Asynchronization of Cell Division is Concurrently Related with Ciliogenesis in Sea Urchin Blastulae

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Abstract
During the early development of the sea urchins, Temnopleurus toreumaticus, T. hardwickii and Hemicentrotus pulcherrimus, the division synchrony in all blastomeres lasted only until the 4th cleavage, and a regional synchrony or a graded activity of cell division appeared. In the midblastula stage prior to hatching, the regional synchrony vanished simultaneously with the formation of cilia; then the division proceeded asynchronously. The analysis of cell pedigrees confirmed that a variable extension of intercleavage times occurred after ciliogenesis. In blastomeres derived from mesomeres of T. toreumaticus embryos, the mean intercleavage time extended from 48 min of the 8th cycle (pre-ciliated) to 115 min of the 9th cycle (ciliated), and the coefficient of variation increased from 15%-39%. Kinetic analysis of cell proliferation, on the basis of the transition probability model of cell cycle control, was attempted. The minimum time required for the completion of the cell cycle was the decisive factor in the cell cycle succession of pre-ciliated blastomeres; a sudden and sharp decrease in the transition probability of the ciliated blastomeres probably interpreted the abrupt slowing and asynchronization of the cleavage cycle at the time of ciliogenesis.