Long-term stability of the bovine block to polyspermy

Effect of environmental factors on estrus synchronization and artificial insemination success in farmers flock in sheep under semi-arid tropical region

Environmental cues in the different seasons might influence the estrus and lambing percentages. The present study was conducted to assess the effect of environmental factors on estrus synchronization (ES) and artificial insemination success in sheep. During the period from 2012 to 2017, a total of 1,197 ewes of Kheri and Malpura breeds of 34 farms originating from 10 villages were synchronized for estrus and then fixed-time artificial insemination (FTAI) was done. Oestrous synchronization was done by intravaginal progesterone sponges and eCG protocol. Fixed-time cervical insemination was performed 48 and 56 hr after sponge removal in ewes exhibiting estrus, using liquid chilled semen containing 100 million sperm per dose of Patanwadi/Malpura rams. Mean sunshine hours, maximum and minimum ambient temperatures, temperature amplitude, mean relative humidity (RH), temperature-humidity index (THI), mean temperature during sponge-in to sponge-out-12-day period, sponge-out and next day, AI day and AI to next 15 days have been calculated. The estrus response and lambing percentage were higher (p < .05) when ES and FTAI were done during hot-humid with rainfall season. Successful (p < .05) ES occur when the minimum temperature was higher in cold-humid season, and the sunshine hour was higher in hot-humid with scanty rainfall season during sponge-in to sponge-out period. The success of the lambing percentage was higher (p < .05) when maximum temperature, minimum temperature, mean temperature and THI was higher on the day of AI and the next 15-day period, irrespective of the season. The success of FTAI indicates the adaptability of the local breed to tolerate the harsh climate of the hot semi-arid region without much hindrance in lambing percentages.

Preventing polyspermy in mammalian eggs-Contributions of the membrane block and other mechanisms

The egg's blocks to polyspermy (fertilization of an egg by more than one sperm) were originally identified in marine and aquatic species with external fertilization, but polyspermy matters in mammalian reproduction too. Embryonic triploidy is a noteworthy event associated with pregnancy complications and loss. Polyspermy is a major cause of triploidy with up to 80% of triploid conceptuses being the result of dispermic fertilization. The mammalian female reproductive tract regulates the number of sperm that reach the site of fertilization, but mammals also utilize egg-based blocks to polyspermy. The egg-based blocks occur on the mammalian egg coat (the zona pellucida) and the egg plasma membrane, with apparent variation between different mammalian species regarding the extent to which one or both are used. The zona pellucida block to polyspermy has some similarities to the slow block in water-dwelling species, but the mammalian membrane block to polyspermy differs substantially from the fast electrical block that has been characterized in marine and aquatic species. This review discusses what is known about the incidence of polyspermy in mammals and about the mammalian membrane block to polyspermy, as well as notes some lesser-characterized potential mechanisms contributing to polyspermy prevention in mammals.

Prophase I mouse oocytes are deficient in the ability to respond to fertilization by decreasing membrane receptivity to sperm and establishing a membrane block to polyspermy

Changes occurring as the prophase I oocyte matures to metaphase II are critical for the acquisition of competence for normal egg activation and early embryogenesis. A prophase I oocyte cannot respond to a fertilizing sperm as a metaphase II egg does, including the ability to prevent polyspermic fertilization. Studies here demonstrate that the competence for the membrane block to polyspermy is deficient in prophase I mouse oocytes. In vitro fertilization experiments using identical insemination conditions result in monospermy in 87% of zona pellucida (ZP)-free metaphase II eggs, while 92% of ZP-free prophase I oocytes have four or more fused sperm. The membrane block is associated with a postfertilization reduction in the capacity to support sperm binding, but this reduction in sperm-binding capacity is both less robust and slower to develop in fertilized prophase I oocytes. Fertilization of oocytes is dependent on the tetraspanin CD9, but little to no release of CD9 from the oocyte membrane is detected, suggesting that release of CD9-containing vesicles is not essential for fertilization. The deficiency in membrane block establishment in prophase I oocytes correlates with abnormalities in two postfertilization cytoskeletal changes: sperm-induced cortical remodeling that results in fertilization cone formation and a postfertilization increase in effective cortical tension. These data indicate that cortical maturation is a component of cytoplasmic maturation during the oocyte-to-egg transition and that the egg cortex has to be appropriately primed and tuned to be responsive to a fertilizing sperm.

Induced seasonal reproductive performance in two breeds of sheep

The objective of the present experiment was to determine whether failure to become pregnant through out-of-season reproductive management regimes is attributable to a failure to stimulate normal corpora lutea (CL) production. Romney ewes and East Friesian Composite (EF) ewes were mated in summer, autumn, winter, and early and late spring after administration of intravaginal progesterone inserts (plus eCG in spring and summer) in a factorial (two breedsxfive mating periods) experimental design. Oestrus rate was determined from mating data, and the numbers of CL were determined by laparoscopy 9 days after ram introduction. Conception rate, pregnancy rate and litter size were determined by transabdominal ultrasonography 62 days after ram introduction. Proportions of ewes displaying oestrus did not differ significantly (75-100%) between seasons or breeds, except for the Romney ewes in summer (67%; P<0.05). Numbers of CL on Day 9 for EF ewes was lowest in autumn (1.5) and winter (1.5), and highest in late spring (2.1; P<0.05). In contrast, Romney ewes had the lowest CL count in summer (1.3), and the highest in autumn (1.8) and winter (1.6; P<0.05). Only in summer was there a difference between breeds (EF 1.8; Romney 1.3; P<0.01). The proportion of ewes that failed to conceive despite having one or more CL present was highest in late spring (EF 41%, Romney 43%), and lowest in autumn (EF 9%, Romney 4%) and winter (EF 14%, Romney 4%; P<0.05). Conception and pregnancy rates followed similar patterns with values for autumn (EF 91%, 91%; Romney 96%, 96%) and June (EF 86%, 82%; Romney 91%, 83%) being significantly (P<0.05) higher than in early (EF 50%, 40%; Romney 54%, 50%) and late spring (EF 44%, 36%; Romney 42%, 36%). Pregnancy rate in summer was higher for EF ewes (60%) than for Romney ewes (39%) but conception rates were not statistically different (EF 68%; Romney 60%). Numbers of foetuses identified at scanning was highest in autumn (1.5) and lowest in late spring (0.5 P<0.001): Litter size in pregnant EF ewes was highest in early spring and lowest in winter (1.8 versus 1.2; P<0.01), but for pregnant Romney ewes was highest in winter and lowest in early spring (1.9 versus 1.3; P<0.001). It was concluded that seasonal differences in the ability of ewes to conceive are not the consequence of failure to display oestrus or to ovulate, but probably are a result of failure of fertilisation or the establishment of pregnancy.

Mammalian membrane block to polyspermy: new insights into how mammalian eggs prevent fertilisation by multiple sperm

To inhibit fertilisation by more than one sperm (a condition known as polyspermy), eggs have developed preventative mechanisms known as blocks to polyspermy. The block at the level of the egg extracellular coat (the zona pellucida in mammals, the vitelline envelope in non-mammals) has been well characterised in many different animal species and the block at the level of the egg plasma membrane is understood in some non-mammalian species. However, virtually nothing is known about the membrane block to polyspermy in mammalian eggs, despite data dating back 50–90 years that provide evidence for its existence. In the present review, we will discuss the background on blocks to polyspermy used by animal eggs and then focus on the membrane block to polyspermy in mammalian eggs. This will include a summary of classical studies that provide evidence for this block in mammalian eggs, assays used to study the mammalian membrane block and what has been elucidated from recent experimental studies about the cellular signalling events that lead to membrane block establishment and the mechanism of how the membrane block may prevent additional fertilisation.

Cellular and molecular mechanisms leading to cortical reaction and polyspermy block in mammalian eggs

Following fusion of sperm and egg, the contents of cortical granules (CG), a kind of special organelle in the egg, release into the perivitelline space (cortical reaction), causing the zona pellucida to become refractory to sperm binding and penetration (zona reaction). Accumulating evidence demonstrates that mammalian cortical reaction is probably mediated by activation of the inositol phosphate (PIP(2)) cascade. The sperm-egg fusion, mediated by GTP-binding protein (G-protein), may elicit the generation of two second messengers, inositol 1,4,5 triphosphate (IP(3)) and diacylglycerol (DAG). The former induces Ca(2+) release from intracellular stores and the latter activates protein kinase C (PKC), leading to CG exocytosis. Calmodulin-dependent kinase II (CaMKII) may act as a switch in the transduction of the calcium signal. The CG exudates cause zona sperm receptor modification and zona hardening, and thus block polyspermic penetration. Oolemma modification after sperm-egg fusion and formation of CG envelope following cortical reaction also contribute to polyspermy block.

Relationship of seminal traits and insemination time to fertilization rate and embryo quality

The nature of subfertility due to the male or inseminate is as complex as that of the female. Fertilization failure or failure in embryogenesis are both documented to be of seminal origin. Males also differ in the numbers of sperm required to reach their maximum fertilization rate. Males requiring more sperm would be considered to have compensable seminal deficiencies. These include a number of known (viability and morphology) and unknown factors (functional or molecular traits) precluding sperm access to the ovum or ability to engage the ovum sufficiently to initiate fertilization and the block to polyspermy. Differences in fertility among males or inseminates independent of sperm dosage are considered uncompensable. These deficiencies would be associated with fertilizing sperm that are incompetent to maintain the fertilization process or subsequent embryogenesis once initiated, with most failures occurring prior to maternal recognition of pregnancy. Such sperm would preempt fertilization by competent sperm. Chromatin aberrations in morphologically normal or near normal spermatozoa from abnormal semen samples appear to be the best candidates for the uncompensable deficiency. However, recognition of uncompensable or incompetent fertilizing sperm has not been achieved. Six-day-old non-surgically recovered bovine ova/embryos have been used to evaluate compensable and uncompensable seminal deficiencies as well as to test reproductive strategies. These ova/embryos provide information on fertilization status and embryo quality as well as quantitative and qualitative data regarding associated accessory sperm. Thus, they permit the separation of reproductive failure by fertilization from that by embryonic development. Accessory sperm number is positively associated with both fertilization rate and embryonic quality. Early insemination results in low fertilization rates (low accessory sperm number), but good embryo quality, whereas, late insemination results in high fertilization rates (high accessory sperm number), but poor embryo quality. Additional studies will be necessary to substantiate this model; however, if true, future research designed to improve results to artificial insemination should be tested by breeding early in estrus where sperm viability is most limiting and embryo quality is best.

Deep uterine insemination of cattle: a fruitful way forward with smaller numbers of spermatozoa

After describing the site of fertilisation and that of the functional sperm reservoir in the female tract, proposals are made concerning a modified site of sperm deposition in cattle. By means of a deep pre-ovulatory insemination into the ipsilateral uterine horn, the chances should be raised of establishing viable spermatozoa in the isthmus where they would undergo a form of physiological encapsulation and storage. Release and activation of such spermatozoa would be prompted by imminent ovulation. Potential advantages of this approach include those of raising the overall fertility of genetically valuable bulls whose non-return rates are sub-optimal; reducing the number of spermatozoa in each insemination dose; using effectively the limited numbers of sex-selected sperm cells (X and Y chromosome bearing spermatozoa) currently available from flow cytometry. Putative disadvantages might include rectal palpation of the ovaries to locate the pre-ovulatory follicle; perforation of the uterine wall by the deep insemination catheter; risk of polyspermic fertilisation; and the inappropriateness of the technique for non-clinically qualified inseminators. Each of these reservations is responded to in a rational manner. Given a change of attitude, a modified technique of insemination would be feasible under commercial conditions and might give a welcome boost to a sagging artificial insemination industry.