Ultrastructural aspects of the development of the hyaline layer and extracellular matrix lining the gastrointestinal tract in embryos and larvae of the starfish Pisaster ochraceus preserved by freeze substitution

Synthesis and secretion of molecules exhibiting the HL1 epitope during development of the hyaline layer of the asteroid Pisaster ochraceus

A complex ECM layer called the hyaline layer (HL) surrounds embryos and larvae of the starfish Pisaster ochraceus. When preserved by freeze substitution, the HL of a bipinnaria larva consists of six sublayers. From the plasmalemma outwards these are the intervillous layer (iv), the H3, H2, H1 sublayers that make up the supporting layer, a boundary layer (b) and the coarse outer meshwork (cm). HL development begins at fertilization when exocytosis of the cortical granules releases ECM into the perivitelline space and elevates the fertilization membrane. Over the course of early development the layers are added in a sequential manner and by hatching the embryo is surrounded by a thin HL containing most if not all of the layers. The layers thicken over the next few days. By the bipinnaria stage the larvae are surrounded by a thick six-layered HL. HL1 is a monoclonal antibody that reacts against an epitope found in all regions of the HL of the bipinnaria larva except the H2 sublayer. Western blots show that it is present on several molecules during HL development. The number and pattern of the HL1-labeled molecules change during development, suggesting that either new molecules are being produced or that some molecules are precursors of others. Light (immunofluorescence) and TEM (immunogold) studies using HL1 in the early stages of development show that HL1-positive material is not present in the corticle granules and that it only begins to be manufactured and secreted in quantity in the blastula stage at 18-20 h. Following this it continues to be secreted at least as far as the bipinnaria stage. Molecules containing the HL1 antigen therefore do not appear to play a major role in early development of the HL but are necessary for later events. The results suggest that, like the sea urchin HL, the starfish HL undergoes a sequential organization of the different HL layers from ECM components, which are released into the perivitelline space.