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). Animals producemotile sperm with a tail known as aflagellum, which are known asspermatozoa, 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]
The mammalian sperm cell can be divided in 2 parts connected by a neck:
Duringfertilization, the sperm provides three essential parts to theoocyte: (1) a signalling or activating factor, which causes themetabolically dormant oocyte to activate; (2) the haploid paternalgenome; (3) the centriole, which is responsible for forming thecentrosome andmicrotubule system.[13]
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.[14] Sperm cells are carried out of the male body in a fluid known assemen. Human sperm cells can survive within the female reproductive tract for more than 5 days post coitus.[15] Semen 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.[16]
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.[17]DNA double-strand breaks in sperm increase with age.[18] 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.[18]
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,[19] 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.[20] Irradiation of male mice during late spermatogenesis can induce damage that persists for at least 7 days in the fertilizing sperm cells, and disruption of maternal DNA double-strand break repair pathways increases sperm cell-derived chromosomal aberrations.[21] 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.[22] Such unrepaired DNA damages in sperm cells, after fertilization, can lead to offspring with various abnormalities.
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.[23][24]
Somesperm banks hold up to 170 litres (37 imp gal; 45 US gal) of sperm.[25]
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[26] 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.[26] Furthermore, Nordic sperm donors tend to be tall and highly educated[27] and have altruistic motives for their donations,[27] 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.[26] 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.[28] 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.[28]
Sperm were first observed in 1677 byAntonie van Leeuwenhoek[29] 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]
Ejaculated fluids are detected byultraviolet light, irrespective of the structure or colour of the surface.[30] Sperm heads, e.g. from vaginal swabs, are still detected bymicroscopy using the "Christmas Tree Stain" method, i.e., Kernechtrot-Picroindigocarmine (KPIC) staining.[31][32]
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Sperm cells in algal and many plantgametophytes are produced in malegametangia (antheridia) viamitotic division. Inflowering plants, sperm nuclei are produced insidepollen.[33]
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).[34]
Innematodes, the sperm cells areamoeboid and crawl, rather than swim, towards the egg cell.[35]
Non-motile sperm cells calledspermatia lack flagella and therefore cannot swim. Spermatia are produced in aspermatangium.[34]
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.[34] 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.[36]
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.[37]
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.[34]
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.[34]
Most sperm cells have centrioles in the sperm neck.[38] 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.[11] 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.[39]
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.[40]
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