Fuel/energy sources of spermatozoa

Abstract

While preparing for successful fertilization, spermatozoa spend a considerable amount of time maturing in the male and female reproductive tracts, which provide them the competency for fertilization. One of the most important changes attained by spermatozoa during epididymal transit is the development of forward progressive motility, which is primarily dependent on energy. The need for energy by spermatozoa comes to the center stage only when spermatozoa are either ejaculated or suspended in an artificial medium, which ultimately provide them the opportunity and the environment to move and become motile. Any transporting system can operate only when provided with an appropriate support of fuel machinery. Spermatozoa utilize energy on motility, which is primarily in the form of intracellular ATP generated by oxidation of substrates, fructose, glucose, sorbitol, lactate, or pyruvate. Spermatozoa motility is generated by the beating of the extremely long flagellum which is more than 90% of the total length of a mammalian sperm cell. Fuel is inevitable for this efficient flagellar movement leading to motility, one of the critical functional capabilities of all spermatozoa. Spermatozoa, either devoid/altered motion characteristics or depleted fuel resources or both, lose the ability to move forward and cannot fertilize the egg. It is thus important to know the energy status of spermatozoa in order to understand their mobility, survival, and the changes they undergo during their entire life cycle. Though spermatozoa of all species require energy for fuelling their movement, different species have adapted different mechanisms for obtaining this energy. The purpose of this chapter is to review our current understanding on substrates for energy production in spermatozoa, energy storage sites, and utilization with respect to the spermatozoon-specialized structure, the uptake of specific substrates, and their metabolic breakdown and the manipulation of energy resources in vitro sustaining spermatozoa motility.