Fertilization Notes KEY Discuss proper behavior while discussing these upcoming topics. There is to be no giggling or other inappropriate comments. Failure to conduct yourselves properly will result in your removal from the classroom. During this topic, there are THREE things I want you to keep in the back of your mind: 1. The female body WANTS to become pregnant. This is vital to the continuation of the species. 2. Sperm and egg cells exist for the sole purpose of reproduction. Their entire reason for being to join together and make a new human 3. Sex feels so good to entice you to reproduce. If sex felt horrible, propagation of the species would not be as likely. Some words you need to be familiar with hearing: a) Sperm - male gamete or sex cell. b) Egg - female gamete or sex cell. c) Vagina - canal extending from the cervix to the outside of the body. d) Cervix - forms a canal that leads from the vagina to the uterus. e) Uterus (or womb) - a hollow organ located in a woman s lower abdomen. This is where child development occurs. f) Ovary - female reproductive gland (site of egg production) located on each side of the uterus. g) Fallopian Tube - transports the egg from the ovary to the uterus. h) Ovulation - the release of an egg from the ovary. The egg then enters into the fallopian tube. i) Ejaculation - the release of sperm. Since the male really only contributes one thing to the process of conception, we will look at the sperm in more detail first. Firstly, sperm are not produced until puberty and they are produced in the male testis. This is why it is so important to protect that area from injury. Any damage could result in the male becoming infertile (unable to have children) Page 1 of 8
- Looks and moves exactly like a tadpole - Smallest cell in the human body o Only about 0.005mm in size o A typed period (. ) is about 0.615mm in size o So a sperm cell is 123x smaller than a typed period! - Sperm move by whipping their tail back and forth o The tail is about 10x the size of the head - About 100 400 million sperm are deposited per ejaculation o Less than 20 million sperm deposited indicates low fertility FUN FACT: A male produces about 1000 sperm per heart beat! - Once males start to produce sperm they do not stop until they die. STOP!!!!? Why, why, why does a male produce so many sperm? What does this tell us about the sperm themselves and the males approach to fertilization? Males produce sperm based on a quantity approach instead of quality. Many sperm are defective, but enough good sperm are made to take a run at the egg. Basically the male approach is this: If enough sperm are directed towards the egg, chances are at least ONE should make it through! Use sports analogy as comparison example: basketball shooting hoops, hockey shooting pucks at net with goalie cutout, baseball swinging at every pitch from a machine That is about all there is to the male side of things. Now let s look at everything the female brings to the table. The Female Reproductive System: Page 2 of 8
Where do you think sperm are deposited? Where sperm are deposited at entrance to the cervix Where do you think fertilization of the egg occurs? High in the fallopian tube, near the ovary Where fertilization occurs Now what about the egg cell itself? What are its properties? - Largest cell in the human body - About 0.1mm in size - That s about 20x the size of a sperm cell - It is actually the size of a written period or the end of your pencil. It is the only human cell that can actually be seen with the naked human eye. But how does the female reproductive system actually work? Let s look at some of the background information first. Page 3 of 8
From puberty onwards, once a month the ovaries are signaled to prepare the release AN egg. Hormones regulate the menstrual cycle: estrogen and progesterone. Every cycle is started on the first day of a female s period. In most healthy females, ONE ova is matured and released every 26 to 28 days from one ovary. This is called ovulation. WAIT!!!! Wait, wait, wait your telling me the male contributes 100-400 MILLION sperm in a single ejaculation while females only contribute ONE tiny egg! What does this tell us about the egg and the female approach to fertilization? The egg was very carefully created during the females development in the womb, before she was even born. There was no rush in its creation (relative to sperm creation that is!) The vast majority of the eggs within the ovaries steadily die, until they are depleted at menopause. - At birth 1 million eggs - Puberty 300,000 eggs remain o Of these, 300 to 400 will be ovulated during a woman's reproductive lifetime. The eggs continue to degenerate during pregnancy, with the use of birth control pills, and in the presence or absence of regular menstrual cycles. The clock runs regardless of the circumstances. This process is not foolproof though. Sometimes during ovulation: o More than one egg may be released from the ovary. o Both ovaries may release an egg. Immediately after ovulation, the uterus begins to prepare for a possible pregnancy. Makes a warm, nutrient rich, and cushy environment by the thickening the lining of the uterus. If sperm do not meet the ova then the party is over The ovum disintegrates after about 24 hours, the uterus releases the thickened lining (after a few weeks) and the female will then get her period. o Then the process starts over again. o Continues until menopause When a woman is approx. age 45 50, their hormones start changing and they stop having the ability to reproduce. WAIT!!!! Sooooo men can reproduce basically until they die, but women can only reproduce until age 50 or so!? That is quite a difference! Why would this be? Page 4 of 8
IF contraception is used, generally it is ovulation that does not occur. However, should a person miss even one pill (or even be a few hours late taking the pill), or misuse one of the other forms of contraception, ovulation can occur o That is often how females become pregnant, even when they are on the pill Now that we have a very basic idea of the setting of sexual reproduction, we can look at everything that has to go right in order for conception to occur. So how far must a sperm travel to find the egg? - About 200 mm (or 20 cm) from entrance of cervix to site of fertilization in the fallopian tube. - That s about 40,000 times the size of the sperm cell!! - If a human takes a step of 1m, and the distance you had to walk was 40,000 times your size, then you would have to walk 40,000m or 40km! This is the distance from CNB to the middle of Wood Lake on the way to Winfield! This distance is certainly doable, but there is a catch! The sperm can travel this distance in only about 1-2 hours!! Can you walk 40km in 1-2 hours?? I don t think so! You would have to be walking at 20km per hour to accomplish this. The average human walking speed is about 5km/hr. So you would really have to pick up the pace! Once the sperm reaches the fallopian tube, how long do you think the sperm can survive and try to find the egg? 2-5 days of searching for something that wants to be found. **The egg releases chemical signals that are designed to attract the sperm towards it. Kind of like a homing beacon! This beacon is short range, but certainly effective! This is all well and good, but how does the sperm actually make the journey? We know that 100-400 million sperm are ejaculated at a single time, but how many even make it close to the egg cell? - 1000-2000 only! - That s a 99.99% mortality or failure rate STOP!!!! What the we were told that BOTH the female and male WANT fertilization to occur, and yet the journey for the sperm must be crazy tough for such a high failure rate. Why is that? For something that is supposed to work, it seems like it barely does! Page 5 of 8
Here are the steps a sperm cell must go through to reach the egg. For each step we will be asking WHY the situation is the way it is! 1. Deposited - Sperm are ejaculated into the vaginal canal at the entrance to the cervix - The speed at which semen is ejaculated has been measured to be at approx. 50 km per hour. To push the sperm as far into the women as possible, making their journey just that little bit easier. 2. Vaginal acidity kills many - The vaginal canal is a very acidic place, making it hostile territory for sperm. - Close to ovulation, the vaginal canal becomes less acidic, making it SLIGHTLY more hospitable to sperm (but still acidic enough to kill millions!) - Semen, which is the fluid ejaculated with the sperm, however, helps to protect sperm by changing the ph level around the swimming sperm. Protect vagina from harmful bacteria Kill any sperm that cannot escape the acidic environment fast enough 3. Swim against the current of fluid coming from the cervix. - The female reproductive system is a self cleaning system. - Fluid is continually being released from the uterus, through the cervix to flush out the vaginal canal. Keep harmful bacteria from entering the uterus 4. Get through the mucus plug - For most of a female s cycle, there is a thick mucus plug that acts like a bodyguard to the cervix. Prevent bacteria from entering the uterus - It is difficult for sperm to get past. - Close to ovulation, however, this plug turns into a thin stretchy liquid that actually HELPS sperm cross the cervix. If a sperm reaches the entrance to the cervix it must be strong enough to survive acidic environment (test #1 passed) Page 6 of 8
5. Cross through the cervix - Through most of a female s cycle, the cervix is tightly closed, making it difficult for sperm to enter the uterus. - Close to ovulation, the cervix is slightly dilated (opened) allowing sperm easy access to the uterus. When ovulation is near the females body WANTS sperm to reach the egg that is about to be released. 6. Swim up through the uterus - Long trek for the remaining sperm: only about one million make it past the cervix. While the distance is only 4 6 inches (10 15 cm), it is like any one of us trying swimming across the lake and back twice and having obstacles in our way. Like swimming blindfolded, in the rain, being windy, etc. - Sperm can live 2 to 5 days inside the female body once they reach this point. If sperm were deposited a few days before ovulation they can still be around to search for the egg days later. 7. Choose the correct fallopian tube - Since typically only one ovary will release an egg each cycle, half of the sperm may head in the wrong direction. - However, scientists have found that eggs give off a chemical sent. The Sperm have chemical receptors in their heads. Sperm follow scent of the chemical to find the egg. Blind but not random venture. The egg WANTS to be found! It can only survive for about a day unfertilized once released. - Sperm that go the wrong direction (to the wrong tube) or just don t have enough energy to make it are just absorbed into the female body as protein. 8. Swim against the cilia - Cilia (tiny hairs) are found in lining both fallopian tubes. This is what helps the egg travel down to the uterus, because it has no other form of movement. - The cilia are all pushing in the opposite direction that the sperm are headed. Page 7 of 8
9. Fertilize the egg (Show image of sperm and ova in book) - As I mentioned earlier, only about 1000 sperm out of several hundred million, will make it close to the egg. - But remember, it only takes one sperm! Once the egg is fertilized, no other sperm can penetrate it. - When the egg becomes fertilized, the egg s shell changes its electrical charge making it impossible for other sperm to enter. If more than one sperm were to enter egg, there would be too many chromosomes. REMINDER: Although it may sound impossible for sperm to reach the egg, it only takes one out of all the hundreds of million sperm. STOP!!!! That is a crazy journey! Why would the female body provide such a crazy difficult journey for the sperm!? This process is supposed to HAPPEN, right? Show recap PowerPoint of a sperms journey! Page 8 of 8