An overview of the available methods for morphological scoring of pre-implantation embryos in in vitro fertilization

Abstract

Assessment of embryo quality in order to choose the embryos that most likely result in pregnancy is the critical goal in assisted reproductive technologies (ART). The current trend in human in vitro fertilization/embryo transfer (IVF/ET) protocols is to decrease the rate of multiple pregnancies after multiple embryo transfer with maintaining the pregnancy rate at admissible levels (according to laboratory standards). Assessment of morphological feathers as a reliable non-invasive method that provides valuable information in prediction of IVF/intra cytoplasmic sperm injection (ICSI) outcome has been frequently proposed in recent years. This article describes the current status of morphological embryo evaluation at different pre-implantation stages.

Keywords: Blastocyst; Cleavage Stage; Embryonic Development; In Vitro Fertilization; Zygote.

Fig 1

A. Classification of pronuclear morphology according to Brezinova et al. (18). Pattern " O", is defined as the same number of small nucleolar precursor bodies (NPBs) distributed in the nucleus or large NPBs with polar distribution between the two pronuclei. B. Classification of pronuclear morphology according to Brezinova et al. (18). Pattern " Other", Zygotes with nonsymmetrical alignments of NPB.

Fig 2

Embryo scoring based on specific time points for embryo cleavage during screening. Image obtained from the article of Brazinova et al. (18).

Fig 3

The presence or absence of a cytoplasmic halo: zygote with cytoplasmic halo (A); zygote without cytoplasmic halo (B). Classification according to Depa-Martynow et al. (1).

Fig 4

Zygote classification according to Scott et al. (19): Z₁ zygote (equal numbers of nucleolar precursor bodies (NPBs) aligned at the pronuclear junction) (A), Z₂ zygote (equal number and size of nucleoli which were scattered equally in the two nuclei) (B), Z₃ zygote (zygotes with unequal numbers or size of nucleoli in just one nucleus and equal number and size of nucleoli in another nucleus) (C) and Z₄ zygote (the pronuclei are located in the periphery or are separated with different sizes) (D).

Fig 5

Examples of zygote scoring according to Senn et al. (25). Scores (1, 2 or 3) assigned to each individual parameter (proximity, orientation and centering of the pronuclei, cytoplasmic halo, number and polarization of nucleolar precursor bodies (NPBs)) are indicated for each zygote. The cumulated pronuclear score (CPNS) is indicated in parentheses. (Bar=10 μm).

Fig 6

A. High quality 8-cell embryo. Embryo grading: 8 cell, grade 4. Grading method according to Advanced Fertility Center of Chicago (24). B. Irregular cells and fragmented 5-cell embryo. Embryo grading: 5 cell, grade 2. Grading method according to Advanced Fertility Center of Chicago (24). C. Severely fragmented and unevenly sized cells embryo. Embryo grading: 6 cell, grade 1. Grading method according to Advanced Fertility Center of Chicago (24). D. Multinucleated 2 cell embryo. Image obtained from the site of Advanced Fertility Center of Chicago (24).

Fig 7

Embryos scoring according to Depa-Martynow et al. (1): grade A (embryos with 8 blastomeres and a maximum 20% of cytoplasmic fragmentation) (A), grade B (embryos with 8 blastomeres and over 20% cytoplasmic fragmentation) (B), grade C (4-6 cell embryos with a maximum 20% fragmentation) (C), grade D (4-6 cell embryos and over 20% fragmentation) (D).

Fig 8

Examples of scored blastocyst according to Gardner et al. (36) and image obtained from our IVF laboratory (27). A. 4AB. 4: well expanded, A: ICM and B: TE. B. 1AB. 1: Cavity <1/2 of the embryo’s volume, A : ICM, B: TE. C. 5AA. 5: Blastocyst hatching out of shell, A: ICM, B: TE. ICM; Inner cell mass and TE; Trophoectoderm.

Fig 9

Embryo evaluation according to Fisch et al. (37). A. 16-18 hours post insemination (stage 1), nucleolar alignment along the pronuclear axi, B. 25-27 hours post insemination (stage 2), demonstrating symmetrical blastomere cleavage and no fragmentation, C. 64-67 hours post insemination (stage 3), demonstrating symmetrical cleavage, eight cells and no fragmentation and D. Expanded blastocyst at ~120 hours post insemination (stage 4) (Image obtained from our IVF laboratory).