Evidence-Based ET: Does corpus luteum (CL) quality in bovine ET recipients matter?

Categories: Evidence-Based ET
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Published on: December 19, 2013

John F. Hasler

Yes, of course it does! Obviously, however, the accuracy of this statement depends on the definition of CL ‘quality’. In many cases, assessing the ‘quality’ of the corpus luteum is not a particularly accurate endeavor whether it is based on gross morphology, rectal palpation or ultrasound examination. When considering the outcome of ET in bovine recipients, the most important single criterion involving the CL is whether or not there is sufficient progesterone to support a pregnancy. Based on this, and for the purposes of this discussion, I am defining CL ‘quality’ as being related to progesterone secretion.  However, it is not practical to determine progesterone levels in recipients at the time of ET.  In addition, numerous studies have demonstrated that there is a very wide range of progesterone levels at the time of ET that successfully support pregnancy. Consequently, ET usually is based on physical characteristics of the CL at the time of transfer.

The early ET program at Colorado State University involved mid-ventral surgical transfers into recipients under general anesthesia.  Recipients were used if a CL was visually identified, irrespective of size.  Animals with CL judged to be small or lacking a crown exhibited progesterone levels that were comparable to females that possessed CL of normal appearance (Hasler et al., 1980). In addition, at the time of transfer there were no significant differences between serum progesterone levels of females which remained pregnant after transfer and those that did not. When flank surgical transfers replaced the mid-ventral approach, ET practitioners continued to have the opportunity to visually inspect the CL at the time of transfer. At Em Tran, Inc., all recipients were rectally palpated prior to transfer and the CL was visually evaluated in some.  There were no differences in pregnancy rates of recipients with CL classified as normal, small or cystic (Hasler, et al. 1987). Similar results, also based on flank surgical transfers, were reported from the Select Embryos ET unit in Ohio (Coleman et al., 1987). Similarly, the ET program at Carnation Genetics failed to find a relationship between the palapable size or cystic status of the CL and progesterone level at the time of transfer (Remsen et al., 1982).

The adoption of nonsurgical transfer, which occurred before the ready availability of real time ultrasound machines and/or the widespread adoption of the technology by cattle ET practitioners, mandated that the presence and suitability of CL were determined by rectal palpation.  In using crossbred recipient cows in Brazil that were rectally palpated prior to ET, no effect of CL size on pregnancy outcome was detected (Benyei et al., 2006). Likewise, in a Florida study involving lactating, heat-stressed Holstein recipients, no ultrasound exams were utilized, and there was no correlation detected between CL category (large with crown, small CL with no prominent crown and poor or cystic and pregnancy rate (Ambrose et al., 1999).

In a very nice study conducted in both VA and KS, Spell et al. (2001) recorded CL measurements with ultrasound and collected blood for progesterone analysis at the time of ET. The study did not find any differences in mean CL diameter, luteal volume or plasma progesterone concentration among recipients that did or did not become pregnant after ET. In addition, a veterinarian performed rectal palpation without knowing the results of transrectal ultrasonography. Embryos were transferred to all recipients deemed “unacceptable” based on rectal palpation due to the presence of a cavity, resulting in a “soft” texture or the absence of a palpable crown, if they possessed a CL >13mm in diameter.  The pregnancy rate in this group was significantly higher (80%) than the group of recipients possessing CL classified as “good or excellent” (70%), based on rectal palpation. This study demonstrated that the minimum progesterone level for establishment of pregnancy was lower than previously reported. More importantly, it also was clear that very suitable recipients would have been rejected on the basis of rectal palpation without a follow-up ultrasonographic evaluation. Looney et al. (2006) stated that CL >10mm in diameter were acceptable for use as ET recipients.

In contrast to the Bos taurus studies quoted above, a study involving Bos indicus cattle in Brazil showed that pregnant ET recipients had slightly larger CL (20.3mm) compared to open recipients (18.6mm), but there was no difference in progesterone levels at the time of transfer (Nogueira et al., 2012). However, in another Brazilian study conducted in Amazonia and involving more than 5,000 Bos indicus recipients, there was no correlation between CL size or type and pregnancy rates following ET of IVF-derived embryos (Florentino et al., 2013. Lastly, in another Brazilian study involving crossbred (Bos taurus X indicus) cow and heifer recipients, synchronized for fixed-time ET and ultrasonically examined prior to transfer, there was no relationship between CL size or cystic status and pregnancy outcome. Mean progesterone levels in recipients that became pregnant were, however, higher than in recipients that failed to maintain pregnancy following transfer.

 

Where does all this information leave the ET practitioner who just wants to improve pregnancy results?  First of all, I am quite sure that many of you have long since developed firm opinions on what constitutes an acceptable CL, whether it is based on palpation or ultrasound. That said, unless you quantitatively analyze a fairly large data set, you may or may not be operating under either a false or accurate ‘clinical impression’. Based on my personal experiences and taking into account the literature discussed above, some of which is contradictory, the following conclusions seem valid: When assessing potential recipients, whether they were observed in estrus or synchronized for fixed time ET, palpation of any structure that resembles a CL, even if small in size, is probably a valid candidate for transfer.  When ultrasound is employed, then females with CL exhibiting no palpable crowns can also be used with confidence, as can those with cystic CL.  I have no explanation for why the Bos indicus CL sizes reported in the papers by Florentino et al. (2013) and Nogueira et al. (2012) were larger than what is routinely reported in Bos taurus cattle.

 

References

Ambrose, J.D., Drost, M., Monson, R.L. et al., 1999. Efficacy of timed embryo transfer with fresh and frozen in vitro produced embryos to increase pregnancy rates in heat-stressed dairy cattle. J Dairy Sci. 82:2369-2376.

Benyei, B.,  Komlosi, I.,  Pecsi, A., et al., 2006. The effect of internal and external factors on bovine embryo transfer results in a tropical environment. Anim. Reprod. Sci. 93:268-279.

Coleman, D.A., Dailey, R.A., Leffel, R.E., Baker, R.D. 1987. Estrous synchronization and establishment of pregnancy in bovine embryo transfer recipients. J Dairy Sci. 70:858-866.

Florentino, C.M., Mariani, A.C.B., Souza, J.F., et al., 2013. Pregnancy rates bovine recipients inovulated with in vitro produced (IVP) embryos in the legal Amazon. J. Anim. Sci. Adv. 3:233-242.

Hasler, J.F., Bowen, R.A., Nelson, L.D. Seidel, G.E. Jr. 1980. Serum progesterone concentrations in cows receiving embryo transfers. J. Reprod Fert 58:72-77.

Hasler, J.F., McCauley, A.D., Lathrop, W.F., Foote, R.H. 1987. Effect of donor-embryo-recipient interactions on pregnancy rate in a large-scale bovine embryo transfer program. Theriogenology 27:139-168.

Ireland, J.J.,  Murphee, R.L., Coulson, P.B. 1980. Accuracy of predicting stages of bovine estrous cycle by gross appearance of the corpus luteum. J Dairy Sci 63:155-160.

Nogueira, E, Cardoso, G.S., Marques, H.R. et al. 2012. Effect of breed and corpus lueum on pregnancy rate of bovine embryo recipients Rev Bras de Zoo 41:1-13.

Remsen, L.G., Roussel, J.D., Karihaloo, A.K. 1982. Pregnancy rates relating to plasma progesterone levels in recipient heifers at day of transfer. Theriogenology 18:365-372.

Spell, A.R., Beal, W.E., Corah, L.R., Lamb, G.C. 2001. Evaluating recipient and embryo factors that affect pregnancy rates of embryo transfer in beef cattle. Theriogenology 56:287-297.

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