Regeneration of the Cell Mass in Larvae of Temnopleurid Sea Urchins

Mechanisms of cell mass (CM) formation were analyzed by microsurgery in two temnopleurid sea urchins, Mespilia globulus and Temnopleurus toreumaticus. The CM in temnopleurids is formed at the early larval stage from the left ectodermal invagination, and with the hydrocoel derived from the mesoderm, forms an adult rudiment. After serial removal of the CM, it was strongly regenerated until its attachment to the hydrocoel, with the same timing as in control larvae. Embryos that had the tip of the archenteron or the coelomic pouches removed formed a CM in the normal manner. Removal of the CM plus the left somatocoel or the hydrocoel allowed CM regeneration with and without adult rudiment formation. A transplanted CM enlarged autonomously but did not contribute to adult rudiment formation, and larvae formed a new CM. Our observations suggest that the hydrocoel recognizes its distance from the CM to induce the growth of the CM and controls the normal timing of adult rudiment formation.