Sperm Morphology
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Sperm morphology is assessed routinely as part of standard laboratory analysis in the diagnosis of human male infertility. This practice has its origins in the work of Mac Leod & Gold (1951) which showed that sperm morphology was significantly different in fertile compared to infertile man.
Despite this standardization, human semen evaluation continues to be influenced by subjectiveness of the investigator and a lack of objective measurements for sperm morphology continues to be a problem.
There is an ongoing debate on which criteria should be applied to define normal spermatozoa and which classification of abnormal forms is most appropriate.
Studies on sperm morphology should concentrate to obtain measurements and biological data of spermatozoa which are functionally active. Only then the definition of normal can be achieved and clinically useful criteria can be adopted.
However, the definition of a normal spermatozoon as described by WHO in 1992 is different from that used by other authors (17,21,22,31). The evaluation of the morphology of human spermatozoa varies widely between and sometimes even within laboratories. While most investigators agree on the appearance of a normal spermatozoon, standardized analysis is difficult because of the use of different staining techniques which are not always suitable for optimal examination from head to tail (Figure 1). For example, the techniques for preparing morphology specimens have been expanded from three to five.
The difficulty in classifying human sperm morphology is mainly caused by the large variety of abnormal forms found in the semen of infertile men. Only certain types of abnormalities can be analyzed objectively (11).
The definition of ‘morphological normal’ is still discussed, as well as the clinical relevant limits for the rate of pathologic forms.
What is a normal spermatozoon ?
The WHO’s definition of a normal spermatozoon is not based on any biological data. As a consequence, the implementation of new standards has resulted in some controversy. However, sperm morphology was found to correlate more closely with fertilization rates than sperm count and motility (2).
Because it is not possible to determine the fertilizing potential of individual human spermatozoa, physiological endpoints other than fertilization must be studied to obtain insight into the mechanisms by which sperm morphology influences the fertilization process, e.g. examination of morphologic characteristics of spermatozoa recovered from cervical mucus and/or those binding to the zona pellucida.
The strict criteria of sperm morphology (e.g. Kruger’s criteria) use the examination of spermatozoa that had penetrated cervical mucus for the definition of normal spermatozoon. Even by using strict criteria it is difficult to determine if results between studies are really comparable.
In order to avoid subjectivity, over the past 20 years numerous studies describe image analysis techniques in the assessment of sperm morphology. These techniques allow objective characterization of different sperm forms. Automated methods may help, but there remains a lack of biological data to support the use of computer-aided semen analysis in clinical settings.
Recent evidence suggests that sperm morphology assessment by relatively simple and inexpensive methods can provide prognostic information similar to that obtained from some of the more elaborate sperm function tests.
Sperm morphology evaluation is considered to be a highly subjective procedure because, unlike the haematopoietic cells for example, the difficulty in classifying human sperm morphology is caused by the large variety of abnormal forms found in the semen of infertile and fertile men. Only certain types of abnormality can be quantitated objectively (11).
Normal sperm morphology needs to consider two points. The first one is the proportion of spermatozoa with normal morphology in semen and the second is the definition and the characterization of the normal spermatozoa.
According to WHO criteria, a normal ejaculate must have at least 30% normal sperm.(36). For the stricter criteria, fertile men have > 14% normal forms in their semen and men with < 4% of normal forms are subfertile. According to Kruger’s criteria, IVF outcome was suboptimal when normal sperm morphology was less than 14% and worst if it was under 4%.(17). The sperm deformity index is a more reliable predictor of the outcome of fertilization in vitro than the proportion of normal sperm morphology.(2).
Kruger’s opinion is that the existing classification of abnormal and normal shaped sperms are in need of revision by those involved in the field. Scanning electron microscope is worth further evaluation as a tool in the accurate scoring of normal sperm morphology.
The advantage of using strict criteria in morphology evaluation is the fact that the measure is reproducible between patients and between different technicians performing the test. It also allows the clinician to classify the patient into one of two specific groups ( < 14% and >14% normal morphology), giving a reliable criteria to plan the approach for future IVF cycles (17).
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computer aided sperm analysis: automatic analysis of sperm motility and morphology for assisted reproduction, veterinary research and toxicology
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