Mitotic index, influx and mean transit time in the hamster cheek pouch epithelium, a partially synchronized cell system.  Presentation of a mathematical model based on a non-stationary probability density function for the transit time in a compartment. 
Mitotic activity was followed in the epithelium of the hamster cheek pouch for about 12 hr in two experiments under different conditions of noise and light intensity.  A mathematical model based on a non-stationary probability density function for the transit time through mitosis was developed, making an analysis of this partially synchronized cell system possible.  The most important result of the two experiments is the indication of a non-stationary mean transit time for cells in mitosis.  In the first experiment (70 dB, 103 lux), which produced high mitotic indices (maximum 2.0%), the influx changed from 0.  to 0.9%/hr (mean of 7-hourly determinations: 0.4%/hr), and the mean transit time from 1.9 to 5.5 hr.  In the second experiment (70 dB, 265 lux), which had a lower level of MI (maximum 1.3%), the influx changed from 0 to 1.8%/hr (mean of 13-hourly determinations: 0.4%/hr), and the mean transit time from 0.3 to 3.3 hr.  It was difficult to say precisely whether the variation in influx or in mean transit time was the main factor in the increase and decrease of MI.  The data could not be simulated with a stationary mean transit time.  It is suggested that changes in MI due to different external circumstances are mainly the result of variation in the mean transit time.