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05/12/2008
On a biological brethe
OK... so whoever designed the human reproductive system was HIGH! Seriously.... let me give you a brief lesson. Hormones are in bold, other scientific words that I need to learn to spell (and that you might not know) are in italics.
Spermatogenisis (the male side)
The hypothalamus (in the brain) releases GnRH (Gonadatrophic Releasing Hormone - Gonads being the sex organs). This travels the short distance south to the Anterior Pituitary Gland. The Anterior Lobe of the Pituitary Gland is stimulated to produce FSH (Follicle Stimulating Hormone) and ISCH (Intersticial Cell Stimulating Hormone - female equivilent is LH). These hormones then travel the long distances through many blood vessels to the testicles.
Now we need some anatomy. The testis is compised of seminiferous tubules (that make sperm) and then cells that make up the tissue between the tubules such as blood vessels and other intersticial cells. ISCH stimulates the intersticial cells to produce testosterone. Testosterone has several functions. The one you will all know is stiumlating the onset of secondary sexual characteristics in prepubescent males (poor things). Testosterone also stiumlate sertoli cells and inhibits the production of GnRH (more on that later).
Back to the seminiferous tubules. The walls of these tubules are comprised of highly specialised sertoli cells that govern the process of spermatogenisis. The outer cell layer of these tubules is a germinal epithelial layer - a source of unspecialised cells that can be transformed into sperm. These cells undergo meiosis, splitting twice to form 4 haploid cells - half the number of chromosomes. The first meiotic split halves the DNA content of the cell and create two haploid gametes (primary spermatocytes), the second split is just a staightforward cloning (as in mitosis) to produce the 4 haploid secondary spermatocytes.
The sertoli cell then governs the specification of these into spermatids and the sperm. This process involves the creation of a long flagellum (tail), production of many mitochondria in the middle piece to fuel the movement of the sperm, the digestion of the cell cytoplasm to reduce bulk and the production of an acrosome layer on the head of the spem which contains enzymes to break through the wall of the developed ovule (technically secondary oocyte).
The process of spermatogenis occurs from the external wall of the seminiferous tubule (the germinal epithelium) inwards towards the lumen (empty space of the tubule) in the centre. In effect average/normal cells go in on the outside and then fully fledged sperm are collected in seminiferous tubules as part of a continuous cycle.
This cycle is regulated by the inhibiting effects of testosterone and the inhibin produced as a by product of the action of sertoli cells. These two hormones stimulate the hypothalamus into *not* producing GnRH and therefore the Anterior Lobe of the Pituitary Gland is not stiumlated to produce ISCH and FSH. Consequently the rate of spermatogensis falls. Once the levels of inhibin and testosterone fall again then the is less (indirect) inhibition of ISCH and FSH and the rate of spermatogenisis rises. This is an example of the negative feedback of certain products (or by-products) of a reaction to maintain homeostasis.
Ok. So we have our sperm. The sperm produced and collected in the seminiferous tubules pass into small ducts called the vasa efferentia and then into the epididymis where they can be stored for up to 18 hours. Prior to ejaculation the sperm are moved the vas deferens (a muscular tube), the seminal vesicle releases mucus whilst the ejaculatory glands release an alkali solution to neutralise vaginal acids (Prostate Gland) and a solution of sugars to provide energy for the sperm on their travels (Coweper's Gland). This is mixture has now formed semen which will be released from the urethra by ejaculation.
Ejaculation is stimulated by increased supply of blood to the spongy tissue in the penis causing it to swell and harden and stiumlating the contaction of errectile tissue. Physical contact stimulates the glans penis which causes the muscles along the ejaculatory tract (the epididymis to the urethra) to contract moving the sperm/semen through the tract via peristalsis and finally the contraction of the urethra results in ejaculation.
Inside the femal the force of ejaculation propels *some* sperm into the uterus (/top of the vagina). In three days the sperm will have "swum" to the oviduct. Out of 500 million sperm only a few hundred reach the oocyte. Only one can ever fertilise.
It is interesting to note that there are also hormones in semen that stimulate muscular contraction of the uterus/oviducts to help the sperm reach the oocyte and improve the probability of fertilisation. There are only 5 million sperm in 5cm^3 of semen. The testies hang outside the body because the reactions involved occur at an optimum temperature of 35 degrees celcius.
WOW hasn't that taken the fun out of it!
And if you think that that might be a little complicated... sometime I might try to explain the female system which is approximately 4.7 times worse.
Anyone would think the intention of the entire process was failure... it certainally causes a significant stumbling block for most A Level students - And this really is the easy one.
I have decided this is a good way to revise. :P
xXx
19:12 Posted in Complete Random Junk! , Life , School , Science | Permalink | Email this
Comments
oh Ruth you dirty talker you.......
O_o
Posted by: stuart Dinge | 05/13/2008
Hardly... I don't think anything could make sex sound less fun!
xXx
Posted by: Ruth | 05/13/2008
*sighs as the horrifically corrosive sarcasm is utterly lost on his otherwise devastatingly beautiful and reasonably intelligent better half*
X
Posted by: Stuart Dingle | 05/13/2008
*pokes respective other half for being hopelessly soppy... and for failing to realise that his "cutting" wit was not worthy of acknowledgment*
xXx
Posted by: Ruth | 05/13/2008