Formation of the intra-embryonic coelom in the early chick embryo.

The formation of the intra-embryonic coelom has been studied by light and electron microcopy. The coelom is formed by the fusion of small spaces that arise between two layers of lateral plate mesoderm. The first spaces appear at stage 6 and are found in the antero-lateral part of the area pellucida. Other spaces appear medial and caudal to the early ones. These small spaces fuse to form a large single cavity by stage 8. Cephalic extension of the coelom is by insinuation of this large cavity between the ectoderm and endoderm of the pro-amnion. Medially, the cavity extends as a ventro-medially directed pouch which meets its counterpart of the alternate side of the area pellucida by stage 9. The two sides are fused by stage 10. Changes occur in the shape of mesoderm cells during the formation of the cavities. The initially multilayered stellate mesoderm cells are arranged into two layers of columnar cells. With the formation of the cavities the cells change from columnar, through cuboidal, to squamous. The process of cell shape changes occurs in a cephalo-caudal and a medio-lateral direction. Also, the somatic mesoderm precedes the splanchnic mesoderm in the flattening out process. Glycosaminoglycans (GAG) are detectable in the coelom following treatment of embryos with cetylpyridinium chloride or tannic acid. The presence of GAG, proline-containing materials and fibronectin is demonstrated in the basement membranes related to the lateral plate mesoderm. The significance of these findings is discussed in relation to the role of the macromolecules in the morphogenesis of the chick coelom.