Stage 8

Key events: rapid phase of germ band extension, to 60% egg length

Stage 8 lasts for approximately 30 min (3:10-3:40 h) and comprises the rapid phase of germ band elongation; it ends with mesodermal parasegmentation.

Mesoderm and endoderm formation have taken place during the previous two stages by invagination of ventral furrow and the amnioproctodeum. The cells that remain at the outer surface of the embryo represent the ectoderm and the amnioserosa.

The gradual extension of the germ band over the dorsal side of the embryo is called germ band eleongation. This movement occurs very rapidly during its initial phase and is quite slowly during the following stages. By the end of stage 8 the elongation has progressed to bring the posterior tip of the germ band to about 60% egg length (0% egg length = posterior pole).

The germ band consists of an inner, mesodermal, and an outer, ectodermal layer. The beginning of germ band elongation coincides with profound modifications of the mesodermal primordium. The ventral furrow closes off shortly after invagination. The cylindrical cells of the ventral furrow are organized in a regular epithelium during stage 7. This architecture is lost at the onset of stage 8, when the epithelium disaggregates and all cells of the mesoderm start dividing within a very short time. During the initial period of germ band elongation, all mesodermal cells perform two consecutive, parasynchronous mitoses, while the ectodermal cells immediately adjacent to them remain mitotically quiescent.

Most of the cells in the walls of the pocket formed by the proctodeal invagination divide at this stage.

Signs of increased mitotic activity are visible within the ectodermal germ layer, mainly anterior to the cephalic furrow, in the territory of the procephalic lobe.

Media list
Stage 8, in vivo (1.5 MB)
Stage 8, invagination (0.3 MB)..........additional information

Genes discussed
Gene product - Domains
twist (twi)
transcription factor - bHLH
required for mesoderm development, high Twi levels block formation of visceral mesoderm and heart and induce somatic myogenesis