Axis formation in the avian embryo is a multi-step process at the molecular and the cellular levels. The unfertilized avian egg shows a distinct radial symmetry, however, during the intra uterine stages the bilateral symmetry is determined by the formation of the polarized area pellucida. This polarity is superimposed after the egg is laid by sequential posterior-to-anterior morphogenetic events that include formation of the marginal zone, Koller`s sickle, the hypoblast and the primitive streak. In the laid egg, the cells in the posterior marginal zone and Koller`s sickle possess the highest potency to induce development of the embryonic axis and determine its location. At later stages this region progressively loses its potency. At these stages, the posterior midline region of the blastoderm, which includes mostly the epiblast, but not of the hypoblast, plays a crucial role in the process axis initiation. The relatively high proportion of cell division observed around this posterior region may be associated with its major developmental potential to initiate the embryonic axis. This region is identified as an axis initiation center and its cells are found to have organizer properties which determine the initiation site of the axis in the avian embryo. Although the early avian blastoderm has a remarkable capacity to regulate, only one embryonic axis initiates and develops in the blastoderm. It is concluded that at these earlier stages of development the avian blastoderm acts as an integrative system programmed to form a single embryonic axis by the coordinated action of inductive and inhibitory effects.
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