Differential growth in the cortical fibrillar system as the trigger for oral differentiation and cell division in Stentor

Abstract

Three lines of evidence suggest that the cell cycle of Stentor is timed in the cortex by the lengthening of the somatic kineties during interphase. Oral differentiation, which triggers cleavage in fully grown cells, is associated with the development of a characteristic relationship between the area of the oral apparatus and the area of the cell body. This oral/somatic ratio declines during interphase from a mean value of 0.12 to a mean value of 0.06 during division. Deviations from the mean are large enough to suggest that the important relationship does not involve surface area but some factor related to it. Further experiments indicate that the “critical” ratio involves the size of all or part of the oral fibrillar system in relation to the length of the somatic f ibrillar system. This “critical” ratio develops because the somatic fibrillar system increases in size during interphase while the oral fibrillar system does not. The process of reorganization (spontaneous replacement of intact oral structures during interphase) results from growth of abnormally small cells which apparently reach the “critical” oral/somatic ratio while still below the size required for division. Reorganization can be produced experimentally by obtaining abnormally small cells using either starvation or microsurgery. As these miniature stentors grow toward division, they reorganize cyclically. The mean oral/somatic ratio of reorganizers is very similar to that obtained for dividers, even though the reorganizers measured were far smaller.