Seminal plasma (SP) purportedly plays a critical role in reproduction, but epididymal spermatozoa are capable of fertilisation following deposition in the uterus, calling into question the biological requirement of this substance.  We have recently shown that SP enhances the ability of epididymal spermatozoa to penetrate the ovine cervix and achieve fertilisation following cervical artificial insemination.  However, the mechanism behind this ability is still largely unknown.  As such, the aim of the current study was to examine the effect of SP on the in vitro characteristics of epididymal spermatozoa.

The motility (HT CASA IVOS II; Hamilton-Thorne, Beverly, MA, USA) and cervical mucus penetration (distance travelled by the vanguard spermatozoon through a capillary tube of oestrous cervical mucus) of epididymal spermatozoa (EPI; n=3 Merino rams, collected via microperfusion of the vas deferens), epididymal spermatozoa exposed to previously collected SP (EPI+SP; 1:1 dilution spermatozoa:SP) and ejaculated spermatozoa (EJAC; n=3 Merino rams) were assessed 0, 3 and 6 h post collection following dilution in UHT milk (final concentration: 25×10⁶ spermatozoa/mL).  Results were analysed using a linear mixed model (Genstat 13^th Ed, VSN Intl). 

There was no significant difference in motility between treatments over the 6 h incubation period (P>0.05).  When pooled over time, the vanguard spermatozoon travelled significantly further for EP+SP than for the EP and EJAC treatments (P<0.05) with the distance for the latter two treatments being similar (P>0.05).

These results suggest that the enhanced cervical transit ability of epididymal spermatozoa is not entirely dependant on sperm motility but rather by some unknown cervical penetration trait conferred by exposure to SP.  Further research is now warranted to identify the components within SP which are responsible for assisting the transit of spermatozoa through the cervix and the mechanisms by which they act.