Fertility, Reproductive Capacity & Sperm

Fertility and Reproductive Capacity – Basic Concepts

Fertility, in general terms, refers to an organism's ability to reproduce. In men, specifically, this means the continuous production of functional sperm and the ability to enable fertilization through sexual intercourse. "Reproductive capacity" is the everyday term for the same concept. When a man persistently lacks sufficient ability to fertilize through his semen, this is referred to as infertility or subfertility — depending on the extent of the impairment.

Male infertility is more common than often assumed: in roughly 40 to 50 percent of all unfulfilled desires to have children, a male factor is a cause or contributing factor. An estimated one in seven men in the Western world is affected by reduced fertility.

Sperm and Germ Cells

Sperm are the male germ cells — highly specialized cells responsible for fertilizing the female egg cell. They are produced in a continuous process (spermatogenesis) in the seminiferous tubules of the testes, which begins at puberty and continues uninterrupted into old age. Sperm maturation takes about 64 to 72 days and is temperature-dependent: the optimal temperature for spermatogenesis is around 34 to 35 °C, slightly below core body temperature — which explains why the testes are located outside the abdomen, in the scrotum.

A mature sperm cell consists of an oval head containing the haploid genetic material (23 chromosomes), a midpiece with the mitochondria necessary for motility, and a long flagellum that drives directional swimming motion. During fertilization, the germ cells of both partners fuse; from the combined 23 plus 23 chromosomes, a new cell with a complete diploid genome is formed — the zygote.

Semen Quality – Assessment and Significance

Semen quality is assessed using several parameters captured in a semen analysis. The WHO reference values (WHO 2021) define the lower limit of the normal range as follows: sperm concentration of at least 16 million per milliliter (or at least 39 million per ejaculate), progressive motility of at least 30 percent, and normal morphology (Kruger criteria) of at least 4 percent. Deviations are described with specific clinical terms — oligozoospermia (too few sperm), asthenozoospermia (reduced motility), teratozoospermia (abnormal morphology), or a combination of these known as OAT syndrome.

Influences on semen quality are wide-ranging. Contrary to common perception, sperm quality in men measurably declines from the fifth decade of life onward, though considerably more slowly than female fertility. Smoking, alcohol, cannabis, and anabolic steroids demonstrably impair spermatogenesis. Heat exposure — for example from frequent sauna use, hot baths, or tight synthetic clothing — raises testicular temperature and can temporarily reduce sperm quality. Environmental pollutants such as endocrine disruptors (e.g. phthalates, bisphenol A) are suspected of impairing sperm quality. Infections and inflammation in the urogenital tract — such as epididymitis or prostatitis — can directly damage sperm function. Oxidative stress also plays a significant role: antioxidants such as vitamin C, vitamin E, and coenzyme Q10 can improve sperm quality under conditions of oxidative stress.

Hormonal Basis of Fertility

Sperm production depends on an intact hypothalamic-pituitary-gonadal axis. FSH (follicle-stimulating hormone) acts on the Sertoli cells and is directly required for spermatogenesis; LH (luteinizing hormone) stimulates the Leydig cells to produce testosterone, which in turn is an indispensable basis for spermatogenesis. Hormonal disorders — including hypogonadism, hyperprolactinemia, or thyroid disease — can impair fertility and must be ruled out diagnostically.

Exogenous testosterone intake — for example as part of TRT or through anabolic steroid misuse — suppresses FSH and LH release via the negative feedback mechanism and can thereby significantly reduce sperm production, temporarily or permanently. Men wishing to have children should therefore avoid TRT and discuss alternative treatment approaches if needed.

Diagnostics

Semen analysis is the basic examination for male fertility. It is obtained by masturbation after an abstinence period of two to seven days and should be analyzed at least twice, four to twelve weeks apart, since semen quality can vary considerably. Additional tests such as baseline hormone diagnostics, testicular ultrasound, genetic testing (karyotype, Y-chromosome microdeletion), and antibody testing may also be useful.

Treatment Options

Depending on the cause, different treatment approaches are available. Hormonal disorders can be treated causally — for example hypogonadism with gonadotropins, or hyperprolactinemia with dopamine agonists. Anatomical causes such as a varicocele can be treated surgically if there is a clinically relevant impairment of semen quality. Causal infections are treated with antibiotics. In pronounced oligozoospermia or azoospermia without a treatable cause, assisted reproduction techniques are available — ranging from intrauterine insemination to in vitro fertilization with ICSI (intracytoplasmic sperm injection). In non-obstructive azoospermia, surgical sperm retrieval (TESE, testicular sperm extraction) may be attempted.