Objectives: To develop quantitative criteria for assessing sperm morphology and to determine the correlation between the percentage of morphologically normal spermatozoa and the outcome of the sperm hypo-osmotic swelling test, sperm acrosin profile, and sperm capacity for fertilization.
Design: The maximal length and width of the sperm head, the length of the midpiece and principal piece of the sperm tail, and the ratio of the surface of the acrosomal region to the total surface of the head were determined in specimens obtained from a group of infertile men and a group of fertile men using a confocal scanning laser microscope. Group A consisted of 53 infertile men who were participating in an IVF program, and group B consisted of 98 fertile men. The mean +/- 2 SD of the morphometric parameters in group B was established as representing the lowest and highest normal values in both groups. A normal spermatozoon was defined as one with morphometric parameters within normal levels. The lowest percentage of morphologically normal spermatozoa, hypo-osmotic swelling test result, and acrosin activity in group B were also taken as the lowest normal values in group A.
Setting: In vitro fertilization program at the Tottori University School of Medicine, Yonago, Japan.
Main outcome measures: Sperm morphometric parameters, percentage of morphologically normal spermatozoa, hypo-osmotic swelling test, and acrosin activity.
Results: The length of the midpiece, ratio (x 100) of the surface of the acrosomal region to the total surface of the sperm head, percentage of morphologically normal spermatozoa, outcome of hypo-osmotic swelling test, and acrosin activity were significantly higher in group B than in group A. The maximal width of the head was significantly lower in group B than in group A. Strongly positive correlations were observed between percentage of morphologically normal spermatozoa or length of midpiece and the proportion of fertilized oocytes in group A and between ratio (x 100) of the surface of the acrosomal region to the total surface of the head and acrosin activity in groups A and B. Sperm morphology showed high positive and negative predictive values for acrosin activity (normal/abnormal) and fertility potential (present/absent).
Conclusions: Using quantitative strict criteria, we found that sperm morphology was an important predictor of sperm fertilizing capacity. The confocal scanning laser microscope provided useful information about the sperm cytoskeleton and its importance in fertilization.