It is estimated that as many as 10% of couples worldwide suffer from infertility or reduced fertility, and that in approximately half of these cases this results from defective spermatogenesis. In 60% of these infertile men, the failure to produce mature germ cells (i.e. azoospermia) or the formation of low numbers of sperm (i.e. oligozoospermia) can be ascribed a genetic aetiology. Few of the loci associated with male infertility have been mapped in humans; however, several genetic models of defective germ-cell development and differentiation have been described in mice. The schematic in Figure 1 summarizes the main features of spermatogenesis in humans.

To prepare for the presence of a fetus, the uterus undergoes a critical process termed decidualisation that, in mice and probably humans, is required for reproductive success. This process involves phenotypic changes in uterine stromal cells that enable them to support implantation and placental development. Accompanying this transformation is the recruitment of a specialised population of maternal immune cells. These cells, termed decidual leukocytes, are a significant presence, composing up to 70% of cells in the uterus throughout early pregnancy. At first glance, the recruitment of immune cells to the site of implantation seems at odds with maternal tolerance. However, the unique characteristics of decidual leukocytes, particularly with regard to their abundance and distinct composition, suggest that they participate in protecting fetal tissues from immune attack. Thus, the mechanisms that recruit decidual leukocytes to the uterine wall provide an important component to successful pregnancy.

Endometriosis is a benign and often progressive disease in which tissue originating in the uterine lining spreads locally by direct invasion, or through the blood vessels to distal sites. The disorder affects as many as 10–20% of women aged 30–40, although many women remain asymptomatic. Between 30% and 45% of infertile women are diagnosed with endometriosis.

Polycystic ovary syndrome (PCOS) is among the most common endocrine disorders in females, reportedly affecting approximately 5–10% of women of reproductive age. The syndrome is characterized by oligo- or amenorrhoea, with clinical or biochemical hyperandrogenism, as evidenced by acne, hirsutism or alopecia; high serum levels of testosterone, androstenedione and dehydroepiandrosterone (DHEA) and its sulphate (DHEAS) and/or low levels of sex-hormone binding globulin (SHBG) are typical biochemical findings. Lack of internationally agreed diagnostic criteria detract from comparisons between studies reported in the literature, the diagnosis in the USA relying more on abnormal ovarian appearances using ultrasound imaging. By contrast, the diagnosis in the UK rests more heavily on clinical and biochemical features with abnormal imaging not regarded as essential for the diagnosis. Polycystic ovaries on ultrasound are defined by the presence of eight or more subcapsular cysts ≤10 mm and increased ovarian stroma. False-negative scan reports may reflect inter-operator variability while the relatively high prevalence of typical polycystic appearances in asymptomatic women adds to the uncertainties of including ultrasound imaging as a major diagnostic criterion.