An important part of any breeding soundness exam is an evaluation of sperm morphology. In the most fundamental case, the size and shape of the head, midpiece and tail are examined. Additional information can be gained by evaluating integrity of the acrosome and sperm membranes.
Sperm from different species vary in size and shape. Bull and human sperm, for example, have paddle-shaped heads, rodent sperm have hook-shaped heads, and the heads of chicken sperm are spindle-shaped and almost difficult to distinguish from the midpiece. The images shown below of rat, bull and chicken sperm are all at the same magnification.
The results of a sperm morphology exam are reported as percent normal. It is always the case that some sperm from an ejaculate are morphologically abnormal, but when that fraction becomes excessive, fertility may decrease. It is also useful to subclassify the abnormal population into the types of abnormality observed. Two types of classification schemes are commonly used:Abnormalities can be classified as affecting the head, midpiece or tail. The most basic type of classification scheme differentiates primary and secondary abnormalities:
- Anatomic site of the defect: The problem can involve the head, midpiece or tail. Some abnormal sperm may have defects in more than one site.
- Primary versus secondary defects: Primary defects are the more severe and are thought to originate while the sperm was still within the semeniferous epithelium of the testis. Secondary defects are less serious and thought to arise during passage thought the epididymis or by mishandling after ejaculation. Some authors question the utility or physiologic basis of this pattern of classification.
Preparing Slides for a Basic Morphology Examination
Many different staining techniques have been devised for examining sperm morphology. An nigrosin-eosin stain is commonly used because it is effective, simple and, in addition to allowing sperm to be readily visualized, it is a so-called "live-dead" stain, allowing one to assess membrane integrity at the same time as morphology. The technique for preparing an eosin-nigrosin-stained slide is as follows:
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The nigrosin-stain stain produces a dark background on which the sperm stand out as lightly colored objects. Normal live sperm exclude the eosin stain and appear white in color, whereas "dead" sperm (i.e. those with loss of membrane integrity) take up eosin and appear pinkish in color, as shown above with sperm from an Asian elephant(courtesy of J. K. Graham).
Another stain that can be used, and is demonstrated below, is toluidine blue. This staining procedure produces very nice preparations, but is more difficult and time consuming that nigrosin-eosin.
Finally, a technique preferred by many for evaluating sperm morphology is to use no stain at all, but to visualize sperm under differential interference contrast microscopy. The sperm are first fixed with glutaraldehyde, and can be stored for prolonged periods in that solution. This procedure is particularly useful for assessing acrosomal integrity.
Examples of Morphological Abnormalities
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