Embryos

Blastomere number

The first cleavage may occur as soon as 25h after insemination. The fast cleaving embryos are at the 4-cell stage on day 2 ( 44-46 h post insemination) and at the 8-cell stage on Day 3 (66-68h post insemination). As cleavages of human embryos are asynchronous, embryos at the 3, 5, 6,7 or 9-cell stage are frequently observed.

Blastomere size

From the 2-cell to the 8-cell stage, there are 3 different sizes of blastomeres, as shown in Figure 1. Identical size of blastomeres is observed only at the 2, 4 and 8-cell stages. Embryos presenting blastomeres of different sizes at the 2, 4 and 8-cell stages are considered as uneven embryos. Embryos at the 3-cell stage with one large and two small blastomeres are even embryos.

Multinucleation

3 states of nucleation of the blastomeres can be observed: one nucleus per blastomere, no nucleus (M-phase, before cleavage) or several nuclei (multinucleation). Multinucleation, that is highly abnormal, is better observed at the 2-cell stage.

Extracellular fragmentation

The degree of fragmentation may vary considerably and is usually expressed as % of embryo volume. Fragmentation can be separated in 3 categories: < 20%, 20-50% and >50%.

Parameters of the EQC test

The EQC test will present you several possible answers for the various characteristics of an embryo. You can make only one choice. You are certainly used to grade the embryos in your day-to-day activity, grading is a convenient way of communicating embryo characteristics both to colleagues and medical staff. In our view, this is an important parameter to monitor in an External Quality Control scheme.

The first mitotic division usually occurs between 20-24 h after fertilization. Cleavages will then continue

Parameters	Morphological aspects
2 – 16	nomber of blastomeres clearly identifiable in an uncompacted embryo

Embryos usually cleave giving rise to blastomeres of equal sizes. Usually, the mitotic divisions occur in an asynchronous manner in each blastomere. In embryos with 2, 4, 8 or 16 blastomeres the sizes of each cell should ideally be equal, but this might not always be the case. In embryos with uneven blastomeres (3, 5, 7, 9, 11, 13, 15) or in those containing a number of blastomeres that is not a multiple of 4, the sizes should also be different.

Parameters	Morphological aspects
Equal	the blastomeres are of equal sized
Unequal	the blastomeres are of unequal sizes

During development of the embryo, some blastomere may expel some material, known as fragments. These fragments may vary in amount and size from one embryo to the other. Ideally, no or very little amount of fragments is visible. In extreme case, the fragments may occupy almost all space within the zona pellucida.

Parameters	Morphological aspects
0%	no fragment appear between the blastomeres
<10%	the volume occupied by the fragments is less than 10% of the total embryo volume
10-<25%	the volume occupied by the fragments is more than 10%, but less than 25%
25-<50%	the volume occupied by the fragments is greater than 50%

Effects of embryo morphology on implantation

The influence of embryo morphology on implantation is summarized in the following table.

Parameters	Morphological aspects
Blastomere number	Beneficial if 4-5 on Day 2 or 7-9 on Day 3
Uneven embryo	Detrimental
Multinucleation	Detrimental
Fragmentation	Detrimental if severe

References

Rienzi et al. Significance of morphological attributes of the early embryo. RBMonline 10: 669-681, 2005
Roux C et al. Morphological classification of human in-vitro fertilization embryos based on the regularity of the asynchronous division process. Human Reprod Update 1: 488-496, 1995.
Van Royen et al. Multinucleation in cleavage stage embryos. Human Reprod 18: 1062-1069, 2003

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