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Introduction
Following the advent of human in vitro fertilization (Steptoe & Edwards, 1978), much attention has been given to identifying first embryo morphology and, later, oocyte morphology as prognostic tools (Elder & Dale, 2011); less attention has been given to sperm morphology. The spermatozoon delivers the haploid male genome to the oocyte, introduces the centrosome and triggers the oocyte egg into activity.
The sperm head may be considered in three parts: (1) the nucleus with a haploid set of chromosomes, in which deoxyribonucleic acid (DNA) is packaged into a volume that is typically less than 10% of the volume of a somatic cell nucleus (Dadoune, 2003; Elder & Dale, 2011); (2) the acrosome, a large Golgi-derived secretory vesicle on the proximal hemisphere of the head that contains an array of hydrolytic enzymes used for digesting the zona pellucida during penetration (Gerton, 2002; Yoshinaga & Toshimori, 2003); and (3) the perinuclear theca, a rigid capsule composed of disulphide bond stabilized structural proteins amalgamated with various other protein molecules (Oko, 1995).
Human spermatozoa exhibit a wide range of shapes. Several studies have indicated that sperm morphology best predicts the outcome of natural fertilization (Kruger et al., 1988; Bartoov et al., 1999), intra-uterine insemination (Berkovitz et al., 1999), conventional in vitro fertilization (IVF) (Kruger et al., 1988; Mashiach et al., 1992) and intracytoplasmic sperm injection (ICSI) (Palermo et al., 1992; Bartoov et al., 2002), and several techniques have been described that provide valuable information on the morphology and pathological features of spermatozoa. In a classical clinical evaluation, human sperm are fixed, stained and analysed by optical microscopy. Recently, several novel techniques have been developed for the identification of more detailed features of the cells. Differential interference contrast microscopy, scanning near-field optical microscopy, electrostatic force microscopy, atomic force microscopy and scanning thermal microscopy (Akaki et al., 2002; Bartoov et al., 2002; Rothery et al., 2003). Most of these techniques involve biochemical processing that requires specific equipment and may also alter the vitality of the sperm analysed.
The sperm cell is almost transparent in conventional bright field microscopy, as its optical proprieties differ slightly from the surrounding liquid, generating little contrast. However, a...