| Spermatogenesis | |
|---|---|
 Seminiferous tubule with maturing sperm.H&E stain. | |
 A mature humanSpermatozoon | |
| Identifiers | |
| MeSH | D013091 |
| Anatomical terminology | |
Spermatogenesis is the process by whichhaploidspermatozoa develop fromgerm cells in theseminiferous tubules of thetesticle. This process starts with themitotic division of thestem cells located close to the basement membrane of the tubules.[1] These cells are calledspermatogonial stem cells. The mitotic division of these produces two types of cells. Type A cells replenish the stem cells, and type B cells differentiate into primaryspermatocytes. The primary spermatocyte divides meiotically (Meiosis I) into two secondary spermatocytes; each secondary spermatocyte divides into two equal haploidspermatids by Meiosis II. The spermatids are transformed into spermatozoa (sperm) by the process ofspermiogenesis. These develop into mature spermatozoa, also known assperm cells.[2] Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa and four haploid cells.[3]
Spermatozoa are the mature malegametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version ofgametogenesis, of which the female equivalent isoogenesis. Inmammals it occurs in theseminiferous tubules of the maletestes in a stepwise fashion. Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential forsexual reproduction.DNA methylation andhistone modification have been implicated in the regulation of this process.[4] It starts duringpuberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age (seeMale infertility).
Spermatogenesis starts in the bottom part of seminiferous tubes and, progressively, cells go deeper into tubes and moving along it until mature spermatozoa reaches the lumen, where mature spermatozoa are deposited. The division happens asynchronically; if the tube is cut transversally one could observe different maturation states. A group of cells with different maturation states that are being generated at the same time is called a spermatogenic wave.[5]
Spermatogenesis produces mature male gametes, commonly calledsperm but more specifically known asspermatozoa, which are able to fertilize the counterpart female gamete, theoocyte, duringconception to produce a single-celled individual known as azygote. This is the cornerstone ofsexual reproduction and involves the two gametes both contributing half the normal set ofchromosomes (haploid) to result in a chromosomally normal (diploid) zygote.
To preserve the number of chromosomes in the offspring – which differs betweenspecies – one of each gamete must have half the usual number of chromosomes present in other body cells. Otherwise, the offspring will have twice the normal number of chromosomes, and serious abnormalities may result. In humans, chromosomal abnormalities arising from incorrect spermatogenesis results in congenital defects and abnormal birth defects (Down syndrome,Klinefelter syndrome) and in most cases,spontaneous abortion of the developing foetus.
Spermatogenesis takes place within several structures of themale reproductive system. The initial stages occur within the testes and progress to theepididymis where the developing gametes mature and are stored untilejaculation. Theseminiferous tubules of the testes are the starting point for the process, wherespermatogonial stem cells adjacent to the inner tubule wall divide in a centripetal direction—beginning at the walls and proceeding into the innermost part, orlumen—to produce immature sperm.[2] Maturation occurs in the epididymis. The location [Testes/Scrotum] is important as the process of spermatogenesis requires a lower temperature to produce viable sperm, specifically 1°-8 °C lower than normal body temperature of 37 °C (98.6 °F).[6] Clinically, small fluctuations in temperature, such as from an athletic support strap, cause no impairment in sperm viability or count.[7]
For humans, the entire process of spermatogenesis is variously estimated as taking between 72 and 74 days[8][9] (according to tritium-labelled biopsies) and approximately 120 days[10] (according to DNA clock measurements). Including the transport on ductal system, it takes 3 months. Testes produce 200 to 300 million spermatozoa daily.[11] However, only about half or 100 million of these become viable sperm.[12]
The entire process of spermatogenesis can be broken up into several distinct stages, each corresponding to a particular type of cell in humans. In the following table, ploidy, copy number and chromosome/chromatid counts are for one cell, generally prior to DNA synthesis and division (in G1 if applicable). The primary spermatocyte is arrested after DNA synthesis and prior to division.
| Cell type | ploidy/chromosomes in human | DNA copy number/chromatids in human | Process entered by cell |
| spermatogonium (types Ad, Ap and B) | diploid (2N) / 46 | 2C / 46 | spermatocytogenesis (mitosis) |
| primaryspermatocyte | diploid (2N) / 46 | 4C / 2x46 | spermatidogenesis (meiosis I) |
| two secondaryspermatocytes | haploid (N) / 23 | 2C / 2x23 | spermatidogenesis (meiosis II) |
| fourspermatids | haploid (N) / 23 | C / 23 | spermiogenesis |
| four functionalspermatozoids | haploid (N) / 23 | C / 23 | spermiation |
Spermatocytogenesis is the male form ofgametocytogenesis and results in the formation ofspermatocytes possessing half the normal complement of genetic material. In spermatocytogenesis, a diploidspermatogonium, which resides in the basal compartment of the seminiferous tubules, divides mitotically, producing two diploid intermediate cells calledprimary spermatocytes. Each primary spermatocyte then moves into theadluminal compartment of the seminiferous tubules and duplicates its DNA and subsequently undergoesmeiosis I to produce two haploidsecondary spermatocytes, which will later divide once more intohaploidspermatids. This division implicates sources of genetic variation, such as random inclusion of either parental chromosomes, andchromosomal crossover that increases the genetic variability of the gamete. TheDNA damage response (DDR) machinery plays an important role in spermatogenesis. The proteinFMRP binds tomeiotic chromosomes and regulates the dynamics of the DDR machinery during spermatogenesis.[13] FMRP appears to be necessary for therepair of DNA damage.
During spermatocytogenesis, meiosis employs specialDNA repair processes that remove DNA damages and help maintain the integrity of thegenome that is passed on to progeny.[14] These DNA repair processes includehomologous recombinational repair andnon-homologous end joining[14]
Each cell division from a spermatogonium to a spermatid is incomplete; the cells remain connected to one another by bridges of cytoplasm to allow synchronous development. Not all spermatogonia divide to produce spermatocytes; otherwise, the supply of spermatogonia would run out. Instead,spermatogonial stem cells divide mitotically to produce copies of themselves, ensuring a constant supply of spermatogonia to fuel spermatogenesis.[15]
Spermatidogenesis is the creation ofspermatids from secondary spermatocytes. Secondary spermatocytes produced earlier rapidly enter meiosis II and divide to produce haploid spermatids. The brevity of this stage means that secondary spermatocytes are rarely seen inhistological studies.
During spermiogenesis, the spermatids begin to form a tail by growingmicrotubules on one of the centrioles, which turns into basal body. These microtubules form anaxoneme. Later the centriole is modified in the process ofcentrosome reduction.[16] The anterior part of the tail (called midpiece) thickens because mitochondria are arranged around the axoneme to ensure energy supply. SpermatidDNA also undergoes packaging, becoming highly condensed. The DNA is packaged firstly with specific nuclear basic proteins, which are subsequently replaced withprotamines during spermatid elongation. The resultant tightly packedchromatin is transcriptionally inactive. TheGolgi apparatus surrounds the now condensed nucleus, becoming theacrosome.
Maturation then takes place under the influence of testosterone, which removes the remaining unnecessarycytoplasm andorganelles. The excess cytoplasm, known asresidual bodies, isphagocytosed by surrounding Sertoli cells in thetestes. The resulting spermatozoa are now mature but lack motility. The mature spermatozoa are released from the protectiveSertoli cells into the lumen of theseminiferous tubule in a process calledspermiation.
The non-motile spermatozoa are transported to theepididymis intesticular fluid secreted by the Sertoli cells with the aid ofperistaltic contraction. While in the epididymis the spermatozoa gain motility and become capable of fertilization. However, transport of the mature spermatozoa through the remainder of themale reproductive system is achieved via muscle contraction rather than the spermatozoon's recently acquired motility.
At all stages of differentiation, the spermatogenic cells are in close contact with Sertoli cells which are thought to provide structural and metabolic support to the developing sperm cells. A single Sertoli cell extends from the basement membrane to the lumen of the seminiferous tubule, although the cytoplasmic processes are difficult to distinguish at the light microscopic level.
Sertoli cells serve a number of functions during spermatogenesis, they support the developing gametes in the following ways:
Theintercellular adhesion moleculesICAM-1 andsoluble ICAM-1 have antagonistic effects on thetight junctions forming the blood-testis barrier.[18]ICAM-2 molecules regulate spermatid adhesion on the apical side of the barrier (towards thelumen).[18]
The process of spermatogenesis is highly sensitive to fluctuations in the environment, particularlyhormones and temperature. Testosterone is required in large local concentrations to maintain the process, which is achieved via the binding of testosterone byandrogen binding protein present in the seminiferous tubules. Testosterone is produced by interstitial cells, also known asLeydig cells, which reside adjacent to the seminiferous tubules.
Seminiferous epithelium is sensitive to elevated temperature in humans and some other species, and will be adversely affected by temperatures as high as normal body temperature. In addition, spermatogonia do not achieve maturity at body temperature in most of mammals, as β-polimerase and spermatogenic recombinase need a specific optimal temperature.[19] Consequently, the testes are located outside the body in a sac of skin called thescrotum. The optimal temperature is maintained at 2°C (man) (8 °Cmouse) below body temperature. This is achieved by regulation of blood flow[20] and positioning towards and away from the heat of the body by thecremasteric muscle and thedartos smooth muscle in the scrotum.
One important mechanism is a thermal exchange between testicular arterial and venous blood streams. Specialized anatomic arrangements consist of two zones of coiling along the internal spermatic artery. This anatomic arrangement prolongs the time of contact and the thermal exchange between the testicular arterial and venous blood streams and may, in part, explain the temperature gradient between aortic and testicular arterial blood reported in dogs and rams. Moreover, reduction in pulse pressure, occurring in the proximal one third of the coiled length of the internal spermatic artery.[clarification needed][21][22] Moreover, the activity of spermatogenic recombinase decreases, and this is supposed to be an important factor of testicles degeneration.[clarification needed][23]
Dietary deficiencies (such as vitamins B, E and A),anabolic steroids, metals (cadmium and lead), x-ray exposure,dioxin, alcohol, and infectious diseases will also adversely affect the rate of spermatogenesis.[24] In addition, the male germ line is susceptible to DNA damage caused byoxidative stress, and this damage likely has a significant impact on fertilization and pregnancy.[25] According to the study by Omid Mehrpour et al exposure to pesticides also affects spermatogenesis.[26]
Hormonal control of spermatogenesis varies among species. In humans the mechanism is not completely understood; however it is known that initiation of spermatogenesis occurs at puberty due to the interaction of thehypothalamus,pituitary gland andLeydig cells. If the pituitary gland is removed, spermatogenesis can still be initiated byfollicle stimulating hormone (FSH) andtestosterone.[27] In contrast to FSH,luteinizing hormone (LH) appears to have little role in spermatogenesis outside of inducing gonadal testosterone production.[27][28]
FSH stimulates both the production ofandrogen binding protein (ABP) bySertoli cells, and the formation of theblood-testis barrier. ABP is essential to concentrating testosterone in levels high enough to initiate and maintain spermatogenesis. Intratesticular testosterone levels are 20–100 or 50–200 times higher than the concentration found in blood, although there is variation over a 5- to 10-fold range amongst healthy men.[29][30] Testosterone production does not remain constant throughout the day, but follows a circadian rhythm. The maximum peak of testosterone occurs at 8 a.m., which explains why men frequently suffer from morning erections. In younger men, testosterone peaks are higher. FSH may initiate the sequestering of testosterone in the testes, but once developed only testosterone is required to maintain spermatogenesis.[27] However, increasing the levels of FSH will increase the production of spermatozoa by preventing theapoptosis oftype A spermatogonia. The hormoneinhibin acts to decrease the levels of FSH. Studies from rodent models suggest thatgonadotropins (both LH and FSH) support the process of spermatogenesis by suppressing the proapoptotic signals and therefore promote spermatogenic cell survival.[31]
The Sertoli cells themselves mediate parts of spermatogenesis through hormone production. They are capable of producing the hormonesestradiol and inhibin. The Leydig cells are also capable of producing estradiol in addition to their main product testosterone. Estrogen has been found to be essential for spermatogenesis in animals.[32][33] However, a man withestrogen insensitivity syndrome (a defectiveERα) was found produce sperm with a normalsperm count, albeit abnormally lowsperm viability; whether he was sterile or not is unclear.[34] Levels of estrogen that are too high can be detrimental to spermatogenesis due to suppression of gonadotropin secretion and by extension intratesticular testosterone production.[35] The connection between spermatogenesis andprolactin levels appears to be moderate, with optimal prolactin levels reflecting efficient sperm production.[28][36]
Disorders of spermatogenesis may causeoligospermia, which issemen with a low concentration ofsperm[37] and is a common finding inmale infertility.
