occurs in two temporal and spatial different phases during embryogenesis.
Up to the 9th cleavage cycle (stage 3) all nuclei share a common cytoplasm.
Subsequently, 3 to 4 polar buds are formed around the nuclei at the posterior pole of the embryo.
After two further cleavages the polar buds pinch off at stage, giving rise to 12 to 14 spherical pole cells. These are the first cells generated during embryogenesis. The pole cells represent the germ line.Thus, with pole cell formation also the cellular seperation between germ line and soma has taken place.
The cellularization of the somatic cells occurs during stage 5, by introgression of membrane furrows to separate single blastoderm nuclei. It is completed, when the furrows reach the yolk.This is a rapid process, and is accomplished within 30 min at 25°C. Blastoderm nuclei are spherical at the onset of cellularization but elongate considerably as the process continues, increasing in length from 3 - 4 µm to 10-15 µm. Blastoderm cells around the perimeter of the entire egg are not completely isolated, since they still maintain connected with the syncytial yolk cytoplasm through wide cytoplasmic bridges. These connections are lost during gastrulation. All blastoderm nuclei and cells have the same shape and do not show any apparent differences between particular egg regions. Both, shape and size, however, will show considerable regional variations during the following gastrulation.
With the formation of the somatic cells, also the 3 germ layers (ectoderm, endoderm and mesoderm) as well as the extraembryonic amnioserosa become determined. Thus, it is possible by different methods to construct a fate map (also called anlagenplan) for this stage.
Stage 3 (0.1 MB)
Stage 3, in vivo (0.4 MB)
Stage 4, in vivo (2.5 MB)
Cellularization in vivo (0.4 MB)
Germ layers (1.2 MB)
Germ layers, interactive (4 MB)