This chapter focuses on the genetic basis of male infertility. The male specific Y (MSY) is a chromosomal material that bridges the two polar pseudoautosomal regions and is unique in the human genome. A karyotype and Y-chromosomal microdeletion assay should be obtained as complementary tests in all non-obstructive azoospermia (NOA) and severely oligospermic men prior to intracytoplasmic sperm injection (ICSI) and the use of testicular sperm. Prognosis and planning are optimized with any informative result. Steroidogenic acute regulatory (StAR) protein is the rate-limiting step for androgen biosynthesis in particular and all steroids in general. The hypothalamic-pituitary-gonadal (HPG) axis controls human sexual maturation and spermatogenesis. The hypothalamus secretes gonadotropin- releasing hormone (GnRH) which regulates the production of Folliclestimulating hormone (FSH) and luteinizing hormone (LH) by the pituitary gonadotropes. Kallmann syndrome results from the failure of the GnRH-releasing neurons to migrate to the olfactory lobe during development.

In vivo, only a species-specific number of ovarian follicles are selected to release their oocytes for fertilisation during each reproductive cycle. While many factors play a role in selecting the follicles for ovulation, the overriding factor is availability and follicular response to the circulating gonadotrophins, folliclestimulating hormone (FSH) and luteinising hormone (LH). Initially, oocyte growth is commensurate with follicle growth. In most species, oocytes are generally thought to have completed their growth phase by the early antral stage of follicular development. While gonadotrophins and steroids have been the focus of much research into follicular development, numerous other growth factors and cytokines have been identified as also having a role in oocyte developmental competence. Some of these factors affect the selection of follicles that proceed to the ovulatory stages while others directly enhance oocyte maturation. Another aspect of oocyte maturation is the epigenetic phenomenon of genomic imprinting.