A spermatozoon is a cell built for a single task: delivering the father’s genetic material to an egg. Sperm biology asks how this cell is constructed, how it becomes able to fertilize, how its quality can be measured and why the process sometimes fails. The questions matter well beyond the laboratory. The World Health Organization (WHO) estimates that about one in six people of reproductive age experience infertility during their lifetime, and it lists problems with sperm production, transport and quality among the recognised causes on the male side.
The archive linked to this page shows how broad the field has been since the 1960s. Studies of human semen sit alongside work on cattle, camels, cats, birds and African elephants, and on subjects that range from the ion pumps involved in sperm movement to the possible effects of exercise, vitamins and common medicines on male reproductive function.
The anatomy of a sperm cell
Developmental biology textbooks describe the mature sperm cell in a few functional parts. The head is made up of the nucleus, which holds the paternal chromosomes, and the acrosome, a cap-like vesicle of enzymes that is released when the sperm meets the egg. Behind the head, the midpiece contains mitochondria, which supply the energy that drives the flagellum, the long tail whose beating moves the cell forward.
Each of these parts is shaped during the final phase of sperm production in the testis, described on the page on the testis and spermatogenesis. Because head, midpiece and tail have different roles, laboratories assess shape and movement separately rather than relying on a single number.
Becoming able to fertilize
Sperm leaving the testis are not yet ready to fertilize. In the laboratory and domestic species that have been studied most, sperm acquire forward movement and fertilizing ability while they pass through the epididymis, the long coiled duct attached to each testis. How much of this applies to men is less clear. A 2016 review of human epididymal physiology pointed out that most knowledge comes from animal models, that the human epididymis has only a limited capacity to store sperm, and that the use of sperm retrieved surgically from different parts of the male ducts in assisted reproduction has raised new questions about what the human epididymis actually contributes.
A second step happens only after ejaculation. In 1951 Min Chue Chang in the United States and Colin Austin in Australia independently reported that sperm must spend time in the female reproductive tract before they can fertilize. This process became known as capacitation. Its discovery helped make in vitro fertilization possible in mammals, and researchers have since worked to explain its molecular basis, a task that is still not complete.
Measuring semen quality
The standard way of assessing sperm in men is semen analysis, and the WHO laboratory manual has been the main international reference for how to carry it out since its first edition in 1980. The sixth edition was published in 2021. Its reference data come from 3,589 men whose partners became pregnant within 12 months. The lower fifth percentile values in that group, meaning the level below which only 5% of these fertile men fell, include:
- a semen volume of 1.4 mL;
- a total of 39 million sperm per ejaculate;
- 30% of sperm showing progressive (forward) movement;
- 4% of sperm with a normal shape.
These figures are often read as pass-or-fail thresholds, but that is not how the current manual presents them. The sixth edition states that a value below the fifth percentile is not, on its own, enough to diagnose male infertility, and it no longer uses some older diagnostic labels for semen results. A critical review published soon after welcomed the more robust laboratory methods but noted that the manual offers no replacement decision limits, so the percentile values are likely to remain in everyday use.
Male factors in infertility
WHO defines infertility as a disease of the male or female reproductive system, marked by failure to achieve a pregnancy after 12 months or more of regular unprotected sexual intercourse. On the male side, it groups the causes into a few broad categories:
- blockages of the reproductive tract that stop semen from being ejected;
- hormonal disorders involving the hypothalamus, the pituitary gland or the testes;
- failure of the testes to produce sperm, for example in association with varicocele or after treatments such as chemotherapy;
- abnormal sperm shape, movement or overall quality.
Possible environmental influences on semen quality, and the long-running argument about whether average sperm counts are falling, are covered on the page on environment, endocrine disruptors and reproduction.
Sperm in evolution and across species
Evolutionary biologists study sperm for a different reason. In 1970 the biologist Geoff Parker defined sperm competition as the contest between the sperm of two or more males to fertilize the same set of eggs. The idea was first developed for insects and has grown into a large research field. Its theoretical models predict, among other things, that males should invest in more sperm when the risk of competing with another male’s ejaculate is high. Because the sperm of one male generally far outnumber the eggs available, researchers also ask whether sperm from the same ejaculate compete with each other, a question that remains relatively poorly understood.
This comparative view explains why work on livestock, wild mammals and birds runs through the archive from its earliest volumes: comparing species helps separate features shared by all sperm from adaptations to a particular way of life.
What is established and what is still debated
Some points are well established: the basic architecture of the sperm cell, the need for further maturation after sperm leave the testis, the requirement for capacitation before fertilization, and standardised laboratory methods for semen analysis. Other questions remain open:
- how far findings on epididymal maturation in animals apply to men;
- the full molecular explanation of capacitation;
- how best to interpret semen results now that the fifth percentile is no longer treated as a diagnostic cut-off, and what should replace it;
- the importance of competition between sperm within a single ejaculate.
This overview draws on public health sources and peer-reviewed reviews. It describes the science and is not medical advice; questions about an individual’s fertility belong with a doctor or fertility clinic.
Sources
- WHO, “Infertility” fact sheet, updated 2025
- Boitrelle F et al., Life (Basel), 2021 — The sixth edition of the WHO manual for human semen analysis: a critical review and SWOT analysis (doi:10.3390/life11121368)
- Gilbert SF, Developmental Biology, 6th edition, Sinauer, 2000 — Structure of the gametes (NCBI Bookshelf)
- Sullivan R, Mieusset R, Human Reproduction Update, 2016 — The human epididymis: its function in sperm maturation (doi:10.1093/humupd/dmw015)
- Gervasi MG, Visconti PE, Molecular Reproduction and Development, 2016 — Chang’s meaning of capacitation: a molecular perspective (doi:10.1002/mrd.22663)
- Sutter A, Immler S, Philosophical Transactions of the Royal Society B, 2020 — Within-ejaculate sperm competition (doi:10.1098/rstb.2020.0066)
References from the journal archive
These articles were published in Reproduction, the Journal of Reproduction and Fertility or Reviews of Reproduction and are still cited at their old addresses on this domain. Each link leads to the publisher’s version through its DOI.
- STUDIES ON THE DILUTION AND STORAGE OF FOWL SEMEN
- EXPERIMENTS ON INTERSPECIFIC SPERM PENETRATION THROUGH THE ZONA PELLUCIDA
- INTERRELATIONSHIPS AMONG THE CHARACTERISTICS OF HUMAN SEMEN AND FACTORS AFFECTING SEMEN-SPECIMEN QUALITY
- PURIFICATION AND PROPERTIES OF HUMAN SEMINAL MALTASE
- STUDIES ON DOG SEMEN
- EFFECT OF FREQUENCY OF EMISSION ON SEMEN OUTPUT AND AN ESTIMATE OF DAILY SPERM PRODUCTION IN MAN
- RELATION BETWEEN FRUCTOSE CONTENT OF SEMEN AND FERTILITY IN MAN
- A TECHNIQUE FOR ELECTRO-EJACULATION IN CHINCHILLAS
- MALE REPRODUCTIVE ORGANS OF THE AFRICAN ELEPHANT, LOXODONTA AFRICANA
- FACTORS AFFECTING SPONTANEOUS EJACULATION OF MALE GUINEA-PIGS
- FRUCTOSE, LACTIC ACID AND CITRIC ACID CONTENT OF THE SEMEN OF ELEVEN SUBHUMAN PRIMATE SPECIES AND OF MAN
- AN EXPERIMENT WITH HETEROSPERMIC INSEMINATION IN CATTLE
- BLOOD PRESSURE AND RESPIRATORY PATTERNS DURING HUMAN COITUS
- IDENTIFICATION AND SUBCELLULAR LOCALIZATION OF THE ENZYMES EFFECTING PENETRATION OF THE ZONA PELLUCIDA BY RABBIT SPERMATOZOA
- INFLUENCE OF HOMOZYGOSITY FOR ROSE COMB ON FUMARASE, ACONITASE, ISOCITRIC DEHYDROGENASE AND MALIC DEHYDROGENASE ACTIVITY IN SPERMATOZOA OF THE DOMESTIC FOWL ( GALLUS DOMESTICUS )
- ELECTROPHORETIC DETECTION OF MULTIPLE FORMS OF TRYPSIN-LIKE ACTIVITY IN SPERMATOZOA OF THE DOMESTIC FOWL
- OCCURRENCE OF AMYLO-1,6-GLUCOSIDASE IN HUMAN SEMINAL PLASMA: ITS PROPERTIES AND SIGNIFICANCE IN SPERM METABOLISM
- A COMPARATIVE STUDY OF COITAL PHYSIOLOGY, WITH SPECIAL REFERENCE TO THE SEXUAL CLIMAX
- EFFECT OF STORAGE AT 5° C ON THE DISC ELECTROPHORETIC PATTERNS OF OVINE AND BOVINE SEMINAL PROTEINS
- GONADAL AND EXTRAGONADAL SPERM RESERVES OF INDIGENOUS CENTRAL AFRICAN BULLS
- POLYSPERMIC FERTILIZATION FOLLOWING TUBAL SURGERY IN PIGS, WITH PARTICULAR REFERENCE TO THE RÔLE OF THE ISTHMUS
- EFFECT OF TESTOSTERONE POLYDIMETHYLSILOXANE IMPLANTS UPON SPERM PRODUCTION, LIBIDO AND ACCESSORY SEX ORGAN FUNCTION IN RABBITS
- ENZYME COMPOSITION OF BUFFALO SEMINAL PLASMA
- VARIATIONS IN THE STRUCTURAL CHARACTER AND STABILITY OF THE NUCLEAR CHROMATIN IN MORPHOLOGICALLY NORMAL HUMAN SPERMATOZOA
- EFFECTS OF LONG-TERM CADMIUM EXPOSURE ON THE SEMINAL VESICLES OF MICE
- ELECTROEJACULATION OF AUSTRALIAN MARSUPIALS AND ANALYSES OF THE SUGARS IN THE SEMINAL PLASMA FROM THREE MACROPOD SPECIES
- FREE N-ACETYLAMINOSUGAR IN THE SEMINAL PLASMA OF EUTHERIAN MAMMALS
- Electroejaculation, semen characteristics and semen preservation of the brindled gnu
- Penetration of hamster and rabbit zona-free eggs by rat and mouse spermatozoa with special reference to sperm capacitation
- The relationship between semen evaluation methods and fertility in the bull
- Assessment of spermicides by a stripping technique against human spermatozoa
- Capacitation of bull spermatozoa and fertilization in vitro of cattle follicular oocytes matured in culture
- Effect of various hexoses on sperm capacitation and penetration of rat eggs in vitro
- Carnitine, acetylcarnitine and the activity of carnitine acyltransferases in seminal plasma and spermatozoa of men, rams and rats
- Sperm head length as a predictor of fecundity in the sandhill crane, Grus canadensis
- Within-subject variability of human semen in regard to sperm count, volume, total number of spermatozoa and length of abstinence
- Association between scrotal circumference, live weight and sperm output in cattle
- A glycolytic product is obligatory for initiation of the sperm acrosome reaction and whiplash motility required for fertilization in the mouse
- Spermatozoa of the Atlantic bottlenosed dolphin, Tursiops truncatus
- In-vitro fertilization in the mouse and the relevance of different sperm/egg concentrations and volumes
- Motility and eosin uptake of formaldehyde-treated ram spermatozoa
- Production, transport, maturation, storage and survival of spermatozoa in the male Japanese quail, Coturnix coturnix
- Sperm transport in the human female reproductive tract in relation to semen analysis characteristics and time of ovulation
- Factors in seminal plasma of bulls that affect the viability and motility of spermatozoa
- Failure of spleen cell migration assays to detect cell-mediated immunity to spermatozoa after natural mating in female mice
- Influence of egg investments on in-vitro penetration of mouse eggs by misshapen spermatozoa
- Ovarian and endocrine responses in the cat after coitus
- Potassium ions modulate expression of mouse sperm fertilizing ability, acrosome reaction and hyperactivated motility in vitro
- Development of an assay to assess the functional integrity of the human sperm membrane and its relationship to other semen characteristics
- Electroejaculation, semen characteristics and serum testosterone concentrations of free-ranging African elephants ( Loxodonta africana )
- Human sperm characteristics during frequent ejaculation
- Observations on the response of human spermatozoa to gravity, boundaries and fluid shear
- Analysis of the direct effects of prostaglandins on human sperm function
- Comparison of acrosome reaction-inducing activities of human cumulus oophorus, follicular fluid and ionophore A23187 in human sperm populations of proven fertilizing ability in vitro
- Individual variations in FSH release after a GnRH challenge and relationship with semen output in young bulls
- On mammalian sperm dimensions
- Semen-induced ovulation in the bactrian camel ( Camelus bactrianus )
- Influence of various substrates on the acetylcarnitine:carnitine ratio in motile and immotile human spermatozoa
- Cellular basis of defective sperm function and its association with the genesis of reactive oxygen species by human spermatozoa
- Minimum and maximum extracellular Ca2+ requirements during mouse sperm capacitation and fertilization in vitro
- Effects of seminal vesicle fluid components on sperm motility in the house mouse
- Pharmacokinetics of non-steroidal anti-inflammatory drugs in male rabbits after acute and chronic administration and effect of chronic treatment on seminal prostaglandins, sperm quality and fertility
- Calcium channel antagonists modulate the acrosome reaction but not capacitation in mouse spermatozoa
- Influence of ejaculation frequency on semen characteristics in chimpanzees ( Pan troglodytes )
- Light and scanning electron microscopy of fallow deer ( Dama dama ) spermatozoa
- Copulatory behaviour and coagulum formation in the female reproductive tract of the Australian hopping mouse, Notomys alexis
- Effect of season on seminal traits and serum hormone concentrations in captive male Siberian tigers ( Panthera tigris )
- Comparative metabolism of spermatozoa from subfertile Delaware and Wyandotte roosters
- Evaluation of the effect of butyl p -hydroxybenzoate on the proteolytic activity and membrane function of human spermatozoa
- Decrease in calmodulin concentrations during heparin-induced capacitation in bovine spermatozoa
- Differential contribution of leucocytes and spermatozoa to the generation of reactive oxygen species in the ejaculates of oligozoospermic patients and fertile donors
- Relationship between iron-catalysed lipid peroxidation potential and human sperm function
- A comparative study of sperm–egg interactions in birds
- Function and culture requirements of snow leopard ( Panthera uncia ) spermatozoa in vitro
- Human zona pellucida recognition associated with removal of sialic acid from human sperm surface
- Localization of cyclooxygenase and production of prostaglandins in bovine spermatozoa
- Activation of hamster zona-free oocytes by homologous and heterologous spermatozoa
- Reactive oxygen species, lipid peroxidation and enzymatic defence systems in human spermatozoa
- Glutathione as a treatment for male infertility
- Quantification of normal head morphometry of stallion spermatozoa
- Investigation of the association between the presence of cytoplasmic residues on the human sperm midpiece and defective sperm function
- Lipid and antioxidant changes in spermatozoa and seminal plasma throughout the reproductive period of bulls
- Reactive oxygen species and sperm physiology
- The human sperm centrosome is responsible for normal syngamy and early embryonic development
- Characteristics of the semen of three endangered species of gazelles ( Gazella dama mhorr , G. dorcas neglecta and G. cuvieri )
- Fertilization promoting peptide: an important regulator of sperm function in vivo?
- Sperm competition in birds
- Origin of DNA damage in ejaculated human spermatozoa
- Effect of long-term supplementation with arachidonic or docosahexaenoic acids on sperm production in the broiler chicken
- Effects of angiotensin II on the acrosome reaction in equine spermatozoa
- Functional processing of fertilin: evidence for a critical role of proteolysis in sperm maturation and activation
- An X-Y paint set and sperm FISH protocol that can be used for validation of cattle sperm separation procedures
- Comparative analyses of semen and endocrine characteristics of free-living versus captive jaguars (Panthera onca)
- Hyperactivation of mammalian spermatozoa: function and regulation
- Role of quinine-sensitive ion channels in volume regulation in boar and bull spermatozoa
- Current status of sexing mammalian spermatozoa
- Detection and location of the enzymes of de novo pyrimidine biosynthesis in mammalian spermatozoa
- Effect of chronic administration of 7α-methyl-19-nortestosterone on serum testosterone, number of spermatozoa and fertility in adult male bonnet monkeys (Macaca radiata)
- Segregation of spermatozoa within sperm storage tubules of fowl and turkey hens
- Energy metabolism and intracellular pH in boar spermatozoa
- Involvement of tyrosine kinase and cAMP-dependent kinase cross-talk in the regulation of human sperm motility
- Motility of spermatozoa at surfaces
- Concepts in sperm heterogeneity, sperm selection and sperm competition as biological foundations for laboratory tests of semen quality
- Insights into the molecular basis of sperm–egg recognition in mammals
- Ligands and receptors mediating signal transduction in sea urchin spermatozoa
- Requirement for an intact cytoskeleton for volume regulation in boar spermatozoa
- Semen-induced luteal phase and identification of a LH surge in the koala ( Phascolarctos cinereus )
- Spermatozoa motility in the Persian sturgeon, Acipenser persicus : effects of pH, dilution rate, ions and osmolality
- The cytosolic sperm factor that triggers Ca2+ oscillations and egg activation in mammals is a novel phospholipase C: PLCζ
- The mechanism of sperm–oocyte fusion in mammals
- A mouse sperm decapacitation factor receptor is phosphatidylethanolamine-binding protein 1
- Adipokines: implications for female fertility and obesity
- Effects of dietary vitamin A intake on acrosin- and plasminogen-activator activity of ram spermatozoa
- Localization of phosphatidylserine in boar sperm cell membranes during capacitation and acrosome reaction
- Molecular physiology and pathology of Ca2+-conducting channels in the plasma membrane of mammalian sperm
- Multiplex determination of murine seminal fluid cytokine profiles
- Seminal plasma regulates ovarian progesterone production, leukocyte recruitment and follicular cell responses in the pig
- Sperm channel diversity and functional multiplicity
- A molecular analysis of the population of mRNA in bovine spermatozoa
- A novel pyruvate kinase (PK-S) from boar spermatozoa is localized at the fibrous sheath and the acrosome
- Establishment of the mammalian membrane block to polyspermy: evidence for calcium-dependent and -independent regulation
- Induction of sperm acrosome reaction by perivitelline membrane glycoprotein ZP1 in Japanese quail ( Coturnix japonica )
- Is sperm evaluation useful in predicting human fertility?
- Long-term storage of mouse spermatozoa after evaporative drying
- Regulatory properties of 6-phosphofructokinase and control of glycolysis in boar spermatozoa
- Channels for water efflux and influx involved in volume regulation of murine spermatozoa
- Chromatin-unstable boar spermatozoa have little chance of reaching oocytes in vivo
- Effects of Kaempferia parviflora extracts on reproductive parameters and spermatic blood flow in male rats
- Innate immunity and disorders of the female reproductive tract
- Multivariate cluster analysis to study motility activation of Solea senegalensis spermatozoa: a model for marine teleosts
- Paternal effect on embryo quality in diabetic mice is related to poor sperm quality and associated with decreased glucose transporter expression
- Presence, processing, and localization of mouse ADAM15 during sperm maturation and the role of its disintegrin domain during sperm–egg binding
- Semen from scrapie-infected rams does not transmit prion infection to transgenic mice
- Sperm competition and sperm cooperation: the potential role of diploid and haploid expression
- β-Defensin 22 is a major component of the mouse sperm glycocalyx
- Acaciaside-B-enriched fraction of Acacia auriculiformis is a prospective spermicide with no mutagenic property
- Ca2+-stores in sperm: their identities and functions
- Inflammatory pathways in female reproductive health and disease
- Osmotic characteristics and fertility of murine spermatozoa collected in different solutions
- Activity of the Na,K-ATPase α4 isoform is important for membrane potential, intracellular Ca2+, and pH to maintain motility in rat spermatozoa
- Paternal DNA packaging in spermatozoa: more than the sum of its parts? DNA, histones, protamines and epigenetics
- Protein kinase A and protein kinase Cα/PPP1CC2 play opposing roles in the regulation of phosphatidylinositol 3-kinase activation in bovine sperm
- Proteomics and the search for biomarkers of female reproductive diseases
- Regulation of hyperactivation by PPP2 in hamster spermatozoa
- Characterization of mouse sperm TMEM190, a small transmembrane protein with the trefoil domain: evidence for co-localization with IZUMO1 and complex formation with other sperm proteins
- Dynamics of flagellar force generated by a hyperactivated spermatozoon
- Key gene regulatory sequences with distinctive ontological signatures associate with differentially endonuclease-accessible mouse sperm chromatin
- Nitric oxide stimulates human sperm motility via activation of the cyclic GMP/protein kinase G signaling pathway
- Sperm acrosin is responsible for the sperm binding to the egg envelope during fertilization in Japanese quail ( Coturnix japonica )
- Sperm proteasome and fertilization
- Sperm quality assessments for endangered razorback suckers Xyrauchen texanus
- Sperm surface changes and physiological consequences induced by sperm handling and storage
- The sperm nucleus: chromatin, RNA, and the nuclear matrix
- Transcriptional dynamics of the sex chromosomes and the search for offspring sex-specific antigens in sperm
- A caged progesterone analog alters intracellular Ca2+ and flagellar bending in human sperm
- Sperm proteasome degrades egg envelope glycoprotein ZP1 during fertilization of Japanese quail ( Coturnix japonica )
- Sperm wars and the evolution of male fertility
- The slower the better: how sperm capacitation and acrosome reaction is modified in the presence of estrogens
- The sperm epigenome and potential implications for the developing embryo
- Vitamin D metabolism, sex hormones, and male reproductive function
- Age-dependent inhibin B concentration in relation to FSH and semen sample qualities: a study in 2448 men
- Copper-zinc superoxide dismutase deficiency impairs sperm motility and in vivo fertility
- Mitochondria functionality and sperm quality
- Sex selection of sperm in farm animals: status report and developmental prospects
- Innate immunity and inflammation of the bovine female reproductive tract in health and disease
- Low amounts of mitochondrial reactive oxygen species define human sperm quality
- Porcine sperm capacitation involves tyrosine phosphorylation and activation of aldose reductase
- Roles of extracellular ions and pH in 5-HT-induced sperm motility in marine bivalve
- SUMO1 in human sperm: new targets, role in motility and morphology and relationship with DNA damage
- Sperm activation by heat shock protein 70 supports the migration of sperm released from sperm storage tubules in Japanese quail ( Coturnix japonica )
- Genetics of male infertility: from research to clinic
- Redistribution of the intra-acrosomal EGFP before acrosomal exocytosis in mouse spermatozoa
- Regulation and roles of Ca2+ stores in human sperm
- Characterization of tetraspanin protein CD81 in mouse spermatozoa and bovine gametes
- A novel hypothesis for histone-to-protamine transition in Bos taurus spermatozoa
- The effects of three different exercise modalities on markers of male reproduction in healthy subjects: a randomized controlled trial
- Antidiabetic therapies and male reproductive function: where do we stand?
- Potential negative effects of anti-histamines on male reproductive function
- Siglec expression on the surface of human, bull and ram sperm