Читать книгу Bovine Reproduction - Группа авторов - Страница 154

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].