Movements and stepwise fusion of endodermal precursor cells in leech.

At the four-cell stage, embryos of glossiphoniid leeches comprise identified blastomeres A, B, C and D. Subsequent cleavages of the A, B and C quadrants yield three large, yolk-rich endodermal precursor cells, macromeres A"', B"' and C"'. Eventually, these cells generate the epithelial lining of the gut via cellularization of a multinucleate syncytium. Meanwhile, cleavage in the D quadrant generates ten teloblasts that give rise to segmental mesoderm and ectoderm via stem cell divisions. Here we show that, during cleavage, macromeres A"', B"' and C"' shift clockwise relative to the D quadrant, while C"' comes to envelop the nascent teloblasts. During gastrulation, derivatives of the teloblasts undergo epibolic movements over the surface of the A"', B"' and C"' macromeres to form the germinal plate, from which segmental tissues arise. We find that the three macromeres fuse in a stepwise manner to initiate formation of the multinucleate syncytium; cell C"' fuses about 25 h after the fusion of A"' and B"', and the teloblasts fuse with the macromere-derived syncytium later still. When macromeres are biochemically arrested by microinjecting them with the A chain of ricin, a further difference among the macromeres is revealed. Biochemical arrest of A"' or B"' slightly retards the rate of germinal plate formation, but arrest of C"' frequently accelerates this process.