Abstract
Recent studies reported morphokinetic indices for optimal selection of embryos in assisted reproductive technology (ART). The morphokinetics in blastocyst stage include the collapse and re-expansion rates after thawing. However, evaluation methods using these morphokinetics have not been established, mainly because the underlying molecular mechanisms remain unclarified. In this study, we focused on the relationship between these morphokinetic observation of the blastocyst behaviour and the number of cells constituting the blastocyst. We evaluated 38 surplus human frozen-thawed blastocysts using time-lapse cinematography and recorded their expansion, contraction, and hatching. A total of 28 blastocysts expanded in culture (cross-sectional area ≥ 5,000 π μm2). In comparison to the ones that did not, the expanded group presented significantly more number of inner cell mass (ICM) and trophectoderm (TE) cells, which eventually develop into the fetus and placenta, respectively (ICM: Expanded 10.2 ± 6.3 vs. Non-Expanded 6.0 ± 12.3, p < 0.05; TE: Expanded 165.7 ± 74.8 vs. Non-Expanded 57.0 ± 29.4, p < 0.05). Moreover, a positive correlation was found between the expansion rate (up to 4 h) and the number of TE cells (r = 0.558, p = 0.0021). Additionally, blastocysts that hatched had a significantly higher number of TE cells than those that did not (hatching 225.2 ± 61.2 vs. no hatching 121.1 ± 48.6, p < 0.0001). The number of TE cells per unit of cross-sectional area correlated negatively with the contraction time (r = –0.601, p = 0.0007). No correlation between the number of ICM cells and these morphokinetics was detected. In conclusion, our study demonstrates that different morphokinetics of frozen-thawed blastocysts reflect the number of TE cells. The differentiation of blastocysts containing sufficient TE cells would be beneficial for implantation and prognosis of a subsequent pregnancy. Thus, evaluation of these morphokinetics can be an effective method to screen good embryos for ART.
Highlights
In 1978, the first child in the world was born with the aid of in vitro fertilization/embryo transfer (IVF/ET)
As Gardner reported, blastocysts in the Expanded group comprised a large number of inner cell mass (ICM) and TE cells [1]
We clarified the morphokinetics of frozen-thawed human blastocysts and demonstrated that the presence of re-expansion, initial expansion rate, occurrence of contraction, and hatching were related to the number of TE cells
Summary
In 1978, the first child in the world was born with the aid of in vitro fertilization/embryo transfer (IVF/ET). The Gardner grading system, based on the degree of expansion of the blastocyst as well as ICM and TE qualities, is widely used to select embryos in ART [1]. Morphological indices have attracted great interest, the extent of blastocyst expansion and the TE grade [2,3]. This type of visual-based evaluation of the embryos can be highly subjective [4]. Morphokinetics observable with TLC show variable re-expansion rate and collapse. Blastocysts showing collapse have been associated with decreased pregnancy rates in ART [10], this is controversial [11]. The mechanisms by which these morphokinetics differ are not clearly understood, making it difficult to interpret the blastocyst behavior and predict outcome of ART
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