Germ Cell Development In Humans And Infertility

Infertility is common among both men and women. Although human reproduction and fertility have been studied for many years, few genes have been identified that contribute to human germ cell production. However, several studies have demonstrated that the age at onset of menopause has a significant genetic component; this property is likely to reflect the quantity and quality of female germ cells that are formed and differentiated. Family history is a significant predictor of early menopause (menopause at age <47 years), and it is reflected by an increased risk of early menopause in women with affected siblings of approximately sixfold (Cramer et al., 1995). In addition, sibling studies have estimated the heritability of the timing of menopause to be high, and several studies have documented the role of discreet regions of the X chromosome in families with a history of early menopause and in those without a family history (Santoro, 2001; Taylor, 2001; Testa et al., 2001; Laml et al., 2002; Shibanuma et al., 2002; Loffler et al., 2003). In addition, a number of genes may be associated with more rare forms of ovarian failure, including the FOXL2 gene, which is clearly associated with blepharophimosis ptosis epicanthus inversus syndrome, a genetic condition that is associated with aberrant eye development and female infertility, as well as EIF-2B and the follicle-stimulating hormone receptor gene, FSHR (Aittomaki et al., 1995; Aittomaki, 1996; Crisponi et al., 2001; Beysen et al., 2004).

Likewise, there are several studies that have investigated the genetic component of sperm production. In these studies, the most common genetic lesions associated with spermatogenic defects are deletions of the Y chromosome, including deletions that encompass the DAZ gene, which are associated with azoospermia (no sperm in the ejaculate) and oligozoospermia (<20 million sperm per mL of ejaculate) (Reijo et al., 1995; Reijo et al., 1996; Vogt, 1997). Rare point mutations and polymorphisms are also linked to male infertility, in several genes and at several loci (Cooke et al., 1998; Cooke, 1999; Matzuk and Lamb, 2002). Nonetheless, the genetic basis for the failure of germ cell development in men and women is not yet well understood.

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