Equine in vitro produced blastocysts: assessing the foaling rate

I have included a second study for context: to provide numbers to the days to blastocyst formation and earlier data on optimal post-ovulation days to implant the embryo into the recipient mare.
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Equine in vitro produced blastocysts: relationship of embryo morphology, stage and speed of development to foaling rate
Reprod Fertil Dev. 2023 Feb;35(4):338-351.
Authors
Niamh Lewis 1 , Heloisa Canesin 2 , Young Ho Choi 3 , Rob Foss 4 , Matheus Felix 5 , Kindra Rader 3 , Katrin Hinrichs 5
Affiliations

1 School of Veterinary Medicine, University College Dublin, Belfield Campus, Dublin 4, Ireland.
2 Equine Embryology Laboratory, CH Equine Reproduction, Adamantina, Brazil.
3 Department of Veterinary Physiology and Pharmacology, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, TX 77843, USA.
4 Equine Medical Services, Inc., Columbia, MO 65201, USA.
5 Department of Veterinary Physiology and Pharmacology, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, TX 77843, USA; and Department of Clinical Studies – New Bolton Center, University of Pennsylvania, 382 W. Street Road, Kennett Square, PA 19348, USA.

Abstract

Context: Information on factors associated with developmental competence of equine in vitro -produced (IVP) blastocysts is lacking.

Aims: To determine the relationships of stage, grade, day of development, and specific morphological parameters of equine IVP blastocysts, to pregnancy and foaling rates.

Methods: Photomicrographs of 316 IVP embryos with known pregnancy outcomes were scrutinised individually by four observers. Inter-observer variation was assessed, and pregnancy outcome evaluated in relation to day of blastocyst development and assigned grade and stage. Individual component analysis was performed to determine the association of specific morphological parameters with foaling rate.

Key results: Overall pregnancy rate was 76.9% and foaling rate was 56.3%. The day of embryo development did not affect pregnancy rate but significantly affected foaling rate. Embryo stage did not affect foaling rate. Embryo grade affected foaling rate only for Day-9 embryos. Some morphological features in the bovine grading system did not predict outcome in equine IVP embryos. Significant individual parameters differed between Stage 5 and Stage 6 equine blastocysts.

Conclusions: Day of blastocyst development is the major factor related to foaling rate for equine IVP embryos. Notably, there was no effect of embryo stage on foaling rate and no evidence that prolonging culture until embryos advance in stage increases foaling rate. The standard bovine grading system is not directly applicable to equine IVP embryos; equine-specific staging and grading systems are proposed.

Implications: This information will allow laboratories to identify embryos with the highest developmental competence. Use of the proposed systems will increase consistency in embryo assessment among laboratories.

Reprod Fertil Dev
2020 Mar;32(5):468-473.
Speed of in vitro embryo development affects the likelihood of foaling and the foal sex ratio
A Claes 1, J Cuervo-Arango 2, S Colleoni 3, G Lazzari 4, C Galli 4, T A Stout 2
Affiliations expand

Abstract
The success of invitro embryo production (IVEP) in horses has increased considerably during recent years, but little is known about the effect of the speed of invitro embryo development. Blastocysts (n=390) were produced by intracytoplasmic sperm injection of IVM oocytes from warmblood mares, cryopreserved, thawed and transferred into recipient mares on Days 3, 4, 5 or 6 after ovulation. The time required for invitro-produced (IVP) embryos to reach the blastocyst stage was recorded (Day 7 vs Day 8). The likelihood of foaling was affected by the speed of invitro embryo development and recipient day after ovulation at transfer. The odds ratio for foaling was ~0.63 for transfer of Day 8 (46%) compared with Day 7 (56%) IVP blastocysts. The highest likelihood of pregnancy (72%) and foaling (60%) was observed when IVP blastocysts were transferred to recipient mares on Day 4 after ovulation. Finally, the sex (colt:filly) ratio was higher after transfer of Day 7 (71%:29%) than Day 8 (54%:46%) IVP blastocysts, suggesting that the speed of embryo development is sex dependent. In conclusion, the speed of invitro embryo development in our IVEP system affects the likelihood of foaling and the sex of the foal.