Comparative evaluation of coelomocytes in Paracentrotus sea urchins: Description of new cell types and insights on spherulocyte maturation and sea urchin physiology

Sea urchins are the best-known models in echinoderm immunology and only four coelomocytes – i.e. phagocytes, vibratile cells, and red and colorless spherulocytes – have traditionally been observed in these animals. However, recent studies have demonstrated that this number could be underestimated, which consequently may hinder a better understanding of sea urchin physiology. Here, we investigated the cells of one of the most known sea urchin genera in the world: the genus Paracentrotus. This genus comprises two species with distinct geographic distributions, thus providing an excellent scenario for comparative analyses. In this context, through an integrative approach consisting of living cells, cytological preparations, and scanning electron microscopy (SEM), we analyzed morphological, morphometric, and cytochemical characteristics of the coelomocytes of Paracentrotus lividus (Lamarck, 1816) and Paracentrotus gaimardi (Blainville, 1825). Seven different coelomic subpopulations were found in these species, including the traditional types and the granular spherulocyte, crystal cell, and progenitor cell. For all spherulocyte subpopulations, a set of morphologically similar cells were observed in both cytological and SEM preparations. Based on morphology, morphometry, and cytochemistry, we observed that these morphotypes could be organized in a sequence, which we interpreted as a maturation process. Then, we raise one hypothesis that explains how spherulocytes mature. Lastly, taking into account the all results achieved by this study, we discuss (1) how the morphological, morphometric, and cytochemical characteristics observed in Paracentrotus coelomocytes may be used to identify the coelomocytes of other sea urchins; (2) the physiological implications of our findings. Considering that P. lividus is one of the most studied sea urchins in the world, the new findings obtained here may shed new light on traditional aspects of sea urchin immunology (e.g. coelomocytes function), as well as stimulate research on new aspects of Echinoidea and even in Echinodermata immunology.