Biology of spermatozoa maturation: an overview with an introduction to this issue

Mammalian spermatozoa undergo morphological, biochemical, and physiological modifications initially in the testis (testicular maturation) and later in the epididymis (epididymal maturation). These maturational changes are commensurate with the functional events that occur in developing germ cells and maturing spermatozoa. This special issue reviews the recent, relevant topics dealing with spermatozoa maturation and focuses on the events that occur in internal components such as the nucleus, the acrosome, the perinuclear theca, the fibrous sheath, and the cytoplasmic droplet as well as the plasma membrane. These structures/elements and the constituent proteins of which they are comprised undergo a variety of sequential modifications starting from their origination in developing germ cells up to epididymal maturation. Several steps of the maturation processes on the sperm plasma membrane are mediated by external enzymes and secretions derived from the epithelium lining of the genital tract. Degradation of some of the constituent proteins and the elimination of defective spermatozoa are controlled by the degradation/recycling system, the ubiquitin system. These maturational modifications are necessary for spermatozoa to become fertilization-competent cells and to be stored safely in the male.