The organic matrix of the skeletal spicule of sea urchin embryos.

Abstract

Abstract. The micromeres that arise at the fourth cell division in developing sea urchin embryos give rise to primary mesenchyme, which in turn differentiates and produces calcareous endoskeletal spicules. These spicules have been isolated and purified from pluteus larvae by washing in combinations of ionic and nonionic detergents followed by brief exposure to sodium hypochlorite. The spicules may be demineralized and the integral matrix dissolves. The matrix is composed of a limited number of glycoproteins rich in aspx, glux, gly, ser, and ala, a composition not unlike that found in matrix proteins of biomineralized tissues of molluscs, sponges, and arthropods. There is no evidence for collagen as a component of the matrix. The matrix contains N-linked glycoproteins of the complex type. The matrix arises primarily from proteins synthesized from late gastrulation onward, during the time that spicule deposition occurs. The mixture of proteins binds calcium and is an effective immunogen. Electrophoresis of the glycoproteins on SDS-conraining acrylamide gels, followed by blotting and immunocytochemical detection, reveals major components of ~47, 50, 57, and 64 kD, and several minor components. These same components may be detected with silver staining or fluorography of amino acid-labeled proteins. In addition to providing convenient molecular marker for the study of the development of the micromere lineage, the spicule matrix glycoproteins provide an interesting system for investigations in biomineralization.W E report here our initial results on purification and characterization of the organic matrix of the endoskeletal spicule of the sea urchin embryo. The pluteus larva has calcareous rods, which are intracellular deposits of calcite (mainly CaCO3 and some MgCO3 on an organic ground substance; Benson et al., 1983). The matrix of the skeletal element is of interest because of its probable role in the biomineralization process and because of the obvious parallel between biomineralization of hydroxylapatite in vertebrates and calcite in marine invertebrates. An equally important consideration is the opportunity the spicule presents to study the determination and differentiation of the micromere lineage, a group of cells whose developmental history is very well known. At the fourth cell division in the sea urchin embryo four micromeres arise at the vegetal pole. These micromeres give rise to primary mesenchyme cells, which in turn differentiate into the skeletal spicules (reviewed by Wilt et al., 1985). Micromeres may be isolated from the embryo and will differentiate autonomously in culture to form spicules (Okazaki, 1975). Hence, information on the matrix of the spicules may be useful in the study of tissuespecific differentiation in sea urchin embryos. We have devised ways to purify the matrix of the spicule, and the data support the conclusion that the matrix is composed of a small number of soluble N-linked glycoproteins that have a strong biochemical similarity to acidic proteins present in calcareous structur...

Transfer of a large gene regulatory apparatus to a new developmental address in echinoid evolution

“…A starting point is at the differentiation gene batteries that generate the biomineral matrix. Earlier evidence indicated that many specific protein components of the biomineral matrix are present in both demineralized embryonic skeletal spicules and demineralized adult endoskeleton, i.e., spines and test plates (9). Analysis of biomineralization genes in the S. purpuratus genome sequence (10) proves this is the case, although, for some families of these genes, certain paralogues are used in adult skeletons, some in embryonic and some in both.…”

SM37, a skeletogenic gene of the sea urchin embryo linked to the SM50 gene

“…These proteins fall into two classes, those that are constituents of the skeletogenic matrix per se, that is, the protein armature of the spicule in which the calcite biomineral is embedded; and those that play other roles in the cellular process of skeletogenesis. The definition of skeletal matrix protein has rested on the painstaking biochemical isolation of spicule matrix components originally carried out by Wilt, Benson and their colleagues (Benson et al 1986;Killian & Wilt 1996). They have shown that even when cleaned of all external components the matrix consists of at least 50 different proteins, but unfortunately only a few of these are so far characterized.…”

Role for platelet‐derived growth factor‐like and epidermal growth factor‐like signaling pathways in gastrulation and spiculogenesis in the Lytechinus sea urchin embryo

“…A species-specific, invariant number of PMCs ingress into the blastocoel , and in Lytechinus spp., 64 PMCs ingress and extend filopodia to form a syncytium of two ventrolateral rings of cells. The PMCs deposit CaC03 and MgC03 crystals and spicule matrix proteins (Benson et al, 1986;Decker et al, 1987;Sucov et al, 1987;Livingston et al, 1991) from which a species-specific triradiate skeleton or spicule is formed.…”