Male germ cell development, called spermatogenesis (synonym: formation of sperm; spermatogenesis), takes place in the testes (testicles) of the male, with development being completed for the first time at the onset of puberty. The entire process takes approximately 70 days. Since it is a complex process, it is first important to understand the anatomy of the testis and the hormones that regulate it. In advance, brief explanations about pubertal development:
- First true sign of puberty in boys is the enlargement of the testis from prepubertal 1-3 ml to > 3 ml Time: > 9-12 (mean) – < 14 years;
- Increase in testicular volume or length begins at about 12 years (range of variation: 10-14 years).
- Pubarche (pubic hair) about 6 months later (at about 12, 5 years; range of variation: 9-15 years).
- Pubertal growth spurt begins together with the axillary hair (about 14 years).
- Pollutions (first ejaculations) take place at about 14.5 years, at this time also voice change* (only after growth spurt); now also in about one-third of cases occurrence of acne vulgaris
* Boys today come on average at 13.1 years already in the voice change. One speaks of pubertas praecox (early puberty) in boys when the onset of puberty occurs before the 9th birthday. Note: Boys with idiopathic pubertas praecox achieve normal adult body size when therapy with a GnRH analogue (drugs used to artificially lower testosterone or estrogen levels in the blood) is started early.
Anatomy of the testis
The male testis consists of two distinct compartments: The tubular compartment contains the seminiferous tubules, or seminiferous tubules. These are lined by the germinal epithelium, which contains the division-active germ cells and the so-called Sertoli cells. The Sertoli cells, as supporting cells, form the architecture of the epithelium, nourish the sperm (seminal cells) and coordinate germ cell development. The interstitial compartment is formed by testosterone-producing Leydig cells, connective tissue cells, macrophages (phagocytes), blood vessels, and nerves.
Endocrinology of spermatogenesis (spermatogenesis)
Spermatogenesis is controlled by a number of hormones (hypothalamic-pituitary-testicular axis):
- Gonadotropin-releasing hormone (GnRH) – The hypothalamus is part of the diencephalon (interbrain) and, as the supreme control center of autonomic body functions, has the task of controlling circulation, respiration, fluid or food intake, and sexual behavior. For this purpose, it secretes a large number of hormones, of which the gonadotropin-releasing hormone (GnRH) influences the formation of the hormones LH and FSH in the pituitary gland. They are important for the regulation of spermatogenesis.
- LH (luteinizing hormone) – This hormone is formed in the pituitary gland (pituitary gland) and stimulates testosterone production in the Leydig cells of the testis in males.
- FSH (follicle stimulating hormone) – This hormone is also produced by the pituitary gland and acts directly on the germinal epithelium of the testis.
Both testosterone and FSH act directly on the Sertoli cells of the testis, which coordinate spermatogenesis.
Spermatogenesis
Spermatogenesis begins with gonocytes, the germ cells that are created during fetal development. After birth, further development occurs into the so-called spermatogonia. These immature germ cells can continuously divide mitotically (reproductive divisions) and form the basis for spermatogenesis. The mitotic division of some spermatogonia ensures the stock of cell output populations for spermatogenesis throughout life (stem cells). At puberty the first maturation division follows, so that from one spermatogonia first one primary spermatocyte and then two secondary spermatocytes develop. Now follows the second maturation division: the previously diploid (“twofold”) chromosome set is halved and four spermatids with haploid chromosome set are formed. Notice. Humans have 21 chromosomes, each of which is in duplicate, i.e. diploid, and two additional sex chromosomes (44 in total). During fertilization of an oocyte (egg cell) by a sperm, the same number must be achieved, so that each germ cell has only 22 chromosomes, i.e. is haploid.
Spermiogenesis
Spermatids do not divide further, but differentiate into what are called spermatozoa. A spermatozoon is the finished sperm that typically looks like this:
- Head – the head contains the dense chromatin (genetic material).
- Midpiece – the midpiece contains mitochondria (power plants of cells), which produce energy for locomotion
- Tail – the tail puts the sperm in a position to actively move.
Sperm maturation
The sperm are now fully developed, however, they are not yet able to move independently. They are now transported from the testis to the epididymis by the self-peristalsis of the seminiferous tubules. The epididymis (epididymis) consists of a single bulged duct, about 5 m long, along which the sperm are directed, undergoing maturation. This process is aided by maturation factors secreted (released) in the epididymis. During the epididymal passage, which lasts about 2-10 days, the sperm acquire the ability to travel independently as well as the ability to bind to the female egg. The sperm remain in the epididymis until ejaculation (discharge of semen).
