Changes in the arrangement of the extracellular matrix, larval shape, and mesenchyme cell migration during asteroid larval development

When fixed in the presence of alcian blue, extracellular matrix (ECM) in the embryonic asteroid blastocoel can be visualized by light and electron microscopy as a fibrous meshwork encrusted with alcianophilic material. In early to mid‐gastrulae, the ECM is associated with the basal laminae underlying the ectoderm and endoderm. It also forms a fibrous meshwork between them in the posterior part of the blastocoel. In early larvae, when mesenchyme cells arrive at the esophagus to differentiate into smooth muscle, very little ECM is associated with the stomach region. In contrast, a meshwork of long ECM strands radiates from the esophageal basal lamina which connects to a dense ECM web associated with the inner aspect of the dorsal ectoderm. This dorsal web is associated, in turn, with numerous long ECM strands which run parallel to the stomodeum. The strands located between the esophagus and the ectoderm appear when the mouth and coeloms form and may be responsible for a constriction of the ectoderm that forms in this region. During late gastrula one population of mesenchyme cells becomes associated with the esophageal region and differentiates into muscle. Most of the other mesenchyme cells stop migrating through the esophageal web at this time. Less alcianophilic material is associated with the esophageal basal lamina, and the ECM adjacent to the esophagus in the late gastrula and early bipinnaria larvae. The arrangement of the ECM elements suggests that they could be involved in controlling the migration of mesenchyme cells, particularly those destined for the esophagus.