Ultrastructural study of the hyaline layer of the starfish embryo,Pisaster ochraceus

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Abstract
The hyaline layer (HL) is a multilayered extracellular matrix (ECM) that coats the external surfaces of sea urchin and starfish embryos. It is thought to protect and lubricate the embryo, stabilize the blastomeres during morphogenesis, and regulate nutrient intake. Ultrastructural studies of chemically fixed embryos have shown the HL to consist of two to four sublayers. However, since chemical fixatives may cause collapse and alter the positions and antigenicity of the extracellular components, fixation methods that exclude chemicals may reveal a picture of the HL closer to what is present in vivo. Freeze substitution, a fixation method whereby tissues are rapidly frozen and dehydrated at low temperatures, has proved useful for fixing material rich in ECM. In this study, embryos of the starfish Pisaster ochraceus were fixed for microscopy using freeze substitution and three chemical methods in order to determine, as accurately as possible, the structure of the HL. Embryos appear to be best preserved by freeze substitution and demonstrate a HL consisting of at least six distinct sublayers. Based on staining with anionic dyes, most sublayers appear to contain glycosaminoglycans. Freeze substituted embryos, which were also stained with monoclonal antibodies raised against their ECM, revealed that some molecules are common to all six sublayers, whereas other molecules may be restricted to specific sublayers. This suggests that each sublayer could have a different function. Additional evidence suggests that microvillus associated bodies, present in other marine invertebrate embryos, may anchor the asteroid HL to the cell surface microvilli.

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