Provisión de los recursos materiales y de infraestructura

A small breeding trial was included as part of the study reported in Chapter 9. Unexpectedly low pregnancy rates were obtained; only 3 of 43 inseminated heifers were pregnant 35 days post-AI. Although we were tempted to conclude that non-steroidal aromatase inhibitor treatment has a direct negative impact on oocyte competence, other factors inherent to the experimental design and resulting from the altered gonadotropin profiles obtained after letrozole treatment need to be considered.

The requirement of estradiol for final follicular maturation and development of oocyte competence in mammals is not fully understood. While the inclusion of estradiol in in vitro maturation protocols impaired bovine oocyte nuclear maturation and subsequent embryo

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development in some studies [269, 270], others report that estradiol is essential for normal in

vitro maturation [271], especially of early antral follicle-derived oocytes [272]. In rhesus

monkeys, aromatase inhibitor treatment during the late follicular phase did not alter the number nor the pattern of growth of follicles, although oocyte activation and in vitro fertility was reduced [139]. However, data on the effect of estradiol deprivation on bovine oocyte maturation in vivo are not available, most likely due to the lack of an efficient animal model or treatment regimen to mimic such a condition. Although not directly assessed in this study, we can presume that follicular environment may have been affected by letrozole treatment, potentially affecting oocyte quality by disturbing meiosis. However, a mouse model demonstrated that treatment with anastrozole did not impair follicular growth, ovulation nor fertilization in vivo and embryo development in vitro [273]. In addition, non-steroidal aromatase inhibitor treatment may have had several important extra-gonadal effects; estradiol produced by the growing pre-ovulatory follicle has been shown to be associated with the development of receptive endometrium, the production of the cervical mucus, and sperm transport [274]. Clinical studies in humans, however, denote that the main advantage of the use of letrozole over other estradiol suppressors used in ovarian stimulation (such as clomiphene citrate, CC) is that uterine maturity does not seem to be compromised [200, 201, 203]. The improved endometrial thickness obtained after letrozole treatment compared to that of CC-treated patients has been attributed to the relatively short half-life of letrozole and the reversible nature of letrozole interaction with the P450arom [200, 201, 203]. An interesting observation made during the experiments included in Chapter 9 supports, in part, the notion of alteration in reproductive organs other than the ovaries after non- steroidal aromatase inhibitor treatment in cattle. Heifers showed normal signs of estrus after

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initial PGF and GnRH treatment (control ovulations), but failed to exhibit estrus signs or behaviour (i.e., mounting, standing to be mounted, vaginal mucus discharge) once letrozole was included to the PGF-GnRH protocol.

Other factors that need to be examined to understand the pregnancy outcomes presented in Chapter 9 include duration of dominance, and incidence of oocyte aging and/or activation. The impact of duration of dominance of the pre-ovulatory follicle on timing of ovulation and subsequent fertility has been reported [265, 292]. Prolonged dominance of the ovulatory follicle has been associated with increased LH pulsatility [266]. Additionally, early activation of oocytes (resumption of meiosis) driven by increased LH pulsatility has been considered the main cause of decreased fertility following the use of progestogens in synchronization protocols [268, 292- 294]. During the breeding trial, heifers in which treatment was initiated between Days 7 and 9 and between Days 15 and 17 (Day 0 = ovulation) may have been at risk of developing an ovulatory follicle which was dominant for approximately 12 days and would likely have had an aged and/or prematurely activated oocyte (Chapter 9). We must also consider the possibility that increased LH secretion (potentially caused by letrozole treatment [39, 40]) may have led to premature activation of oocytes and a reduction in fertility, even in those heifers in which duration of dominance of the pre-ovulatory follicle was within normal range (1 to 5 days, [268]). Finally, another factor which could have accounted for the low pregnancy rates is the occurrence of short-lived CL, previously discussed. Unfortunately, post-AI ultrasound examinations were not performed until pregnancy check 35 days later; hence, no information on CL lifespan was available for this set of animals.

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Deficiencies in the experimental design and limited resources (number of heifers available) introduced additional confounding factors to the interpretation of the results of the AI trial included in Chapter 9. The lack of a letrozole-free control group made it difficult to determine the impact that factors such as AI technician, semen quality and semen handling may have had on pregnancy rates. Although semen quality was deemed to be satisfactory in terms of post-thaw motile cells and progressive motility (49% and 44%, respectively) ensuring the present of at least 12 million progressively motile sperm cells per insemination dose, previous evidence of the fertility of the semen was not available. Another important factor to consider is the timing of the inseminations. Heifers were inseminated 24 h after GnRH and this was based on an earlier study in which it was reported that ovulations occurred between 24 and 32 h after GnRH treatment [94, 98]. However, it is unknown if letrozole treatment alters the timing between GnRH treatment and ovulation. Daily ultrasound examinations did not allow the determination of the time of ovulation precisely during the studies included in Chapter 9. In the Ovsynch protocol, the second GnRH treatment is normally administered 48 h post PGF, and cows are inseminated 16 to 18 h later [295]. A study showed that GnRH treatment 24 h post-PGF resulted in lower pregnancy rates that when GnRH was given 48 h post-PGF, with both groups being inseminated 24 h post- second GnRH [94]. As discussed previously, it is conceivable that increasing the interval between PGF and GnRH from 24 h to 48 h could be beneficial not only to prevent short-lived CL as previously discussed, but also for oocyte competence and fertility. Furthermore, adjusting the timing of insemination using a letrozole-PGF-GnRH based protocol may also result in improved pregnancy rates.

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