Changes in structure and location of extracellular matrix of the blastocoel during development of the starfish Pisaster ochraceus

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Abstract
The extracellular matrix in the blastocoel of embryos of the starfish Pisaster ochraceus is easily visualized at both the light microscope and transmission electron microscope levels prior to the appearance of mesenchyme cells if the anionic dye alcian blue is included in the fixative. In the early gastrulae, the extracellular matrix is located close the ectodermal and endodermal cells and has four distinct forms: small visicles, amorphous material, granules of densely stained material, and short strands of intermediate staining density. The latter two types are usually associated with one another. A short time later, the extracellular matrix begins to organize into longer strands of intermediate staining density with dense granules located along their length and vesicles and amorphous materials in close association. Initially the strands appear to be unbranched; however, by mid gastrula they have formed an extensive, branched meshwork filling the blastocoel area adjacent the tip of the archenteron. This meshwork consists primarily of the dense granules arranged along the intermediate staining filaments. Crescent-shaped regions of dense granules are occasionally seen in the blastocoel adjacent the ectodermal cells during this period. Similarly shaped regions consisting primarily of vesicles and amorphous material are seen adjacent the endodermal cells. Shortly after the mesenchyme cells appear in the blastocoel, the strands of the meshwork located in the presumptive mouth region are arranged in a radial manner and appear thicker; the dense material is more evenly distributed, and small regularly spaced protrusions are found along their length. The results suggest that a meshwork of strands is gradually built up, probably by self-assembly of components derived from both the ectoderm and endoderm, before the mesenchyme cells enter the blastocoel. The organization of this meshwork of strands appears to be altered after the appearance of the mesenchyme cells which migrate through it.