Measurement of the size distribution of zymogen granules from rat pancreas

Zymogen granules are obtained in pure form and processed for electron microscopy. Thin sections are photographed and diameters measured with a Zeiss particle size analyzer. Since sectioning cuts any given particle in random way, these diameters are not the true diameters of the particles. The true size distribution is obtained by comparing the observed diameter distribution with a generated diameter distribution. The generated distribution is constructed from an assumed parent distribution (of true diameters) by the Monte-Carlo technique. "Goodness of fit" is judged by the value of "chi-squared" resulting from the comparison. Appropriate adjustments of the parameters of the true distribution are made on the basis of minimizing chi-square. A result of this process is that the zymogen granules follow a normal distribution: mean = 0.984 +/-0.005 mum, SD = 0.190 +/-0.005 mum. A second preparation of granules was made and diameters were measured directly with a scanning electron microscope. The distribution was again found to be normal, thus supporting the first result.