How to Evaluate Equine Embryos

1. Introduction

Personnel involved in the equine embryo transfer industry need to be able to properly identify and evaluate embryos. Accurate assessment of developmental stage, quality, and size of embryos is critical. It is also important to be able to differentiate between an embryo, an unfertilized oocyte (UFO), and non-embryonic structures that may be recovered during an embryo flush procedure.
The objectives are to review developmental stages of equine embryos and assignment of embryo quality score or grade.

2. Materials and Methods

Initial detection of an embryo in the search dish should be followed by a systematic evaluation of the embryo. In most instances, the evaluation will not take a long period of time. However, in some cases, a decision as to whether or not a structure is an embryo, an unfertilized oocyte, or a non-embryonic structure may take several minutes. It is recommended that the evaluation consist of developmental stage, quality score (grade), and size.
A good-quality microscope, with an eyepiece micrometer for measurement of embryo diameter, is essential for proper embryo evaluation. Embryos are typically washed through three or more drops of either flush medium or holding medium before evaluation. The initial flush fluid in the search dish may contain a significant amount of cellular debris. The goal is to transfer the embryo from that initial medium and sequentially remove as much debris as possible during the wash procedure. At the conclusion of the wash steps, the embryo is maintained in holding medium and evaluated before transfer, shipment to another facility for transfer, or cryopreservation.

3. Results

Developmental stages of equine embryos are presented in Table 1 (Figs. 1–4). [...]