Abstract
Mechanisms that regulate the number of cell division rounds during embryogenesis have remained largely elusive. To investigate this issue, we used the ascidian, which develops into a tadpole larva with a small number of cells. The embryonic cells divide 11.45 times on average from fertilization to hatching. The number of cell division rounds varies depending on embryonic lineages. Notochord and muscle consist of large postmitotic cells and stop dividing early in developing embryos. Here we show that conversion of mesenchyme to muscle cell fates by inhibition of inductive FGF signaling or mis-expression of a muscle-specific key transcription factor for muscle differentiation, Tbx6, changed the number of cell divisions in accordance with the altered fate. Tbx6 likely activates a putative mechanism to halt cell division at a specific stage. However, precocious expression of Tbx6 has no effect on progression of the developmental clock itself. Zygotic expression of a cyclin-dependent kinase inhibitor, CKI-b, is initiated in muscle and then in notochord precursors. CKI-b is possibly downstream of tissue-specific key transcription factors of notochord and muscle. In the two distinct muscle lineages, postmitotic muscle cells are generated after 9 and 8 rounds of cell division depending on lineage, but the final cell divisions occur at a similar developmental stage. CKI-b gene expression starts simultaneously in both muscle lineages at the 110-cell stage, suggesting that CKI-b protein accumulation halts cell division at a similar stage. The difference in the number of cell divisions would be due to the cumulative difference in cell cycle length. These results suggest that muscle cells do not count the number of cell division rounds, and that accumulation of CKI-b protein triggered by tissue-specific key transcription factors after cell fate determination might act as a kind of timer that measures elapsed time before cell division termination.
Highlights
The mechanisms by which embryos regulate the number of cells constituting the body are a key issue in developmental biology [1]
Numbers of cell division rounds in various tissues Ascidian embryos provide a good system for studying the regulation of cell division rounds during embryogenesis
The numbers of cell division rounds during embryonic development vary according to cell lineage
Summary
The mechanisms by which embryos regulate the number of cells constituting the body are a key issue in developmental biology [1]. Control of the number of cell division rounds in specific tissues or organs is important for proper embryonic development, but its nature has remained elusive. Halocynthia roretzi, develop into simple tadpole larvae with a relatively small number of cells: approximately 2800. Embryonic cells divide 11.45 times on average after fertilization. Notochord and muscle cells stop dividing early during embryogenesis. The numbers of cell division rounds in muscle and notochord are conserved among several distantly related ascidian species, Ciona intestinalis and Ascidia ahodori, with various egg sizes [3]
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