The Avian Marginal Zone and Its Role in Early Development

Abstract

The term avian marginal zone is descriptive but in a way misleading, as it automatically reminds us of the same term used for a nonhomologous structure in amphibian development. In the amphibian blastula the marginal zone is a belt whose cells are already determined to invaginate during gastrulation and to form the different mesodermal components. In the chick the marginal zone (MZ) is also a belt-like region at the periphery of the area pellucida, but its contours start to show up at the posterior end at a stage earlier than the blastula (stage X E.G&K: Eyal-Giladi and Kochav, 1976), and become completely delineated at stage XIII E.G&K which is comparable to the amphibian blastula (Waddington, 1933; Eyal-Giladi, 1984, 1991). A stage XIII blastoderm can be divided according to morphological criteria into two components:

• 1) The central, blastular part senso stricto, which is double layered. The upper layer - the epiblast - can be compared to the amphibian animal hemisphere, while the lower layer - the hypoblast - can be compared to the amphibian vegetal hemisphere. The narrow space separating the two layers is comparable to the blastocoelic cavity.

• 2) The peripheral ring surrounding the blastular part, includes the marginal belt adjacent to the central part, which in turn is encircled by the area opaca (Fig. 1).