Ultrastructural studies of the early events of the human sperm acrosome reaction as initiated by human follicular fluid

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Abstract
It has been suggested that the morphology of the human sperm acrosome reaction (AR) is markedly different from that of other mammalian sperm. The present study examines the fine structural events of the early stages of the human sperm AR as initiated by preovulatory human follicular fluid. Human sperm, capacitated in vitro for 6 hr at 40°C (90% motility) were diluted with equal volumes of follicular fluid before fixing in cacodylate-buffered glutaraldehyde at 5, 10, 15, 20, and 180 sec. Fixed sperm were treated with either tannic acid or thiocarbohydrazine and OsO4. Over 2,000 sperm were viewed. By 5 sec, 2% of the sperm had initiated the AR. By 15 sec, 8% of the sperm were in some stage of the reaction, and after 180 sec 40% of the sperm had completed the acrosome reaction. The initial stages of the human AR are characterized by a swelling or decondensation of the acrosomal matrix. The fusion between the plasma membrane and outer acrosomal membrane begins at the most anterior tip of the head and progresses toward the equatorial segment. Fusion and fenestration ended at the equatorial segment. With thiocarbohydrazine + OsO4 fixation the fused membranes are distinct hybrid vesicles with the outer acrosomal membrane being far more electron dense. The acrosomal matrix is retained by 20 sec, but by 180 sec the matrix is completely dispersed, even when viewed after tannic acid fixation. Also by 180 sec, vesicles were being progressively lost. It is therefore concluded that the morphology of the human sperm AR, as initiated by human follicular fluid, is not unique, but is similar to that described for other mammalian sperm.