Insulin-Induced Outgrowth of Pseudopodial Cables from Cultured Micromere-Derived Cells Isolated from Sea Urchin Embryos at the 16 Cell Stage, with Special Reference to the Insulin-Receptor.

Cultured cells derived from micromeres isolated from sea urchin embryos at the 16 cell stage are known to show outgrowth of pseudopodial cables followed by spicule rod formation when cultured in the presence of horse serum. Micromere-derived cells cultured with bovine insulin showed pseudopodial cable growth but did not produce spicule rods. Micromere-derived cells reversibly bound to insulin through out the period between 3 and 20 hr of culture. The dissociation constant of insulin with these cells was about 5.1 × 10⁻¹⁰M during the whole culture period examined. Horse serum, as well as blastocoelic fluid obtained from early gastrulae, concentration-dependently reduced the amount of insulin bound to these cells, but the bound insulin was scarcely replaced by any proteins tested, such as bovine serum albumin. The micromere-derived cells were bound to have an insulin-binding protein, that may be the receptor for insulin or insulin-like proteins. The insulin-binding protein had a smaller molecular weight than the insulin receptor of mammalian cells. The binding of insulin with this protein in micromere-derived cells probably results in pseudopodial cable growth.