Metabolic Strategy of Human Spermatozoa 1011 Introduction

The primary function of the spermatozoon is to carry and introduce the intact DNA into the oocyte. For that purpose, after ejaculation the spermatozoon must propel itself from the vagina into the uterus in order to meet the oocyte at the site of fertilization in the ampullar region of the oviduct. Prior to oocyte fertilization, sperm must undergo in the isthmic region of the oviduct a series of biochemical changes known as capacitation that allow sperm to bind and penetrate the oocyte in vivo. Therefore, the spermatozoon could be considered as a self-propelled specialized DNA carrier. However, in order to fully accomplish its function of delivering the paternal genome to the oocyte, the fertilizing spermatozoon, through a phospholi-pase Czeta that carries in its nuclear membrane, must first activate the oocyte and trigger the expression of homeobox genes that encode proteins responsible for binding specific DNA sequences of the zygote that ultimately determines embryo and fetal development.

The ability of sperm to display motion and to propel itself in vivo and in vitro is highly dependent on its capability of producing ATP which is used as the substrate by dynein ATPase to transduce chemical energy into mechanical work by the contractile proteins of the axoneme. The ability of sperm to maintain its characteristic motility pattern is not going to depend only on the normal function of the contractile proteins of the axoneme and its ability to generate ATP, but also on the availability of metabolic substrate in the female reproductive tract to produce ATP.

In addition to the biochemical changes that experiment during the process of capacitation, sperm are exposed to significant environmental changes as they move from the testis to the distal portion of the oviduct where fertilization normally takes place. Exposure of epididymal sperm to the seminal plasma during ejaculation reinitiates motility and sharply increases sperm energy metabolism. Further environmental changes occur in the female reproductive tract as sperm move out from the seminal plasma into the uterine and oviductal fluids. Under these conditions, sperm survival is going to depend on the efficient utilization and control of metabolic of nutrients present in the extracellular medium of the female reproductive tract.

The purpose of this chapter is to examine the metabolic strategy of human spermatozoa with special emphasis on glycolysis and oxidative metabolism and the impact of this strategy on sperm motility and fertilization.

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