Typical spermatozoa of the rat, human and stallion are shown in Fig. 1 and the important elements of a spermatozoon are depicted in Fig. 2 . A partial list of spermatozoal attributes essential for fertility is presented in Table 1. Collectively, these attributes depend on the normal development and function of the genomic package, the mitochondria, dense fibers and microtubular elements of the axoneme, the acrosome and enzymes therein, and the multi-compartmentalized plasma membrane. Seminal plasma is the fluid portion of an ejaculum, but only one of several distinctly different fluids to which sperm are exposed. Spermatozoa are transmitted from the seminiferous epithelium in a fluid milieu, and the solutes therein are removed and replaced within the efferent ducts and epididymis.
Some species have a complete array of accessory sex glands, including the above three types.
Other species lack the bulbourethral glands or vesicular glands. Certain proteins and other molecules in the secretion of one or more accessory sex glands are identical to some components of cauda epididymal fluid, or even blood plasma, and others are unique products of that gland. Thus, seminal plasma includes a broad spectrum of chemical constituents contributed by the epididymis and the accessory sex glands. The relative contributions of the epididymis or different accessory sex glands to the seminal plasma of a given ejaculum are dependent on many factors including the interval of sexual abstinence, duration of foreplay, pathophysiological processes in the male, and the species. Because semen is a mixture of spermatozoa and fluids moved by emission from the cauda epididymidis and vas deferens with fluids from the accessory sex glands, the sperm to fluid ratio is quite variable.