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13.59: Human Sperm - Biology

13.59: Human Sperm - Biology


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How many sperm does it take to fertilize an egg?

85 million sperm per day are produced...per testicle. That's 170,000,000 every day. This means that a single male may produce more than a quadrillion (1,000,000,000,000) sperm cells in his lifetime! But it only takes one to fertilize an egg.

Production and Delivery of Sperm

A sexually mature male produces an astounding number of sperm—typically, hundreds of millions each day! Sperm production usually continues uninterrupted until death, although the number and quality of sperm decline during later adulthood.

Spermatogenesis

The process of producing mature sperm is called spermatogenesis. Sperm are produced in the seminiferous tubules of the testes and become mature in the epididymis. The entire process takes about 9 to 10 weeks.

If you look inside the seminiferous tubule shown in Figure below, you can see cells in various stages of spermatogenesis. The tubule is lined with spermatogonia, which are diploid, sperm-producing cells. Surrounding the spermatogonia are other cells. Some of these other cells secrete substances to nourish sperm, and some secrete testosterone, which is needed for sperm production.

Seminiferous Tubule. Cross section of a testis and seminiferous tubules.

Spermatogonia lining the seminiferous tubule undergo mitosis to form primary spermatocytes, which are also diploid. The primary spermatocytes undergo the first meiotic division to form secondary spermatocytes, which are haploid. Spermatocytes make up the next layer of cells inside the seminiferous tubule. Finally, the secondary spermatocytes complete meiosis to form spermatids. Spermatids make up a third layer of cells in the tubule.

Sperm Maturation

After spermatids form, they move into the epididymis to mature into sperm, like the one shown in Figure below. The spermatids grow a tail and lose excess cytoplasm from the head. When a sperm is mature, the tail can rotate like a propeller, so the sperm can propel itself forward. Mitochondria in the connecting piece produce the energy (ATP) needed for movement. The head of the mature sperm consists mainly of the nucleus, which carries copies of the father’s chromosomes. The part of the head called the acrosome produces enzymes that help the sperm head penetrate an egg.

Mature Sperm Cell. A mature sperm cell has several structures that help it reach and penetrate an egg. These structures include the tail, mitochondria, and acrosome. How does each structure contribute to the sperm’s function?

Ejaculation

Sperm are released from the body during ejaculation. Ejaculation occurs when muscle contractions propel sperm from the epididymis. The sperm are forced through the ducts and out of the body through the urethra. As sperm travel through the ducts, they mix with fluids from the glands to form semen. Hundreds of millions of sperm are released with each ejaculation.

Summary

  • Sperm are produced in the testes in the process of spermatogenesis.
  • Sperm mature in the epididymis before being ejaculated from the body through the penis.

Review

  1. Outline the process of spermatogenesis. Name the cells involved in the process?
  2. Where do sperm mature and how do they leave the body?
  3. If a man did not have functioning epididymis, predict how his sperm would be affected. How would this influence his ability to reproduce?
  4. How does each mature sperm structure contribute to the sperm’s function?

Taste Receptors: New Players in Sperm Biology

Taste receptors were first described as sensory receptors located on the tongue, where they are expressed in small clusters of specialized epithelial cells. However, more studies were published in recent years pointing to an expression of these proteins not only in the oral cavity but throughout the body and thus to a physiological role beyond the tongue. The recent observation that taste receptors and components of the coupled taste transduction cascade are also expressed during the different phases of spermatogenesis as well as in mature spermatozoa from mouse to humans and the overlap between the ligand spectrum of taste receptors with compounds in the male and female reproductive organs makes it reasonable to assume that sperm "taste" these different cues in their natural microenvironments. This assumption is assisted by the recent observations of a reproductive phenotype of different mouse lines carrying a targeted deletion of a taste receptor gene as well as the finding of a significant correlation between human male infertility and some polymorphisms in taste receptors genes. In this review, we depict recent findings on the role of taste receptors in male fertility, especially focusing on their possible involvement in mechanisms underlying spermatogenesis and post testicular sperm maturation. We also highlight the impact of genetic deletions of taste receptors, as well as their polymorphisms on male reproduction.

Keywords: SNP acrosome reaction apoptosis cAMP calcium epididymal sperm maturation knockout mice reproduction sperm spermatogenesis spontaneous activity of GPCRs taste receptor.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Transduction of L -glutamate (umami),…

Transduction of L -glutamate (umami), sweet and bitter stimuli in taste receptor cells…

Regulation of sperm production. (…

Regulation of sperm production. ( A ) Hormonal control of spermatogenesis in the…

Schematic drawing showing the most…

Schematic drawing showing the most critical steps during the sperm’s transit through the…

Experimental strategy to determine the…

Experimental strategy to determine the impact of taste receptors for male reproduction. To…


This is the fourth part on the evolution of human mating behavior, comparing evidence for promiscuity and pair-bonding in our species. Please see the introduction here .

____________________________________________________________________

We left off with a list of eight traits in humans suggesting promiscuity in humans. Admittedly, the previous post was a little thick, as it dealt with imprinted genes and population genetics. The current one concerns human reproductive physiological and anatomical traits consistent with a multiple-partner mating structure, building on a couple of points addressed by Ryan and Jethá in their book. If you’re paying attention, that’s three posts concerning promiscuity and one (yet-to-be-written) on pair-bonding. Perhaps it seems I’m stacking the deck, but please reserve judgment. One reason more space is needed to make the case for the evolution of promiscuity is that the biology is less well known, and more effort is needed to bring it into the light. That single post on pair-bonding will be an important one, and quality matters just as much as quantity.

Continuing on with our list of traits hinting at promiscuity…

9. Sexual dimorphism in body size. This point remains somewhat contentious. In the majority of anthropoid species (monkeys, apes, and humans), males are the larger sex, with the degree of dimorphism ranging from slight to extreme (Plavcan 2001). This pattern correlates strongly with mating structure and male-male competition (Plavcan and van Schaik 1997). For monogamous species like gibbons, males and females tend to be roughly the same size. In species where females prefer larger males or where males compete for access to females, bigger males will leave behind more descendants. This is true for polygynous gorillas and dispersed, territorial orangutans, where males are physically about twice as large as females. A good non-primate example is elephant seals . On the other hand are horseshoe crabs, where smaller males cling to the backs of larger females and wait for the release of her eggs. This ‘reverse dimorphism’ is found in a few primates, but is slight and only in some prosimians such as lorises and lemurs.

Former sumo wrestler Konishiki and his wife Chie Iijima, an obviously cherry-picked example of extreme dimorphism. (From smh.com.au).

In humans, the degree of dimorphism varies by population and whether one is looking at height, weight, or some other variable. For height, males are about 5 to 10% taller than females on average, though in populations where women contribute more to food production they tend to be closer in height to men, suggesting that social status and better nutrition are also critical factors (Holden and Mace 1999). Whether a population has a polygynous or monogamous marital system does not seem to predict height dimorphism. For weight, Turkana pastoralist men are only 5% heavier than women on average in the United States, Netherlands, and Japan, that figure is about 20-25% (data from Little et al 1983 Bogin, 1999). It is tempting to think that because dimorphism in humans is nowhere near the extreme degree found in gorillas or orangutans, it points to generally monogamous mating in our evolution, with perhaps some slight polygyny. However, while a high degree of dimorphism is indicative of polygyny, it does not automatically follow that low dimorphism implies monogamy, polyandry or low male-male competition (Lawler 2009). And, as Ryan and Jethá remind us, the range of dimorphism seen in humans is within the range found in multi-male/multi-female chimpanzees and bonobos (15-20%).

Variation and sexual dimorphism in height across populations. (Data from Holden and Mace 1999).

They contend that this is overlooked for a couple of reasons. One is that it is often forgotten that reproductive competition among males is not limited to aggression, territoriality, or female mate preference, but may also take place post-copulation within the female reproductive tract through sperm competition. However, sperm competition can only occur if females copulate with more than one male in a relatively short time period, and that notion of female sexual assertiveness defies the culturally accepted view of females as demure and coy. To Ryan and Jethá, we have a cultural blind spot that predisposes us to consider monogamy or polygyny the only possible human mating strategies because they are more compatible with the image of males as sexually forward and females as reticent. This in turn causes us to overlook the possibility that we likely shared the more promiscuous behaviors found in both of our closest primate cousins.

It is also possible that this blind spot is partly attributable to parental investment theory and the (generally correct) observation that males have more to gain in terms of genetic fitness from having multiple partners due to differences between gametes and gestation. Here’s an example of this logic:

The fact that males produce numerous and cheap sperm, while females produce few and expensive eggs, has two important consequences. First, … a male may potentially fertilize many females, whereas females are limited by the number of eggs, and thus offspring, that they can produce and raise. Males, thus, will benefit by trying to fertilize as many females as possible, and will compete for access to reproductive females. In contrast, females will benefit by mating with the best mate possible.” (Mills et al, 2010: 544).

However, it can’t be that simple, or every sexually reproducing species would have the same basic reproductive behavior, and Dr. Tatiana would then be out of a job. Certainly, having multiple partners can benefit females too. In Meredith Small’s words: “the most striking feature of female primate sexuality is the consistent orientation toward novelty and variation, sometimes to the point of promiscuity” (1993:176). She cites several possible advantages of sexual novelty and promiscuity for female primates, including strengthening social bonds, being better able to evaluate male quality through experience, preventing infanticide by males by confusing paternity, and avoiding inbreeding. Another benefit might be potential paternal care from multiple males. Additionally, a female may mate with multiple males simply to increase the odds of conception because, as Small phrases it: “primate males, both human and nonhuman, can’t mate time and time again in succession. They need time off. And this vacation might be frustrating to a female monkey in heat” (p. 179). For a female primate, the window of opportunity for pregnancy may be brief, and an individual male might not be able to get the job done. Is any of this relevant to humans? Possibly.

10. Reproductive Physiology. Human sperm count averages somewhere between 280 and 480 million per ejaculate, and is closer to that of chimps and bonobos (603) than to orangutans (67) or gorillas (51) (Ryan and Jethá, p. 230) 1 . This is consistent with an evolutionary scenario of human multimale/multifemale mating, and selection for higher counts via sperm competition. 2 Why should this be?

In species where females mate with a single male (i.e., monogamy, polygyny, or dispersed), the effect of natural selection on sperm counts is relaxed. This is seen most readily in the bizarre example of seahorses and pipefish, where a female leaves her eggs in the male’s brood pouch, and the chance of fertilization by another male is nil. As a result, sperm counts are “vanishingly low” in these species, as it would be a waste of resources to produce more than is necessary (Judson, 2002: 23). At the other extreme is the highly promiscuous Australian fairy-wren, where females copulate with many males, who in turn are able to produce 8.3 billion (yes, billion with a ‘B’) sperm per ejaculate (Tuttle et al 1996). It helps to have multiple lottery tickets if there are many players involved.

Bonobo testes. The outcome of sperm competition.

Obviously, we are not fairy-wrens or pipefish. But we probably have more in common with the former, as well as chimpanzees and bonobos, at least in this regard. Consistent with this is the large, true scrotum found in humans, chimps and bonobos, but not in orangutans or gorillas. This places testes in a position susceptible to injury, but also facilitates sperm longevity by keeping them at lower temperatures (also see Smith, 1984). As Plavcan (2001:39) wrote: “there is overwhelming evidence that testicular volume, density of seminiferous tubules, and seminal vesicle size are greater in species with multimale mating systems, where females mate with more than one male.”

It is difficult to conceive of another reason that human males should produce more sperm than is needed for fertilization in a monogamous mating structure other than some form of sperm competition. It is true that human testes size and sperm counts are not on the order of magnitude found in chimpanzees or bonobos (or possibly woolly spider monkeys- see Milton 1985). But in chimpanzees, females have been observed to copulate as many as 50 times a day with a dozen different males, putting a premium on sperm competition (cited in Smith 1984: 82). Still, human sperm counts are well above that found in primates where females mate with only one male. There could be another more proximate explanation for this, though I’m just not aware of it.

Ryan and Jethá address other traits in human biology consistent with an evolutionary scenario of promiscuity, which I can’t cover in depth here. These include: the chemical contents of semen in split ejaculates 3 , the potential for multiple female orgasms and copulatory vocalizations (maybe), and the conspicuous size of penes and breasts in humans (both larger than in any other primate species). As a counter-argument, C. Owen Lovejoy referred to humans as “the most epigamically adorned primate,” with our exaggerated genitalia and secondary sexual characteristics, unusual body and facial hair patterning, and sex differences in body shape and fat patterning (1981:346). However, he felt that this was consistent with monogamous mating, as a pair-bond would be strengthened if both sexes were ornate and attractive to each other, rather than only one sex, as seen in lions, mandrills, peacocks, etc. It’s an interesting idea, and he did cite some evidence for this in male and female herons which both have ‘elaborate display plumes.’

The mandrill, another epigamically adorned primate (from wikipedia).

But in the big picture, I think it is impossible to ignore the many traits in our biology that suggest strongly that lifetime monogamy does not come easily to humans. 4 Theodosius Dobzhansky famously wrote that “nothing in biology makes sense except in the light of evolution.” What do we make of imprinted genes, promiscuity in chimpanzees and bonobos, multiple lifetime partners and infidelities across cultures, high sperm counts, results from population genetics hinting at polygyny, et cetera? This is not to say that biology is destiny (I wouldn’t be a very good anthropologist to suggest otherwise). But these things are a part of our biology, passed down over millions of years, and they can’t be swept under the rug. We also have the ability to love very deeply, and that has its own biological correlates. What we need is a more complex synthesis, incorporating both of these facts. Somebody smarter than me should do it.

A Musical Interlude. (Link)

Part 5, Pair-Bonding and Romantic Love. ( Link )

1. These figures are generally consistent with those reported in a summary by Smith (1984), but reported a lower figure for humans (175 million per ejaculate). I imagine that like just about everything else in human biology, there is a wide range in human sperm count across individuals and populations. For example, Kate Clancy explained why there is so much variation in the menstrual cycle.

2. There are other forms of sperm competition than sheer numbers. In promiscuous fruit flies, males have evolved sperm that contain toxins that kill that of other males, often to the detriment of the female’s health and life expectancy (sperm with frickin’ laser beams on their heads). Closer to home, humans also seem to genetically resemble chimpanzees and bonobos for semen coagulation or ‘mating plugs,’ which could slow any subsequent male’s sperm from traveling through the female reproductive tract (see Eric M. Johnson’s post on this). Finally, as Ryan and Jethá discuss, the shape of the human penis, in particular its rather large size – compared to other primates – and the mushroom shaped glans, makes it particularly effective at displacing semen left by other males from the vaginal tract. Yes, people have tested this (via Jesse Bering ). All for science. You should see what they do with ducks. Unbelievable. ( Ed Yong ).

3. Note: despite its putative beneficial effects never suggest semen as a Valentine’s gift , especially if you are the editor of an academic journal.

4. I thought Shmuley Boteach, commenting on Dan Savage’s NYTimes article on non-monogamy, made an astounding admission for a rabbi when he wrote : “Let’s be clear. Yes, monogamy is challenging and does not come naturally.” He had other important points to say about choices and responsibility, but still.

References :

Bogin B. 1999. Patterns of Human Growth. Cambridge. ( Link )

Holden C, Mace R. 1999. Sexual dimorphism in stature and women’s work: A phylogenetic cross-cultural analysis. American Journal of Physical Anthropology 110: 27-45. ( Link )

Judson O. 2002. Dr. Tatiana’s Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex. Metropolitan Books. ( Link )

Lawler RR. 2009. Monomorphism, male-male competition, and mechanisms of sexual dimorphism. Journal of Human Evolution. 57: 321-5.

Little MA, Galvin K, Mugambi M. 1983. Cross-sectional growth of nomadic Turkana pastoralists. Human Biology 55: 811-30.

Lovejoy OC. 1981. The origin of man. Science. 211(4480): 341-50.

Mills DS, Marchant-Forde JN, McGreevy PD (Eds). 2010. The Encyclopedia of Applied Animal Behaviour and Welfare. Oxfordshire: CABI. ( Link )

Milton K. 1985. Mating patterns of woolly spider monkeys, Brachyteles arachnoids: implications for female choice. Behavioral Ecology and Sociobiology 17: 53-9. ( Link )

Plavcan MJ. 2001. Sexual dimorphism in primate evolution. Yearbook of Physical Anthropology. 44: 25-53. ( Link )

Plavcan MJ, van Schaik CP. 1997. Intrasexual competition and body weight dimorphism in anthropoid primates. American Journal of Physical Anthropology. 103: 37-68. ( Link )

Ryan C, Jethá C. 2010. Sex at Dawn: The Prehistoric Origins of Modern Sexuality. Harper. ( Link )

Small M. 1993. Female Choices: Sexual Behavior of Female Primates. Cornell.

Tuttle EM, Pruett-Jones S, Webster MS. 1996. Cloacal protuberances and extreme sperm production in Australian fairy-wrens. Proceedings of the Royal Society of London B 263: 1359-64 . ( Link )


Evolutionary and reproductive biologists, and ecologists, and andrologists, in biology departments and medical schools.

Sperm Biology: An Evolutionary Perspective
1. Three Centuries of Sperm Research
2. The Evolutionary Origin and Maintenance of Sperm: Selection for a Small, Motile Gamete Mating Type
3. Sperm Morphological Diversity
4. The Evolution of Spermatogenesis
5. Sperm Motility and Energetics
6. Sperm Competition and Sperm Phenotype
7. Ejaculate-Female and Sperm-Female Interactions
8. The Evolutionary Significance of Variation in Sperm-Egg Interactions
9. Sperm and Speciation
10. Evolutionary Quantitative Genetics of Sperm
11. Sperm Proteomics and Genomics
12. Drive and Sperm: The Evolution and Genetics of Male Meiotic Drive
13. Unusual Gametic and Genetic Systems
14. Sperm and Conservation
15. Sperm, Human Fertility and Society


Sperm DNA: How your genes are passed on

Sperm cells are sophisticated packages for delivering DNA to the egg to create a new genetically unique person. As awesome of a specimen as you are, Mother Nature does not simply pass along an exact duplicate of your genes on to your offspring. Instead, your DNA is rearranged into millions of unique combinations, so that each sperm cell carries a slight (and hopefully improved) variation of you.

To understand how, you need to take a closer look at your own DNA. You have 46 chromosomes that contain all your genes. Each chromosome is paired, so there are 23 sets or pairs of chromosomes. One set of 23 chromosomes are given to you by your father and the other set by your mother. Each cell in your body contain all 23 pairs.

In the normal life of the cell, the DNA is unwound in a long chain of spaghetti so that the cell can use it to create proteins that enable the body to function properly. You can think of genes as instructions to the cells for how to build proteins. Slight alterations in genes cause slight alterations to the proteins they create. For example, there are several genes that instruct the cells in your scalp to create proteins that become the hair on your head. Alterations to those genes will alter color, texture and shape of your hair. They can also cause disposition to baldness. Each of your cells contains two copies of every gene, one from your mom and one from your dad. When the genes are different from each other the cell has two options. It can follow the instructions given by a dominant gene or it can blend the instructions given by the both genes much as a cook improvises using two recipes to make a new dish.

When sperm cells are created, DNA is taken through two steps. First the two pairs of chromosomes are brought together, chopped up and reassembled causing some of the chromosomes to have a mixed set of genes coming from both parents. Then, the pairs are divided so that one of the chromosomes goes into one sperm cell and the other goes into a second sperm. The resulting sperm will end up with some genes that are entirely from your mother, some from your father, and some that are a blend of the two. The last chromosome is known as the sex chromosome and contains an X from your mom and a Y from your dad. When the pairs split during the formation of sperm, the X goes into one sperm and the Y goes into another. The means that half of the sperm are female and half are male.


Contents

Function

The main sperm function is to reach the ovum and fuse with it to deliver two sub-cellular structures: (i) the male pronucleus that contains the genetic material and (ii) the centrioles that are structures that help organize the microtubule cytoskeleton. [ clarification needed ]

Anatomy

The mammalian sperm cell can be divided in 2 parts:

  • Head: contains the nucleus with densely coiled chromatin fibers, surrounded anteriorly by a thin, flattened sac called the acrosome, which contains enzymes used for penetrating the female egg. It also contains vacuoles. [4]
  • Tail: also called the flagellum, is the longest part and capable of wave-like motion that propels sperm for swimming and aids in the penetration of the egg. [5][6][7] The tail was formerly thought to move symmetrically in a helical shape. However, a 2020 study by the University of Bristol stated that the tail moves in a more complicated manner, combining asymmetrical standing and travelling waves as well as rotating the entire body to achieve a perceived symmetry. [8][9]

The neck or connecting piece contains one typical centriole and one atypical centriole such as the proximal centriole-like. [10] [11] The midpiece has a central filamentous core with many mitochondria spiralled around it, used for ATP production for the journey through the female cervix, uterus and uterine tubes.

During fertilization, the sperm provides three essential parts to the oocyte: (1) a signalling or activating factor, which causes the metabolically dormant oocyte to activate (2) the haploid paternal genome (3) the centriole, which is responsible for forming the centrosome and microtubule system. [12]

Origin

The spermatozoa of animals are produced through spermatogenesis inside the male gonads (testicles) via meiotic division. The initial spermatozoon process takes around 70 days to complete. The process starts with the production of spermatogonia from germ cell precursors. These divide and differentiate into spermatocytes, which undergo meiosis to form spermatids. 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 a spermatozoan. [13] Sperm cells are carried out of the male body in a fluid known as semen. Human sperm cells can survive within the female reproductive tract for more than 5 days post coitus. [14] Semen is produced in the seminal vesicles, prostate gland and urethral glands.

In 2016, scientists at Nanjing Medical University claimed they had produced cells resembling mouse spermatids from mouse embryonic stem cells artificially. They injected these spermatids into mouse eggs and produced pups. [15]

Sperm quality

Sperm quantity and quality are the main parameters in semen quality, which is a measure of the ability of semen to accomplish fertilization. Thus, in humans, it is a measure of fertility in a man. The genetic quality of sperm, as well as its volume and motility, all typically decrease with age. [16] (See paternal age effect.)

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. [17] (see e.g. 8-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. [18] 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. [19] Treatment of male mice with melphalan, 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. [20] Such unrepaired DNA damages in sperm cells, after fertilization, can lead to offspring with various abnormalities.

Sperm size

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. [21] [22]

Market for human sperm

Some sperm banks hold up to 170 litres (37 imp gal 45 US gal) of sperm. [23]

In addition to ejaculation, it is possible to extract sperm through TESE.

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 [24] 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. [24] Furthermore, Nordic sperm donors tend to be tall and highly educated [25] and have altruistic motives for their donations, [25] partly due to the relatively low monetary compensation in Nordic countries. More than 50 countries worldwide are importers of Danish sperm, including Paraguay, Canada, Kenya, and Hong Kong. [24] However, the Food and Drug Administration (FDA) of the US has banned import of any sperm, motivated by a risk of transmission of Creutzfeldt–Jakob disease, although such a risk is insignificant, since artificial insemination is very different from the route of transmission of Creutzfeldt–Jakob disease. [26] 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 the blood-testis barrier to make transmission possible. [26]

History

Sperm were first observed in 1677 by Antonie van Leeuwenhoek [27] using a microscope. He described them as being animalcules (little animals), probably due to his belief in preformationism, which thought that each sperm contained a fully formed but small human. [ citation needed ]

Forensic analysis

Ejaculated fluids are detected by ultraviolet light, irrespective of the structure or colour of the surface. [28] Sperm heads, e.g. from vaginal swabs, are still detected by microscopy using the "Christmas Tree Stain" method, i.e., Kernechtrot-Picroindigocarmine (KPIC) staining. [29] [30]

Sperm cells in algal and many plant gametophytes are produced in male gametangia (antheridia) via mitotic division. In flowering plants, sperm nuclei are produced inside pollen. [31]


Sperm shape, also known as sperm morphology, is extraordinarily diverse across the animal kingdom. In fact, sperm is the most diverse type of animal cell on planet Earth.

Pretty much all sperm has a head, a middle piece, and a tail (flagellum) that enables the sperm to swim. But beyond that, sperm shape and length can wildly vary among and within different species. The head of human sperm is rounded and smooth, but some rats’ sperm have hooked heads and different tail lengths. Certain birds even have corkscrew-shaped sperm.

Scientists have devised different hypotheses to explain why sperm has evolved differently across species. The new study discusses a hypothesis that focuses on fertilization mode, or the environment in which sperm fertilizes an egg.

Other theories on sperm morphology focus on how competition between sperm and the size of animals impact the length of sperm:

  • Longer sperm often compete with each other to fertilize the egg
  • Sperm competition typically occurs when a female mates with several males to increase the likelihood of successful fertilization from multiple sperm
  • Sperm competition is still widely debated in humans

Basically, there is a trade-off between the size of the sperm and the number of sperm produced, according to the Nature study. For example, when external fertilization occurs in aquatic environments, the size of the sperm matters less for successful reproduction than the quantity of sperm that gets produced.

However, it’s a different story for animals where sperm fertilizes the egg inside the female’s body.

Larger female mammals, such as elephants, have bigger reproductive tracts. A 2015 study found that in these bigger animals, sperm can get lost in the reproductive tract and may never reach the egg. When sperm is longer, less sperm will be produced. Therefore, it’s not advantageous for males in these species to produce larger sperm.

Instead, researchers learned that large male animals, like elephants, tend to produce a greater quantity of smaller sperm to increase the chances of successful fertilization.

Research on sperm competition can explain why elephants evolved to produce shorter sperm compared to smaller animals rats, which likely have longer sperm because the females’ reproductive tracts are not as big.


The epigenetic alterations of human sperm cells caused by heroin use disorder

The molecular mechanisms of drug use on sexual health are largely unknown. We investigated, the relationship between heroin use disorder and epigenetic factors influencing histone acetylation in sperm cells. The volunteers included twenty-four 20- to 50-year-old men with a normal spermogram who did not consume any drugs and twenty-four age- to BMI-matched men who consume only the drug heroin for more than last four months. HDAC1 and HDAC11 mRNA expression levels in spermatozoa and miR-34c-5p and miR-125b-5p expression levels in seminal plasma were measured. The heroin-user group showed significantly increased white blood cell counts and decreased sperm motility and survival rates (8.61 ± 1.73, 21.50 ± 3.11, 69.90 ± 4.69 respectively) as compared to the control group (1.49 ± 0.32, 38.82 ± 3.05, 87.50 ± 0.99 respectively) (p ≤ .001). An increase in DNA fragmentation index (DFI) (heroin-user group: 41.93 ± 6.59% and control group: 10.14 ± 1.43%, p = .003), a change in frequency of HDAC1 (heroin-user group: 1.69 ± 0.55 and control group: 0.45 ± 0.14, p = .045) and HDAC11 (heroin-user group: 0.29 ± 0.13 and control group: 2.36 ± 0.76, p = .019) in spermatozoa and a significant decrease in seminal miR-125b-5p abundance (heroin-user group: 0.37 ± 0.11 and control group: 1.59 ± 0.47, p = .028) were reported in heroin consumers. Heroin use can lead to male infertility by causing leukocytospermia, asthenozoospermia, DFI elevation in sperm cells and alterations in seminal RNA profile.

Keywords: DNA fragmentation index Epigenetic Alterations Heroin user Spermatozoa.


2. Endocannabinoid System

This consists of endocannabinoids, enzymes involved in their synthesis and degradation, a putative membrane transport system and receptors through which they elicit their physiological functions [18]. Endocannabinoids are synthetised on demand from membrane precursors and are not stored [18].

NAEs are produced by enzymatic degradation of NAPEs. The major pathway is catalysed by a specific membrane-associated phospholipase D (NAPE-PLD) enzyme [20] which is regulated by a signalling mechanism [21]. N-Arachidonylethanolamide (AEA), N-palmitoylethanolamide (PEA) and N-oleoylethanolamide (OEA) are enzymatically released together from membrane phospholipid precursors when cells are stimulated by depolarizing agents, neurotransmitters and hormones [6,21,22,23]. NAEs are lipophylic molecules with a very low solubility in water [24], which are likely bound in biological fluids to albumin [24,25]. Moreover, recent data could suggest an interaction also with lipoproteins [26].

2-AG is synthetised by two enzymes: a specific phospholipase C (PLC), which hydrolyses inositol phospholipids generating diacylglycerol (DAG), and a sn-1-DAG lipase, which converts DAG to2-AG [3]. Degradation of NAEs requires their transport within the cell, but the transport mechanism is still debated. However, the presence of an endocannabinoid membrane transporter (EMT), was suggested for the uptake of extracellular AEA [10].

Once inside the cells, NAEs are quickly metabolized by a fatty acid amide hydrolase (FAAH), that breaks the amide bond and releases free fatty acid and ethanolamine [27]. FAAH is an integral membrane protein (located in endoplasmic reticulum) whose active site was supposed to be accessed by NAEs via the bilayer [28]. However a recent study identified a system of carrier proteins that transport AEA from the plasma membrane to FAAH in the rough endoplasmic reticulum [29]. Although PEA is hydrolysed by FAAH, it is preferentially demolished by the cysteine amidase N-acylethanolamine-hydrolyzing acid amidase (NAAA) [30,31] which is located in lysosomes [32]. 2-AG is degraded to arachidonic acid and glycerol by a specific monoacylglycerol lipase in the cytosol [33].

A recent study [10] demonstrated that endogenous AEA is present in human spermatozoa, together with the active AEA-synthase NAPE-PLD, the AEA-hydrolase FAAH and a purported carrier EMT. The possibility that OEA and PEA are co-released together with AEA by spermatozoa has been not yet examined.

Cannabinoid receptors belong to the superfamily of G-protein-coupled receptors, producing an inhibition of adenylate cyclase activity and inhibition of calcium channel activation by depolarization [34]. Two main subtypes have been cloned and characterized. Cannabinoid receptor 1 (CB1) was originally cloned from rat and human brain [35,36]. Quite recent papers demonstrated that it is widely distributed in neural and nonneural cells in reproductive and other peripheral organs [9,37,38]. Functional CB1 receptors are expressed in male and feminine human reproductive tract [35,39,40,41,42,43,44,45,46], included human sperm [37,47].

Cannabinoid receptor 2 (known as CB2) was originally cloned from human promyelocytic leukemia HL 60 cells [48] and has important roles in modulating immune responses [22]. Functional CB2 receptors are expressed in some male and feminine tissues of human reproductive tract [39,41,43,49], but a functional CB2 receptor was not identified in human sperm [10], although it was isolated in porcine sperm [47]. It was demonstrated that CB receptors signalling, by differential activation of G-protein subtypes regulating multiple signal transduction pathways, can modulate capacitation and fertilizing potential of human and boar sperm in vitro [9,37,47,50]. AEA has also been shown to bind to the type-1 vanilloid receptor (TRPV1) [51] which is also expressed in human sperm, where it controls sperm/oocyte fusion [10].

More recently two G protein-coupled receptors (GPR) have been shown as novel cannabinoid receptors: GPR119 and GPR55. The former is activated by OEA and is strongly implicated in the regulation of energy balance and body weight, while the latter is activated by multiple different cannabinoid ligands, included PEA [13], and it is associated with pain signalling in wild-type animals [52]. The possibility that mammalian sperm also express these receptors remains to be investigated.

The localization of ECS components in human sperm was the post-acrosomal region for TRPV1, and the post-acrosomal region and the midpiece for NAPE-PLD, CB1 and FAAH [10].

ECS is involved in fertility not only in humans and mammalians but also in non mammalian vertebrates and invertebrates, being the system highly conserved from evolutionary view point [53]. Schuel et al. [54] were the first to show in the sea urchin Strongylocentrotus purpuratus that cannabinoids directly affect the process of fertilization by reducing the fertilizing capacity of sperm.


Contents

Fertilization

Depending on the species, spermatozoa can fertilize ova externally or internally. In external fertilization, the spermatozoa fertilize the ova directly, outside of the female's sexual organs. Female fish, for example, spawn ova into their aquatic environment, where they are fertilized by the semen of the male fish.

During internal fertilization, however, fertilization occurs inside the female's sexual organs. Internal fertilization takes place after insemination of a female by a male through copulation. In most vertebrates, including amphibians, reptiles, birds and monotreme mammals, copulation is achieved through the physical mating of the cloaca of the male and female. [1] In marsupial and placental mammals, copulation occurs through the vagina. [2]

Human semen

Composition

During the process of ejaculation, sperm passes through the ejaculatory ducts and mixes with fluids from the seminal vesicles, the prostate, and the bulbourethral glands to form the semen. The seminal vesicles produce a yellowish viscous fluid rich in fructose and other substances that makes up about 70% of human semen. [3] The prostatic secretion, influenced by dihydrotestosterone, is a whitish (sometimes clear), thin fluid containing proteolytic enzymes, citric acid, acid phosphatase and lipids. [3] The bulbourethral glands secrete a clear secretion into the lumen of the urethra to lubricate it. [4]

Sertoli cells, which nurture and support developing spermatocytes, secrete a fluid into seminiferous tubules that helps transport sperm to the genital ducts. The ductuli efferentes possess cuboidal cells with microvilli and lysosomal granules that modify the ductal fluid by reabsorbing some fluid. Once the semen enters the ductus epididymis the principal cells, which contain pinocytotic vessels indicating fluid reabsorption, secrete glycerophosphocholine which most likely inhibits premature capacitation. The accessory genital ducts, the seminal vesicle, prostate glands, and the bulbourethral glands, produce most of the seminal fluid.

Seminal plasma of humans contains a complex range of organic and inorganic constituents.

The seminal plasma provides a nutritive and protective medium for the spermatozoa during their journey through the female reproductive tract. The normal environment of the vagina is a hostile one (c.f. sexual conflict) for sperm cells, as it is very acidic (from the native microflora producing lactic acid), viscous, and patrolled by immune cells. The components in the seminal plasma attempt to compensate for this hostile environment. Basic amines such as putrescine, spermine, spermidine and cadaverine are responsible for the smell and flavor of semen. These alkaline bases counteract and buffer the acidic environment of the vaginal canal, and protect DNA inside the sperm from acidic denaturation.

The components and contributions of semen are as follows:

Gland(s) Approximate fraction Description
testes 2–5% Approximately 200 million – 500 million spermatozoa (also called sperm or spermatozoans), produced in the testes, are released per ejaculation. If a man has undergone a vasectomy, he will have no sperm in the ejaculation.
seminal vesicles 65–75% Amino acids, citrate, enzymes, flavins, fructose (2–5 mg per mL semen, [5] the main energy source of sperm cells, which rely entirely on sugars from the seminal plasma for energy), phosphorylcholine, prostaglandins (involved in suppressing an immune response by the female against the foreign semen), proteins, vitamin C.
prostate 25–30% Acid phosphatase, citric acid, fibrinolysin, prostate specific antigen, proteolytic enzymes, zinc. (The zinc level is about 135 ± 40 μg/mL for healthy men. [6] Zinc serves to help to stabilize the DNA-containing chromatin in the sperm cells. A zinc deficiency may result in lowered fertility because of increased sperm fragility. Zinc deficiency can also adversely affect spermatogenesis.)
bulbourethral glands < 1% Galactose, mucus (serve to increase the mobility of sperm cells in the vagina and cervix by creating a less viscous channel for the sperm cells to swim through, and preventing their diffusion out of the semen. Contributes to the cohesive jelly-like texture of semen), pre-ejaculate, sialic acid.

A 1992 World Health Organization report described normal human semen as having a volume of 2 mL or greater, pH of 7.2 to 8.0, sperm concentration of 20×10 6 spermatozoa/mL or more, sperm count of 40×10 6 spermatozoa per ejaculate or more, and motility of 50% or more with forward progression (categories a and b) of 25% or more with rapid progression (category a) within 60 minutes of ejaculation. [7]

A 2005 review of the literature found that the average reported physical and chemical properties of human semen were as follows: [8]

Property Per 100mL In average volume (3.4 mL)
Calcium (mg) 27.6 0.938
Chloride (mg) 142 4.83
Citrate (mg) 528 18.0
Fructose (mg) 272 9.25
Glucose (mg) 102 3.47
Lactic acid (mg) 62 2.11
Magnesium (mg) 11 0.374
Potassium (mg) 109 3.71
Protein (mg) 5,040 171
Sodium (mg) 300 10.2
Urea (mg) 45 1.53
Zinc (mg) 16.5 0.561
Buffering capacity (β) 25
Osmolarity (mOsm) 354
pH 7.7
Viscosity (cP) 3–7
Volume (mL) 3.4
Values for average volume have been calculated and rounded to three significant figures. All other values are those given in the review.

Appearance and consistency

Semen is typically translucent with white, grey or even yellowish tint. Blood in the semen can cause a pink or reddish colour, known as hematospermia, and may indicate a medical problem which should be evaluated by a doctor if the symptom persists. [9]

After ejaculation, the latter part of the ejaculated semen coagulates immediately, [10] forming globules, [11] while the earlier part of the ejaculate typically does not. [12] After a period typically ranging from 15 to 30 minutes, prostate-specific antigen present in the semen causes the decoagulation of the seminal coagulum. [13] It is postulated that the initial clotting helps keep the semen in the vagina, [10] while liquefaction frees the sperm to make their journey to the ova. [10]

A 2005 review found that the average reported viscosity of human semen in the literature was 3–7 cP. [8]

Quality

Semen quality is a measure of the ability of semen to accomplish fertilization. Thus, it is a measure of fertility in a man. It is the sperm in the semen that is the fertile component, and therefore semen quality involves both sperm quantity and sperm quality.

Quantity

The volume of semen ejaculate varies but is generally about 1 teaspoonful or less. A review of 30 studies concluded that the average was around 3.4 milliliters (mL), with some studies finding amounts as high as 5.0 mL or as low as 2.3 mL. [8] In a study with Swedish and Danish men, a prolonged interval between ejaculations caused an increase of the sperm count in the semen but not an increase of its amount. [14]

Increasing semen volume

Some dietary supplements have been marketed with claims to increase seminal volume. Like other supplements, including so-called herbal viagra, these are not approved or regulated by the Food and Drug Administration (as licensed medications in the US would be), and none of the claims have been scientifically verified. Similar claims are made about traditional aphrodisiac foods, with an equal lack of verification.

Storage

Semen can be stored in diluents such as the Illini Variable Temperature (IVT) diluent, which have been reported to be able to preserve high fertility of semen for over seven days. [15] The IVT diluent is composed of several salts, sugars and antibacterial agents and gassed with CO2. [15]

Semen cryopreservation can be used for far longer storage durations. For human sperm, the longest reported successful storage with this method is 21 years. [16]

Disease transmission

Semen can transmit many sexually transmitted diseases and pathogens, including viruses like HIV [17] and Ebola. [18] Swallowing semen carries no additional risk other than those inherent in fellatio. This includes transmission risk for sexually transmitted diseases such as human papillomavirus (HPV) or herpes, especially for people with bleeding gums, gingivitis or open sores. [19] [20] Viruses in semen survive for a long time once outside the body. [ medical citation needed ]

Blood in semen (hematospermia)

The presence of blood in semen or hematospermia may be undetectable (it can only be seen microscopically) or visible in the fluid. Its cause could be the result of inflammation, infection, blockage, or injury of the male reproductive tract or a problem within the urethra, testicles, epididymis or prostate. It usually clears up without treatment, or with antibiotics, but if persistent further semen analysis and other urogenital system tests might be needed to find out the cause.

Semen allergy

In rare circumstances, humans can develop an allergy to semen, called human seminal plasma sensitivity. It appears as a typical localized or systemic allergic response upon contact with seminal fluid. There is no one protein in semen responsible for the reaction. Symptoms can appear after first intercourse or after subsequent intercourse. A semen allergy can be distinguished from a latex allergy by determining if the symptoms disappear with use of a condom. Desensitization treatments are often very successful. [21] [22]

Benefits to females

Females may benefit from absorbing seminal fluid. Such benefits include male insects transferring nutrients to females via their ejaculate in both humans and bovines, the fluid has antiviral and antibacterial properties and useful bacteria such as Lactobacillus have been detected in fluid transferred from birds and mammals. [17]

Qigong

Qigong and Chinese medicine place huge emphasis on a form of energy called 精 (pinyin: jīng, also a morpheme denoting "essence" or "spirit") [23] [24] – which one attempts to develop and accumulate. "Jing" is sexual energy and is considered to dissipate with ejaculation so masturbation is considered "energy suicide" amongst those who practice this art. According to Qigong theory, energy from many pathways/meridians becomes diverted and transfers itself to the sexual organs during sexual excitement. The ensuing orgasm and ejaculation will then finally expel the energy from the system completely. The Chinese proverb 一滴精,十滴血 (pinyin: yì dī jīng, shí dī xuè, literally: a drop of semen is equal to ten drops of blood) illustrates this point.

The scientific term for semen in Chinese is 精液 (pinyin: jīng yè, literally: fluid of essence/jing) and the term for sperm is 精子 (pinyin: jīng zǐ, literally: basic element of essence/jing), two modern terms with classical referents.

Indian philosophy

In the Indian system of medicine called Ayurveda semen is said to be made from 40 drops of blood. It is considered as the end of the food digestion cycle. [25]

One of the key aspects of Hindu religion is abstinence called Brahmacharya. It can be lifelong or during a specific period or on specific days. Brahmacharya attaches a lot of importance to semen retention.

Many yogic texts also indicate the importance of semen retention and there are specific asanas and Bandhas for it like Mula Bandana and Aswini Mudra. [26]

Greek philosophy

In Ancient Greece, Aristotle remarked on the importance of semen: "For Aristotle, semen is the residue derived from nourishment, that is of blood, that has been highly concocted to the optimum temperature and substance. This can only be emitted by the male as only the male, by nature of his very being, has the requisite heat to concoct blood into semen." [27] According to Aristotle, there is a direct connection between food and semen: "Sperms are the excretion of our food, or to put it more clearly, as the most perfect component of our food." [28]

The connection between food and physical growth, on the one hand, and semen, on the other, allows Aristotle to warn against "engag[ing] in sexual activity at too early an age . [since] this will affect the growth of their bodies. Nourishment that would otherwise make the body grow is diverted to the production of semen. Aristotle is saying that at this stage the body is still growing it is best for sexual activity to begin when its growth is 'no longer abundant', for when the body is more or less at full height, the transformation of nourishment into semen does not drain the body of needed material." [29]

Additionally, "Aristotle tells us that the region round the eyes was the region of the head most fruitful of seed ("most seedy" σπερματικώτατος), pointing to generally recognised effects upon the eyes of sexual indulgence and to practices which imply that seed comes from liquid in the region of the eyes." [30] This may be explained by the belief of the Pythagoreans that "semen is a drop of the brain [τὸ δε σπέρμα εἶναι σταγόνα ἐγκέφαλου]." [31]

Greek Stoic philosophy conceived of the Logos spermatikos ("seminal word") as the principle of active reason that fecundated passive matter. [32] The Jewish philosopher Philo similarly spoke in sexual terms of the Logos as the masculine principle of reason that sowed seeds of virtue in the feminine soul. [33]

The Christian Platonist Clement of Alexandria likened the Logos to physical blood [34] as the "substance of the soul", [35] and noted that some held "that the animal semen is substantially foam of its blood". [36] Clement reflected an early Christian view that "the seed ought not be wasted nor scattered thoughtlessly nor sown in a way it cannot grow." [37]

Women were believed to have their own version, which was stored in the womb and released during climax. Retention was believed to cause female hysteria. [38]

In ancient Greek religion as a whole, semen is considered a form of miasma, and ritual purification was to be practised after its discharge. [39]

Sacred semen

In some pre-industrial societies, semen and other body fluids were revered because they were believed to be magical. Blood is an example of such a fluid, but semen was also widely believed to be of supernatural origin and effect and was, as a result, considered holy or sacred. The ancient Sumerians believed that semen was "a divine substance, endowed on humanity by Enki", the god of water. [40] : 28 [41] The semen of a god was believed to have magical generative powers. [40] : 49 In Sumerian mythology, when Enki's seed was planted in the ground, it caused the spontaneous growth of eight previously-nonexistent plants. [40] : 49 [42] Enki was believed to have created the Tigris and Euphrates rivers by masturbating and ejaculating into their empty riverbeds. [40] : 32, 49 The Sumerians believed that rain was the semen of the sky-god An, [43] which fell from the heavens to inseminate his consort, the earth-goddess Ki, [43] causing her to give birth to all the plants of the earth. [43]

Dew was once thought to be a sort of rain that fertilized the earth and, in time, became a metaphor for semen. The Bible employs the term "dew" in this sense in such verses as Song of Solomon 5:2 [44] and Psalm 110:3, declaring, in the latter verse, for example, that the people should follow only a king who was virile enough to be full of the "dew" of youth. [45]

The orchid's twin bulbs were thought to resemble the testicles, which is the etymology of the disease orchiditis. There was an ancient Roman belief that the flower sprang from the spilled semen of copulating satyrs. [46]

In a number of mythologies around the world, semen is often considered analogous to breast milk. In the traditions of Bali, it is considered to be the returning or refunding of the milk of the mother in an alimentary metaphor. The wife feeds her husband who returns to her his semen, the milk of human kindness, as it were. [47]


Watch the video: 3D animation of how IVF works (November 2022).