CHAPTER OUTLINE

Normal male reproductive function – the manufacturing of semen, the ability to create and sustain penile erection and to ejaculate – is dependent on the physiological integration of the hypothalamo–pituitary–testicular axis. Production of androgens, normal responses in the tissues on which these hormones act and intact neurological pathways required for ejaculation are crucial for full operation of this system, to make human reproduction possible.

The testes have two distinct roles: androgen production and spermatogenesis. These are achieved by two main units – the interstitial cells and the seminiferous tubules – under the control of the hypothalamo–pituitary axis.

Gonadotrophin releasing hormone (GnRH) is secreted from the hypothalamus in a pulsatile fashion, with peaks occurring approximately every 90 min. It regulates the release of LH, which is crucial for the regulation of Leydig cell number and function, and hence production of testosterone, and follicle stimulating hormone (FSH), which stimulates Sertoli cell division and growth and is therefore responsible for testicular enlargement at puberty. Faster pulse frequencies favour LH release and slower ones, FSH. The pulsatility of GnRH secretion is important in the regulation of gonadotrophin secretion, as continuous exposure results in down-regulation of the GnRH receptor. Luteinizing hormone secretion is also highly pulsatile, which is reflected in measured plasma concentrations. Plasma concentrations of FSH are less variable, presumably as a result of slower metabolic clearance.

Follicle stimulating hormone is also regulated by inhibin B, which is produced by Sertoli cells and inhibits FSH production to regulate spermatogenesis. Conversely, activin works to enhance FSH action in the gonads, increasing spermatogenesis. Both are secreted in the seminal fluid and are thought to work locally in addition to centrally on the pituitary. Inhibin B can be used as a marker for male factor infertility as concentrations directly reflect the function of the Sertoli cells: plasma inhibin B concentrations are significantly higher in fertile men than in infertile men.

Gonadotrophin secretion is affected by a diverse range of regulators, including stress and nutritional status. Recent studies have shown that low concentrations of leptin, an adipose-derived hormone, or decreased sensitivity to leptin, such as occurs in obese individuals, are associated with decreased gonadotrophin concentrations and decreased fertility. This is mediated via hypothalamic kisspeptin neurons rather than directly via GnRH neurons. Kisspeptin is a neurohormone thought to have a key role in puberty, although this is yet to be clearly defined. Patients with mutations in the kisspeptin gene fail to go through puberty owing to hypogonadotropic hypogonadism. In animal models, treatment with kisspeptin can restore GnRH pulsatility and gonadotrophin secretion.

Testicular cancer is the commonest malignancy of men aged 20–39. Histologically, it is commonly a germ cell malignancy – either a seminoma or, more often, a teratoma. Patients are likely to present with a testicular mass, and as this cancer is relatively easily detectable with regular self-inspection/examination, and has a high cure rate if detected prior to metastatic spread, there have been several public campaigns to raise awareness amongst young men.

Endocrine disruptors are environmental substances that interfere with normal hormonal systems. They are very diverse and can be plastics, chemicals, pesticides and fungicides. They are often able to interact with steroid hormone receptors and can act as androgens and anti-androgens. Exposure may be occupational, but also through water supply, soil and food. There may be a lag between exposure and clinical presentation and there may be trans-generational effects if mutations or modifications of gene expression occur. The developmental stage at which a person is exposed may be crucial. The observations of a declining sperm count, and increasing rates of testicular cancer throughout the last century prompted researchers to ask if these epidemiological trends could be explained by endocrine disruptors. In animal models, exposure to phthalates is significantly related to oligospermia, and epidemiological evidence suggests this may be the case for humans. Endocrine disruptors have also been linked to cryptorchidism, hypospadias, early or delayed puberty and prostatic hyperplasia. More detail on this topic is beyond the scope of this chapter, but the interested reader will find more material in the further reading section, below.

The laboratory evaluation of testicular function may be conveniently divided into assessment of spermatogenesis and assessment of endocrine gonadal function.

Semen analysis is used both in the evaluation of male fertility and the follow-up of treatment regimens for male subfertility. It must be noted that there is wide inter-laboratory variation in the results, although this has been reduced in recent years by automated systems using electro-optics or computer-assisted analysis. Furthermore, there is marked variation in sperm output on a day-to-day basis. It is often more useful in clinical practice to assess the actual fertilization capacity of sperm in vitro.

A semen sample is collected by masturbation and should ideally be analysed within 1 h. Basic semen analysis measures the number of spermatozoa (per unit volume and per ejaculate), motility and morphology. These are assessed according to World Health Organization (WHO) criteria, updated in 2010, although it should be noted that individual laboratories will also have developed their own reference ranges. The WHO reference ranges were determined by the analysis of semen from fertile men whose partners became pregnant within one year of trying to conceive. Average sperm count is > 60 million/mL and a count of < 15 million/mL is considered subfertile. There should be > 39 million sperm in a given ejaculate and semen volume should be 1.5 mL or greater.

Motility is defined as the percentage of moving sperm and is subdivided into the following categories:

• non-progressive motility (NP): all other patterns of motility with an absence of progression

• immotility (IM): no movement.

The WHO reference values state that ≥ 40% of the sperm should exhibit either PR or NP motility and ≥ 32% should exhibit PR motility. At least 58% of sperm should be alive – this is especially important for samples with a PR motility of < 40% and must be assessed within 1 h of ejaculation.

Morphology, describing defects in the appearance of spermatozoa, is most commonly assessed according to the WHO morphology standards. The WHO lower reference limit for normal morphology is 4%. Few samples exhibit > 25% normal spermatozoa.

Determination of basal plasma LH and FSH concentrations is a routine part of the assessment of the hypothalamo–pituitary–gonadal axis. As LH and, to a lesser extent, FSH are secreted in a pulsatile fashion, more accurate interpretation may be possible if serial samples are taken over a period of time. However, in routine clinical practice, the measurement of basal plasma concentrations of LH and FSH with testosterone provides most of the information required.

Elevated LH and FSH in the presence of low testosterone concentrations suggest primary testicular failure; low or normal LH and FSH associated with low testosterone concentrations suggest a hypothalamo–pituitary disorder. It is important to recognize that the latter may be caused by any acute illness. Rarely, LH alone may be elevated with a low testosterone (the Sertoli-only syndrome) and, in patients with abnormal spermatogenesis, FSH may be raised with a normal testosterone. It should be remembered that pituitary and gonadal dysfunction may coexist, as in haemochromatosis. In this situation, LH, FSH and testosterone concentrations are low.