Cellular Elements of the Test Plates in the Sea Urchin Stronglyocentrotus intermedius

Abstract

Cellular elements of the test plates in the sea urchin Strongylocentrotus intermedius were examined using light and electron microscopes to promote an understanding of their roles in the growth of test plates. Seven types of cells were identified in the interstitial tissue of the test stereom: sclerocyte, spherule, rounded, phagocytic, vacuolated, strongly eosinophilic, and cuboidal undifferentiated. Small basophilic cells accumulated at the sutural areas, i.e., the growth sites of test plates, and were closely associated with the trabeculae. Ultrastructural aspects of the cells indicated apossibility that they participate in the trabecula formation. Therefore, the small basophilic cells should be sclerocytes. Morphological aspects of the spherule, rounded, and phagocytic cells of the test corresponded to those of the spherule, vibratile, and phagocytic coelomocytes, respectively. They are probably migrating coelomocytes. The vacuolated cells were distributed throughout the interstitial tissue and even in test epidermis. The strongly eosinophilic cells were located near the trabecule base and the suture, where they might have some association with collagen fibers. The cuboidal cells appeared to be undifferentiated, progenitor cell. The vacuolated, strongly eosinophilic, and cuboidal undifferentiated cells might originate in situ in the test tissue or may be forms of tansforming coelomocytes.