Monthly Archives: March 2012

11.4c Birth

Childbirth

During pregnancy, progesterone inhibits secretion of oxytocin by the pituitary gland and also inhibits contractions by the uterus walls, the myometrium. At the end of pregnancy, hormones produced by the fetus signal the placenta to stop secreting progesterone so oxytocin is secreted. This hormone causes contractions of the muscle fibers in the myometrium which are detected by stretch receptors that signal to the pituitary gland to increase oxytocin secretion. An increase oxytocin makes the contractions more frequent and more vigorous causing even more oxytocin secretion. This is a type of positive feedback system which is a very unusual control system in humans. In this case it has the advantage of causing a gradual increase in myometrium contractions and allow the baby to be bron with the minimum intensity of contraction. Relaxation of muscle fibers in the cervix cause it to dilate and uterine contraction then bursts in the amniotic sac and the amniotic fluid passes out. Further uterine contractions which usually occur over hours than minutes finally push the baby out through the cervix and vagina. The umbilical cord is broke and the baby can take its first breathe and achieve independence from its mother.

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11.4b Fertilization

Fertilization

Sperm that are released into the vagina during intercourse are stimulated to swim by calcium ions diffusing from vaginale fluids into their tail. The membranes of sperms can detect chemicals released by the egg by their receptor. This allows them to swim towards the egg. Once the egg is reached many reactions take place.

1. The acrosome reaction 

The zona pellucida is glycoproteins that surround the egg. The acrosome is a large membrane-bound sac of enzymes in the head of the sperm. When the sperm makes contact with the zoa pellucida, the contents inside the acrosome are released and the enzymes then loosen the zona pellucida which allows sperm to force their way through by beating their tail.

2. Penetration of the egg membrane

The acrosome reaction exposes an area of membrane on the tip of the sperm that has proteins that bind to the egg membrane. The first sperm that gets pass the zona pellucida binds and the membranes of the sperm and egg fuse together. The sperm nucleus enters the egg cell and this is when fertilization occurs.

3. The cortical reaction 

The sperm brings parental genes and causes the egg to become active. This causes an effect on the cortical granules which are vesicles located near the egg membrane. There are plenty of these vesicles and when activation of the egg occurs, the contents are released from the egg by exocytosis. The chemicals in the cortical granules interact with the glycoproteins in the zona pellucida turing into an impenetrable fertilization membrane.

Data-based question: maternal age and pregnancy 

1. From ages 12-19 to 20-24 there is a rise. After this, at 20-24, births are most successful and continue to be around the 20s. Once a person is in their 30s, there is a significant drop and once they are older than 44, the successful birth rates are very low.

2. Mothers in their 20s are more suitable to have children as they are younger. As they get older, they may not be built to have babies anymore therefore successful births are very low.

3. This could be due to social factors. They may not be educated as much as others and not have jobs therefore are not sure how to take care of children and get the right medical treatment. Also, their bodies are yet developing therefore might not be ready to give birth.

4. One difference is that the education of a mother is better at an older age (35 or more) so this a better environment for children to be born it. Another difference is that mothers at an older age have less successful pregnancies than younger women because their bodies may not be as equip at younger mothers.

5. The ideal age to have children may be 20-29. This is because the live births rate is higher than any other age. Also, the miscarriage rate is lower than others. Though mothers that are older are more educated, people are this age are still educated and it is safer to have a baby at this age therefore this would be the ideal age.

11.4a Gametogenesis

Spermatogenesis 

Spermatogenesis is the product of sperm that occurs in the testes. The testes is composed of narrow tubes known as seminiferous tubules with small groups of cells filing the gaps between the tubules. The gaps are known as interstices so the cells inside them are known as interstitial cells or sometimes called Leydig cells. Seminiferous tubules are also made of cells with the out layer known as germinal epithelium. This is where the production of sperm occurs. Cells in different stages of sperm production are found inside the germinal epithelium and the most mature stages are closest to the fluid-filled center of the seminiferous tubules. Cells that have developed tails are called spermatozoa which is frequently abbreviated to sperm. Sertoli cells are also in the walls of the tubules.

Oogenesis

Oogenesis is the production of egg cells in the ovaries. There are two types of ovary tissues a central tissue called the medulla and the surrounding tissue known as the cortex. Oogenesis starts in the ovaries of a female fetus. It was thought that the outer layer of the ovary was the germinal epithelium but this is probably not true Nevertheless, germ cells in the fetal ovary divide by mitosis and the cells formed move to separate themselves through the cortex. When the fetus is four to five months old the cells grow and divide by meiosis and by the seventh month, they are still in the first division of meiosis and a single layer of cells called follicle cells have formed around them. No further development takes place until puberty has occurred. The cells that started to divide by meiosis and the surrounding follicle cells is called a primary follicle together. There are about 400,000 ovaries at birth and no more primary follicles are produced. But at the start of each menstrual cycle a small batch are stimulated to develop by FSH. One usually goes on to become a mature follicle and contains a secondary oocyte.

Gametogenesis- comparing male and female 

Male and female gametes have very different structures . THey are also different in the amount produced and the timing of formation and release.

Number of female gametes and release

In females the first division of meiosis produces one large cell and one small cell. THe small cell is the first polar body and degenerates and dies. The large cell goes on to the second division of meiosis and completes it after fertilization. Again one large and small cell is produced and the small cell dies. The process happens once per menstrual cycle and usually only one egg is produced.During the years of puberty to menopause, only a few hundred female gametes are likely to be produced.

Number of male gametes and release 

From puberty onwards the testes produce sperm continuously. At any time there are millions of sperm at different stages of development. When the sperm are fully formed though they are still non-motile they pass out of the testis, through narrow tubes, and into the epididymis which is a long coiled duct adjacent to the testis. The sperm become mature and capable of motility in this tube. They are stored in the epididymis and the timing of their release depends on the sexual activity of the man. Whenever the male ejaculates, millions of sperm are moved by muscle along the sperm duct from the epididymis to the urethra. Fluid is added by the seminal vesicle and prostate gland to produce a volume of 3.5 ml of semen. The fluid secreted by the seminal vesicle includes alkali to help the sperm cope with the acidity of the vagina, fibrinogen to coagulate the semen, and fructose for sperm cell respiration. The secretion from prostate gland contains critic acid and hydrolytic enzymes such as fibrinolysin which liquefies the coagulated sperm in the vagina.

Data-based question: sperm fonts and male fertility

1a.

1b.

2a.

2b.

3.

6.6 Males and IVF

IVF- getting the gametes 

The natural method of fertilization for humans is in vivo which means inside living tissues of the body. Fertilization can also occur outside the body in carefully controlled laboratory conditions. This is called in vitro fertilization, abbreviated to IVF. In this method, firstly down-regulation occurs which is when a women takes drugs each day to stop her pituitary gland from secreting FSH and LH. Next intramuscular injections of FSH and LH are given everyday for ten days to cause many follicles to develop. Thirdly, the follicle is matured by an injection of HCG. Next, a micropipette is mounted on an ultrasound scanner and passed through the uterus wall to wash eggs out of the follicles. Next the man produces semen and is it collected in a sample bottle carefully labelled. Lastly, the semen is processed and injected below a layer of sterile fluid and after a hour, the semen that have swum up into the fluid are removed.

IVF- fertilization and implantation 

The mixing of sperm and egg cells are done in sterile conditions in hospitals or fertility centers. If in vitro fertilization works, one or more embryos are put in the uterus. If they implant and continue to grow, there is not difference between this pregnancy and one that occurred by natural conception. In this procedure, firstly each egg is mixed with 50,000 to 100,000 sperm cells in a shallow dish incubated at 37 degrees C the next day. Next, fertilization is detected. Next, the embryos are checked and healthy four-cell embryos are scheduled to transfer when they are 48 hours old. Up to three embryos are drawn into a plastic tube and pushed into the uterus and the embryos are released. Extra progesterone is given as a tablet in the vagina to ensure that there is a uterus lining. Lastly, two weeks after the embryo transfer has occurred, a pregnancy test can be taken to see if an embryo is developing.

6.6a Reproduction

Reproduction

Reproduction for humans involves the fusion of a sperm and an egg from the male and female respectively. If the gene tdf is present in the resulting embryo, the embryonic gonads will produce testes in the eighth week of pregnancy and start producing testosterone until the fifteenth week. At puberty, the secretion of testosterone increases and causes secondary sexual characteristics or the sex drive in men to develop.

Female reproductive system

Babies develop as females if the gene tdf is not present. This gene is located on the Y chromosome so with two X chromosomes, the baby will not have this gene. Therefore, ovaries develop and testosterone will not be secreted. The two female hormones estrogen and progesterone are always present in pregnancy and are secreted frist by the mother’s ovaries then bu the placenta. Without testosterone present, and instead estrogen and progesterone, female genitalia develop.

Hormonal control of the menstrual cycle 

Some mammals like deer are only able to breed at specific seasons however humans can reproduce at any time. In females, this involves the production of an egg (oocyte) inside a fluid-filled sac called a follicle. The endometrium or the lining of the uterus also thickens. If no embryo has formed, the thickening breaks down and is passed out of the body as menstruation and the process repeats itself. This cycle is known as the menstrual cycle. During the first part of the menstrual cycle, a group of follicles with eggs in them are stimulated to grow. While follicles are developing, the endometrium thickens as well. The most developed follicle breaks open and releases an egg into the oviduct and the other follicles are now degenerate. The follicle that released the egg becomes known as the corpus luteum. As the endometrium develops further, it is prepared to the implantation of an embryo. If fertilization does not happen, the corpus luteum breaks down and then the thickening of the endometrium breaks down as well is shed during menstruation.

Data-based question: the female athlete triad