Mosaic and Regulation Phenomena during the Early Formation of the Chick Blastoderm
(a): Section through a hemiembryo after 2 days of culture. A hemi-primitive streak (with groove, indicated by arrowhead) is seen close to the cut edge; the unilateral mesoblast mantle (M) extends laterally (at the right of the figure) to form some coelomic vesicles (C) which form a dome over blood islands (B) in close association with the gut endoderm (EN). E, epiblast. Hematoxylin-eosin. Bar = 100 m. (b): Magnification of the medial part of Figure 8(a), showing the hemi-primitive streak region in detail. The hemi-primitive groove (arrowhead) is formed by unilateral ingrowth of thickened upper layer cells (UL) forming mesoblast strands (M) in the depth; E, epiblast (note the different aspect of the thicker undifferentiated upper layer and the thinner skin forming epiblast). Hematoxylin-eosin staining. Bar = 25 m. (c): Section through the cranial part (neural plate: NP) of a chicken hemi-embryo after 2 days of culture, at low magnification. Only a row of rudimentary coelomic vesicles (arrowheads) has developed in the unoperated side, forming the most cranial part of the coelomic cavity. Note the large intraembryonic cavity (G), empty at the operated side. No heart tube and no pericard are seen even in the unoperated side (right of the figure). Hematoxylin-eosin. Bar = 100 m. (d): Higher magnification of median part of the unoperated side, seen in 8C. S, somite; C, coelomic vesicles between the superficial parietopleura and the deeper splanchnopleura (forming blood islands) and adherent to the deep layer (D); bar = 25 m. (e): Section through the head region of a hemi-embryo after 2 days of culture, showing a neural groove (G) with median structures (derived from Hensen’s node); floor plate (F) and accompanying notochord (C), definitive endoderm (arrowheads); M, cranial mesoblast; hematoxylin-eosin. Bar = 50 m.
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