Everything about Spermatozoa totally explained
A
spermatozoon or
spermatozoan (
pl. spermatozoa), from the
ancient Greek σπέρμα (seed) and ζῷον (living being) and more commonly known as a
sperm cell, is the
haploid cell that's the male
gamete. It
joins an
ovum to form a
zygote. A zygote is a single cell, with a complete set of chromosomes, that normally develops into an
embryo.
Sperm cells contribute half of the
genetic information to the
diploid offspring. In mammals, the
sex of the offspring is determined by the sperm cells: a spermatozoon bearing a Y
chromosome will lead to a
male (XY) offspring, while one bearing an X chromosome will lead to a
female (XX) offspring (the
ovum always provides an X chromosome). Sperm cells were first observed by a student of
Anton van Leeuwenhoek in
1677.
Mammalian spermatozoan structure, function, and size
Humans
The human sperm cell is the reproductive cell in males. Sperm cells come in two types; "male" and "female." Sperm cells that give rise to female (XX) offspring after fertilization differ in that they carry an X chromosome, while sperm cells that give rise to male (XY) offspring carry a Y chromosome. The "female" sperm cell also differs phenotypically in that it has a larger head in comparison to the "male" sperm cell .
In male humans, sperm cells consists of a head 5
µm by 3 µm and a tail 50 µm long. The
Reynolds number associated with spermatozoa is in the order of 1, so it's known that the spermatozoa exhibit
laminar flow. Spermatozoan stream lines are straight and parallel. The tail
flagellates, which propels the sperm cell (at about 1-3 mm/minute in humans) by whipping in an elliptical cone.
.
Semen has an alkaline nature, and they don't 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. In this time,
fibrinogen from the
seminal vesicles forms a clot, securing and protecting the sperm. Just as they become hypermotile,
fibrinolysin from the
prostate dissolves the clot, allowing the sperm to progress optimally.
The spermatozoon is characterized by a minimum of
cytoplasm and the most densely packed DNA known in eukaryotes. Compared to
mitotic chromosomes in
somatic cells, sperm DNA is at least sixfold more highly condensed.
During fertilization, the sperm's
mitochondria are destroyed by the egg cell, and this means only the mother is able to
provide the baby's mitochondria and
mitochondrial DNA, which have an important application in tracing maternal
ancestry. However it has been recently discovered that mitochondrial DNA can be recombinant
.
Avoidance of immune system response
Glycoprotein molecules on the surface of sperm cells are recognised by all human immune systems, and interpreted as a signal that the cell shouldn't be rejected. The male immune system might otherwise attack sperm whilst in the testes, and the female immune system would attack sperm in the
reproductive tract. The specific glycoproteins coating sperm cells are also utilized by some cancerous and bacterial cells, some parasitic worms, and HIV-infected white blood cells, in order to avoid an immune response from the
host organism.
Spermatozoa in other organisms
Animals
Fertilization relies on spermatozoa for most sexually reproductive animals.
The
fruit fly(External Link
) has the largest known spermatozoon relative to its size.
Drosophila melanogaster produces sperm that can be up to 1.8 mm in size, which is longer than the adult fly. The incredibly long tail is thought to block other sperm from entering the egg. The entire sperm, tail included, gets incorporated into the
oocyte cytoplasm.
Sea urchins such as
Arbacia punctulata—are the workhorses of sperm research, because they spawn large numbers of sperm
into the sea, making them well-suited as model organisms for research experiments.
Plants, algae and fungi
The
gametophytes of
bryophytes,
ferns and some
gymnosperms produce motile
sperm cells, contrary to
pollen grains employed in most gymnosperms and all
angiosperms. This renders sexual reproduction in the absence of
water impossible, since water is a necessary medium for sperm and egg to meet. Algae and lower plant sperm cells are often multi-flagellated (see image) and thus morphologically different from animal spermatozoa.
Some algae and fungi produce non-motile sperm cells, called spermatia. In higher plants and some algae and fungi, fertilization involves the migration of the sperm nucleus through a fertilization tube (for example
pollen tube in higher plants) to reach the egg cell.
Spermatozoa production in mammals
Spermatozoa are produced in the
seminiferous tubules of the
testes 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 a
Fallopian tube or the
uterus.
Spermatozoa Activation
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 intiial 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 ion 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
hyaluronidase to digest cumulus cells surrounding the oocyte and exposing acorsin attatched to the inner membrane of the sperm. 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.
Acrosin digests the zona pellucida and membrane of the oocyte. Part of the sperm's cell membrane then
fuses with the egg cell's membrane, and the contents of the head sink into the egg. ZP3, one of the proteins that make up the zona pellucida, binds to a partner molecule on the sperm. 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.
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 cell membranes renders them impenetrable shortly after the first sperm enters the egg.
Further Information
Get more info on 'Spermatozoa'.
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