Bovine Reproduction. Группа авторов
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tail (center).
Knobbed Acrosome
The most common morphological aberration affecting the acrosome is the KA defect. There are two forms of this defect: indented and beaded (Figure 9.14). The indented form is the most common presentation, showing up in spermiograms from a few to most of the sperm being affected. Instead of having the gentle convex curve that typifies the top or apex of the head, sperm with indented KAs will have a flattened to indented apex and variable thickening of the acrosomal ridge. A smaller proportion of sperm will not show the flattened apex but instead have a small white (eosin‐nigrosin stained smear) semicircular structure just below the apex. These sperm often serve as an indication that the other sperm should be carefully scrutinized to look for flattening and indentation. A few sperm with the indented form may be noted in spermiograms with other defects and have been shown to appear three weeks following a thermal (heat) shock to spermatogenesis [33]. When differential counts showing 30%, 50%, or greater proportions of KA are present the bull should be considered suspect and further investigation is warranted. The indented form occurs in several breeds of cattle. To date, the indented form of the KA has not been conclusively proven to be heritable, but evidence suggests that it likely is.
Figure 9.14 Sperm with the knobbed acrosome defect: indented form (left); beaded form (right).
The beaded presentation is relatively rare and believed to be heritable. Unlike the indented form, there is usually no flattening of the apex. The acrosome has become enlarged and stretched and then folded back on itself over the head of the sperm. Eosin stain does not penetrate the folded, thickened acrosome, resulting in a very distinctive semicircular white structure covering the apex of the sperm head. Typical beaded KA spermiograms contain high percentages of affected sperm and few to no other defects.
KAs seem to have variable, almost confusing effects on fertility. When considering the impact of a high percentage of KAs on fertility it is helpful to consider the two forms. Bulls producing the beaded form are extremely subfertile. Although there were normal‐appearing sperm in their ejaculates, it appears that the beaded KAs were not compensable, or that there was an undetected abnormality affecting even the normal sperm. Results of breeding trials using bulls producing high numbers of indented KAs have been variable. In single sire mating situations and artificial insemination studies little to no differences in pregnancy rates were recorded; however, when similarly affected bulls were used in competitive mating trials the KA bulls sired significantly fewer calves. These results implied that normal appearing sperm coexisting with the KA sperm were not as capable of fertilization when competing with normal sperm produced by unaffected bulls [34].
Midpiece Defects
Detached Heads
Detached heads, also termed free heads or loose heads, are commonly found in low numbers in semen samples collected during late winter and early spring, suggesting that their occurrence is due to senescence. In these cases, the sperm take up the eosin stain uniformly, indicating that they were dead before ejaculation. Detached heads, both normal and abnormal forms, are also found in variable numbers with other defects following an insult to spermatogenesis. Large numbers of detached normal heads may be seen in cases of sperm accumulation associated with a failure to void aging sperm in the urine. Detached heads may occasionally be encountered that stain alive, usually in the presence of a variety of other defects, indicating an insult to spermatogenesis that has affected the formation of the connection between the head and the midpiece. Detached heads are not a normal phenomenon. Although a few are acceptable they should always be included in the differential count. The one notable exception is when assessing the true proportion of knobbed acrosomes (KAs) in a sample. Dead sperm eventually lose the acrosome; therefore acrosomal anomalies will not be visible. Disregarding dead, detached heads in this case will give a more accurate proportion of KAs present in the spermiogram. (Editor's note: a normal detached or free head may be classified as a midpiece defect because it results from separation at the junction of the midpiece and head.)
Proximal Droplets
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.
Distal Midpiece Reflexes
Occurring in all breeds of cattle, the DMR is the most common midpiece defect (Figures 9.16 and 9.17). The classic appearance is that of a “J” hook involving the very distal end of the midpiece causing the rest of the tail to be whipped around 180°. Cytoplasmic droplet material is usually trapped in the loop but not always; in the absence of droplet material, examiners should look for the classic “J” hook. Other manifestations include a less‐severe 90° bend in the midpiece and second bends involving the principal piece that cause the midpiece and principal piece to loop in a circle. The DMR defect occurs in the epididymis associated with a hormonal chain reaction which ultimately causes a decline in testosterone levels. Sperm are stored in the cauda epididymis prior to ejaculation and epididymal sperm possess a distal cytoplasmic droplet which gets trapped in the reflex and is unable to be shed. Both heat and stress may cause a decline in testosterone and the appearance of DMRs. The DMR defect is most often associated with stress insults to spermatogenesis, appearing within just a few days of a stress occurrence. Common stressful events include severe weather, injuries, and illness of at least three or four days duration. With more severe or long‐standing events, other defects will appear in succession following the DMR according to where they occur during spermatogenesis. In cold, northern regions DMRs are frequently recorded in semen samples collected during the late winter and early spring, suggesting that photoperiod and/or nutrition may be a cause. This defect is seen in all breeds; however, some animals within a breed appear to be more affected than others