An optical model for fluorescence of mammalian sperm in flow cytometry

When flat sperm heads that have been stained to fluorescence are examined in a flow cytometer, unexpectedly, skewed pulse height distributions are obtained despite the apparent homogeneity of the samples. This anomaly has been ascribed to an optical artifact that arises when the cells are oriented in flow. We have extended our model for fluorescent scattering to spheroids and here explore some aspects for oblate spheroids which serve to model sperm heads. Although computational limitations have restricted these studies to oblate spheroids about 1.5 micrometer in diameter and an eccentricity of 0.1, the results clearly show effects of particle size, shape, optical properties and particularly of orientation on the differential scattering cross-sections. This plethora of information contained in the fluorescent signals may suggest further experiments.