Figure 2. The remarkable anatomy and life history of a sperm cell.

(A) Spermatogenesis takes place within the germinal epithelium of the testes. Spermatogonia pass across the blood-testes barrier, enter meiosis and differentiate into spermatocytes and ultimately haploid round spermatids. The latter then metamorphose into spermatozoa in a remarkable process known as spermiogenesis, which achieves the dramatic morphological transformation of the germ cell in the complete absence of de novo gene transcription. (B) The fully differentiated spermatozoon comprises a sperm head (containing the nucleus and a secretory vesicle, the acrosome, which plays a key role in fertilization), the neck (containing a centriole which orchestrates all cell division in the embryo), the midpiece (containing the mitochondria, which contribute to the energy demands of the cell but are destroyed in the oocyte following fertilization), the principle piece, which propels the spermatozoon towards the oocyte, changing both the amplitude and frequency of the flagellar beat pattern as it does in order to generate a specialized form of motility in the immediate vicinity of the oocyte (hyperactivation), which facilitates penetration of the vestments surrounding the oocyte) and the tail end piece. (C) Spermatozoa leaving the testes are immature and completely lacking in functional competence. Functionality is only acquired as these cells pass through a complex excurrent duct system known as the epididymis. In this organ, post-translational changes to the protein composition of the spermatozoa confer upon these cells the potential to exhibit vigorous motility and achieve fertilization. (D) Realization of this potential for fertilization involves a further series of changes in the spermatozoa as these cells ascend the female reproductive tract. These changes are termed “capacitation” and allow the spermatozoa to express receptors for the shell that encases the oocyte, the zona pellucida, respond to this unique cell-cell recognition event by undergoing the acrosome reaction and express the hyperactivated motility needed to penetrate through the zona pellucida to the vitelline membrane of the egg, where sperm-egg fusion occurs.