With an odyssey defined as a long journey during which many things happen, it would seem that the spermatozoon epitomizes this term.  For spermatologists and clinicians, however, the voyage of a spermatozoon might be best described as a mystery expedition.  Our understanding of spermatozoal physiology remains fairly superficial despite an exhaustive number of manuscripts relating to the spermatozoon (over 63,000 entries on PubMed alone when sorting by the terms “spermatozoon” or “spermatozoa”).  The uninformed individual might describe a spermatozoon as a highly specialized, but simple, cell with only one role to fulfill…that of fertilization.  While fertilization might be considered the endpoint of spermatozoal function, this cell must be extremely sophisticated and adaptable to achieve this task, and the process involves a series of highly coordinated cellular and molecular events.  Following is a list of some requirements ascribed to a mammalian spermatozoon: loss of most organelles and cytoplasm during formation the testis and maturation in the epididymis (requires a host of intracellular and intercellular signaling events); remodeling of spermatozoal chromatin within the epididymis as a protective mechanism against environmental injury (requires repackaging of nuclear DNA into a highly condensed form through the aid of specialized proteins termed protamines); plasma membrane alterations within the epididymis to yield proteins important to fertilization (requires various enzymatic-linked alterations of existing proteins, as well as uptake of proteins from epididymal fluid or from the epididymal epithelium); passage through the uterus and uterotubal junction of the female at the time of insemination (requires activated flagellar movements, protection against immunologic attack, and post-translational modification of sperm-derived proteins); binding to oviductal epithelial cells to form a spermatozoal reservoir (requires specific cell-cell attachment, possibly mediated through spermatozoal surface carbohydrate-binding proteins, termed lectins); acquisition of additional maturational changes, collectively termed capacitation, that permit a spermatozoon to fertilize an oocyte (requires an assortment of signal transduction cascades); release from oviductal epithelial cells and passage to the vicinity of the oocyte at the isthmic-ampullar junction of the oviduct (requires a coordinated spermatozoal-release mechanism, hyperactivated motility, and probably chemotaxis); penetration through the extracellular matrix of the oocyte cumulus (possibly mediated by hyperactivated motility and redistribution/unmasking of surface-associated hyaluronidase, as the cumulus matrix is rich in hyaluronic acid); binding to the zona pellucida, a highly glycosylated protein matrix surrounding the oocyte (may involve specific affinity between spermatozoal surface molecules and the zona pellucida components); acquisition of the acrosome reaction, a regulated form of exocytosis (requires reorganization of the outer acrosomal and overlying plasma membranes necessary for fusion and vesiculation); penetration of the zona pellucida (release of acrosomal contents is required for this event to occur); binding and fusion with the oolemma (requires specific region-dependent molecular interactions); dispersion of nuclear contents (requires specific fusogenic alterations of the lipid membranes of the spermatozoon and oocyte); oocyte activation (a spermatozoon-derived factor is required for activation of the oocyte and embryonic development); pronucleus formation (requires decondensation of the highly compact spermatozoal nucleus); organization of the mitotic spindle after pronuclear formation (requires contribution of proximal centriole from the spermatozoon); and contribution to post-fertilization events (through transfer of mRNA and microRNAs to the oocyte).

After viewing this lengthy list of functions, one becomes quite appreciative of the highly complex and specialized features of a spermatozoon.  In fact, the biochemical and biophysical features are so sophisticated that many of the cellular and molecular mechanisms remain unresolved to this day.  Furthermore, spermatozoa (and oocytes) represent some of the most highly differentiated cells in the mammalian body; yet, when sperm and oocyte are combined, they retain their potential for totipotency (ability to divide and produce all cell types of the body) through creation of a zygote.

This presentation offers as cursory overview of spermatozoal development, function, and transport through the eyes of an equine veterinarian. My professional background is predominantly clinical in nature, but my fascination with spermatozoal function and preservation has led to a fairly sizeable review of the scientific literature over the years in hopes of extracting laboratory findings that have application to my daily activities in the clinical arena.