FERTILIZING-CAPACITY OF RAT SPERMATOZOA IS CORRELATED WITH DECLINE INSTRAIGHT-LINE VELOCITY MEASURED BY CONTINUOUS COMPUTER-AIDED SPERM ANALYSIS - EPIDIDYMAL RAT SPERMATOZOA FROM THE PROXIMAL CAUDA HAVE A GREATER FERTILIZING-CAPACITY IN-VITRO THAN THOSE FROM THE DISTAL CAUDA ORVAS-DEFERENS

Rat spermatozoa recovered from different regions of the excurrent duct s of 10 adult males (proximal cauda epididymidis [PC], distal cauda ep ididymidis [DC], and vas deferens [VD]) were assessed by in vitro fert ilization (IVF) using limited sperm numbers, and by continuous evaluat ion of motility parameters during 5 hours of incubation in vitro with automated computer-aided sperm analysis (CASA). Spermatozoa from the P C region fertilized (68 +/- 6%) a significantly greater (P less than o r equal to 0.005) number of oocytes than those from the DC (44 +/- 5%) or VD (47 +/- 7%). For pooled samples from all three regions, the mea n fertilization rate (51 +/- 14%) was less than for spermatozoa from t he PC (P less than or equal to 0.05) but was not significantly differe nt from spermatozoa from the DC or VD. For each time point and sample, 1,592 +/- 428 sperm tracks were analyzed. CASA was verified by compar ison with manual still-frame analysis of video recordings, by repeated analysis of the same or different samples of spermatozoa, and by exam ination of computer tracks. The coefficients of variation for various motion parameters suggested that the CASA obtained a high degree of pr ecision. There were no significant differences in motility parameters for spermatozoa recovered from equivalent regions of the left or right tract or in motility parameters for spermatozoa from differents regio ns of the tract immediately after recovery. However, during incubation in vitro, spermatozoa from the DC or VD regions exhibited a marked de cline in straight-line velocity (VSL) compared with spermatozoa from t he PC region, The reduction in VSL (combined values from right and lef t tract) for DC or VD.spermatozoa compared with PC spermatozoa was sig nificant at 2.5 hours of incubation (P less than or equal to 0.05) and highly significant (P less than or equal to 0.005) by the end of the incubation period. Differences in average path velocity (VAP) were als o apparent after 4 hours (P less than or equal to 0.05), but no signif icant differences were observed for measurements of curvilinear veloci ty (VCL) or lateral head displacement (ALH). Overall, the decline in V SL over 5 hours was highly correlated (P less than or equal to 0.001) with the outcome of fertilization in vitro. In contrast, initial VSL a nd changes in VCL of spermatozoa were not correlated with fertilizatio n rate. These results indicate that the in vitro fertilizing capacity of rat spermatozoa is correlated with 1) the decline in straight-line velocity (VSL) as measured by repeated CASA during incubation in vitro and 2) with the site of recovery of mature rat spermatozoa from the d istal excurrent duct. it is suggested that the deterioration of the qu ality of rat spermatozoa in the distal epididymidis and vas deferens d uring storage may occur sooner than previously realized, and therefore care must be taken when recovering samples for fertility assessment. In keeping with findings in other species, immediate ''snapshot'' anal ysis of rat motility was a poor predictor of sperm fertility. in contr ast, continuous CASA provided significant information for determining sperm fertilizing capacity and will be a useful technique for reproduc tive toxicology.