Movements and stepwise fusion of endodermal precursor cells in leech |
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Authors: | N-J L Liu Deborah E Isaksen C M Smith D A Weisblat |
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Institution: | (1) Department of Molecular and Cell Biology, 385 LSA, University of California, Berkeley, CA 94720-3200, USA, US |
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Abstract: | 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.
Received: 14 October 1997 / Accepted: 4 February 1998 |
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Keywords: | Annelida Leech Cell fusion Endoderm Macromeres |
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