Time‐lapse system can provide a culture environment to observe the development of embryos continuously. There are many morphokinetic markers to help us to find out the best quality of embryos. We review the studies to clarify the relationship of markers between implantation potential and embryo chromosome status. Surprisingly, most of markers are controversial or no significant effect on implantation potential and pregnancy rate. We suppose that some uncertain factors may influence embryonic implantation and pregnancy. Here we provide a new method for selecting optimal quality of embryos by many morphokinetic markers in the time‐lapse system. Therefore, we can expect that the time‐lapse system helps us to choose the good quality embryos for subsequent embryos transfer to improve implantation potential, euploid chromosome and pregnancy rate. Furthermore, studies need to understand the other maternal physical conditions correlation with embryos implantation.
- cleavage embryo
- morphokinetic markers
The morphology of embryo is the most widespread method to select the embryo with high implantation potential in assisted reproductive technology (ART). Conventionally, embryo development was daily observed after insemination, which could assist the embryologists to select the optimal embryo to transfer for elevating live birth rate eventually. However, the daily observation is considered as a disadvantage for embryo development because of the frequent transfer between incubator and atmospheric environment. Thus, a new and powerful tool, time‐lapse monitor (TLM), was developed to estimate the morphokinetic markers of embryos. Currently, TLM can be used to evaluate the embryo growing status from the time of insemination to blastocyst formation. The sequential assessment of pronuclear, cleavage stage, and blastocyst morphology can continuously evaluate the morphology of embryos through automatically obtaining images in every 5–20 min. Besides, TLM offers a steady culture condition due to bypassing the daily observation. Here, we discuss the timing of embryo cleavage and the following effects of implantation potential in this chapter.
2. Morphokinetic markers
Generally, there are many milestones (Figure 1), including pronucleus appearance, pronucleus breakdown, first division, second division and blastulation, during the period of fertilization to blastocyst formation. The TLM fails to obtain the pictures at every minute since the capturing period was limited. Although the limitation of the time lapse is obvious, currently, it is still the most practical manner to evaluate the timing of embryo development rather than daily observation. Here, we listed the morphokinetic markers and discussed the timing of different time point during the development of embryos and the effect of clinical outcomes.
(1) The timing of second polar body extrusion (tPB2): the time of the second polar body extrusion is 2.9 ± 0.1 h after Intra-cytoplasmic sperm injection (ICSI). The range of extrusion time is around 0.7–10.15 h. If the oocytes from female age >38 years old, the timing of second polar body extrusion was significantly delayed but no other effects were observed in further embryo development . The mean time of tPB2 is 3.9 h in euploid and 4.0 h in aneuploid embryos, respectively. The chromosome integrity of embryos is irrelevant to the timing of second body extrusion .
(2) The timing of pronuclear appearance (tPNa): the time of pronuclear appearance is 8.4 ± 2.4 h in the implantation group and 8.2 ± 1.9 h in the non‐implantation group . In euploid embryos, the mean time of tPNa is 10.2 h and 10.1 h in aneuploid embryos . Therefore, the timing of pronuclear appearance has no significant effect on implantation potential and chromosome status.
(3) The timing of pronuclear fading (tPNf): longer time taken in pronucleus (PN) breakdown might be beneficial for live birth. Azzarello et al.  claimed that the timing of tPNf was longer in live birth group (24.9 ± 0.6 vs. 23.3 ± 0.4 h), and there was no live birth if the timing of PN breakdown was less than 20 h. The timing of PN breakdown was equal between implanted and non‐implanted embryos [3, 5]. The mean time of tPNf is 24.4 h in euploid embryos and 24.8 h in aneuploid embryos . The timing of pronuclear fading has no significant difference in embryo implantation and chromosome status but no live birth when tPNf is less than 20 h.
(4) Time from insemination to the 2‐cell stage (t2): it is still controversial in the period. Meseguer et al.  presented that the t2 of implanted embryos group was shorter than non‐implanted embryos (25.6 ± 2.2 vs. 26.7 ± 3.8 h). Chamayou et al.  showed no significant difference in implanted and non‐implanted embryos (26.9 ± 3.2 vs. 27.0 ± 4.0 h). Kirkegaard et al.  claimed that t2 was similar in the pregnancy and non‐pregnant groups. Curiously, t2 is shorter when embryo was incubated in single culture medium than sequential culture medium (27.36 ± 4.12 vs. 29.09 ± 4.86 h) . The mean time of t2 is no significant between euploid (28 h) and aneuploid embryos (28.4 h) . The development of the 2‐cell stage may be faster in implanted embryos but no significant in chromosome status.
(5) Time from insemination to the 3, 4, 5 cells (t3, t4, t5): some studies have shown that the enhanced implantation potential has been observed in shorter t3, t4 and t5. The time periods of t3, t4 and t5 were significantly shorter in implanted embryos than non‐implanted embryos. The times of t3 (37.4 ± 2.8 h), t4 (38.2 ± 3.0 h) and t5 (52.3 ± 4.2 h) are significant difference in implanted embryos compared with the times of t3 (38.4 ± 5.2 h), t4 (40.0 ± 5.4 h) and t5 (52.6 ± 6.8 h) in non‐implanted embryos . However, Chamayou et al.  and Kirkegaard et al.  demonstrated that there was no difference in embryo implantation and pregnancy rate. The embryo development is faster in single culture medium than in sequential culture medium (t3, 37.75 ± 6.64 vs. 39.53 ± 6.15 h; t4, 40.07 ± 5.98 vs. 41.45 ± 6.07 h; t5, 48.77 ± 9.49 vs. 52.22 ± 9.34 h) . The mean time of t3 (37.4 vs. 37.2 h) and t5 (50.4 vs. 50.6 h) is no significant difference between euploid and aneuploid embryos, but the mean time of t4 (40 h) is significant difference between the euploid (40 h) and aneuploidy (41.1 h) blastocysts . Consequently, faster embryo development of t3, 4, 5 is beneficial for implantation, but only t4 might influence the euploid rate of blastocysts.
(6) Time from insemination to the 6, 7, 8, 9 cells (t6, t7, t8, t9): according to the previous report, although the time from insemination to the 8 cells exhibited faster in implanted embryos (54.9 ± 5.2 vs. 58.0 ± 7.2 h) , the other report showed that there are no statistical difference between the implanted and nonimplanted embryos at t6 (54.3 ± 5.8 vs. 54.5 ± 8.2 h), t7 (57.4 ± 8.6 vs. 57.6 ± 9.8 h), t8 (61.0 ± 10.8 vs. 60.8 ± 11.5 h) and t9 (77 ± 8.5 vs. 76 ± 11.3 h) . In addition, Kirkegaard et al.  also proved that the pregnant rate was irrelevant to the period. In euploid embryos, the t6 (53.9 h), t7 (57.8 h), t8 (61.9 h) and t9 (76.1 h) are similar to the time in aneuploid embryos . Statistically, the t6, t7, t8 and t9 have no significant difference between the implanted and non‐implanted embryos and between the euploid and aneuploid embryos.
(7) Time from insemination to morula (tM): morula is defined as all cells fused together. There is no difference that the tM is 86 ± 9.1 and 84.4 ± 11.4 h in implanted and non‐implanted embryos, respectively . The tM of euploid (94.4 h) and aneuploid (95.3 h) are insignificant . Therefore, statistically, the tM does not involve in the implantation potential and chromosome status.
(8) Time from insemination to starting blastulation (tSB): the initiation of blastulation means the time point of the blastocoel cavity observation. There is no significant difference in the mean time of tSB in implantation and pregnancy [3, 5]. Therefore, the time from insemination to starting blastulation does not affect embryo implantation potential and pregnancy rate. However, the mean time of tSB (103.4 h) in euploid embryos is significant shorter than aneuploid embryos (103.4 h,
(9) Time period from insemination to expanded blastulation (tEB): expanded blastulation means the diameter of blastocyst had increased by more than 30%, the expanding results in a thin zona pellucida . There is no
(10) Time period between 2‐cell and 3‐cell stage (t3‐t2, cc2): cleavage cycle 2, time of the second cycle is also known as the time between 2‐cell and 3‐cell stage. The mean of cc2 is 11.4 h in implanted embryos and 11.8 h in non‐implanted embryos . Meseguer et al.  found the same cc2 (11.8 h) in implanted and non‐implanted embryos. The mean of cc2 (11 h) is also no
(11) Time period between 5‐cell and 3‐cell stages (t5‐t3, cc3): it is also defined as cleavage cycle 3 by Chamayou et al. . They presented that the median of cc3 was significant longer in implanted embryos than nonimplanted embryos (14.4 and 13.0 h, respectively). As a result, longer cc3 may be beneficial for embryo development.
(12) Time of synchrony of the second cell cycle (s2, t4‐t3): time between 4‐cell and 3‐cell stages or 3‐cell stage also means s2. The mean of s2 is 2 h in implanted embryos and 1.7 h in non‐implanted embryos . It also has no significant difference between pregnancy and non‐pregnancy groups . However, the mean of s2 is significant smaller in euploid embryos than aneuploid embryos (2.6 vs. 4.2 h) . Therefore, the mean of s2 might be used for predicting the chromosome status of embryos.
(13) Time of synchrony of the third cell cycle (s3, t8‐t5): S3 also signifies the time between 8‐cell and 5‐cell stages. It includes the sum of 5‐cell, 6‐cell and 7‐cell stages. There is no difference in the mean of s3 between implanted embryos and non‐implanted embryos (8.0 vs. 8.1 h) . Kirkegaard et al.  also found no difference between pregnancy and non‐pregnancy groups. There are no data compared with the mean of s3 in aneuploid and non‐aneuploid embryos. Hence, the effect of s3 on implanted potential and pregnancy rate remains no significantly.
3. Special markers in time‐lapse system
Some morphokinetic markers are only revealed in the time‐lapse system because the continuously and frequently recording system. Traditional observation has difficulty in observing these transitory phenomena. Following this, we listed these morphokinetic markers and conclude the effect of embryos.
The continuously morphokinetic change of embryo development is the main characteristic of time‐lapse system. We can observe many milestones of embryos development and calculate the time intervals to understand the relationship of implantation potential, chromosome status and pregnancy rate. Unfortunately, all the morphokinetic markers could not predict implantation potential, chromosome status and pregnancy rate exactly. Most of markers are controversial or no significant effect. Conventionally, embryos with quicker development would be recommended for transfer to raise the pregnancy rate. However, after reviewing all the data, not all markers can support this principle.
The reason of controversial descriptions of the markers is very incomprehensive. We suppose that some factors might influence embryos implantation and pregnancy. Obviously, maternal and physical conditions, such as endometrial receptivity, endometrial polyps, endometrial or endocervical infection, hydrosalpinx, immune disorder, subclinical hypothyroidism etc., can also impede the embryos implantation and the following pregnancy. We also know that aneuploid embryos show poor implantation rate or result in spontaneous abortion. Although some markers correlate with higher rate of euploid embryos, it still cannot be used for predicting euploid embryos precisely. If people want to know the chromosome status of embryos, pre‐implantation genetic screening (PGS) is still the first choice.
Therefore, the time‐lapse system can help us to evaluate the quality of embryos. We can use more precise morphokinetic markers to distinguish the embryos quality. The embryos with good quality have higher rate of implantation potential and normal chromosome. Currently, PGS is the optimal manner to find out the euploid embryos. However, the good quality of euploid embryo is not a guarantee of embryo implantation and pregnancy. It is the basic condition for better embryo implantation. We have to consider many other maternal and physical situations which greatly affect embryo implantation to promote the implantation and pregnancy rate. It also needs further studies to clarify the mystery of implantation process.