Nowadays, the main criterion for select blastocyst to transfer is morphological feature, such as the grades for ICM and TE based on Gardner system. However, some essential morphological parameters may be neglected, i.e. cytoplasmic strings among cells. It is reported that some TE cells at the polar-mural junction extend cell projections to the surface of the ICM cells through the cytoplasmic strings by observing the ultrastructure of blastocysts . Salas-Vidal and Lomeli (2004) reported that filamentous actin was detected in the cytoplasmic extensions or filopodia in mouse blastocysts . There existed two kinds of filopodia: 1) short filopodia extend both from the ICM and the mural TE into the blastocoel cavity; 2) long, thin filopodia traverse the blastocoel from the mural TE to a central ICM cell. The short filopodia exist abundantly in blastocoel; and about 40% blastocysts could be observed with the long filopodia.
The cytoplasmic strings between ICM and mTE cells could commonly be observed at blastocyst stage by time lapse system. It is reported that 55–60% of blastocele could obverse this phenomenon . In present study, even 79.08% transferred blastocysts could be observed with cytoplasmic strings between ICM and mTE cells. The reason that present percentage is higher in our study may be the cases were transferred blastocysts, rather than all blastocysts. Besides, these blastocysts were selected as the most likely to get pregnancy, so we may conclude the cytoplasmic strings were more visible among good quality blastocysts. Therefore, it is crucial to explore the origin and effect of these strings.
The cytoplasmic strings may be caused due to the immigration from polar TE cells to mural TE . Salas-Vidal and Lomelı́ (2004) reported that the cytoplasmic strings between ICM and mTE cells may suggest cellular activity . Besides, filopodia are dynamic, appearing to extend and retract in coordination with cell division . The strings were also verified by immunolocalization of the FGFR2 and ErbB3 receptors to indicate a direct communication between the mural TE and the ICM cells . It is also reported that the cytoplasmic string could receive the required mitotic signals from the ICM. Other studies found that filopodia was abundant of E-cadherin, and determined cell elongation behaviors required by compaction. When α-catenin and β-catenin were knocked down, the filopodia quality was reduced dramatically[21, 23]. So, E-cadherin may be the main ingredient of filopodia. Besides, these cytoplasmic strings not only exist in vitro culture, but also in uteri of volunteers . So, cytoplasmic strings between ICM and mTE cells is a common physiological phenomenon that occurs during cell growth. The presence is conducive to the communication between the mural TE and the ICM cells, conducive to blastocyst development.
In present study, the group of presence of cytoplasmic strings between ICM and mTE cells was more visible with a higher grade of ICM and TE. Therefore, we may conclude that cytoplasmic strings between the mTE and the ICM cells may be an indicator of better quality blastocysts. This may due to the enhanced cellular communication between ICM and mTE cells. Therefore, it is interesting to explore the effect of these strings on the clinical and perinatal outcome of blastocysts.
In order to compare the pregnancy outcomes between blastocysts with present and absent cytoplasmic strings between ICM and mTE cells, clinical pregnancy, live birth and abortion rates were evaluated. A higher clinical pregnancy and live birth rates were observed (P < 0.05), but abortion rate was not significantly different (P > 0.05). In Thomas Ebner’s study, string-positive blastocysts and string-negative blastocysts have similar clinical pregnancy rate (46.3% versus 41.2%, P > 0.05) and live birth rate (42.7% versus 30.9%, P > 0.05). Though the live birth rate of string-positive blastocysts is much higher than string-negative blastocysts, no significant difference was observed. This may due to the quantity of involved cases, only 82 cycles in string-positive group and 68 cases in string-negative group . If Ebner’s study could enlarge the case quantity, a similar conclusion may be summarized. In our study, abortion rate was 10.65% in present stings group and 12.5% at absent stings group (P > 0.05). But, among the three groups of string classification, the abortion rate ranged from 8.08–20%. Due to the few cases of abortions, no significant difference was observed (P > 0.05). However, this also reminds us to take notice that whether the high abortion was caused by too many strings. For perinatal outcome, the neonatal birth weight was statistically similar in all groups, from 3.31 to 3.37 kg (P > 0.05). So, the cytoplasmic strings between ICM and mTE cells could increase the pregnancy and live birth rates, but did not influence the abortion and birth weight. This may due to the selective and bidirectional communication between mitotic mTE cells and ICM cells induced by cytoplasmic strings. Though the communication between mTE and the ICM could be accomplished by diffusion of secreted molecules into the fluid, this may also bring these signals to cells that have already ceased proliferation. So, cytoplasmic strings between ICM and mTE cells lead to communication more effectively and enable a more efficient delivery of substances and participate in the exploration of the extracellular environment. Therefore, we suspect for the first time that cytoplasmic strings between ICM and mTE cells are a positive phenomenon, which will increase the pregnancy and live birth rates clinically.