In vitro maturation of horse oocytes

Can Nicotinamide Adenine Dinucleotide (NAD+) and Sirtuins Be Harnessed to Improve Mare Fertility?

Years of sire and dam selection based on their pedigree and athletic performance has resulted in a reduction in the reproductive capability of horses. Mare age is considered a major barrier to equine reproduction largely due to an increase in the age at which mares are typically bred following the end of their racing career. Nicotinamide adenine dinucleotide (NAD+) and its involvement in the activation of Sirtuins in fertility are an emerging field of study, with the role of NAD+ in oocyte maturation and embryo development becoming increasingly apparent. While assisted reproductive technologies in equine breeding programs are in their infancy compared to other livestock species such as cattle, there is much more to be learnt, from oocyte maturation to early embryo development and beyond in the mare, which are difficult to study given the complexities associated with mare fertility research. This review examines what is already known about the role of NAD+ and Sirtuins in fertility and discusses how NAD+-elevating agents may be used to activate Sirtuin proteins to improve equine breeding and embryo production programs both in vivo and in vitro.

Optimization of recovery and maturation methods for cumulus-oocyte complexes in jennies

Embryo production in donkeys is inefficient compared with that in other livestock. Obtaining a sufficient number of MII oocytes is the first step to solving this problem. In this study, the number, morphology and maturation rates of cumulus-oocyte complexes (COCs) obtained from abattoir-derived ovaries or live jennies were compared. The diameter of follicles from abattoir-derived ovaries was measured and divided into group 1 (2-6 mm), group 2 (6-10 mm), group 3 (10-20 mm), group 4 (20-28 mm) and group 5 (>28 mm). The results showed that the number of follicles per ovary in group 2 (3.6 ± 0.28) and 3 (4.2 ± 0.90) was higher than that in the other groups (p < .05). The recovery rate in group 3 was higher than group 1 (48.8% vs. 26.8%, p = .00), but lower than group 5 (48.8% vs. 76.5%, p = .025). The percentage of grade A COCs in group 3 was higher than group 2 (59.3% vs. 39.5%, p = .00) and group 1 (59.3% vs. 26.7%, p = .00). Moreover, the percentage of grade A COCs in group 4 (55.0%, p = .710) and group 5 (46.2%, p = .351) was reduced compared with that in group 3. From the above results, the developing follicles (group ovum pick-up [OPU], 10-20 mm) and preovulation follicles (group OPU-Preov, >35 mm) were aspirated from live jennies using OPU. Although there was no difference in the recovery rates of COCs between group 3 and OPU (48.8% vs. 43.0%, p = .184), the percentage of grades A COCs in group OPU was higher than group 3 (72.5% vs. 59.3%, p = .036). There was no difference in the maturation rate between group 3 and OPU (60.3% vs. 69.3%, p = .171) after the COCs matured in vitro. The rates of recovery (72.2%) and maturation (92.3%) in group OPU-Preov were higher than those in other groups (p < .05). Moreover, the effects of maturation time and serum type on maturation rates were evaluated in groups B44 (44 h, FBS), B36 (36 h, FBS) and D44 (44 h, foetal donkey serum, FDS). These results indicated that the maturation rate in group B36 was lower than group B44 (13.1% vs. 47.0%, p = .00) and group D44 (13.1% vs. 53.3%, p = .00). In conclusion, the quality of donkey COCs from OPU was higher than that from abattoir-derived ovaries, the suitable time of donkey in vitro maturation (IVM) was 44 h, and FBS could be replaced with FDS in donkey IVM medium.

Time of first polar body extrusion affects the developmental competence of equine oocytes after intracytoplasmic sperm injection

Assisted reproduction techniques (ARTs) have become widespread in the equine breeding industry. In particular, the combination of oocyte recovery from live mares followed by IVM and intracytoplasmic sperm injection (ICSI) has increased markedly among the ARTs used with valuable or low-fertility animals. There is currently no consensus among research groups regarding the optimal oocyte maturation period to produce high-quality embryos. In this study, we report the maturation dynamics of equine oocytes at different time points, from 20 to 40h (Experiment 1). In addition, in Experiment 2, equine ICSI blastocysts were produced from oocytes that exhibited early (up to 24h) or late (28-30h) extrusion of the first polar body (PB). Blastocyst rates and diameter were recorded and embryo quality was assessed by analysing the number of apoptotic cells and Yes-associated protein 1 (YAP1) expression. By 20h of IVM, 42% of oocytes were mature, and the remaining oocytes matured within the next 17h of IVM. Although no differences were found in cell apoptosis or the number of YAP1-positive cells between groups exhibiting early and late PB extrusion, embryos from the early group (Group I) exhibited an improved total cell number and blastocyst rate compared to embryos from the late group (Group II) (18.60% vs 10.17% respectively).

Fertilisation in the horse and paracrine signalling in the oviduct

The mammalian oviduct plays a crucial role in the preparation of gametes for fertilisation (transport and final maturation) and fertilisation itself. An increasing number of studies offers a comprehensive overview of the functions of the oviduct and its secretions, but this topic has had limited investigation in the horse. Limited data are available on the final oocyte maturation in the equine oviduct. However, in vitro and in vivo systems have been established to analyse the influence of equine oviduct epithelial cells (OEC) during maturation on the potential of oocytes for fertilisation and development. Most studies focus on the role of the oviduct in equine sperm function, such as spermatozoa transport, attachment to oviduct epithelium, viability, motility and capacitation. Moreover, some possible candidate molecules for sperm-oviducal interactions have been identified in the horse. Finally, the low efficiency of conventional in vitro fertilisation and the in vivo fertilisation of equine oocytes transferred into the oviduct of an inseminated mare predicted an influence of oviduct in equine fertilisation. Actually, in vivo and in vitro experiments demonstrated a role of the oviduct in equine fertilisation. Moreover, recent studies showed a beneficial effect of homologous and heterologous OEC on equine in vitro fertilisation, and some candidate molecules have been studied.

Collection, evaluation, and use of oocytes in equine assisted reproduction

Assisted reproductive techniques have been developed to obtain pregnancies from subfertile mares and stallions and to salvage gametes after death. In recent years, these procedures have been used for clinical cases with repeated success. Although new developments occur, the basis for the success and future development of assisted reproductive techniques is our ability to collect and handle the equine oocyte successfully. This article focuses on important clinical aspects of oocyte collection and evaluation and briefly discusses the clinical use of assisted reproductive procedures in the horse.

In vitro maturation and fertilization of equine oocytes recovered during the breeding season

The aim of this study was to develope an efficient and reproducible procedure for in vitro maturation (IVM) and fertilization (IVF) in the horse. Cumulus-oocyte complexes (COCs) recovered from the ovaries of mares slaughtered during the breeding season were morphologically evaluated, and those showing a compact cumulus and homogeneously appearing cytoplasm were selected for culture. Effects on the maturation of estrous mare serum (EMS) versus estrous cow serum (ECS) as medium supplement were also evaluated (Experiment 1). In Experiment 2, the fertilization of in vitro matured oocytes with frozen-thawed semen separated by swim-up and treated with heparin was carried out to determine the effects on fertilization of 1) increasing sperm concentrations (1x 10(6), 5 x 10(6) and 1 x 10(7)sperm cells/ml), 2) IVM medium supplementation with EMS or ECS and 3) partial cumulus mass removal before insemination. Forty-nine percent of the collected oocytes (335 683 ) showed a compact cumulus and homogeneous ooplasm and thus were selected for culture. In Experiment 1, high nuclear maturation rates were observed in both EMS (82%,32 39 ) and ECS (87.5%,56 64 ) groups, with no statistically significant difference. In Experiment 2, the percentage of normal fertilization (2 polar bodies, 2 pronuclei and sperm tail) was similar for all 3 tested sperm concentrations (12.9%,4 31 ; 15.2%,9 59 and 15.5%,9 58 ). No advantage in using the homologous serum in IVM medium was noted in terms of fertilization (12.2%, 5 41 with EMS vs 12.9%, 4 31 for ECS). However, significantly higher fertilization rates were obtained after partial cumulus removal compared with that of oocytes fertilized with a whole cumulus (32.6%, 14 43 vs 12.2%, 5 41 ; P < 0.05). The incidence of polyspermic fertilization was low under all culture conditions (0 to 2.4%). In a replicate in which the oocytes fertilized after the cumulus removal were further cultured for 72 h two embryos, one at the 2-cell stage and the other at the 4-cell stage, could be obtained. These results indicate that, in the horse, the cumulus can be partially removed to increase the fertilization of compact-cumulus oocytes recovered during the breeding season using frozen-thawed, heparin-treated semen.

Qualitative and quantitative analysis of goat ovaries, follicles and oocytes in view of in vitro production of embryos

Goat ovaries were collected from the slaughterhouse and categorized as right, left, corpus luteum (CL)-present and -absent group and evaluated on the basis of weight (g), length (cm), width (cm), number of follicles, follicles aspirated and number and state of cumulus-oocyte-complexes (COCs). Comparatively higher weight [(0.66+/-0.02) vs (0.64+/-0.02) g], length [(1.17+/-0.02) vs (1.11+/-0.02) cm] and width [(0.77+/-0.02) vs (0.74+/-0.02) cm] were found in right ovaries than those of left. On the other hand significantly (P<0.05) higher weight [(0.71+/-0.03) vs (0.64+/-0.01) g] and width [(0.76+/-0.03) vs (0.75+/-0.01) cm] were found in CL-present group than those of CL-absent group of ovaries. The left ovaries contained comparatively higher number of normal COCs [(1.06+/-0.09) per ovary] than right ovaries [(1.03+/-0.10) per ovary] and the similar trend was found in total number of follicles [(4.51+/-0.25) vs (4.30+/-0.23) per ovary] and follicles aspirated [(2.55+/-0.14) vs (2.52+/-0.12) per ovary]. But the total COCs per ovary was almost similar in both ovaries [right and left: (1.85+/-0.12) and (1.85+/-0.11) per ovary, respectively]. Higher number of total COCs [(1.87+/-0.09) vs (1.76+/-0.16) per ovary], total number of follicles [(4.45+/-0.19) vs (4.16+/-0.37) per ovary], follicles aspirated [(2.55+/-0.10) vs (2.48+/-0.21) per ovary] and normal COCs [(1.12+/-0.07) vs (0.76+/-0.14) per ovary] were found in CL-absent group than those of CL-present group of ovaries.

Effect of initial cumulus morphology on meiotic dynamic and status of mitochondria in horse oocytes during IVM

The aim of this investigation was to examine the chromatin configuration of the nucleus, pattern of mitochondrial aggregation and mitochondrial activity in parallel studies in the same horse oocytes. Horse oocytes recovered by ultrasound-guided follicle aspiration in vivo were classified according to two main initial cumulus morphologies as having compact or expanded cumulus. The percentage of oocytes with a diplotene meiotic configuration at the time of recovery from the follicles was highest in compact oocytes. Oocytes with expanded cumulus layers at the time of recovery matured more rapidly in vitro and reached a proportion >50% at the metaphase II stage (M 2) sooner during in vitro maturation (IVM), than did compact oocytes. The mitochondrial aggregation pattern changed from finely distributed (Type 1) through crystalline (Type 2) to an aggregated, granulated appearance (Type 3) during IVM. The pattern of mitochondrial aggregation at the time of recovery was associated with the initial cumulus morphology of the oocyte, in that compact oocytes had a higher proportion of Type 1 aggregation, whereas expanded oocytes had a higher proportion of Type 3. The fluorescence intensity of metabolic active mitochondria, measured by fluorescence intensity (Em 570) per oocyte after MitoTracker CMTM Ros orange labelling, increased in the oocytes during IVM and depended on initial cumulus investment. Oocytes with the granulated type of aggregated mitochondria Type 3 had the highest level of metabolic activity and were in more progressed stages of meiosis (A 1-M 2). Oocytes initially having expanded layers of cumulus reached significantly higher levels of mitochondrial activity after IVM than did oocytes initially having compact cumuli. During resumption of meiosis the mitochondrial activity of oocytes with initially expanded cumulus increased continuously up to M 2, whereas in oocytes from compact cumulus-oocyte complex (COC), the activity declined after A 1/T 1 stages of meiosis.

The Development and Application of the Modern Reproductive Technologies to Horse Breeding

Although the horse was probably the first animal to experience and benefit from artificial insemination, it trailed the field somewhat with regard to the application of embryo transfer and other oocyte and embryo-related modern breeding technologies. But with a late run it is now back in mid-field and gaining fast on the other large domestic species in the application of the many technological advances of the past 20 years to sound breeding practice. Improvements in extenders and cryoprotectants have resulted in a veritable upsurge in the transport and insemination of cooled and frozen stallion semen, and parallel improvements in ovulation induction and synchrony, exogenous gonadotrophic stimulation of multiple fertile ovulations and simplified, more efficient methods for non-surgical transfer of embryos to recipient mares, coupled with relaxation of breed society registration restrictions, have together contributed to a similar upsurge in the application of embryo transfer to all breeds and athletic types of horses worldwide, with the continuing and notable exception of the Thoroughbred. Although conventional in vitro fertilization remains something of an unjumped fence in equids, other modern breeding technologies like hysteroscopic low-dose insemination, fluorescence-activated sex sorting of stallion spermatozoa, between-species embryo transfer, embryo freezing and bisection, transvaginal ultrasound-guided oocyte collection, intracytoplasmic sperm injection for fertilization (ICSI), gamete intrafallopian transfer (GIFT) and now nuclear transfer (cloning), have all been applied to equids with encouraging success. Cloning, especially, holds enormous promise for the Sporthorse industry to re-create champion geldings in stallion form for breeding purposes.

Effect of the in vitro maturation medium on equine oocytes: comparison of follicular fluid and oestrous mare serum

The present study evaluated the effect of supplementing the medium used to mature equine oocytes in vitro with oestrous mare serum (EMS) or horse follicular fluid (HFF). To this end, 144 ovaries were obtained from mares aged 16-21 months and transported to the laboratory in Dulbecco's phosphate buffered saline (D-PBS) at 30 degrees C. Oocytes were harvested from the ovaries by slicing, and then selected for in vitro maturation (IVM) according to the number of cumulus cell layers and the characteristics of the cytoplasm. The selected oocytes were washed three times in TCM199 medium plus HEPES (TCM-199H) or in the same medium plus glutamine (TCM-199G), then matured in vitro in six study groups established according to the in vitro maturation (IVM) treatment to see possible interactions between HEPES and glutamine on other supplements: Ten percent EMS was added to two of these media (TCM-199H+EMS and TCM-199G+EMS) and 10% HFF was added to the media in two other groups (TCM-199H+HFF and TCM-199G+HFF). IVM was performed at 38.5 degrees C for 40 h in a controlled atmosphere (5% CO2, 95% relative humidity). The findings indicate that the presence of EMS or HFF in the TCM-199H medium gives rise to the best results in terms of the proportions of oocytes reaching maturity (37.7% and 36.8%, respectively). The values obtained with EMS and HFF were statistically similar to each other but differed from the other treatments. The media containing glutamine led to the highest proportions of degenerated oocytes.

Changes in cumulus-oocyte complexes of pregnant and non-pregnant camels (Camelus dromedarius) during maturation in vitro

The aim of the present study was to examine the cumulus morphology and the oocyte chromatin quality of camel cumulus-oocyte complexes (COCs) at the time of recovery, and to monitor changes in oocyte chromatin configuration and apoptosis in cumulus cells from camel COCs during in vitro maturation (IVM) (0, 12, 24, 32, 36, 42, and 48 p.IVM) depending on pregnancy of donors. A total of 1023 COCs were isolated from sliced ovaries after slaughtering of 47 pregnant and 43 non-pregnant camels in an abattoir. The mean number of COCs per donor was 10.3 in pregnant and 12.5 in non-pregnant donors. The cumulus morphology of COCs was independent of the type of donor and was divided in COCs with compact (26.9 and 28%), dispersed (39.3 and 46%), corona radiata cumulus investment (27.9 and 21.7%) and without cumulus (6 and 4.2%), respectively for pregnant and non-pregnant donors. The highest proportion of COCs exhibited dispersed cumulus (P<0.05). Oocytes with meiotic stages of diplotene >50% were found only in compact (55 and 56.5%) and in dispersed COCs (58.4 and 60%), respectively for pregnant and non-pregnant donors. During IVM (0-48h) the first significant onset of specific meiotic stages were different in oocytes from pregnant donors: metaphase 1 (24-32h), metaphase 2 (36-42h), versus oocytes from non-pregnant donors: metaphase 1 (24h), metaphase 2 (32-48h) (P<0.05). The level of apoptotic cells in cumuli of matured COCs increased during IVM and was higher in matured COCs from non-pregnant donors for each time point during IVM (P<0.01). Camel oocytes meiosis during IVM is accompanied by a drastic increase of apoptosis in the surrounding cumulus cells 0-32 and 0-24h during IVM, respectively for pregnant and non-pregnant donors. The oocytes of pregnant camels require 36h of maturation to reach levels of >50% metaphase 2 stage in comparison to oocytes from non-pregnant donors where 32h are sufficient. The earlier onset of apoptosis in the COCs derived from non-pregnant donors possibly determines the faster progression of the oocytes through the final stages of meiosis.

Pregnancies from vitrified equine oocytes collected from super-stimulated and non-stimulated mares

The objectives were to compare embryo development rates after transfer into inseminated recipients, vitrified thawed oocytes collected from super-stimulated versus non-stimulated mares. In vivo matured oocytes were collected by transvaginal, ultrasound guided follicular aspiration from super-stimulated and non-stimulated mares 24-26 h after administration of hCG. Oocytes were cultured for 2-4 h prior to vitrification. Cryoprotectants were loaded in three steps before oocytes were placed onto a 0.5-0.7 mm diameter nylon cryoloop and plunged directly into liquid nitrogen. Oocytes were thawed and the cryoprotectant was removed in three steps. After thawing, oocytes were cultured 10-12 h before transfer into inseminated recipients. Non-vitrified oocytes, cultured 14-16 h before transfer, were used as controls. More oocytes were collected from 23 non-stimulated mares (20 of 29 follicles), than 10 super-stimulated mares (18 of 88 follicles; P < 0.001). Of the 20 oocytes collected from non-stimulated mares, 12 were vitrified and 8 were transferred as controls. After thawing, 10 of the 12 oocytes were morphologically intact and transferred into recipients resulting in one embryonic vesicle on Day 16 (1 of 12 = 8%). Fourteen oocytes from super-stimulated mares were vitrified, and 4 were transferred as controls. After thawing, 9 of the 14 oocytes were morphologically intact and transferred into recipients resulting in two embryonic vesicles on Day 16 (2 of 14 = 14%). In control transfers, 7 of 8 oocytes from non-stimulated mares and 3 of 4 oocytes from super-stimulated mares resulted in embryonic vesicles on Day 16. The two pregnancies from vitrified oocytes resulted in healthy foals.

Equine oocyte maturation with epidermal growth factor

Epidermal growth factor (EGF) has been shown to have a positive effect during oocyte in vitro maturation in several species. This study was performed to establish the capacity of equine oocytes to undergo nuclear maturation in the presence of EGF and to localise its receptor in the equine ovary by immunohistochemical methods. Oocytes were obtained by aspiration and subsequent scraping from equine follicles (15-25 mm diameter) and cultured in 3 different treatment groups for 36 h: control Group (modified TCM 199 with 0.003% BSA), EGF Group (TCM-199 supplemented with 50 ng/ml EGF) and EMS Group (TCM 199 supplemented with 10% v/v oestrous mare serum). Each group was divided further into 3 treatments with tyrphostin A-47, a specific tyrosine kinase inhibitor, at 0, 10(-4) and 10(-6) mmol/l. Maturation was determined as the percentage of oocytes reaching metaphase II stage at the end of the culture period. Immunohistochemical detection of EGF-receptor (EGFR) was performed using a streptoavidin-biotin method. The recovery rate and oocyte retrieval were 84.6% (recovered oocytes/follicles aspirated) and 6.55 (oocytes/mare), respectively. Treatment with EGF significantly (P<0.05) increased the incidence of metaphase II stage compared with the control group (69.4 vs. 26.9% in controls, respectively). The specific-tyrosine kinase inhibitor A-47 was effective in suppressing EGF-effect on EGF-cultured oocytes; no significant differences were observed in EMS-supplemented oocytes when cultured with A-47. EGF-receptor was localised in follicles, with localisation being more prominent in the cumulus than in mural granulosa cells. This finding, together with the increase of oocyte nuclear maturation rate when using EGF in culture media and the inhibition of maturation by tyrphostin A-47, suggests a physiological role for EGF in the regulation of equine oocyte maturation. The results should help successful development of assisted reproductive technology in the horse.

Organisation of the cytoskeleton during in vitro maturation of horse oocytes

Meiotic maturation of mammalian oocytes is a complex process during which microfilaments and microtubules provide the framework for chromosomal reorganisation and cell division. The aim of this study was to use fluorescence and confocal laser scanning microscopy to examine changes in the distribution of these important cytoskeletal elements and their relationship to chromatin configuration during the maturation of horse oocytes in vitro. Oocytes were cultured in M199 supplemented with pFSH and eLH and, at 0, 12, 24, and 36 hr after the onset of culture, they were fixed for immunocytochemistry and stained with markers for microtubules (a monoclonal anti-alpha-tubulin antibody), microfilaments (AlexaFluor 488 Phalloidin) and DNA (TO-PRO(3)). At the germinal vesicle stage, oocyte chromatin was amorphous and poorly condensed and the microfilaments and microtubules were distributed relatively evenly throughout the ooplasm. After germinal vesicle breakdown, the microtubules were aggregated around the now condensed chromosomes and the microfilaments had become concentrated within the oocyte cortex. During metaphase I, microtubules were detected only in the meiotic spindle, as elongated asters encompassing the aligned chromosomes, and, as maturation progressed through anaphase-I and telophase-I, the spindle assumed a more eccentric position and gradually rotated to assist in the separation of the homologous chromosomes and in the subsequent formation of the first polar body. During metaphase II, the meiotic spindle was a symmetrical, barrel-shaped structure with two poles and with the chromosomes aligned along its midline. At this stage, microtubules were found intermingled with chromatin within the polar body and, although, the bulk of the microfilaments remained within the oocyte cortex, a rich domain was found overlying the spindle. Thus, during the in vitro maturation of horse oocytes both the microfilament and microtubular elements of the cytoskeleton were seen to reorganise dramatically in a fashion that appeared to enable chromosomal alignment and segregation.

Effect of homologous preovulatory follicular fluid on in vitro maturation of equine cumulus-oocyte complexes

The objective of this study was to test the hypothesis that incubating equine cumulus-oocyte complexes (COCs) in medium containing 50% or 100% homologous preovulatory follicular fluid would improve cumulus expansion and nuclear maturation. Oocytes were incubated in one of three media: 1) supplemented TCM-199 (control), 2) 50% (v/v) follicular fluid in control medium or 3) 100% follicular fluid. Cumulus expansion was evaluated subjectively, and nuclear maturation was evaluated by staining oocytes with Hoechst 33258. The hypothesis that incubating COCs in medium containing follicular fluid would improve cumulus expansion was supported. More (P < 0.05) compact COCs incubated in 50% or 100% follicular fluid developed a moderately to completely expanded cumulus after 24 and 36 h of incubation and more (P < 0.05) expanded COCs incubated in 100% follicular fluid developed a moderately to completely expanded cumulus after 36 h of incubation compared to control medium. The hypothesis that incubating COCs in medium containing follicular fluid would improve nuclear maturation was not supported. Although more (P < 0.05) compact COCs incubated in 50% follicular fluid reached polar body-stage compared to those in control medium, the nuclear maturation rate in the control medium was lower than it was when the same medium was used in a preliminary experiment (described in main text); therefore, the apparent superiority of 50% follicular fluid must be interpreted cautiously. Based on these results, future studies are warranted to further address the value of adding preovulatory follicular fluid to equine IVM culture systems.

Effect of sperm cryopreservation and treatment with calcium ionophore or heparin on in vitro fertilization of horse oocytes

Little information is available on methods of sperm capacitation for IVF in the horse. In this study, we summarized results of several independent trials that compared acrosome reaction, hyperactivation and chromatin integrity of fresh or cryopreserved stallion spermatozoa after treatment with heparin or with calcium ionophore. We also examined the influence of spermatozoa storage (fresh vs. cryopreserved), capacitation treatment, oocyte maturation time and cumulus morphology on the penetration rate and fertilization rate. We recovered cumulus-oocyte-complexes (COCs) from ovaries by ultrasound guided follicle aspiration or by scraping of follicles from ovaries obtained at a slaughterhouse. Upon recovery, we evaluated the cumulus morphology, and the COCs were matured in vitro for 18 to 24 or 26 to 40 h. Fresh semen and cryopreserved semen were treated either with heparin (200 microg/mL) or calcium ionophore (7.14 microM). Overall, 28.4% (99/349) of the oocytes were penetrated, and 12.9% (45/349) were fertilized. Fresh spermatozoa treated with calcium ionophore showed a higher penetration rate than cryopreserved spermatozoa (36.0 vs. 0%). Fresh and heparin-treated spermatozoa showed a penetration rate of 29.1%, and the same treatment for cryopreserved spermatozoa showed a penetration rate of 33.7%; none of these differences was significant (P>0.05). Fertilization rates after the calcium and heparin treatment followed the same trend and also showed no significant differences. Prolonged maturation period resulted in higher penetration (P<0.05) and fertilization rates in compact (26 to 40 h: 37.7 and 13.1% vs. 18 to 24 h: 13.1 and 2.8%) and in tendency in expanded COCs (26 to 40 h: 40.0 and 30.3% vs. 18 to 24 h: 29.4 and 13.5%). In oocytes with only a few cumulus cells, the rates tended to be higher after the shorter incubation (18 to 24 h: 33.5 and 18.8% vs. 26 to 40 h: 17.2 and 6.5%). We observed hyperactivation more frequently in fresh than in cryopreserved semen after different treatments (43.2, 39.1 and 35.4% for heparin, calcium ionophore and control vs. 15.7, 10.8 and 5.7%, respectively). We observed significant changes in the acrosome reaction of fresh spermatozoa after heparin treatment (62.6 vs. 48.2%, P<0.05), as well as in cryopreserved spermatozoa after calcium ionophore treatment (31.7 vs. 17.6%, P<0.05). The chromatin integrity was significantly reduced after heparin treatment of fresh spermatozoa, in comparison to control and calcium ionophore (81.0 vs. 87.3 and 86.6, P<0.02). We also observed a similar reduction of chromatin quality after heparin treatment in cryopreserved spermatozoa, but the difference was significant only between heparin and calcium ionophore treatment [77.4 vs. 86.4 (P<0.02) and 84.9]. The results in the this retrospective study show that capacitating fresh spermatozoa with calcium ionophore, or using heparin in cryopreserved spermatozoa, results in higher penetration and fertilization rates of in vitro matured horse oocytes. A prolonged maturation time of 26 to 40 h is necessary for compact cumulus oocyte complexes to achieve the fertilization capacity. Further investigation is needed to show the developmental capacity of these fertilized oocytes.

Increasing culture time from 48 to 96 or 144 hours increases the proportions of equine cumulus oocyte complexes with negative or fragmented nucleus morphology

The objective was to test the hypothesis that increasing equine oocyte culture time from 48 to 96 or 144 h increases nucleus maturation of equine oocytes. The hypothesis was not supported because condensed chromatin-stage oocytes decreased (P<0.01) from 33/126 (26.2%) at 48 h or 34/95 (35.8%) at 96 h to 11/117 (9.4%) at 144 h, and polar body-stage oocytes decreased (P<0.01) from 65/126 (51.6%) at 48 h to 25/95 (26.3%) at 96 h and (P<0.01) to 1/117 (0.9%) at 144 h. Negative (non-staining) oocytes increased (P<0.01) from 16/126 (12.7%) at 48 h or 15/95 (15.8%) at 96 h to 39/117 (33.3%) at 144 h. Fragmented oocytes (with and without fluorescent areas) increased (P<0.01) from 4/126 (3.2%) at 48 h to 20/95 (21.1%) at 96 h and increased again to 60/117 (51.3%) at 144 h. When fragmented oocytes having 1 fluorescent area were defined as condensed chromatin-stage and fragmented oocytes having 2 fluorescent areas were defined as polar body-stage, condensed chromatin-stage oocytes increased (P < 0.05) from 34/126 (27.0%) at 48 h to 38/95 (40.0%) at 96 h, but decreased (P<0.05) to 19/117 (16.2%) at 144 h. Polar body-stage oocytes decreased (P<0.01) from 66/126 (52.4%) at 48 h to 27/95 (28.4%) at 96 h and decreased again to 7/117 (6.0%) at 144 h. Fragmented oocytes without any fluorescent areas increased (P<0.01) from 2/126 (1.6%) at 48 h to 14/95 (14.7%) at 96 h and increased again to 46/117 (39.3%) at 144 h. Under the conditions of this experiment, the hypothesis that increasing the culture time of equine oocytes from 48 to 96 or 144 h would increase oocyte maturation was not supported. We propose that the culture system needs to be improved before this hypothesis can be adequately tested, because prolonged culture significantly increased the proportions of negative and fragmented equine oocytes.

Confocal microscopy of germinal vesicle-stage equine oocytes

The objectives were to compare cumulus type with nucleus form in equine cumulus oocyte complexes (COCs), to define the percentage of germinal vesicle (GV)-stage oocytes within a population of mares, and to further define GV nucleus shapes of equine oocytes. Cumulus types were as follows: 1) compact (56/208, 26.9%), 2) slightly expanded (37/208, 17.8%), 3) moderately expanded (27/208, 13.0%), 4) greatly expanded (15/208, 7.2%), or 5) denuded (73/208, 35.1%). One hundred thirty of 208 COCs (62.5%) were GV-stage, 21/208 (10.1%) were condensed chromatin-stage, 8/208 (3.8%) were polar body-stage, 40/208 (19.2%) were negative (nonstaining), and 9/208 (4.3%) were fragmented. Cumulus types were associated with nucleus forms because higher proportions (P < 0.05) of GV-stage oocytes occurred in compact (42/56, 75.0%), slightly expanded (30/37, 81.1%), moderately expanded (16/27, 59.3%), or denuded (40/73, 54.8%) COCs than in greatly expanded (2/15, 13.3%) COCs. In contrast, lower proportions (P<0.05) of condensed chromatin-stage oocytes occurred in compact (3/56, 5.4%), slightly expanded (0/37, 0.0%), moderately expanded (3/27, 11.1%) or denuded (9/73, 12.3%) COCs than in greatly expanded (6/15, 40.0%) COCs, and lower proportions (P < 0.05) of polar body-stage oocytes occurred in compact (0/56, 0.0%) or denuded (2/73, 2.7%) COCs than in greatly expanded (3/15, 20.0%) COCs. Germinal vesicle-stage equine oocytes had 4 distinct shapes, with higher proportions (P<0.05) having large-regular (54/130, 41.5%) than scattered (10/130, 7.7%), small-round (29/130, 22.3%), or large-irregular (37/130, 28.5%) shapes. Lower proportions (P<0.05) of large-regular GVs occurred in compact (11/42, 26.2%) COCs than in slightly expanded (15/30, 50.0%), or moderately expanded (12/16, 75.0%) COCs. Therefore oocytes with the large-regular GV shape are probably more advanced in development.

Embryo development rates after transfer of oocytes matured in vivo, in vitro, or within oviducts of mares

Objectives of the present study were to use oocyte transfer: 1) to compare the developmental ability of oocytes collected from ovaries of live mares with those collected from slaughterhouse ovaries; and 2) to compare the viability of oocytes matured in vivo, in vitro, or within the oviduct. Oocytes were collected by transvaginal, ultrasound-guided follicular aspiration (TVA) from live mares or from slicing slaughterhouse ovaries. Four groups of oocytes were transferred into the oviducts of recipients that were inseminated: 1) oocytes matured in vivo and collected by TVA from preovulatory follicles of estrous mares 32 to 36 h after administration of hCG; 2) immature oocytes collected from diestrous mares between 5 and 10 d after aspiration/ovulation by TVA and matured in vitro for 36 to 38 h; 3) immature oocytes collected from diestrous mares between 5 and 10 d after aspiration/ovulation by TVA and transferred into a recipient's oviduct <1 h after collection; and 4) im mature oocytes collected from slaughterhouse ovaries containing a corpus luteum and matured in vitro for 36 to 38 hours. Embryo development rates were higher (P < 0.001) for oocytes matured in vivo (82%) than for oocytes matured in vitro (9%) or within the oviduct (0%). However, neither the method of maturation nor the source of oocytes affected (P > 0.1) embryo development rates after the transfer of immature oocytes.

Effect of time during transport of excised mare ovaries on oocyte recovery rate and quality after in vitro maturation

In the mare only a limited number of oocytes can be successfully collected in vivo, so that when large numbers of oocytes are needed for experimentation, ovaries harvested from slaughtered mares must be used. The resulting temperature changes and time intervals mandated by handling and transport of ovaries from the slaughterhouse to the laboratory adversely affect the rate of oocyte recovery and their quality after IVF and maturation. We chose to study the effect of temperature and time in transit of excised ovaries by evaluating rate of oocyte recovery, nuclear maturation stage reached before, and cleavage rate reached after IVF, following short (1.5 to 4 h) and long (6 to 8 h) storage. Temperatures in the storage container decreased from 37-C to 32 degrees and 27.5 degrees C during the short and long interval, respectively. The cumulus-oocytes complexes (COCs) were classified as having a compact cumulus, completely or partially surrounding the oocyte (compact); those having only a corona radiata surrounding the oocyte (corona); those having a completely or partially expanded cumulus, showing a cellular or sparsely cellular, gelatinous cloud around the oocyte (expanded); and those that were completely denuded of both cumulus and corona cells (denuded). All COCs, except the denuded ones, which were discarded, were matured in vitro for 30 h at 38.5 degrees C in 5% CO2. The recovery rate of oocytes was significantly higher after long vs short storage (48 vs 35%; P < 0.01), but the distribution of the collected COCs into the 4 classes was not affected by the storage time. After in vitro maturation nuclear maturity was not affected by the storage time, but oocytes with intact cytoplasmic membranes were more frequently found after short than after long storage (54 vs 34%; P = 0.07), and fully matured oocytes were more often seen with intact membrane (P < 0.01). Moreover, oocytes with intact membranes in metaphase II (MII) were associated with short storage intervals and the corona COC class, while damaged membranes and incomplete maturation were associated with the long storage and the compact COC class.

Effect of mare's age and recovery methods on the recovery rate of equine follicular oocytes for IVM procedures

Mares (n = 39) were classified according to age as young (less than 1.5 yr, n = 17) or old (more than 1.5 yr, n = 22) and sacrificed. Ovaries were measured and weighed, and the number of follicles and CL were counted. Follicle size and distribution were recorded (external: > 20 mm, < 20 mm; internal: > 5 mm, < 5 mm). External follicles were aspirated while internal follicles were sliced. The number and Type of oocytes recovered using each method were recorded. Oocyte recovery rates (oocytes/ovary) resulted in a mean of 0.92 oocytes by aspiration and 1.36 oocytes by additional slicing. The mean numbers of available follicles (8.11 and 5.02) oocytes recovered (4.94 and 3.02) and oocytes selected per ovary (3.29 and 2.32) were not significantly different in the young or old mares, respectively.

Comparison of different methods for the recovery of horse oocytes

The object of this study was to compare 4 different methods of oocyte recovery from mares; 1) transvaginal follicle aspiration in vivo; 2) follicle aspiration in vitro; 3) oocyte recovery by isolation of follicles in vitro and 4) follicle scraping in vitro. Oocyte recovery was highest after follicle scraping (71.1%) and follicle isolation and rupture (61.3%). Follicle aspiration in vitro and in vivo yielded oocytes on 31.2% and 19.3% of occasions, respectively. The output of different types of cumulus-oocyte-complexes was different among the methods; the portion of compact cumulus-oocyte-complexes was significantly higher with follicle scraping (50.7%) and follicle isolation (44.5%) than with aspiration in vivo (31.9%) and in vitro (23.7%). The recovery rate of oocytes from small follicles (<15 mm) was significantly higher than from larger follicles (P<0.05) using transvaginal follicle aspiration. The proportion of oocytes that were degenerate (exhibited shrunken, dense or visibly damaged ooplasm) ranged from 1.2% after follicle scraping, to 17.2% after aspiration in vivo. These results indicate that, for the recovery of horse oocytes in vitro, follicle scraping and follicle isolation give the highest recoveries of cumulus-intact oocytes.

Equine oocyte-cumulus morphology as affected by follicular size

From the ovaries of 256 slaughtered mares a total of 1713 follicles were isolated from which 1641 (95.8%) oocytes were recovered (6.4/mare). A total of 564 follicles and oocytes were evaluated for the degree of vascularisation of the follicle wall, the appearance of the follicular fluid and the location and morphology of the cumulus-oocyte-complex. Follicles with a diameter of >10 mm displayed more numerous, well branched and more pronounced blood vessels than the smaller ones (4-10 mm diameter) and most of them contained clear, yellowish fluid with few granulosa cells. The percentage of oocytes with compact cumuli increased significantly with an increasing diameter of the follicle, being 233%, 43.9%, 55.6% and 64.2% (P<0.01) for the follicles with diameters of 4-10, 11-15, 16-20 and 21-35 mm, respectively. The percentage of oocytes attached to the follicle wall also increased with increasing follicle size, being 48.0%, 59.6%, 81.5% and 90.1% (P<0.01), respectively. On the contrary, the percentage of oocytes floating in the follicular fluid decreased with increasing follicle diameter, from 52.0% in the smallest follicles to 9.9% in the biggest ones. A significantly greater percentage of oocytes found on the follicular wall than in the follicular fluid had a compact cumulus (56.6 versus 21.3%; P<0.01). For in vitro culture were accepted 30.4%, 54.3%, 60.7% and 77.8% (P<0.01) of oocytes from the follicles with diameters of 4-10, 11-15, 16-20 and 21-35 mm, respectively. After culture for 28-40 h in TCM 199 medium, 90 of a total of 165 (54.5%) oocytes reached the metaphase II stage of maturation.

Cumulus expansion, chromatin configuration and meiotic competence in horse oocytes: a new hypothesis

When recovered from the follicle, horse oocytes may be categorised as having either a compact or an expanded cumulus. Cumulus expansion is strongly associated with follicle atresia. Oocytes with expanded and compact cumuli have similar proportions in the germinal vesicle stage when recovered from the follicle. However, during in vitro culture, a higher proportion of oocytes with expanded cumuli mature, and they do so more quickly, than do oocytes with compact cumuli. Using Hoechst 33258 to label chromatin, in the germinal-vesicle stage horse oocytes can be divided into those in which the nucleus fluoresces diffusely (FN), and those in which the chromatin is condensed as a mass within the germinal vesicle (condensed chromatin; CC). The CC configuration is more common in oocytes with expanded rather than compact cumuli. The occurrence of the condensed chromatin configuration in oocytes with compact cumuli rises with increasing follicle size to the point that, in follicles >20 mm diameter, almost 80% of oocytes with compact cumuli have condensed chromatin. This suggests that the chromatin of fully grown oocytes condenses prior to the gonadotrophin stimulation that induces ovulation. The number of oocytes maturing in vitro correlates strongly with the presence of the condensed chromatin configuration at the beginning of culture for both cumulus types. This suggests that the condensed chromatin configuration represents the meiotically competent oocyte. In contrast, FN oocytes appear to be meiotically incompetent, although 61% of oocytes from small, apparently viable follicles, have the FN configuration. This leads to the hypothesis that the FN configuration represents viable germinal vesicle stage oocytes that have yet to acquire meiotic competence. As oocytes achieve meiotic competence, the chromatin appears to condense to the CC configuration. This change occurs during growth of the preovulatory follicle but it can also occur during follicular atresia. Information about the inherent meiotic competence of different populations of oocytes will assist in the selection of oocytes for experiments involving in vitro maturation and fertilisation in equids.

Intracytoplasmic sperm injection (ICSI) versus conventional IVF on abattoir-derived and in vitro-matured equine oocytes

Conventional IVF as well as several assisted microfertilization techniques have shown limited success in the horse. After recent positive results achieved with intracytoplasmic injection of a single spermatozoon (ICSI) in human IVF, we chose to try the method in the horse. We compared conventional IVF to ICSI by fertilization rates of oocytes with compact and expanded cumuli and by developmental potential of the resulting embryos. Cumulus-oocyte complexes (COCs) were obtained by aspirating the follicular fluid from the ovaries of slaughtered mares. Complexes showing complete cumulus investment, either compact or expanded, were randomly assigned to IVF or ICSI trials and separately cultured for IVM. Frozen-thawed stallion spermatozoa were prepared for IVF with a swim-up procedure conducted in Talp-Hepes with heparin or for ICSI in Earle's balanced salt solution (EBSS) supplemented with human serum albumin (HSA). Oocytes for IVF were partially decumulated by pipetting, whereas those for ICSI were totally denuded with 80 UI/ml hyaluronidase. Oocytes were fixed, stained and examined for signs of fertilization the day after IVF or ICSI. The percentage of normally fertilized oocytes showing 2 pronuclei or cleavage was significantly higher with ICSI than IVF (29.8%, 17/57 vs 8.7%, 9/103 ; P < 0.01). Significantly higher fertilization rates were observed in oocytes retrieved with an expanded cumulus when submitted to ICSI procedure as compared with IVF (52.2%, 12/23 vs 17.1%, 6 35 ; P < 0.01), whereas in oocytes recovered with a compact cumulus, fertilization rates were low (14.7%, 5/34 with ICSI and 4.4%, 3 68 with IVF; NS). Embryonal development did not occur after culture following IVF, as indicated by absence of cleavage in any of the 93 inseminated oocytes. Following ICSI, 7 of 55 injected oocytes cleaved, 5 of which had shown expanded cumuli; of the 5, 2 were at the 16-cell stage and one each at the 8-, 3- and 2-cell stage, respectively. The other 2 fertilized oocytes, originating from compact cumuli, reached 4- and 8- cell stages, respectively. These results indicate that ICSI can be applied successfully to in-vitro matured equine oocytes to increase the fertilization rates. In addition, it seems that in vitro cytoplasmic maturation of oocytes issuing from a compact cumulus may not be complete enough to lead to a successful fertilization and that ICSI may be a tool to evaluate ooplasmic maturation.

Maturation and fertilization of equine oocytes

Equine oocytes obtained either by transvaginal ultrasound-guided follicular aspiration or from slaughterhouse ovaries can be matured in vitro. This generally requires culture in TCM-199 containing serum and hormones for 30 to 36 hours. With this protocol, approximately 50% to 60% of the oocytes are at metaphase-II at the end of the culture period. At least some of these oocytes appear viable based on production of fertilized eggs either through in vitro fertilization or fertilization in vivo of a recipient mare. The success of producing equine embryos in vitro is still extremely low. More than likely the conditions for in vitro oocyte maturation are not optimized, and the techniques for capacitating equine spermatozoa are not adequate. The stallion sperm would appear more difficult to capacitate and many of the approaches used in other species have not worked in horses. To date, the only fertilization that has occurred with in vitro matured oocytes has been with sperm treatments containing the calcium ionophore A23187. Increased success with in vitro production of equine embryos may be gained through the use of assisted reproductive techniques such as partial zona dissection or intracytoplasmic sperm injection.