Ontogenesis of testis development and function in humans

Functional gonads are mandatory for sexual reproduction and survival of higher animal species. However, at the level of the individual subject, acquired or inherited gonadal dysfunction and infertility are not commonly associated with severe life-threatening phenotypes. Medical progress and increased societal interest have led to more prioritized agendas for reproductive health problems. Increasing attention is focused on disorders of sex development, fertility and sexual function. Despite this engagement, our understanding of the detailed molecular and cellular adverse events behind such problems is still incomplete. Critical early steps, such as determination of the gonads, occur at precise temporal windows of development. The sex chromosomes are obvious critical contributors, but many other human chromosomes also contribute to sex differentiation, engaging multiple genes and proteins. The aim of this review is to give an up-to-date and comprehensive summary of the events required for gonadal ontogenesis in the human male, from the stage of embryonic sex determination to postnatal maturation including puberty. The principal genes involved in these processes are tabulated and discussed. Morphological events relevant for human gonadal development are covered, in particular in connection with early germ cell maturation and spermatogenesis. Consequences of maldevelopment caused by, e.g. cryptorchidism, are discussed.