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Proximal (cytoplasmic) droplets appear in stained preparations as dense white circular structures obscuring the proximal midpiece (Figure 9.15). During the epididymal phase of sperm maturation proximal droplets normally migrate to the distal position and are then shed at or around the time of ejaculation. Presence of proximal droplets in a semen smear is indicative of a disturbance of spermatogenesis at the epididymal or testicular level. Within approximately one week of an insult to spermatogenesis, proximal droplets appear in the ejaculate. The short, less than 10‐day, time to first appearance of proximal droplets suggests an epididymal origin; however, the reoccurrence of proximal droplets three or four weeks after an insult to spermatogenesis and the well‐documented association of high numbers of proximal droplets with sexual immaturity in bulls suggest the involvement of abnormal spermatogenesis at the testicular level as well. Sperm motility is unrelated to the presence of proximal droplets, yet it has been shown that affected sperm lack the ability to bind to ova. This inability to induce a zona reaction would suggest that normal‐appearing sperm should be able to compensate for those with droplets, but that is not the case. Normal‐appearing sperm produced at the same time as high numbers of sperm with proximal droplets have been shown to have an impaired ability to bind to and penetrate ova. Breeding studies utilizing bulls expressing high numbers of proximal droplets have reported poor conception rates. Proximal cytoplasmic droplets are a serious sperm defect with a profound negative effect on fertility. Clinicians must resist the temptation to give a satisfactory classification to a young bull with a high percentage of proximal droplets in his spermiogram because the amount of time required to clear the defect varies between bulls.

Figure 9.15 Proximal cytoplasmic droplets.