Spermatozoon
A
spermatozoon or
spermatozoan (
pl. spermatozoa), from the
ancient Greek σπερμα (seed) and
ζων (alive) and more commonly known as a
sperm cell, is the
haploid cell that is the male
gamete. It
joins an
ovum to form a
zygote. A zygote can grow into a new
organism, such as a human being.
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
Antoni van Leeuwenhoek in
1677.
Humans
The human sperm is the main reproductive cell in males. The sperms differ in that each carry a set of chromosomes dividing each into either a male, or female sperm. The females differ in that they carry a XX gene, while the male sperm carry a XY gene. The female sperm also differ phenotypically in that they have a larger head in comparison to the male sperms. This contributes to the male sperm being lighter, and therefore faster and stronger swimmers than their female counterparts (although statistically there is still a 50% chance of an either XY or XX embryo forming).
Males
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 is known that the spermatozoa exhibits
laminar flow. Spermatozoan stream lines are straight and parallel. The tail
flagellates, which we now know propels the sperm cell (at about 1-3 mm/minute in humans) by rotating like a propeller, not side to side like a whip. The cell is characterized by a minimum of
cytoplasm. During fertilization, the sperm's
mitochondria gets destroyed by the egg cell, and this means only the mother is able to provide the baby's mitochondria and
mitochondrial DNA, which has an important application in tracing maternal
ancestry. However it has been recently discovered that mitochondrial DNA can be recombinant.
Females
A new scientific breakthrough may lead to
women in future being able to produce sperm.
Scientists in
England have turned
stem cells from an
embryo into sperm which are capable of producing offspring.
The breakthrough is likely to lead to new advances in treating male infertility and even the possibility that women could manufacture sperm.
The researchers at
Newcastle University say that the advance, when developed further, could help men with certain types of
infertility to become fertile and even one day could enable a
lesbian couple to have children that genetically would be their own.
The experiment used embryo cells to produce seven baby
mice, six of whom lived into
adulthood, although the survivors suffered adverse events of the kind seen in
cloning experiments.
The researchers isolated embryonic stem cells from an embryo only a few days old consisting of a cluster of cells. The cells were grown in a laboratory and screened to isolate the spermatogonial stem calls which were grown and then injected into female mouse eggs and grown in early stage embryos.
The research team says its project will aid the understanding of the biological process through which sperm is produced, which should help in the future treatment of infertility.
It is hoped that this new knowledge could be translated into treatments for men whose sperm is dysfunctional, although could be some years into the future.
The research was published in the journal
Developmental Cell.
In other animals
The largest spermatozoa belongs to the
fruit fly.
[1]The working horse for sperm researchers are sea urchins such as Arbacia punctulata which spawn their sperminto the sea at high numbers making them a perfect study tool for experiments.
Fertilization relies on sperm cellsfor most (all?) sexually reproductive animals.
Even some plants, such as bracken fern, use sperm for sexual reproduction(see papers by Brokaw).
It is clear that the odds of the sexes can be manipulated in differing species. For example, temperature affects the sex of the egg in the majority of reptiles (cool temperatures produce males while warm temperatures produce female offsprings). Stress and environment are also crucial in sex determination.
As it stands, there is no definitive scientific evidence on whether or not if such factors affect the sex of human embryos. Research in the field is currently ongoing with hints of some sex determining factors.
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.
|
Acrosome reaction on a Sea Urchin cell |
Mammalian sperm cells become even more active when they approach an egg cell. They swim faster and their tail movements become more forceful and erratic. This behaviour is called "hyperactivation."
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 CatSperchannels 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 first barrier to sperm is made up of so-called cumulus cells embedded in a gel-like substance made primarily of hyaluronic acid. The cumulus cells develop in the ovary with the egg and support it as it grows.
The second barrier coating the
oocyte is a thick shell formed by glycoproteins called the
zona pellucida. 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.
When a sperm cell reaches the egg the acrosome releases its enzymes. These enzymes weaken the shell, allowing the sperm cell to penetrate it and reach the plasma membrane of the egg. Part of the sperm's cell membrane then
fuses with the egg cell's membrane, and the sperm cell sinks into the egg (at which point the sperm tail falls off).
Upon penetration the membrane of the egg cell undergoes a change and becomes impenetrable, preventing further
fertilization of the ovum.
*
sperm competition*
sperm heteromorphism*
semen*
spermatogenesis*
The Handbook of Andrology*
Sperm hyperactivity*
Women could make sperm*
Slower conception 'leads to boys'
