Everything about Spermatogenesis totally explained
Spermatogenesis is the process by which male
spermatogonia develop into mature
spermatozoa. Spermatozoa are the mature male
gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of
gametogenesis. In
mammals it occurs in the male
testes and
epididymis in a stepwise fashion, and for humans takes approximately 64 days. Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for
sexual reproduction. It starts at
puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age. The entire process can be broken up into several distinct stages, each corresponding to a particular type of cell:
Purpose
Spermatogenesis produces mature male gametes, commonly called
sperm but specifically known as
spermatozoa, which are able to fertilize the counterpart female gamete, the
oocyte, during
conception to produce a single-celled individual known as a
zygote. This is the cornerstone of
sexual reproduction and involves the two gametes both contributing half the normal set of
chromosomes (
haploid) to result in a chromosomally normal (
diploid) zygote.
To preserve the number of chromosomes in the offspring, which differs between
species, each gamete must have half the usual number of chromosomes present in other body cells. Otherwise, the offspring will have twice the normal number of chromosomes, and serious abnormalities may result. In humans, chromosomal abnormalities arising from incorrect spermatogenesis can result in
Down Syndrome,
Klinefelter's Syndrome, and
spontaneous abortion. Most chromosomally abnormal zygotes won't survive for long after conception; however, plant reproduction is a little more robust, and viable new species may arise from cases of
polyploidy.
Location
Spermatogenesis takes place within several structures of the
male reproductive system. The initial stages occur within the testes and progress to the
epididymis where the developing gametes mature and are stored until
ejaculation. The
seminiferous tubules of the testes are the starting point for the process, where stem cells adjacent to the inner tubule wall divide in a centripetal direction—beginning at the walls and proceeding into the innermost part, or
lumen—to produce immature sperm. Maturation occurs in the epididymis and involves the acquisition of a tail and hence motility.
Stages
Spermatocytogenesis
Spermatocytogenesis is the male form of
gametocytogenesis and results in the formation of
spermatocytes possessing half the normal complement of genetic material. In spermatocytogenesis, a diploid
spermatogonium divides
mitotically to produce two diploid intermediate cell called a
primary spermatocyte. Each primary spermatocyte duplicates its DNA and subsequently undergoes
meiosis I to produce two haploid
secondary spermatocytes. This division implicates sources of genetic variation, such as random inclusion of either parental chromosomes, and
chromosomal crossover, to increase the genetic variability of the gamete.
Each cell division from a spermatogonium to a spermatid is incomplete; the cells remain connected to one another by bridges of cytoplasm to allow synchronous development. It should also be noted that not all spermatogonia divide to produce spermatocytes, otherwise the supply would run out. Instead, certain types of spermatogonia divide to produce copies of themselves, thereby ensuring a constant supply of gametogonia to fuel spermatogenesis.
Spermatidogenesis
Spermatidogenesis is the creation of
spermatids from secondary spermatocytes. Secondary spermatocytes produced earlier rapidly enter meiosis II and divide to produce haploid spermatids. The brevity of this stage means that secondary spermatocytes are rarely seen in histological preparations.
Spermiogenesis
During spermiogenesis, the spermatids begin to grow a tail, and develop a thickened mid-piece, where the
mitochondria gather and form an
axoneme. Spermatid
DNA also undergoes packaging, becoming highly condensed. The DNA is packaged firstly with specific nuclear basic proteins, which are subsequently replaced with
protamines during spermatid elongation. The resultant tightly packed
chromatin is transcriptionally inactive. The
Golgi apparatus surrounds the now condensed nucleus, becoming the
acrosome. One of the
centrioles of the cell elongates to become the tail of the sperm.
Maturation then takes place, which removes the remaining unnecessary
cytoplasm and
organelles. The excess cytoplasm, known as
residual bodies, is
phagocytosed by surrounding Sertoli cells in the
testes. The resulting spermatozoa are now mature but lack motility, rendering them sterile. The mature spermatozoa are released from the protective
Sertoli cells into the lumen of the
seminiferous tubule in a process called
spermiation.
The non-motile spermatozoa are transported to the
epididymis in
testicular fluid secreted by the Sertoli cells with the aid of
peristaltic contraction. Whilst in the epididymis they acquire motility and become capable of fertilisation. However, transport of the mature spermatozoa through the remainder of the
male reproductive system is achieved via muscle contraction rather than the spermatozoon's recently acquired motility.
Role of Sertoli cells
At all stages of differentiation, the spermatogenic cells are in close contact with Sertoli cells which are thought to provide structural and metabolic support to the developing sperm cells. A single Sertoli cell extends from the basement membrane to the lumen of the seminiferous tubule, although the cytoplasmic processes are difficult to distinguish at the light microscopic level.
Sertoli cells serve a number of functions during spermatogenesis, they support the developing gametes in the following ways:
- Maintain the environment necessary for development and maturation via the blood-testis barrier
- Secrete substances initiating meiosis
- Secrete supporting testicular fluid
- Secrete androgen-binding protein, which concentrates testosterone in close proximity to the developing gametes
- Testosterone is needed in very high quantities for maintenance of the reproductive tract, and ABP allows a much higher level of fertility
- Secrete hormones effecting pituitary gland control of spermatogenesis, particularly the polypeptide hormone, inhibin
- Phagocytose residual cytoplasm left over from spermiogenesis
- They release Antimullerian hormone which prevents formation of the Mullerian Duct / Oviduct.
Influencing factors
The process of spermatogenesis is highly sensitive to fluctuations in the environment, particularly
hormones and temperature. Testosterone is required in large local concentrations to maintain the process, which is achieved via the binding of testosterone by
androgen binding protein present in the seminiferous tubules. Testosterone is produced by interstitial cells, also known as
Leydig cells, which preside adjacent to the seminiferous tubules.
Seminiferous epithelium is sensitive to elevated temperature in humans and some other species, and will be adversely affected by temperatures as high as normal body temperature. Consequently, the testes are located outside the body in a sack of skin called the
scrotum. The optimal temperature is maintained at 2
°C (
man) - 8°C (
mouse) below body temperature. This is achieved by regulation of blood flow and positioning towards and away from the heat of the body by the
cremasteric muscle and the
dartos smooth muscle in the scrotum.
Dietary deficiencies (such as vitamins B, E and A),
anabolic steroids, metals (cadmium and lead), x-ray exposure,
dioxin, alcohol, and infectious diseases will also adversely affect the rate of spermatogenesis.
Hormonal control
Hormonal control of spermatogenesis varies among species. In humans the mechanism are not completely understood, however it's known that initiation of spermatogenesis occurs at puberty due to the interaction of the
hypothalamus,
pituitary gland and
Leydig cells. If the pituitary gland is removed, spermatogenesis can still be initiated by
follicle stimulating hormone and
testosterone.
Follicle stimulating hormone stimulates both the production of
androgen binding protein by Sertoli cells, and the formation of the
blood-testis barrier.
Androgen binding protein is essential to concentrating testosterone in levels high enough to initiate and maintain spermatogenesis, which can be 20-50 times higher than the concentration found in blood. Follicle stimulating hormone may initiate the sequestering of testosterone in the testes, but once developed only testosterone is required to maintain spermatogenesis. However, increasing the levels of follicle stimulating hormone will increase the production of spermatozoa by preventing the
apoptosis of
type A spermatogonia. The hormone inhibin acts to decrease the levels of follicle stimulating hormone.
The Sertoli cells themselves mediate parts of spermatogenesis though hormone production. They are capable of producing the hormones
estradiol and
inhibin. The Leydig cells are also capable of producing
estradiol in addition to their main product testosterone.
Further Information
Get more info on 'Spermatogenesis'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://spermatogenesis.totallyexplained.com">Spermatogenesis Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
