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In the present study, to achieve quantitative assessment of human sperm acrosome reaction by using fluorescein isothiocyanate conjugated Concanavalin A (FITC-Con A), the optimum conditions for microscopic observation were established by comparing the fluorescent images between the purified human sperm with the intact acrosome (non-acrosome reacted sperm; non-AR sperm) and those with exposed inner acrosomal membrane (acrosome reacted sperm; AR sperm). FITC-Con A stained not only the inner acrosomal membrane but also the other portions. These non-specific bindings were excluded by the competitive dissociation with alpha-D-mannose (Man) solution, the specific labeling of the inner acrosomal membrane with FITC-Con A was accomplished in the presence of 0.5 mg/mL Man. The influence of methanol fixation on the acrosome status were examined. The assessment of the acrosome reaction should evaluate whether the inner acrosomal membrane is exposed or masked by the plasma membrane, and methanol fixation is often employed in the histochemical stain. By methanol fixation, the fluorescent profiles of AR sperm were not altered before and after the treatment, bright fluorescence were found in the acrosomal regions. On the other hand, non-AR sperm gave weak fluorescence on the sperm heads before the treatment, the acrosomal regions fluoresced brightly after the treatment similar to those of AR sperm. Methanol fixation caused false positive results, it might denaturate the plasma membranes and facilitate permeability of FITC-Con A. FITC-Con A fluorescent stain and the modified triple stain (rose bengal stain) gave good positive correlation to evaluate AR sperm. This result concluded that the optimum conditions with FITC-Con A stain in the present procedure might be an useful tool for observation of human sperm acrosome reaction.

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