In vitro differentiation of male germ line cells from sea urchin testis.

Fragments of sea urchin testicular tissue were cultured at 15 degrees C in serum-supplemented seawater for 5 weeks. Cells whose DNA had been pulse-labelled with 3H-thymidine at the beginning of the experiment were followed by autoradiography, and counts were made of the proportions of each male germ-line cell type from sections of tissue at several time points. Differentiation of spermatogonia to mid-spermatids occurred within the first 10 days, with a decline of about 40% in the total number of spermatogonia and an increase of 250% in the number of mid-spermatids. Thereafter, no changes occurred in the proportions of germ-line cells, although tissue integrity was maintained throughout. The results indicate that sea urchin male germ-line cells can complete meiosis and the first stages of spermiogenesis including nuclear condensation and cytoplasmic reduction in culture with kinetics similar to those in vivo. The system should permit analysis of factors responsible for male germ-cell differentiation.