expose embryos to retinol-containing medium in vitro because it might mask and/or confound effects of in vivo exposure of oocytes/embryos to exogenous retinol that had been administered to ewes. Hence, the serum component of the medium was twice stripped, since a single charcoal treatment did not remove all detectable retinol. Along with removing the remainder of retinol from the serum, it is possible the second charcoal stripping removed additional beneficial low molecular weight compounds [27]. We chose to not culture embryos in microdrops covered with mineral oil because of the potential for the oil to take up the hydrophobic retinol from the embryos. Hence, embryos were cultured in 3 ml of medium in organ culture dishes to minimize evaporation and temperature change during handling. The relatively large volume may have diminished cooperative interactions of embryos that have been reported by some to improve development in vitro [28, 29]. In addition, for similar reasons, other procedures that improve development such as feeder cells [30–32] were not used. We emphasize that all embryos were cultured under identical conditions and that differences in their developmental competence can be explained only by the different treatments administered to the ewes.

Administration of retinol to superovulated ewes improved subsequent embryonic development through the critical transition from maternal to embryonic genome control and development from morula to blastocyst. The switch from maternal to embryonic gene expression is characterized by loss or degradation of maternal mRNA, activation of embryonic transcriptional machinery, and alteration in protein synthesis (reviewed in [33]). This stage is often associated with developmental arrest or block in embryos cultured in vitro. Sheep and cattle undergo this transition at the 8- to 16-cell stage [34, 35]. The majority of 1- to 4-cell embryos from vehicle-treated ewes blocked at the 8-cell stage (60%), whereas only 6% of embryos from retinol-treated ewes failed to make the transition. In vitro development of morula collected from ewes treated with retinol likewise exhibited greater competence to form blastocysts. The transition from morula to blastocyst is complex; it involves numerous structural and functional alterations, including the processes of compaction, blastocoele formation, and differentiation of trophectoderm, that will contribute to the placenta and inner cell mass giving rise to the fetus (reviewed in [36]).

The mechanism by which retinol administration contributes to critical transitions in early embryonic development is unknown, but the fact that retinol was given prior to ovulation may indicate that it affects the oocyte within the follicle. Maternal factors stored in the egg influence development of the embryo before and after zygotic gene activation [37–39]. Sirard and Blondin [39] demonstrated differences in developmental competence of oocytes matured in vivo or in vitro that were expressed several days later in the ability of the resultant embryos to develop beyond the 16-cell stage [40, 41] and form blastocysts [39]. It was suggested that the intrinsic differences in oocyte competence may result from factors affecting the oocyte during late folliculogenesis. Our results support this hypothesis and suggest that retinol may be one factor that influences oocyte competence.

Superovulation has been shown to alter ovarian function in cattle and sheep, resulting in abnormal follicular steroidogenesis [42, 43], premature and aberrant oocyte maturation [42, 44], and anomalies within the oocyte [43]. In the present study, retinol treatments were applied to superovulated animals, and it is possible that the exogenous retinol compensated for or diminished some aberrant ovarian activities resulting from superovulation.

Schweigert and Zucker [45, 46] associated bovine follicular fluid retinol concentrations with follicular health. Retinol concentrations were highest in healthy follicles, lowest in atretic follicles, and highly correlated with estradiol concentrations. Similarly, we have immunolocalized the binding proteins for retinol (RBP and CRBP) in the thecal cells of healthy but not atretic antral follicles in the ewe and suggested a model in which CRBP accumulates retinol from the blood plasma and RBP transports it across the basement membrane into the follicle fluid where it could influence oocyte maturation/development [17]. In addition, retinoids (ROH and RA) have been shown to stimulate steroidogenesis by granulosa cells in vitro and synergistically enhance the ability of FSH to induce LH receptors and stimulate cAMP and progesterone production [47]. Together, these data suggest that retinoids play a role in normal follicular development and function.

In summary, results from this study demonstrate that retinol treatment in combination with superovulation improves the developmental competence of resultant embryos. This study, taken together with others, provides evidence to indicate that retinoids may play a role in follicular development that positively influences embryonic development but does not preclude potential effects in the oviduct and uterus. These results suggest that retinol administration has the potential to positively impact reproductive efficiency and assisted reproduction protocols in domestic animals.


   FOOTNOTES
 
1 This work was supported, in part, by USDA NRI CGP #93-37201-8980 and The Tennessee Agricultural Experiment Station. Back

2 Correspondence. FAX: 423 974 4359; jgodkin{at}utk.edu Back

Accepted: January 25, 1999.

Received: September 8, 1998.


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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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