Embryonic shield

After about 8-1 Oh of incubation, the embryonic shield or posterior marginal zone has formed, and the position from which the primitive streak will arise is determined. The disk now has polarity dorso/ventrally and antero/posteriorly as well as left/right. 

Fig. 4. (A) A disk with virtually no landmarks to indicate the future polarity of the embryo. (B) Stage 1 after Hamburger and Hamilton (HH), the prestreak, incubation for 4-8 h. After several hours of incubation, the formation of the hypoblast appears as an area of higher density roughly shaped as a triangle rising from the posterior marginal wall. This is called the embryonic shield or Roller s sickle. (C) Stage 2 (HH), the initial streak (6-10 h), a short, squat, thickening of the blastoderm at the posterior marginal zone.

Shields G, Sang JH 1970 Characteristics of five cell types appearing during in vitro culture of embryonic material from Drosophila melanogaster. J Embryol Exp Morphol 23 53-69... 

As regards the function of this electronegative shield in some cell-types, membrane sialic acids prevent aggregation due to electrostatic repulsion in, for example, blood platelets, erythrocytes, and carcinoma cells in culture,418 whereas, in others, for example, chick, embryonic muscle-cells,422 aggregation is facilitated, most probably by Ca24 bridges. Sialic acid was also shown to be involved in the attachment both of endothelium and epithelium to glomerular basement-membranes of rat kidney.423... 

Antibiotics. Embryonic development of Drosophila is rapid, being complete by 21 hours postfertilization at 25 C. Since dissected preparations are cultured from extended germ band (>7.5 hours AEL) and most applications require culturing for only a limited period (<12 hours), antibiotics should not be necessary. If added, penicillin G (0.03 g/liter) and streptomycin sulfate (0.1 g/liter) can be used as in dispersed cell cultures (Shields and Sang 1977). 

Shields G. and Sang J. 1977. Improved medium for culture of Drosophila embryonic cells. Drosophila Inform. Serv. 52 161. 

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