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Fig. 6. Differentiation stages during early PGC development leading to motility, polarization and guided migration. Following their specification, a process which is controlled by maternally provided germ plasm (Hashimoto et al., 2004; Knaut et al., 2000; Yoon et al., 1997), the PGCs appear morphologically similar to somatic cells, yet they show expression of characteristic markers and can be specifically labelled with GFP. During the next stage, the PGCs assume a complex non-polarized morphology, express high levels of E-cadherin on their membrane and do not respond to guidance cues provided by the chemokine SDF-1a. Further differentiation depends on de novo transcription in the zygote since inhibition of transcription by 
-amanitin leads to a complete developmental arrest of PGCs at the second phase. The transition to the third stage relies on the function of the Dead end protein. This stage is characterized by moderate down regulation of E-cadherin and the competence of PGCs to polarize and migrate in response to directional cues.
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