- Introduction
- Procedure
- Result/Discussion
- Conclusion
- Figures
- Protocols
- Sources
Results
The twinning process was successful in the procedure described above. At each successive twinning event, four-celled embryos were divided into two-celled embryos (Figure 1). At the completion of the twinning process, four samples were developed: a group that had undergone one twinning event, a group that had undergone a second twinning event, a group that had undergone a third-twinning event, and a control. Pluteus larva were observed in both the control and the "first-twinning" sample (Figure 2). Continued development was observed in the second-twinning and third-twinning samples, however, none of the embryos in these samples reached the pluteus stage (Figure 3).
Discussion
The results of this experiment indicate that the twinning of sea urchin embryos is possible at an early stage. However, the process is not efficient; using a large number of sea urchin embryos, only a very few survived to the pluteus stage. This relatively low survival rate may be due to the mosaic nature of sea urchin development. Any maternally contributed cytoplasmic determinants may have been insufficiently proportioned to allow further development in the twinned embryos. The mechanical stress of the twinning process may have also been a factor in the low success rate among the twinned samples: all embryos were subjected to extreme hypertonic conditions, and the twinning process itself &emdash;the act of being forced through a nylon mesh and being temporarily removed from the ASW solution as it passed through the mesh more quickly than the embryos&emdash;may have placed a great deal of stress on the embryos. These two experimental factors, the mosaic nature of development and the mechanical stress upon the embryos, would account for the arrest in development as the number of twinning events increased.
It is possible to hypothesize, based on the results of this experiment, that embryo twinning is possible in embryos of other non-regulatively developing species such as tunicates. Since mosaic and regulative development are not discrete classifications, but rather exist as a continuum, it remains to be seen exactly how far along the continuum towards completely mosaic development the twinning process is feasible. The general procedure outlined in this paper could be adapted for use in other species in order to determine the extent to which a set of maternal determinants effect viability in twinned embryos: one could hypothesize that the more mosaic in nature a species' development is, the less likely the embryo is to survive a twinning event.
Although the differences between mosaic and regulative development has implications on the viability of twinned embryos in any species, it is only one of many factors effecting the success and the possibility of twinning. One important factor in the viability of twinning an embryo is the presence of yolk in the zygote. For example, in the case of zebrafish, Rario danio, twinning is not feasible due to the presence of a large yolk pole which, if divided, would disintegrate and destroy the embryo (Gilbert, 2003). Similar complications present challenges in the embryonic cloning of chicks. In human embryos, the early activation of embryonic cytoplasmic factors presents a challenge to high success rates in embryonic twinning (Pence, 1998).
Difficulties aside, embryo twinning clearly has potential for use in a clinical setting, such as in vitro fertilization (IVF): currently dozens of eggs are removed from a women to be fertilized in vitro. The unused eggs then pose an ethical dilemma: they cannot be thrown away (that would be destroying life) and current federal legislation prohibits the use of these embryos for research and development purposes. Therefore, most often they are left to "freeze to death" before being discarded. However, a reliable form of embryo twinning may some day provide a solution for this problem: only a few eggs could be removed and fertilized, then twinned, then half of the twinned batch could be inserted into the mother. If those failed to implant, the twinning process could occur again and the process be repeated. Using embryonic twinning, it could be possible to refine the IVF process such that fewer embryos would be left after implantation was achieved.