This chapter discusses the key events occurring at different stages of development in the normal human male and how this impacts the reproductive function in adulthood. Development of the normal male reproductive system can be divided into five periods: fetal, neonatal, infancy/childhood, puberty, adulthood. Normal hormonal functioning of the fetal testis is essential for masculinization and for development of a male reproductive system. In neonatal male and female babies there is activation of the hypothalamic-pituitary axis and consequent increase in circulating levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). During the neonatal period, Sertoli cells continue to proliferate and this is arguably the most important period in life in terms of the magnitude of increase in Sertoli cell numbers. Sertoli cells start expressing androgen receptor (AR) at the onset of puberty and this is considered one sign of maturation of these cells, as they terminally differentiate and cease proliferating.

Stem cell and gene therapies may involve either permanent or temporary insertion of stem cells or genetic material. Both stem cell therapies and gene therapies can have reproductive effects in the sense that the genetic changes introduced are transmitted in the germline or the stem cells can give rise to germ cells. Some stem cell or gene therapies may therefore actualise the continuing treatment-enhancement debate; that is, whether there is an ethical difference between treating disease and enhancing normal function and whether such a difference makes some or all enhancement morally problematic. Many stem cell and gene therapies will have to be given in the fetal period or during early childhood to be effective or to have their full effect. This raises the issue of proxy consent for very young children. A final set of issues arises in relation to resource allocation within the healthcare system.