High proliferation rate characterizes the site of axis formation in the avian blastula-stage embryo

Localized zones of high cell proliferation have been thought to be important in determining several phases of axis formation at early stages of chick development. It was suggested that a developmental center, a center of cellular activity such as proliferation and movement, is located in the posterior half of the area pellucida in the unincubated chick blastoderm. In the work reported here, we have used the bromodeoxyuridine (BrdU) incorporation procedure followed by immunoperoxidase detection to assess the rate of cell proliferation at particular sub-regions of pregastrulating chick blastoderms (stages X-XIII). Examination of whole-mount and histological sections of stages X through XII blastoderms, pulsed with BrdU, showed no distinguishable difference in labeled cells between particular regions of these blastoderms and also that there are no specific zones of high cellular proliferation in either the hypoblast or the epiblast layers of the area pellucida. However, our observations have shown a striking difference in the stage XIII blastoderm, in which a relatively high amount of labeled cells were detected all around the posterior region of the area opaca, the marginal zone, Koller's sickle and the epiblast. The relatively high proportion of cell divisions observed at the posterior end of a stage XIII blastoderm, the blastula stage of the avian embryo, may be associated with the major developmental ability of this region to initiate an embryonic axis. Directional axis formation, therefore, may be attributed to a region of proliferation in the posterior side of a stage XIII blastoderm.