Evaluation of Equine Embryos

Introduction

Embryos are routinely evaluated after collection and prior to transfer into a recipient mare. The evaluation usually takes only a few moments to perform and can provide valuable information as to the probability that a given embryo may survive after transfer. The primary goal is to determine embryo quality, but the evaluation may also identify embryos with significant abnormalities, differentiate unfertilisedoocytesfromsmallembryos,anddetermineifthe developmental stage is consistent with embryo age.

Developmental stage

The developmental stage of an embryo is normally directly related to age or the number of days after ovulation (Table 1). After fertilisation, a series of cleavage divisions yields an embryo with multiple individual cells called blastomeres. A morula is an embryo with ≥32 tightly compacted blastomeres surrounded by a thick zona pellucida. An equine embryo is usually at the morula stage of development when it first enters the uterus approximately 6 days after ovulation.

As embryonic development continues, the morula transitions into a blastocyst as cell divisions proceed and a small fluid filled cavity or blastocoele begins to form within the centre of the embryo. The blastocyst consists of two populations of cells – an outer rim of trophoblast cells and a distinct inner cell mass embedded within the wall of trophoblast cells. The trophoblast layer will eventually form the placenta and the inner cell mass will form the embryo proper.

Embryo grade or quality

Grade refers to the quality of an embryo. A four-point grading system may be used to evaluate equine embryos. Characteristics used to determine a quality score or grade include: shape of embryo (spherical, oval, collapsed, etc.); thickness of zona pellucida; uniformity of blastomeres (size, colour, structure); presence/absence of extruded or degenerated blastomeres; compactness of blastomeres; degree of cytoplasmic granulation or fragmentation; presence/absence and size of perivitelline space; evidence of dehydration or shrinkage of embryo; presence/absence of abnormalities of the blastocoele; presence/absence of damage to the zona pellucida or capsule (Table 2).

Embryo size

The size of an embryo is also typically related to the age of the embryo. A day 6 equine embryo is approximately 0.2 mm in diameter, while day 7, 8 and 9 embryos are about 0.4, 1.1 and 2.2 mm in diameter, respectively. As the embryo expands, the zona pellucida becomes thinner and eventually is lost. A thin, transparent membrane called the capsule forms under the zona pellucida and becomes visible in the expanding embryo after the zona pellucida is lost. The rapid increase in size is clinically significant, as only small embryos (<0.3 mm) are suitable for freezing and large expanded blastocysts (>1.5 mm) are more easily damaged during flushing, washing and transfer procedures.

[...]