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. The nuclear DNA in sperm cells is
haploid; that is, they contribute only one copy of each paternal
chromosome pair.
Mitochondria in human sperm contain no or very little
DNA because
mtDNA is degraded while sperm cells are maturing, hence they typically do not contribute any genetic material to their offspring. In mammals, sperm cells normally come in two types, "female" and "male", named for the resulting sex of the fertilized zygote each produces after fusing with the ovum. Sperm cells that produce female (
karyotype XX) offspring carry an X-chromosome, while sperm cells that produce male (
XY) offspring carry a Y-chromosome. Errors of meiosis may lead to the formation of sperm containing different arrangements of sex chromosomes, either altogether missing (
monosomy, designated "
0"), or in multiples (
trisomy), such as "
XX", "
XY", etc... some of the conditions known as
Disorders of Sex Development (DSD) are the result of fertilization by such defective sperm. The
sperm cell of
Homo sapiens is the small
reproductive cell produced by males, and can only survive in warm environments; upon leaving the body, it starts to degrade, thereby decreasing the total
sperm quality.
Semen has an alkaline nature and the spermatozoa do not reach full motility (hypermotility) until they reach the
vagina, where the alkaline pH is neutralized by acidic vaginal fluids. This gradual process takes 20–30 minutes. During this period,
fibrinogen from the
seminal vesicles forms a clot, securing and protecting the sperm. Just as they become hypermotile,
fibrinolysin from the
prostate gland dissolves the clot, allowing the sperm to progress optimally.
DNA damage and repair DNA damages present in spermatozoa 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 spermatozoa are particularly vulnerable to free radical attack and the generation of oxidative DNA damage. (see e.g.
8-Oxo-2'-deoxyguanosine) Exposure of males to certain lifestyle, environmental or occupational hazards may increase the risk of
aneuploid spermatozoa. In particular, risk of aneuploidy is increased by tobacco smoking, and occupational exposure to benzene, insecticides, and perfluorinated compounds. Increased aneuploidy of spermatozoa often occurs in association with increased DNA damage.
DNA fragmentation and increased in situ DNA susceptibility to denaturation, the features similar to these seen during
apoptosis of somatic cells, characterize abnormal spermatozoa in cases of
male infertility. Although
DNA repair has long been considered impossible in human spermatozoa due to the high level of DNA compaction in these cells, human spermatozoa possess a truncated
base excision repair pathway that is mediated by
8-oxoguanine DNA glycosylase 1 (OGG1). Thus mature spermatozoa appear to have a limited capacity to mount a DNA repair response to
oxidative stress. The
blood-testis barrier, maintained by the tight junctions between the
Sertoli cells of the seminiferous tubules, prevents communication between the forming spermatozoa in the testis and the blood vessels (and immune cells circulating within them) within the
interstitial space. This prevents them from eliciting an immune response. The blood-testis barrier is also important in preventing toxic substances from disrupting spermatogenesis.
Anatomy The mammalian sperm cell can be divided in 2 parts connected by a neck: • Head: contains the
nucleus with densely coiled chromatin fibers, surrounded anteriorly by a thin, flattened sac called the
acrosome, formed by modification of the
Golgi body, which contains enzymes such as
spermlysin (
hyaluronidase, corona-penetrating enzyme, zona lysin, or
acrosin) used for penetrating the female egg. It also contains vacuoles. As the spermatozoon approaches the ovum, it undergoes the
acrosome reaction in which the membrane surrounding the acrosome fuses with the plasma membrane of the sperm's head, exposing the contents of the acrosome. The head of a human sperm is disc shaped, and approximately . • Tail: also called the
flagellum, is the longest part, at approximately . It is capable of wave-like motion that propels sperm for swimming and aids in the penetration of the egg. The flagellum propels the sperm cell at about . The tail was formerly thought to move symmetrically in a
helical shape. • Neck: also called connecting piece contains one typical centriole and one atypical centriole such as the
proximal centriole-like. The proximal centriole is retained in the mature spermatozoon; the distal centriole disappears after
axoneme assembly. The proximal centriole enters into the ovum, which has no centriole, and starts the first cleavage division of the zygote thus formed. The distal centriole gives rise to the axial filament which forms the tail and has a (9+2) arrangement. A transitory membrane called the
Manchette lies in the
midpiece. • Midpiece: It has 10–14 spirals of mitochondria surrounding the axial filament in the cytoplasm. It provides motility, and hence is called the powerhouse of the sperm. It also has a ring centriole (annulus) that form a diffusion barrier between the midpiece and the principal piece and serve as a stabilizing structure for tail rigidity. Sperm have an olfactory
guidance mechanism, and after reaching the
fallopian tubes, must undergo a period of
capacitation before penetration of the ovum. During
fertilization, the sperm provides three essential parts to the
oocyte: (1) a signalling or
oocyte-activating factor (OAF), 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. It may also contribute with paternal
messenger RNA (mRNA), also contributing to embryonic development. File:Spermatozoa-human-3140x.jpg|Electron
micrograph of human spermatozoa magnified 3140 times. File:Sperms (urine) - Spermler (idrar) - 01.png|Sperm cells in the urine sample of a 45-year-old male patient who is being followed with the diagnosis of
benign prostate hyperplasia. The human spermatozoon contains at least 7500 different
proteins. Human sperm genetics has been associated with
human evolution, per a 2020 study. In humans, recombination rates differ between maternal and paternal DNA: •
Maternal DNA: Recombines approximately
42 times on average. •
Paternal DNA: Recombines approximately
27 times on average.
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.
Sperm activation cell Approaching the egg cell is a rather complex, multistep process of
chemotaxis guided by different chemical substances/stimuli on individual levels of phylogeny. One of the most significant, common signaling characters of the event is that a prototype of professional chemotaxis receptors,
formyl peptide receptor (60,000 receptor/cell) as well as the activator ability of its ligand formyl Met-Leu-Phe have been demonstrated in the surface membrane even in the case of human sperms. Mammalian sperm cells become even more active when they approach an egg cell in a process called
sperm activation. Sperm activation has been shown to be caused by
calcium ionophores in vitro,
progesterone released by nearby
cumulus cells and binding to
ZP3 of the
zona pellucida. The cumulus cells are embedded in a gel-like substance made primarily of
hyaluronic acid, and developed in the ovary with the egg and support it as it grows. The initial change is called "hyperactivation", which causes a change in spermatozoa motility. They swim faster and their tail movements become more forceful and erratic. A recent discovery links hyperactivation to a sudden influx of calcium ion into the tails. The whip-like tail (flagellum) of the sperm is studded with
ion channels formed by proteins called
CatSper. These channels are selective, allowing only calcium ions to pass. The opening of CatSper channels is responsible for the influx of calcium. The sudden rise in calcium levels causes the flagellum to form deeper bends, propelling the sperm more forcefully through the viscous environment. Sperm hyperactivity is necessary for breaking through two physical barriers that protect the egg from fertilization. The second process in sperm activation is the
acrosome reaction. This involves releasing the contents of the acrosome, which disperse, and the exposure of enzymes attached to the inner acrosomal membrane of the sperm. This occurs after the sperm first meets the egg. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of different species from fusing. There is some evidence that this binding is what triggers the
acrosome to release the enzymes that allow the sperm to fuse with the egg. ZP3, one of the proteins that make up the zona pellucida, then binds to a partner molecule on the sperm. Enzymes on the inner acrosomal membrane digest the zona pellucida. After the sperm penetrates the zona pellucida, part of the sperm's cell membrane then
fuses with the egg cell's membrane, and the contents of the head diffuse into the egg. Upon penetration, the oocyte is said to have become
activated. It undergoes its secondary meiotic division, and the two haploid nuclei (paternal and maternal) fuse to form a
zygote. In order to prevent
polyspermy and minimise the possibility of producing a
triploid zygote, several changes to the egg's zona pellucida renders them impenetrable shortly after the first sperm enters the egg.
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. Human sperm cells can survive within the female reproductive tract for more than 5 days post coitus. Mammalian sperm cells are
ejaculated through the
penis in a fluid known as
semen, which 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. Spermatozoa are produced in the
seminiferous tubules of the
testicles in a process called
spermatogenesis. Round cells called
spermatogonia divide and differentiate eventually to become spermatozoa. During
copulation, the
cloaca or
vagina gets
inseminated, and then the spermatozoa move through
chemotaxis to the ovum inside an
oviduct. ==Assisted reproductive technology==