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#development stages

2 public questions tagged with this topic.

Chick development-ll

The primitive streak appears first at the:

Time-lapse imaging of chick gastrulation shows primitive streak first becomes morphologically visible at posterior marginal zone near Koller's sickle as cells accumulate expressing Brachyury epiblast marker. Posterior site coincides with Vg1 and Wnt8c expression domain inducing streak, then extension proceeds anteriorly driven by convergent extension and planar cell polarity. Appearance at anterior would invert polarity and misposition node. Normal development therefore initiates streak posteriorly, with Hensen's node forming later at anterior tip. Posterior origin ensures caudal to cranial development sequence and proper axis orientation. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Gilbert, Developmental Biology 12th ed., Chapter 12: Posterior origin and extension of primitive streak.

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Chick development-ll

Which region becomes the future posterior end of chick embryos?

Koller's sickle is thickened posterior marginal zone crescent at edge of area pellucida rich in cells expressing Brachyury, Nodal and Vg1. Fate mapping shows its cells contribute to posterior amnion and extraembryonic mesoderm but critically its signaling determines posterior identity of embryo, inducing primitive streak at its location. Thus region becomes future caudal end before streak extends. Area pellucida becomes central embryo, Hensen's node anterior tip, area opaca extraembryonic. Therefore Koller's sickle marks presumptive posterior end establishing axis polarity before gastrulation movements initiate. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Bachvarova et al., Koller's sickle progenitors and posterior axis in chick, Development 1998.

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