Introduction

Spermatozoa must be motile for their journey to the egg. In species that undergo external fertilization, such as sea urchins and fishes, the initiation of sperm motility occurs at spawning and movement is further regulated by factors released by oocytes. In mammals, the swimming ability is progressively acquired by spermatozoa during the epididymal transit even though motility is initiated only after dilution in seminal plasma. Factors from the female genital tract will later on trigger the hyperactivated motility that is needed for sperm penetration of the zona pellucida.

In this review, we will first examine the structure of the flagellum giving a special attention to the axonemal motility apparatus. Then, we will briefly discuss some of the factors that influence the initiation, maintenance and evolution of sperm motility.

Experimental approaches to study sperm movement

Before describing the flagellum, we will briefly mention some technical aspects of motility measurements and the most important experimental approaches or models that are used to evaluate the role, and mode of action, of specific structures in flagellar motility.

High speed videomicrography is an essential tool for the measurement of parameters such as the percentage of moving cells, the flagellar beat frequency (the number of complete beats per second; by stroboscopic illumination), the surface of flagellar beat envelope (surface covered by the flagellum over a complete beat) and the maximal wave amplitude (width of the beating envelope) (Brokaw and Kamiya, 1987; Cosson et al., 1999; Gingras et al., 1998).