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Evaluating Sperm Morphology

Smear evaluation technique
  • Two slides should be prepared and at least 200 sperm should be counted per slide to minimize sampling error. A 100X oil immersion (OI) objective should be used (1000X OI magnification) to more clearly identify sperm detail, although it is NOT necessary to report all variations in head size and shape or various midpiece and principal piece defects.
  • Use a systematic approach to prevent biased selection of sperm. Assess every intact sperm that is viewed within a field (having both a head and a tail) but is not overlapping other sperm. Systematically select areas of the slide to view.
  • General morphology guidelines
    Normal sperm morphology requires that the head (including the neck) and tail (including the midpiece and principal piece) of the sperm are normal.
    Head staining characteristics and morphology
    The heads of normal mature sperm are slightly oval in shape and measure approximately 4.0 to 5.5µm in length and 2.5-3.5µm at the widest part. The acrosomal region should comprise 40-70% of the head area. The acrosomal region stains pale blue and the post-acrosomal region stains dark blue with Papanicolaou, Shorr, and Diff-Quik stains. The acrosomal region of the head should contain no large vacuoles and not more than two small vacuoles.
    Tail staining characteristics and morphology
    Sperm have a long tail which is slightly thicker near the head than at the end. Sperm tails measure about 50µm.
    A normal midpiece is slender, regular, and about the same length as the sperm head; it does not bulge out or contain excess cytoplasm. The midpiece may show some red staining.
    The principal piece has a uniform thickness, which is thinner than the midpiece. It can be looped so that it is bent back on itself, although a sharp angle would be abnormal as would coiled tails (>360°).

    An example of a normal sperm is represented on the right.