or not, then it probably is not. Tapered heads and pyriform heads frequently appear together in semen smears. As mentioned in the discussion of normal sperm, it is quite common for individual bulls to produce sperm with narrow heads. Studies have shown that moderately narrow sperm heads do not have any effect on fertility; however, the fate of pyriform heads depends on the severity of the aberration. Severely misshapen pyriform heads are not capable of fertilization and are likely compensable. Mildly pyriformed heads, on the other hand, are capable of fertilization but with a much greater odds of embryonic loss. Pyriformed and tapered heads often appear with other sperm defects within a few weeks following a heat‐ or stress‐related insult to spermatogenesis [2] (Figure 9.9).
Figure 9.9 Tapered head and a detached, pyriform head.
Nuclear Vacuoles
Vacuoles arise from invaginations into the inner nuclear membrane that extend into the sperm nucleus. On eosin‐nigrosin stain smears, vacuoles appear as dark areas on the sperm varying in size and location according to their type. Small pale or white‐colored spots are occasionally seen that are most often shed distal droplet material or other artifact. There are three types of vacuoles: (i) diadem vacuoles (Figures 9.10 and 9.11); (ii) apical vacuoles; and (iii) large confluent vacuoles (Figure 9.12). Diadem vacuoles are the most common type, appearing as a partial or complete arrangement of vacuoles on the equatorial region of the sperm. When the vacuoles form a complete line across the sperm the arrangement has the appearance of a necklace, hence the name diadem which means diamond necklace. Apical vacuoles occur as single or multiple small vacuoles appearing most commonly in the apex of the sperm head. Apical vacuoles may occur alone, affecting few to several sperm, but most often appear in spermiograms with diadem vacuoles. Confluent vacuoles are the most easily visualized of the vacuoles as they are made up of several vacuoles that become joined together, usually distorting the shape of the sperm head.
Figure 9.10 Sperm with diadem vacuoles (presentation 1).
Figure 9.11 Sperm with diadem vacuoles (presentation 2).
Figure 9.12 Sperm with confluent vacuoles and sperm with distal midpiece reflexes.
Diadem and apical vacuoles are easily missed on eosin‐nigrosin smears. Clean objectives, a quality microscope, good quality smears, and close examination of the sperm heads looking specifically for vacuoles will improve the examiner’s success. Feulgen stained smears are superior for identifying nuclear vacuoles where they appear as pale circles (diadem and apical) or large pale areas (confluent) on the sperm heads. Oftentimes if a few vacuoles are noted on an eosin‐nigrosin stained smear, many more will be seen on the Feulgen stained smear.
The effect of apical vacuoles on fertility is inconclusive, whereas there is substantial evidence condemning the diadem as a significant cause of infertility. The diadem vacuole is not a compensable defect. Studies evaluating the effect of purely large confluent vacuoles on fertility are limited, but available information does indicate that fertility is impaired by a high proportion of these vacuoles being present [2].
Stress, heat, toxins, and genetics have all been proven or suspected to be associated with the appearance of diadem vacuoles in bull sperm. Some bulls have been shown to have variable proportions of diadem vacuoles in their spermiograms throughout their lives, while others have small numbers that only appear following a stressful event. Bulls having modest to large numbers of sperm with diadem vacuoles should be considered suspect and at the very least monitored for several months to see if the condition resolves. In the opinion of the author, this may occur with some bulls following the resolution of a stressful event, but there is no certainty the problem will not reoccur sometime later in life.
Microcephalic and Macrocephalic Heads
Sperm heads that are clearly smaller or larger than others are called microcephalic or macrocephalic (Figure 9.13), respectively. Representing a mistake made during spermatogenesis, they are believed to contain either too little or too much nuclear chromatin. These sperm are not believed to be capable of fertilization themselves, but when they are present in a spermiogram they account for no more than 2–5% of the differential and are probably compensable.
Figure 9.13 Macrocephalic sperm with double 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
