Stage 9

Key events: germ band elongation to 70% egg length, early neuroblast delamination

Stage 9 lasts for approximately 40 min (3:40-4:20 h) and is initiated by a transient parasegmentation of the mesodermal layer.

By the form of bulges that protrude slightly into the yolk sac, the mesoderm shows a metameric pattern. In the ectoderm only a slight and also transient parasegmentation becomes visible at this time.

The proctodeal opening reaches 70% egg length (0% egg length = posterior pole). The end of stage 9 is defined by the onset of foregut invagination – the stomodeal invagination.

In stage 9 the first neuroblasts delaminate from the ectoderm; therefore, by the end of stage 9 the germ band exhibits a clearly three-layered architecture: mesoderm - neuroblasts - outer ectoderm.

At the beginning of stage 9, mitotic divisions in the mesoderm cease; the mesodermal cells then reorganize, changing from the loose arrangement into a monolayer.

The ectoderm of the germ band in stage 9 is divided in two clearly defined regions. One is ventromedial and consists of large cells; the neuroblasts and the ventrolateral epidermis derive from the cells of this region (also called neurogenic region). The other ectodermal region is located laterally, and consists of a palisade of narrow, tightly packed cells; the dorsal epidermis and the tracheal placodes will develop from this lateral region.

Media list
Stage 9, in vivo (1.8 MB)
Stage 9, Parasegmentation of mesoderm
Stage 9, anti-Crumbs..........additional information
Stage 9 (0.4 MB)
Stage 9, interactive (4 MB)
Stage 9, Mesoderm (0.3 MB)..........additional information

Genes discussed
Gene product - Domains
crumbs (crb)
transmembrane -EGF repeats - laminin A homolog
involved in epithelial polarity, expressed in the apical membrane of ectodermal cells
twist (twi)
transcription factor - bHLH
required for mesoderm development, high Twi levels block formation of visceral mesoderm and heart and induce somatic myogenesis