MECHANISMS OF GASTRULATION AND TAIL FORMATION IN ASCIDIANS

Ascidian embryos are useful for examining how events that occur during fertilization and cleavage affect gastrulation because they gastrulat e early in development, during the seventh cleavage. In ascidians, bot h dorsal-ventral and anterior-posterior axes are determined before fir st cleavage. The dorsal-ventral axis is fixed along the animal-vegetal axis of the fertilized egg following the first phase of ooplasmic seg regation, perhaps due to determinants moved to the vegetal pole in con cert with the myoplasm and plasma membrane components. The first oopla smic movements appear to be driven by the actin network in the cortica l myoplasm. The anterior-posterior axis becomes apparent after the sec ond phase of ooplasmic segregation, when the cortical myoplasm becomes detached from the egg membrane, and moves to the posterior pole of th e embryo. This movement is dependent on microtubules and has been attr ibuted to the formation and movement of the sperm aster. A major compo nent of the cortical myoplasm, p58, is co-localized along the microtub ules emanating from the sperm aster. Gastrulation begins during the se venth cleavage with the invagination of the large endodermal cells at the vegetal pole of the embryo. The neural plate appears as a thickeni ng of the epidermis on the dorsal side of the larva during the ninth c leavage; then the neural folds are formed, join, and close, elaboratin g the neural tube. Following neurulation, the tail is elongated as the neural tube and notochord cells intercalate at the midline of the emb ryo. Investigations using anural (tailless) ascidian larvae suggest th at some of the processes underlying elongation can be restored by the zygotic genome. Although ascidian larvae contain fewer cells and cell types than vertebrate embryos, ascidian gastrulation and morphogenesis appear to employ similar mechanisms to those in vertebrate embryos. T he extent of our current knowledge about the mechanisms involved in ga strulation and tail formation is summarized, and further experiments a re suggested to explore the molecular mechanisms underlying these proc esses. (C) 1993 Wiley-Liss, Inc.