Bovine Reproduction. Группа авторов
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the aberrant sperm with other sperm on the smear will facilitate classification (Figure 9.8).
Figure 9.8 A sperm with a bowed midpiece and a sperm displaying hypotonic shock (looped tail).
Other debris includes white blood cells, shed droplets, and spheroid cells (shown in Figure 9.3) as well as epithelial cells, bacteria, and red blood cells.
Morphological Aberrations that Are Counted as Normal Sperm Cells
Arguably, sperm cells displaying a distal cytoplasmic droplet or abaxial tails are not normal; however, the presence of very high proportions of either of these aberrations has not been shown to negatively impact fertility. Distal droplets are shed after a period of exposure to vesicular fluid and will often be seen as tiny white circular structures on the smear (Figure 9.3). If there are no other defects visible, sperm displaying a distal droplet or an abaxial tail should be counted as normal (Table 9.3).
Head Defects
Pyriform and Tapered Heads
Pyriform (pear‐shaped) heads are commonly recorded in spermiograms, usually with other sperm defects. The pyriform shape is very distinctive, with varying degrees of severity. The apex of the sperm head is very rounded and the sides are sharply tapered all the way to the base of the sperm head. Many severely pyriformed heads are much smaller overall than normal sperm. Appearing narrow from the apex to the basal plate, truly tapered heads are not as common as pyriform heads. A good rule of thumb is that if the examiner must wonder if a sperm is tapered 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