The biology of infertility: research advances and clinical challenges

Abstract

Reproduction is required for the survival of all mammalian species, and thousands of essential 'sex' genes are conserved through evolution. Basic research helps to define these genes and the mechanisms responsible for the development, function and regulation of the male and female reproductive systems. However, many infertile couples continue to be labeled with the diagnosis of idiopathic infertility or given descriptive diagnoses that do not provide a cause for their defect. For other individuals with a known etiology, effective cures are lacking, although their infertility is often bypassed with assisted reproductive technologies (ART), some accompanied by safety or ethical concerns. Certainly, progress in the field of reproduction has been realized in the twenty-first century with advances in the understanding of the regulation of fertility, with the production of over 400 mutant mouse models with a reproductive phenotype and with the promise of regenerative gonadal stem cells. Indeed, the past six years have witnessed a virtual explosion in the identification of gene mutations or polymorphisms that cause or are linked to human infertility. Translation of these findings to the clinic remains slow, however, as do new methods to diagnose and treat infertile couples. Additionally, new approaches to contraception remain elusive. Nevertheless, the basic and clinical advances in the understanding of the molecular controls of reproduction are impressive and will ultimately improve patient care.

Figure 1

Simple molecular pathway for sex determination in the mammalian gonads. In SRY-positive bipotential gonads, SRY binds to multiple gonad-specific enhancer elements of the SOX9 promoter to upregulate its expression, along with DAX1 and SF1. SOX9 in turn represses SRY in a negative feedback loop, whereas fibroblast growth factor-9 (FGF9) stabilizes SOX9 expression, as confirmed in FGF9-null XY sex-reversed mice. During testis differentiation, SRY, SOX9 or both also act to downregulate the ovarian pathway by suppressing RSPO1 during a key window in early development. In the absence of SRY, levels of RSPO1 rise and cause increased WNT4 activity and β-catenin (β-Cat) signaling by inhibiting internalization of the WNT co-receptor, low-density lipoprotein receptor–related protein-6 (LRP6; ref. 222), resulting in an ovary. WNT4 antagonizes the expression of activin B (INHBB) and induces the activin B antagonist, follistatin (FST), theoretically through β-catenin, preventing testis vasculature development. Other male genes downstream of SRY include GATA-binding protein-4 (GATA4), Zinc finger protein, multitype-2 (ZFPM2, also know as FOG2), Wilm's tumor-1 (WT1) and NR0B1 (DAX1), whereas the gonadal target genes of SOX9 include AMH, FGF9, desert hedgehog (DHH), prostaglandin D synthase (PTGDS), and VANIN1 (VNN1). Solid lines, known effect; dashed lines, postulated effect. Arrows, positive regulation; blunted-end lines, negative regulation.

Figure 2

Sex differentiation in humans. The presence of a fetal testis that secretes both testosterone (T) and anti-Mullerian hormone (AMH) results in the induction of the Wolffian duct into the future vas deferens, epididymis, and seminal vesicles and the regression of the Müllerian duct, respectively. In females, the fetal ovary does not secrete either of these substances, so the Wolffian duct regresses, whereas the Müllerian duct gives rise to the fallopian tubes (oviducts), uterus and the upper portion of the vagina. Androgens (including testosterone) have a key role in the development of the male genital tract but are not the only signaling pathways involved. For a sperm and oocyte to meet in vivo, millions of spermatozoa leave the seminiferous tubules of the testis to mature in the epididymis before traveling through the vas deferens and urethra to enter the female, where they transverse the vagina, cervix and uterus before typically encountering a single oocyte in one of the fallopian tubes. Surgical contraception involves closing this pathway by removing segments of the two vas deferens (that is, vasectomy) in a man or both fallopian tubes (that is, tubal ligation) in a woman.

Figure 3

Neuroendocrine control of pituitary and gonadal function. The hypothalamus, which has a number of nuclei and pathways that affect reproductive behavior, secretes a key decapeptide, GnRH, that binds to its receptor, GnRHR, on the gonadotropes and is involved in induction of sexual maturity through its regulation of the synthesis and secretion of the pituitary gonadotropins FSH and LH. Kisspeptin (KISS1), secreted from neurons whose cell bodies are located in the anteroventral periventricular (AVPV) and arcuate (ARC) nuclei of the hypothalamus, signals through its receptor (KISSR1) to regulate pulsatile secretion of GnRH from additional hypothalamic neurons and thus affects the pathway at a higher level. FSH and LH have key roles on the gonads in both sexes, being involved in folliculogenesis, ovulation and steroidogenesis in females while functioning in gonadal growth, steroidogenesis and spermatogenesis in males. During pregnancy, human chorionic gonadotropin (hCG) production from the early placenta takes over the role of LH, stimulating the ovarian corpus luteum to produce progesterone, which, in turn, stimulates the uterus and maintains pregnancy. Equally important are a number of peptide (for example, inhibin (INH)) and steroidogenic (that is, estradiol and testosterone) feedback systems from the gonads to the pituitary and hypothalamus. Multiple mutations in this axis have been identified in humans and mice (Supplementary Tables 1 and 2).

Figure 4

Genetic dissection of female fertility pathways in mice. An update of the figure from our review six years ago shows a marked increase in the gene products that have key roles at various states of ovarian folliculogenesis and after ovulation (the newly identified genes since the last review are in blue). Women have a pool of resting oocytes in the form of primordial follicles. Once these follicles are depleted, a woman can no longer have children naturally. Several master oocyte-specific transcriptional regulators interact to control primordial follicle formation and follicle maintenance and recruitment into the growing pool, as indicated. At later steps in folliculogenesis and through ovulation, paracrine factors (for example, KIT ligand, GDF9 and BMP15), autocrine factors (for example, activins and inhibins) and endocrine hormones (for example, FSH, LH, estradiol and progesterone) play key parts. After ovulation, proteins of the zona pellucida and oocyte maternal factors permit proper fertilization and the substantial changes in gene expression necessary for early embryogenesis. Ovarian prostaglandins and steroids are also essential to initiate a cascade of events in the uterus that readies it for implantation of a healthy embryo. Multiple proteins and factors are required for placentation and maternal behavior.

Figure 5

Mouse models of male reproductive defects provide new insights into the causes of male infertility. This figure, updated from Matzuk and Lamb, reveals the genes known today that influence testicular and sperm function in the mouse. The genes highlighted in ref. 1 are in black, with new genes identified since then and others not shown previously in blue. Communication between each cellular compartment within the testis (seminiferous tubules, interstitial cells and blood vessels, as well as between individual cell types (germ cells, Sertoli cells, peritubular myoid cells, Leydig cells and macrophages)) play essential parts in mitosis, meiosis and differentiated function. It is noteworthy that the genes fall into specific categories of function, such as those involved in signal transduction, homologous recombination or energy production. Gene targeting in the mouse models has provided new insights into potential etiologies of male infertility (see Supplementary Table 2). OT; oligoteratozoospermia; OAT, oligoasthenoteratozoospermia; Morph., morphology defects; Mot., motility defects.