to the site of fertilization, or that can travel but cannot penetrate the ovum, can be compensated for by motile sperm – compensable defects. Defective sperm, which are able to penetrate the zona pellucida and initiate the zona block are uncompensable [31, 32]. Noteworthy examples of uncompensable sperm are those with diadem vacuoles or abnormal DNA condensation. Although fertilization by a sperm with an uncompensable defect is equally as possible as a normal sperm, the risk of early embryonic loss is much greater with the uncompensable sperm defect.
As mentioned previously, differentially counting sperm defects to produce a spermiogram is a much less confusing approach to assessing sperm morphology. There are approximately 25 recognized sperm abnormalities – some are artifactual and some have no effect on fertility. Table 9.2 lists 19 defects; however, some defects have been grouped together. Examiners should regularly review abnormal sperm types. Comparing differential counts with colleagues and experts in sperm morphology are good ways to ensure that one is performing consistent, quality evaluations. Taking the time to compare a single sperm with a questionable appearance with images of known defects is an effective way to learn.
Table 9.2 Listing of recognized sperm defects.
| Pyriform headsTapered headsMicrocephalic/macrocephalic headsVacuoles – diadem, single vacuoles, confluent vacuolesClumped DNADetached (loose) heads – normal, abnormalDecapitated defectRolled‐crested‐giant head syndromeTeratoidsKnobbed acrosome – beaded, flattenedRuffled and detached acrosomesDistal midpiece reflexMitochondrial sheath defectsDag defectStump tail defectCorkscrew defectPseudodroplet defectCoiled principal pieceProximal cytoplasmic droplets |
Table 9.3 Abnormal sperm morphologies, artifacts, and other cells not included in a sperm morphology differential count.
| Morphology | Artifact | Other cells |
|---|---|---|
| Abaxial tailsDistal droplets | Bowed midpieceSimple bent tail(hypotonic or cold shock) | White blood cellsMedusa cellsSpheroidsEpithelial cells |
A good quality, well‐maintained microscope and some knowledge on how to optimize image quality are important. Oil easily captures dust and debris, so it is good practice to clean the oil immersion objective regularly. A solution of 70% ethanol on a lens‐appropriate cloth works well.
Sperm Abnormalities
Several of the most common or noteworthy sperm defects are presented below.
Normal Sperm
Nearly all novice examiners will be able to recognize a normal sperm at first glance, but if questioned they will be unsure. An effective technique is to develop a system so that each sperm is thoroughly examined. Normal sperm vary in shape both within a single smear and more so between bulls. Heads may appear short and wide or elongated and narrow. Sometimes a sperm head may look excessively narrow, but if there are no other, more obvious, disturbances in head shape and most of the sperm in the ejaculate appear similar then these sperm can be considered to be normal (Figure 9.7).
Figure 9.7 Normal sperm stained with eosin‐nigrosin. The dark staining sperm is dead.
Sperm Morphology – Minimums and Maximums
The widely accepted minimum proportion of morphologically normal sperm is 70%. Below this threshold it is expected that fertility will be impaired and a bull cannot be classified as a satisfactory potential breeder. Generally, bulls that are at or near this threshold have at least two or three sperm defects; however, as a precaution, it is recommended that the proportion of sperm with nuclear defects or proximal droplets should not exceed 20% and the proportion with acrosome or midpiece abnormalities should not exceed 25%.
Artifacts and Debris
Commonly found artifacts include bowed midpieces (also called bowed tails) and bent tails (also called simple bent tails and hypotonic shock). Neither of these anomalies should be counted as abnormal in the differential count. Bowed midpieces are easy to recognize as rainbow‐shaped, C‐shaped, or U‐shaped. A key characteristic is that the tails (midpiece and principal piece) do not fold as is the case with the distal midpiece reflex. The occurrence of bowed midpieces is associated with faulty slide preparation, most often when slides and the stain are cold. Examination of motile sperm on a wet mount can be used to confirm that this aberration is artifact [27].
Hypotonic shock is usually manifested as terminal whipping or looping of the principal piece and is caused by prolonged exposure to hypotonic stains (eosin‐nigrosin and eosin‐aniline blue) before drying. Drying is often prolonged when stains and slides are cold. The looping may appear similar to the distal midpiece defect (DMR) except that there will be no trapped droplet material. Closely examining the terminal portion of the midpiece and comparing 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
