Spermiogenesis and spermatozoa

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The primary function of the spermatozoon is to provide the male pronucleus, and in many species the centrosome, for the fertilized egg (99). To achieve this objective, the mammalian spermatozoon has developed a highly specialized morphology which allows it to protect its DNA during migration to the fertilization site and to recognize, penetrate, fuse with and activate the receptive ovum (20). Spermatogenesis is an extraordinarily intricate process of cellular differentiation that takes place in the seminiferous tubules of the testis, resulting in the production of spermatozoa. The male testis seminiferous tubule is composed of two cell types: Sertoli cells which mainly function to create a favourable environment for the differentiation of the other cell type, the germ cells, which give rise to the mature spermatozoon (22). The primordial germ cells are the spermatogonia which are diploid and located in close association to the basal membrane inside the seminiferous tubules, embedded between the Sertoli cells. Spermatogonia divide mitotically either to replenish themselves or to produce spermatocytes. Spermatocytes undergo meiosis and produce haploid cells named spermatids. During this meiotic division random separation of homologous chromosomes and crossing over of genetic material occurs, events that contribute to the diversity required for the survival of the species (7). Spermiogenesis is the final step of spermatogenesis and consists of a dramatic biochemical and morphological differentiation process in which the haploid round spermatid is transformed into the highly polarized spermatozoon, a process that takes approximately 22 days in humans. Spermiogenesis is a tightly synchronised process involving many physiological changes that are unique to this cell type. For a more detailed study, as well as schemes on mammalian spermatogenesis, the reader is referred to Barratt (7) and de Kretser and Kerr (22).

The mammalian spermatozoon is organized into two major parts, the flagellum concerned with energy production and the motility, and the head containing the paternal DNA and the structures required for ovum recognition, sperm-zona penetration, sperm-oolemma fusion and activation (20, 103, 104). Each of these two elements has distinct structures required for the spermatozoon function. Thus, in the apical part of the head we find the acrosome, a cap-like vesicular formation that contains a number of proteolytic and glycolytic enzymes and holoenzymes necessary to assist the spermatozoon in its passage through the oocyte vestments (1, 103). Also in the head, the perinuclear theca is a condensed layer of selected cytoplasmic proteins that is sandwiched between the nuclear envelope and the inner acrosomal membrane apically, and between the nuclear envelope and the plasma membrane caudally. It is assumed to play a pivotal role in acrosomal-nuclear docking and nuclear shaping during spermiogenesis, and in the sperm-oocyte interactions at fertilization (6, 95). As shown in Figure 7, the sperm tail can be divided into four major regions: the connecting piece provides anchoring of the flagellum to the sperm head and houses the sperm proximal centriole in non-rodent mammals; the middle piece contains the helically wrapped mitochondria that supply the energy for flagellar movement; the principal piece is the longest portion of the tail; and finally the end piece which basically consists of a plasma membrane surrounding the microtubule doublets of the axoneme. The mammalian spermatozoa tail has also developed a series of cytoskeletal elements, namely the outer dense fibers (ODF) and the fibrous sheath (FS) designed to regulate motility and provide structural support to the sperm tail during its movement (67, 105). ODF surround the axoneme in the middle piece and principal piece of the sperm tail. In the middle piece nine ODF associate to corresponding microtubule doublets of the axoneme. In the principal piece, ODF 3 and 8 are substituted by or converted into two longitudinal columns of the FS which are bridged by FS ribs and together surround the seven remaining ODF (24, 66).