The biology of spermatogenesis: the past, present and future

Abstract

The physiological function of spermatogenesis in Caenorhabditis elegans, Drosophila melanogaster and mammals is to produce spermatozoa (1n, haploid) that contain only half of the genetic material of spermatogonia (2n, diploid). This half number of chromosomes from a spermatozoon will then be reconstituted to become a diploid cell upon fertilization with an egg, which is also haploid. Thus, genetic information from two parental individuals can be passed onto their offspring. Spermatogenesis takes place in the seminiferous epithelium of the seminiferous tubule, the functional unit of the mammalian testis. In mammals, particularly in rodents, the fascinating morphological changes that occur during spermatogenesis involving cellular differentiation and transformation, mitosis, meiosis, germ cell movement, spermiogenesis and spermiation have been well documented from the 1950s through the 1980s. During this time, however, the regulation of, as well as the biochemical and molecular mechanisms underlying these diverse cellular events occurring throughout spermatogenesis, have remained largely unexplored. In the past two decades, important advancements have been made using new biochemical, cell and molecular biology techniques to understand how different genes, proteins and signalling pathways regulate various aspects of spermatogenesis. These include studies on the differentiation of spermatogonia from gonocytes; regulation of spermatogonial stem cells; regulation of spermatogonial mitosis; regulation of meiosis, spermiogenesis and spermiation; role of hormones (e.g. oestrogens, androgens) in spermatogenesis; transcriptional regulation of spermatogenesis; regulation of apoptosis; cell-cell interactions; and the biology of junction dynamics during spermatogenesis. The impact of environmental toxicants on spermatogenesis has also become an urgent issue in the field in light of declining fertility levels in males. Many of these studies have helped investigators to understand important similarities, differences and evolutionary relationships between C. elegans, D. melanogaster and mammals relating to spermatogenesis. In this Special Issue of the Philosophical Transactions of the Royal Society B: Biological Sciences, we have covered many of these areas, and in this Introduction, we highlight the topic of spermatogenesis by examining its past, present and future.

Figure 1.

The biology of spermatogenesis in the rat. (a) Schematic drawing of the seminiferous epithelium from a seminiferous tubule in the adult rat testis, illustrating the morphological features of different germ cells during development and their intimate relationship with the Sertoli cell. Leydig cells that produce testosterone and oestradiol-17β via steroidogenesis are restricted to the interstitium. The blood–testis barrier (BTB) comprising tight junctions, basal ectoplasmic specializations and desmosome-gap junctions physically divides the seminiferous epithelium into a basal and an apical (adluminal) compartment. (b) Cross-section of a stage XIV tubule from an adult rat testis during which time meiosis occurs. Several newly formed spermatids from secondary spermatocytes at anaphase are clearly visible in this stage. (c) Drawing depicting the process that occurs in male germ cells during spermatogenesis.