The extracellular matrix of the early sea urchin embryo is known to have an important functional role in morphogenesis and in the regulation of cell type specific gene expression. We have undertaken an immuno-cDNA screen to identify the constituents of the embryonic blastocoelic-extracellular matrix. Here we describe a newly identified member of the extracellular matrix that we have designated ECM 3. The transcript encoding ECM 3 is approximately 9.5 kb in length and partial DNA sequence contains no significant similarity to other sequences in the Genbank. This transcript is present in eggs and early embryos, and early in gastrulation the transcript accumulation increases approximately 25 fold. In situ RNA hybridization shows that the mRNA is present uniformly throughout eggs and early embryos, but beginning at mesenchyme blastula stage, RNA accumulation is selective to cells of the ectoderm except at the animal pole, where ECM 3 mRNA is greatly reduced. In this species, Lytechinus variegatus, the animal pole ectoderm is the site of fusion with the invaginating endoderm during formation of the mouth. In situ analysis of protein expression using a monospecific polyclonal antisera made against recombinant ECM 3 polypeptides shows that during gastrulation the ECM 3 protein accumulates selectively in the basal lamina and blastocoelar regions adjacent to the ectoderm in all regions except for the ectoderm at the animal pole. The ECM 3 protein is not detected in other regions of the blastocoel e.g. adjacent to the endoderm. ECM 3 is also contributed maternally; the ECM 3 protein is synthesized during oogenesis and stored in oocytes within membrane-bound vesicles in the vicinity of Golgi complexes. Following fertilization ECM 3 is selectively secreted basally into the nascent blastocoel. No accumulation is detected in apical regions of the blastomeres or in the hyaline layer/apical lamina. This newly described molecule of the extracellular matrix thus demonstrates expression regulated both by secretion and by cell type specific gene expression, and shows a correlation between a microenvironment of the extracellular matrix and a morphogenetic event.
Keywords: basal lamina; extracellular matrix; gastrulation; regulated secretion.