The axoneme: the propulsive engine of spermatozoa and cilia and associated ciliopathies leading to infertility.

To van Leeuwenhoek who first examined spermatozoa, ca. 1677 [1], and to Gray in 1955 [2] who began to study invertebrate sperm motility, the sperm cell appeared seemingly simple—a head (containing the condensed haploid nucleus) and a flagellum that propels the head to the egg by the propagation of bending waves at nearly 100 Hz in water. By contrast, mammalian spermatozoa are astonishingly complex in their morphology and development from germ cells in the seminiferous epithelia, under the direction of Sertoli cells [3, 4], into their fully formed but functionally inactive state. The inactive spermatozoa then pass through a series of ducts lined by ciliated epithelia followed by maturation in the epididymis. In the female tract, spermatozoa undergo capacitation [5] and self-propulsion through the ciliated oviduct. For reproductive biologists, clinicians, genetic counselors, and general readers, this article will review the advances in our understanding of sperm flagellar and ciliary engines, the axoneme, and some of the defects that cause certain forms of infertility.