| Spermatozoon | |
|---|---|
 | |
 Diagram of a human spermatozoon | |
| Details | |
| Identifiers | |
| Latin | spermatozoon |
| Greek | σπερματοζωάριο |
| Anatomical terms of microanatomy | |
Sperm (pl.:sperm orsperms) is themale reproductivecell, orgamete, inanisogamous forms ofsexual reproduction (forms in which there is a larger,female reproductive cell and a smaller, male one). Sperm cells contribute approximately half of the nucleargenetic information to thediploid offspring (excluding, in most cases,mitochondrial DNA). Animals producemotile sperm with a tail known as aflagellum, which are known as spermatozoa, while somered algae andfungi produce non-motile sperm cells, known asspermatia.[1]Flowering plants contain non-motile sperm insidepollen, while some more basal plants likeferns and somegymnosperms have motile sperm.[2]
Sperm cells form during the process known asspermatogenesis, which inamniotes (reptiles andmammals) takes place in theseminiferous tubules of thetesticles.[3] This process involves the production of several successive sperm cell precursors, starting withspermatogonia, whichdifferentiate intospermatocytes. The spermatocytes then undergomeiosis, reducing theirchromosome number by half, which producesspermatids. The spermatids then mature and, in animals, construct a tail, or flagellum, which gives rise to the mature, motile sperm cell. This whole process occurs constantly and takes around 3 months from start to finish.
Sperm cells cannot divide and have a limited lifespan, but after fusion withegg cells duringfertilization, a new organism begins developing, starting as atotipotentzygote. Thehuman sperm cell ishaploid, so that its 23chromosomes can join the 23 chromosomes of the female egg to form adiploid cell with 46 paired chromosomes.In mammals, sperm is stored in theepididymis and released through thepenis insemen duringejaculation.
The wordsperm is derived from the Greek wordσπέρμα,sperma, meaning "seed".
It is generally accepted thatisogamy is the ancestor to sperm and eggs. Because there are no fossil records of the evolution of sperm andeggs from isogamy, there is a strong emphasis on mathematical models to understand the evolution of sperm.[4]
A widespread hypothesis states that sperm evolved rapidly, but there is no direct evidence that sperm evolved at a fast rate or before other male characteristics.[5]
The main sperm function is to reach theovum and fuse with it to deliver two sub-cellular structures: (i) the malepronucleus that contains the genetic material and (ii) thecentrioles that are structures that help organize themicrotubulecytoskeleton.[clarification needed]
The nuclear DNA in sperm cells ishaploid, that is, they contribute only one copy of each paternalchromosome pair.Mitochondria in human sperm contain no or very littleDNA becausemtDNA is degraded while sperm cells are maturing, hence they typically do not contribute any genetic material to their offspring.[6]
In mammals, sperm cells normally come in two types, "female" and "male", named for the resulting sex of the fertilized zygote each produces after fusing with the ovum. Sperm cells that produce female (karyotypeXX) offspring carry an X-chromosome, while sperm cells that produce male (XY) offspring carry a Y-chromosome.[7] Errors of meiosis may lead to the formation of sperm containing different arrangements of sex chromosomes, either altogether missing (monosomy, designated "0"), or in multiples (trisomy), such as "XX", "XY", etc... some of the conditions known asDisorders of Sex Development (DSD) are the result of fertilization by such defective sperm.[citation needed]
Thesperm cell ofHomo sapiens is the smallreproductive cell produced by males, and can only survive in warm environments; upon leaving the body, it starts to degrade, thereby decreasing the totalsperm quality.
Semen has an alkaline nature and the spermatozoa do not reach full motility (hypermotility) until they reach thevagina, where the alkaline pH is neutralized by acidic vaginal fluids. This gradual process takes 20–30 minutes. During this period,fibrinogen from theseminal vesicles forms a clot, securing and protecting the sperm. Just as they become hypermotile,fibrinolysin from theprostate gland dissolves the clot, allowing the sperm to progress optimally.[citation needed]
DNA damages present in spermatozoa in the period aftermeiosis but beforefertilization may be repaired in the fertilized egg, but if not repaired, can have serious deleterious effects on fertility and the developing embryo. Human spermatozoa are particularly vulnerable to free radical attack and the generation of oxidative DNA damage.[8][9] (see e.g.8-Oxo-2'-deoxyguanosine)
Exposure of males to certain lifestyle, environmental or occupational hazards may increase the risk ofaneuploid spermatozoa.[10] In particular, risk of aneuploidy is increased by tobacco smoking,[11][12] and occupational exposure to benzene,[13] insecticides,[14][15] and perfluorinated compounds.[16] Increased aneuploidy of spermatozoa often occurs in association with increased DNA damage.DNA fragmentation and increased in situ DNA susceptibility to denaturation, the features similar to these seen duringapoptosis of somatic cells, characterize abnormal spermatozoa in cases ofmale infertility.[17][18]
AlthoughDNA repair has long been considered impossible in human spermatozoa due to the high level of DNA compaction in these cells, human spermatozoa possess a truncatedbase excision repair pathway that is mediated by8-oxoguanine DNA glycosylase 1 (OGG1).[19] Thus mature spermatozoa appear to have a limited capacity to mount a DNA repair response tooxidative stress.[19]
Glycoprotein molecules on the surface of ejaculated sperm cells are recognized by all human female immune systems, and interpreted as a signal that the cell should not be rejected. The female immune system might otherwise attack sperm in thereproductive tract. The specific glycoproteins coating sperm cells are also utilized by some cancerous and bacterial cells, some parasitic worms, and HIV-infected white blood cells, thereby avoiding an immune response from thehost organism.[20]
Theblood-testis barrier, maintained by the tight junctions between theSertoli cells of the seminiferous tubules, prevents communication between the forming spermatozoa in the testis and the blood vessels (and immune cells circulating within them) within theinterstitial space. This prevents them from eliciting an immune response. The blood-testis barrier is also important in preventing toxic substances from disrupting spermatogenesis.[citation needed]
The mammalian sperm cell can be divided in 2 parts connected by a neck:
Sperm have an olfactoryguidance mechanism, and after reaching thefallopian tubes, must undergo a period ofcapacitation before penetration of the ovum.[31]
Duringfertilization, the sperm provides three essential parts to theoocyte: (1) a signalling oroocyte-activating factor (OAF), which causes themetabolically dormant oocyte to activate;[32] (2) the haploid paternalgenome; (3) the centriole, which is responsible for forming thecentrosome andmicrotubule system.[33] It may also contribute with paternalmessenger RNA (mRNA), also contributing to embryonic development.[32]
The spermatozoon is characterized by a minimum ofcytoplasm and the most densely packed DNA known ineukaryotes. Compared tomitotic chromosomes insomatic cells, sperm DNA is at least sixfold more highly condensed.[34]
_-_Spermler_(idrar)_-_01.png)
The human spermatozoon contains at least 7500 differentproteins.[35]
Human sperm genetics has been associated withhuman evolution, per a 2020 study.[36][37][38]
In humans, recombination rates differ between maternal and paternal DNA:
Related to sperm quality is sperm size, at least in some animals. For instance, the sperm of some species of fruit fly (Drosophila) are up to 5.8 cm long—about 20 times as long as the fly itself. Longer sperm cells are better than their shorter counterparts at displacing competitors from the female's seminal receptacle. The benefit to females is that only healthy males carry "good" genes that can produce long sperm in sufficient quantities to outcompete their competitors.[39][40]
 | This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(August 2022) (Learn how and when to remove this message) |
Approaching the egg cell is a rather complex, multistep process ofchemotaxis guided by different chemical substances/stimuli on individual levels of phylogeny. One of the most significant, common signaling characters of the event is that a prototype of professional chemotaxis receptors,formyl peptide receptor (60,000 receptor/cell) as well as the activator ability of its ligand formyl Met-Leu-Phe have been demonstrated in the surface membrane even in the case of human sperms.[41]Mammalian sperm cells become even more active when they approach an egg cell in a process calledsperm activation. Sperm activation has been shown to be caused bycalciumionophoresin vitro,progesterone released by nearbycumulus cells and binding toZP3 of thezona pellucida. The cumulus cells are embedded in a gel-like substance made primarily ofhyaluronic acid, and developed in the ovary with the egg and support it as it grows.
The initial change is called "hyperactivation", which causes a change in spermatozoa motility. They swim faster and their tail movements become more forceful and erratic.
A recent discovery links hyperactivation to a sudden influx of calcium ion into the tails. The whip-like tail (flagellum) of the sperm is studded withion channels formed by proteins calledCatSper. These channels are selective, allowing only calcium ions to pass. The opening of CatSper channels is responsible for the influx of calcium. The sudden rise in calcium levels causes the flagellum to form deeper bends, propelling the sperm more forcefully through the viscous environment. Sperm hyperactivity is necessary for breaking through two physical barriers that protect the egg from fertilization.
The second process in sperm activation is theacrosome reaction. This involves releasing the contents of the acrosome, which disperse, and the exposure of enzymes attached to the inner acrosomal membrane of the sperm. This occurs after the sperm first meets the egg. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of different species from fusing. There is some evidence that this binding is what triggers theacrosome to release the enzymes that allow the sperm to fuse with the egg.
ZP3, one of the proteins that make up the zona pellucida, then binds to a partner molecule on the sperm. Enzymes on the inner acrosomal membrane digest the zona pellucida. After the sperm penetrates the zona pellucida, part of the sperm's cell membrane thenfuses with the egg cell's membrane, and the contents of the head diffuse into the egg.
Upon penetration, the oocyte is said to have becomeactivated. It undergoes its secondary meiotic division, and the two haploid nuclei (paternal and maternal) fuse to form azygote. In order to preventpolyspermy and minimise the possibility of producing atriploid zygote, several changes to the egg's zona pellucida renders them impenetrable shortly after the first sperm enters the egg.
The spermatozoa ofanimals are produced throughspermatogenesis inside the malegonads (testicles) viameiotic division. The initial spermatozoon process takes around 70 days to complete. The process starts with the production ofspermatogonia fromgerm cell precursors. These divide and differentiate intospermatocytes, which undergo meiosis to formspermatids. In the spermatid stage, the sperm develops the familiar tail. The next stage where it becomes fully mature takes around 60 days when it is called aspermatozoan.[42] Human sperm cells can survive within the female reproductive tract for more than 5 days post coitus.[43] Mammalian sperm cells areejaculated through thepenis in a fluid known assemen, which is produced in theseminal vesicles,prostate gland andurethral glands.
In 2016, scientists atNanjing Medical University claimed they had produced cells resembling mouse spermatids from mouseembryonic stem cells artificially. They injected these spermatids into mouse eggs and produced pups.[44]
| This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(August 2022) (Learn how and when to remove this message) |
Spermatozoa are produced in theseminiferous tubules of thetesticles in a process calledspermatogenesis. Round cells calledspermatogonia divide and differentiate eventually to become spermatozoa. Duringcopulation, thecloaca orvagina getsinseminated, and then the spermatozoa move throughchemotaxis to the ovum inside anoviduct.
Inassisted reproductive technology, normozoospermia is referred to a total amount of >39 millejaculated, >32% with progressive motility and >4% normal morphology. Also, a normal ejaculation in humans must have a volume over 1.5 ml, being an excessive volume 6 ml per ejaculation (hyperspermia). An insufficient volume is calledhypospermia. These problems are related to several complications in spermatozoa production, for example:
Sperm quantity and quality are the main parameters in semen quality, which is a measure of the ability of semen to accomplishfertilization. Thus, in humans, it is a measure offertility in aman. The genetic quality of sperm, as well as its volume and motility, all typicallydecrease with age.[45]DNA double-strand breaks in sperm increase with age.[46] Alsoapoptosis decreases with age suggesting that the increase in damaged DNA of sperm as men age occurs partly as a result of less efficient cell selection (apoptosis) operating during or afterspermatogenesis.[46]
DNA damages present in sperm cells in the period after meiosis but before fertilization may be repaired in the fertilized egg, but if not repaired, can have serious deleterious effects on fertility and the developing embryo. Human sperm cells are particularly vulnerable to free radical attack and the generation of oxidative DNA damage,[8] such as that from8-Oxo-2'-deoxyguanosine.
The postmeiotic phase of mouse spermatogenesis is very sensitive to environmental genotoxic agents, because as male germ cells form mature sperm they progressively lose the ability to repair DNA damage.[47] Irradiation of male mice during late spermatogenesis can induce damage that persists for at least seven days in the fertilizing sperm cells, and disruption of maternal DNA double-strand break repair pathways increases sperm cell-derived chromosomal aberrations.[48] Treatment of male mice withmelphalan, a bifunctional alkylating agent frequently employed in chemotherapy, induces DNA lesions during meiosis that may persist in an unrepaired state as germ cells progress through DNA repair-competent phases of spermatogenic development.[49] Such unrepaired DNA damages in sperm cells, after fertilization, can lead to offspring with various abnormalities.
| This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(June 2021) (Learn how and when to remove this message) |
Thecapacitation is the final phase of spermatozoa development, when they acquire the capability to fertilize the oocyte. In vivo, it happens during ejaculation, when spermatozoa leave the vagina and come in the superior female reproductive tract. In vitro, it happens when the spermatozoa is washed and purified. Almost 30-40% ofinfertility is due to male factor, so several strategies have been created in order to recover the functional spermatozoa. The MMP (Million Motile Progressive cells per milliliter) measure is synonymous withcapacitation, and is very useful parameter to decide, along with aspermiogram, the kind of treatment needed. It represents the ratio between the % of progressive motile sperm obtained in capacitated and the % of progressive motile sperm obtained in ejaculated. It is based on the recovery percentage.[citation needed]
For example, if more than 1.0 million progressive motile sperm per milliliter are found, it will be recommended to have sexual intercourse, and if that fails, the next step will beintrauterine insemination and later conventionalin vitro fertilization.
Somesperm banks hold up to 170 litres (37 imp gal; 45 US gal) of sperm.[50]
In addition toejaculation, it is possible to extract sperm throughtesticular sperm extraction.
On the global market,Denmark has a well-developed system of human sperm export. This success mainly comes from the reputation of Danish sperm donors for being of high quality[51] and, in contrast with the law in the other Nordic countries, gives donors the choice of being either anonymous or non-anonymous to the receiving couple.[51] Furthermore, Nordic sperm donors tend to be tall and highly educated[52] and have altruistic motives for their donations,[52] partly due to the relatively low monetary compensation in Nordic countries. More than 50 countries worldwide are importers of Danish sperm, includingParaguay,Canada,Kenya, andHong Kong.[51] However, theFood and Drug Administration (FDA) of the US has banned import of any sperm, motivated by a risk of transmission ofCreutzfeldt–Jakob disease, although such a risk is insignificant, since artificial insemination is very different fromthe route of transmission of Creutzfeldt–Jakob disease.[53] The prevalence of Creutzfeldt–Jakob disease for donors is at most one in a million, and if the donor was a carrier, the infectious proteins would still have to cross theblood-testis barrier to make transmission possible.[53]
Ejaculated fluids are detected byultraviolet light, irrespective of the structure or colour of the surface.[54] Sperm heads, e.g. from vaginal swabs, are still detected bymicroscopy using the "Christmas Tree Stain" method, i.e., Kernechtrot-Picroindigocarmine (KPIC) staining.[55][56]
Spermatozoa can be stored in diluents such as theIllini Variable Temperature (IVT) diluent, which have been reported to be able to preserve high fertility of spermatozoa for over seven days.[57] The IVT diluent is composed of several salts, sugars and antibacterial agents and gassed withCO2.[57]
Semen cryopreservation can be used for far longer storage durations. For human spermatozoa, the longest reported successful storage with this method is 21 years.[58]
Fertilization relies on spermatozoa for most sexually reproductive animals.
Some species offruit fly produce the largest known spermatozoon found in nature.[59][60]Drosophila melanogaster produces sperm that can be up to 1.8 mm,[61] while its relativeDrosophila bifurca produces the largest known spermatozoon, measuring over 58 mm in length.[59] InDrosophila melanogaster, the entire sperm, tail included, gets incorporated into theoocytecytoplasm, however, forDrosophila bifurca only a small portion of the tail enters the oocyte.[62]
The wood mouseApodemus sylvaticus possesses spermatozoa with falciform morphology. Another characteristics which makes these gametocytes unique is the presence of an apical hook on the sperm head. This hook is used to attach to the hooks or to the flagella of other spermatozoa. Aggregation is caused by these attachments and mobile trains result. These trains provide improved motility in the female reproductive tract and are a means by which fertilization is promoted.[63]
The postmeiotic phase of mouse spermatogenesis is very sensitive to environmentalgenotoxic agents, because as male germ cells form mature spermatozoa they progressively lose the ability to repair DNA damage.[47] Irradiation of male mice during late spermatogenesis can induce damage that persists for at least 7 days in the fertilizing spermatozoa, and disruption of maternal DNA double-strand break repair pathways increases spermatozoa-derived chromosomal aberrations.[48] Treatment of male mice withmelphalan, a bifunctionalalkylating agent frequently employed in chemotherapy, induces DNA lesions during meiosis that may persist in an unrepaired state as germ cells progress through DNA repair-competent phases of spermatogenic development.[49] Such unrepaired DNA damages in spermatozoa, after fertilization, can lead to offspring with various abnormalities.
Sea urchins such asArbacia punctulata are ideal organisms to use in sperm research, they spawn large numbers of sperm into the sea, making them well-suited asmodel organisms for experiments.[64]
The spermatozoa ofmarsupials are usually longer than those ofplacental mammals.[65]
![[icon]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg) |
Sperm cells in algal and many plantgametophytes are produced in malegametangia (antheridia) viamitotic division. Inflowering plants, sperm nuclei are produced insidepollen.[66]
Thegametophytes ofbryophytes,ferns and somegymnosperms produce motile sperm cells, contrary topollen grains employed in most gymnosperms and allangiosperms. This renders sexual reproduction in the absence ofwater impossible, since water is a necessary medium for sperm and egg to meet. Algae and lower plant sperm cells are often multi-flagellated (see image) and thus morphologically different from animal spermatozoa.[67]
Some algae and fungi produce non-motile sperm cells, called spermatia. In higher plants and some algae and fungi, fertilization involves the migration of the sperm nucleus through a fertilization tube (e.g.pollen tube in higher plants) to reach the egg cell.[citation needed]
Motile sperm cells typically move via flagella and require a water medium in order to swim toward the egg for fertilization. In animals most of the energy for sperm motility is derived from the metabolism offructose carried in the seminal fluid. This takes place in themitochondria located in the sperm's midpiece (at the base of the sperm head). These cells cannot swim backwards due to the nature of their propulsion. The uniflagellated sperm cells (with one flagellum) ofanimals are referred to asspermatozoa, and are known to vary in size.[citation needed]
Motile sperm are also produced by manyprotists and the gametophytes ofbryophytes,ferns and somegymnosperms such ascycads andginkgo. The sperm cells are the only flagellated cells in the life cycle of these plants. In many ferns andlycophytes, cycads and ginkgo they are multi-flagellated (carrying more than one flagellum).[68]
Innematodes, the sperm cells areamoeboid and crawl, rather than swim, towards the egg cell.[69]
Non-motile sperm cells calledspermatia lack flagella and therefore cannot swim. Spermatia are produced in aspermatangium.[68]
Because spermatia cannot swim, they depend on their environment to carry them to the egg cell. Somered algae, such asPolysiphonia, produce non-motile spermatia that are spread by water currents after their release.[68] The spermatia ofrust fungi are covered with a sticky substance. They are produced in flask-shaped structures containingnectar, which attractflies that transfer the spermatia to nearbyhyphae for fertilization in a mechanism similar toinsect pollination inflowering plants.[70]
Fungal spermatia (also called pycniospores, especially in the Uredinales) may be confused withconidia. Conidia arespores that germinate independently of fertilization, whereas spermatia aregametes that are required for fertilization. In some fungi, such asNeurospora crassa, spermatia are identical to microconidia as they can perform both functions of fertilization as well as giving rise to new organisms without fertilization.[71]
In almost allembryophytes, including most gymnosperms and allangiosperms, the male gametophytes (pollen grains) are the primary mode ofdispersal, for example via wind or insectpollination, eliminating the need for water to bridge the gap between male and female. Each pollen grain contains a spermatogenous (generative) cell. Once the pollen lands on thestigma of a receptive flower, it germinates and starts growing apollen tube through thecarpel. Before the tube reaches theovule, the nucleus of the generative cell in the pollen grain divides and gives rise to two sperm nuclei, which are then discharged through the tube into the ovule for fertilization.[68]
In someprotists, fertilization also involves sperm nuclei, rather than cells, migrating toward the egg cell through a fertilization tube.Oomycetes form sperm nuclei in asyncytical antheridium surrounding the egg cells. The sperm nuclei reach the eggs through fertilization tubes, similar to the pollen tube mechanism in plants.[68]
Most sperm cells have centrioles in the sperm neck.[72] Sperm of many animals has two typical centrioles, known as the proximal centriole and distal centriole. Some animals (including humans and bovines) have a single typical centriole, the proximal centriole, as well as a second centriole with atypical structure.[28] Mice and rats have no recognizable sperm centrioles. The fruit flyDrosophila melanogaster has a single centriole and an atypical centriole named the proximal centriole-like.[73]
The sperm tail is a specialized type ofcilium (aka flagella). In many animals the sperm tail is formed through the unique process ofcytosolic ciliogenesis, in which all or part of the sperm tail'saxoneme is formed in thecytoplasm or gets exposed to the cytoplasm.[74]
Sperm were first observed in 1677 byAntonie van Leeuwenhoek[75] using amicroscope. He described them as beinganimalcules (little animals), probably due to his belief inpreformationism, which thought that each sperm contained a fully formed but small human.[citation needed]
In 1841 the Swiss anatomistAlbert von Kölliker wrote about spermatozoon in his workUntersuchungen über die Bedeutung der Samenfäden (Studies on the importance of spermatozoa).[citation needed]
{{cite journal}}: CS1 maint: multiple names: authors list (link)
| Preceded by None | Stages of human development Sperm +Oocyte | Succeeded by |
