title: Non-circadian periodicity in the duration of the cell cycle and the G1 phase in the hindlimb epidermis of the anuran tadpole, Rana pipiens.
abstract: Using the percentage labeled mitoses method, seven cell cycle determinations were initiated at 6-hr intervals over a 36-hr span in order to see if the cell cycle in the tadpole hindlimb epidermis varied with time or showed rhythmicity. There was a pattern of two long cell cycles followed by a shorter one. Total cell cycle length (Tc) and the length of the G1 phase plus one-half of the mitotic time (TG1 + 1/2M) fluctuated the most, although only TG1 + 1/2M varied significantly with the Chi-square test. The proportion of TC spent in each phase was also calculated. Only TG1 + 1/2M/TC had statistically significant fluctuations with time. Rhythmicity was analyzed by a computer program using the method of least squares for cosine curve fitting. Statistically significant ultradian rhythms of 18.4 hr in TC, 18.5 hr in TG1 + 1/2M and 18.6 hr in TG1 + 1/2M/TC and the length of the DNA synthetic phase/total cell cycle length (TS/TC) were found. Circadian rhythmicity was not observed. The acrophases of the ultradian rhythms of TC and TG1 + 1/2M coincided, suggesting that the rhythm of TC was due mainly to variation in TG1 + 1/2M. In the absence of significant variation in TS, the longest phase of the cell cycle, whenever G1 + 1/2M was short, TS/TC increased, so that the 18.6 hr rhythm in TS/TC was also a result of the periodicity in TG1 + 1/2M.
mesh_terms: D000818:Animals; D002453:Cell Cycle; D002940:Circadian Rhythm; D000078404:Epidermal Cells; D004817:Epidermis / Q000502:physiology; D006614:Hindlimb / Q000166:cytology / Q000502:physiology; D007814:Larva; D011894:Rana pipiens / Q000502:physiology*; D013936:Thymidine
chemical_list: D013936:Thymidine