Regulation and the modification of axial properties in partial embryos of the nemertean, Cerebratulus lacteus

Embryos acquire axial properties (e.g., the animal-vegetal, dorsoventral and bilateral axes) at various times over the course of their normal developmental programs. In the spiral-cleaving nemertean, Cerebratulus lacteus, lineage tracing studies have shown that the dorsoventral axis is set up prior to the first cleavage division; however, blastomeres isolated at the two-cell stage will regulate to form apparently perfect, miniature pilidium larvae. We have examined the nature of axial specification in this organism by determining whether partial embryos retain the original embryonic/larval axial properties of the intact embryo, or whether new axial relationships are generated as a consequence of the regulatory process. Single blastomeres in two-cell stage embryos were injected with lineage tracer, and were then bisected along the second cleavage plane at the four-cell stage. Thus, the relationship between the plane of the first cleavage division and various developmental axes could be followed throughout development in the 'half-embryos'. While some embryo fragments appear to retain their original animal-vegetal and dorsoventral axes, many fragments generate novel axial properties. These results indicate that axial properties set up and used during normal development in C. lacteus can be completely reorganized during the course of regulation. While certain embryonic axes, such as the animal-vegetal and dorsoventral axes, appear to be set up prior to first cleavage, these axes and associated cell fates are not irreversibly fixed until later stages of development in normal intact embryos. In C. lacteus, the process whereby these properties are ultimately determined is apparently controlled by complex sets of cell-cell interactions.