Physiological and induced apoptosis in sea urchin larvae undergoing metamorphosis

Paracentrotus lividus embryos at the early pluteus stage undergo spontaneous apoptosis. Using a TUNEL (TdT-mediated dUTP Nick-End Labelling) assay of whole mount embryos, we showed that there was a different distribution of the apoptotic cells in different optical sections. Not more than 20% of cells in plutei was spontaneously apoptotic, as confirmed by the counts of dissociated ectoderm and intestine cells. Observation of larva stages closer to metamorphosis confirmed that apoptosis is a physiologic and necessary event for the development of the adult. In particular, larvae at different developmental stages, showed apoptotic cells in the oral arms, intestine, ciliary band, apical and oral ganglia. Moreover, the number of apoptotic cells decreased in later larva stages, in agreement with the known fact that, in the organism approaching metamorphosis, a smaller number of cells needs to be eliminated. Furthermore, combined TPA and heat shock treatment enhanced apoptosis by increasing the number of cells involved in the phenomenon.