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05/13/2008
A Dominant pedal in Biology!
Ok. Here is the other "half" of the story... this has taken me 2 hours. I did not do this set of notes from memory (the first bit on the Menstural Cycle was). It is 3 pages long in MSWord and I don't expect anyone to read it properly. I am just rather proud of it as it is the most work I've done in biology in 18 months!
It is also a good retort to any idiot male who complains he can't understand women - of course he can't! It is written into our bloody physiology!!!!!!!!
I'm not kidding when I say anyone would think we weren't meant to reproduce.
Hormones are in bold, couldn't be bothered to put scientific words into italics, instead *my* comments are initalics.
Oogenisis – the female side
Ok. So here is the complicated one… *breathes deeply*… and begins.
Once again the hypothalamus has overall control over this process – being the link between the nervous and chemical systems in the body, that sort of makes sense. Once again GnRH is released and travels the short distance to the anterior lobe of the pituitary gland. Again two hormones are released: LH (Lutenising Hormone (=ISCH)) and FSH (Follicle Stimulating Hormone).
Right now things begin to alter slightly. The female process of oogenisis (generation of the egg cell) is not continuous but cyclical and begins before birth. As the ovaries develop in the female embryo, meiosis one begins in the germinal epithelial layer and is halted in prophase one. The ovaries also produce follicle cells which surround the oogonia to form primary follicles.
Basically the soon-to-be-egg cells begin to divide and at frozen part way through the process with the DNA condensed into chromosomes and grouped into homologous (same-sized) pairs. The cells are then surrounded by a layer of other, smaller cells.
Meiosis one is halted at birth and the process cannot resume until puberty occurs and the menstrual cycle begins. (JOY!) This cycle varies between females, but for ease of communication the cycle is “averaged” out over 28 days.
OK, back to Biology. FSH is released from the anterior lobe of the pituitary gland and travels in the blood to the ovaries where it stimulates the continuation of meiosis one. The layers of surrounding cells builds up and a theca layer developes (from the tissue of the ovary) to create a primary oocyte within a primary follicle. The theca layer secretes oestrogen which has a number of functions including:
• Promotes secondary sexual characteristics in females
• Inhibits FSH production to prevent the development of a second oogonia.
• Builds up and prepares the endometrial layer (blood layer) in the uterus
The first meiotic split is completed and one of the haploid cells degenerates into a polar body that has no known purpose. The follicle continues to mature into a Graafian Follicle and oestrogen levels continue to rise. At day 14 of the cycle the oestrogen levels peak and (once over a threshold level) stimulate (not inhibit) the release of FSH and LH.
The production of a Graafian Follicle occurs moving in towards the centre of the ovary; LH stimulates the eruption of the primary oocyte out from the ovary into the oviduct (Fallopian Tube). This can apparently cause a little bleeding as the oocyte is the largest cell in a human body and some women claim to be able to feel the release occurring.)
Here the story splits into two:
The remainder of the Graafian Follicle develops into a Corpus Luteum (or Yellow Body) that produces progesterone (and oestrogen). Progesterone helps to maintain the endometrial layer and also inhibits FSH and LH so no new oogonia are developed.
The primary oocyte travels along the oviduct to the uterus and can survive only a couple of days, without fertilisation, before it degenerates. And then the story splits again….
If fertilisation does not occur, then the primary oocyte dies and shortly afterwards the corpus luteum degenerates. Progesterone and Oestrogen levels fall again and FSH and LH are no longer inhibited. FSH levels rise and stimulate the development of another oogonia and primary follicle to mature to a Graafian Follicle. The endometrial layer decomposes and menstruation occurs.
On The Other Hand…
Long lines of mucus in the uterus provide lines for sperm to swim easily along and guide their travel to the oviducts. The contraction of the uterus (presumably through oxytocin release) also aids this movement. If a sperm meets the primary oocyte, the acrosome layer breaks through the wall of the primary oocyte and stimulates the second division – meiosis two – to create a secondary oocyte. The secondary polar body produced here is again redundant material. The wall of the ovum becomes impermeable to other sperm to prevent double fertilization. The genetic material from the single sperm cell is incorporated into the DNA of the secondary oocyte to form an ovum.
Interesting… sperm are 50um long whilst a secondary oocyte is 140um in diameter! (that is visible to some people!)…. Just visualise it!
This ovum travels down the oviduct to the uterus and (hopefully) imbeds in the endometrial layer. The cell releases hCG (human chorionic gonadotrophin) that prevents the degeneration of the corpus luteum for roughly 12 weeks until the placenta is fully developed. After 12 weeks the placenta takes over the role of producing oestrogen and progesterone that maintain the endometrial layer and prevent the development of another oocyte by inhibiting FSH and LH. Progesterone also relaxes the muscles in the uterus wall to prevent damage to the foetus and potential miscarriage.
Back to placenta: the embryo developing a Chorion layer that protrudes into the uterus wall and forms finger-like protrusions called Choronic villi that have microvilli on the external side of the epithelial cell (outside) layer. Inside these villi a network of blood vessels bring the foetal blood close to the mother’s blood supply in the endometrial layer, so that diffusion can occur – note the two blood supplies never mix.
The foetal heart pumps (faster than the mother) deoxygenated blood out along the umbilical arteries. Gas exchange and exchange of nutrients / hormones / antibodies / urea occurs and fresh blood is transported back to the developing foetus along umbilical veins. HPL (Human Placental Lactogen) is involved in the development of breasts during pregnancy and adjusts the mother’s glucose and fat respiration to the advantage of the foetus. Most bacteria cannot cross the placental barrier but viruses such as Rubella and HIV can.
The foetus develops surrounded by a protective amniotic sac containing amniotic fluid. After roughly 38 weeks the level of progesterone decreases rapidly whilst the level of oestrogen increases. This makes the uterus more susceptible to oxytocin which is a neurotransmitter / hormone produced by the posterior lobe of the pituitary gland. This causes the uterus to contract and the cervix begins to dilate (over a time period of up to 12 hours). A mucus plug that has blocked the cervix during pregnancy detaches and passes out through the vagina and the amniotic sac bursts. Hopefully the foetus has had the sense (and kindness) to rotate around so they can leave the uterus headfirst! (The opposite is a breach-birth and is even more painful!) A rare example of positive feedback: the high levels of oxytocin in the blood during labour, stimulates the release of more oxytocin. The rate of contractions increases steadily and when the cervix has dilated to 10cm diameter then head “engages” and the baby begins to emerge.
(I am slightly confused here: the baby’s skull is not yet fused together… so how does pushing it repeatedly very hard against the cervix and the pelvis not cause damage??)
Once the baby is in air they (We NEED a better gender-neutral pronoun!) begins to breathe and the umbilical cord is cut and tied off. Final contractions of the uterus cause the placental structures to detach from the endometrial layer and pass out of the vagina. Over the following weeks the (deciduous) endometrial layer decomposes and is also lost as progesterone and oestrogen levels fall again.
The final part of the female reproductive cycle is lactation. During pregnancy the presence of HPL (Human Placental Lactogen) allows oestrogen to stimulate the development of the duct systems in the breasts and progesterone to stimulate the development of milk glands. The high levels of progesterone and oestrogen also inhibit the production of prolactin.
After birth, as progesterone and oestrogen levels fall PRF (Prolactin Releasing Factor) is released from the hypothalamus and stimulates the anterior lobe of the pituitary gland to produce prolactin that is responsible for the production of milk. The milk contains lactose (glucose + galactose) along with fat, minerals, vitamins that are easy to digest. There is also a selection of the mother’s antibodies and viruses such as HIV (if she is positive). The suckling action of the baby stimulates nerves in the nipples that send messages to the hypothalamus to release PRF and to the posterior pituitary gland to produce oxytocin. Oxytocin causes the involuntary muscles around the milk glands to contract to force milk through the ducts and out through the nipple. PRF also causes the release of prolactin so the production of milk is maintained.
This process obviously only occurs when the baby is suckling.
xXx
21:02 Posted in Life , School , Science | Permalink | Email this
Comments
[FX: Ripple of Applause]
Posted by: Steve | 05/13/2008
ah,
oxytocin,
a truly wonderful hormone
;-)
X
Posted by: stuart | 05/15/2008