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Spermatogenesisandoogenesisbothinvolvemitosis,cell growth,twodivisionsofmeiosisanddifferentiation Oogenesis Oogenesis is the production the egg cell in the ovaries. The production starts and ends before the birth of the girl: 1. Oogonia cells (cells out of which the ova will grow) start to divide by mitosis (DNA gets duplicated) over and over again. 2. These cells will grow to larger cells, then they are called primary oocytes. 3. The primary oocytes start to divide by meiosis, the process is arrested (stopped) for some years though. This division is called Meiosis I. 4. Some other cells within the ovaries undergo cell division too. They cover the primary oocytes with a cell layer called the follicle. The (still dividing) primary oocytes and their follicle are called primary follicles. At birth, the girl has around 400 000 primary follicles in its ovaries. They won’t change/develop until puberty. 5. At the start of each menstrual cycle, some primary oocytes finally divide. The two cells formed aren’t even close to being equal in size. The smaller one is called the first polar body and acts as a reservoir of the surplus chromosomes. The big cell is called secondary oocyte. Usually only one becomes a mature follicle, containing a secondary oocyte. 6. The follicle cells around the (now) secondary oocyte start to develop and form a double-layered follicle. There’s the secondary oocyte. Then there’s the first layer of the follicle surrounding the oocyte. Then there’s a fluid and then there’s the second layer of the follicle. This is called a Graafian follicle. 7. At ovulation the secondary oocyte and the inner follicle layer will be released. If it don’t get fertilized, the cell will remain an secondary oocyte until it dies. If it does get fertilized it will be doing meiosis II. Meiosis Mitosis Follicle Primary follicle oogonia cells oocyte Primary oocyte Secondary oocyte First polar body Cell division, DNA isn’t duplicated Cell division, DNA is duplicated Cell layer around the egg cell Egg cell and follicle Cells out of which the ova will grow female reproductive cell prior to fertilization Oocyte before meiosis I Oocyte after meiosis I The void cell created during meiosis I ! ! !!! !!! !!! !!! !!! !!! !!! Spermatogenesis Spermatogenesis is the production of sperm. It happens in the testes, which are composed of a mass of narrow tubes, called seminiferous tubules. The seminiferous tubules are made out of cells – the outer layer of these cells is called germinal epithelium. Spermatozoa is the actual word for sperm, this word is almost always abbreviated as sperm though. 1. Outside of the tubules are germ cells called spermatogonium. 2. They all grow, duplicate the DNA and then divide through mitosis. The same thing happens again and again. They divide through mitosis to increase their number. A man can produce one million sperms per day. 3. At some point, they divide through meiosis: They first had 23 pairs of chromosomes, now they have 23 single chromosomes. This called a haploid number of chromosomes. 4. Out of each of these divisions, 4 mature sperm heads are created 5. They will stay in the inside of the testes for some time until they have developed other features: flagellum (tail) for movement and an acrosome which contains the enzymes needed for fertilization. 6. The developing sperms need nutrients for their development. They therefore attach to a Sertoli cell until they are fully developed. Seminiferous tubules Germinal epithelium Spermatozoa Haploid Acrosome Sertoli cell Tubules in the testes in which sperm is produced. !!! Outermost layer of the seminiferous tubules !!! = sperm 23 single chromosomes instead of 23 pairs of chromosomes – half the number of chromosomes contains the enzymes needed for fertilization Provides sperm with nutrients for development ! Processesofspermatogenesisandoogenesisresultin differentnumbersofgameteswithdifferentamountof cytoplasm. The ovum (secondary oocyte) is the largest cell in most animals. It contains a lot of cytoplasm and nutrients needed to provide the future baby with nutrients. Mitosis replaces germinal cells daily The two divisions of meiosis result in 4 haploid sperms. The sperm in contrast are very small cells, containing a flagellum and mitochondria for ATP production and no cytoplasm. Both of the cells are perfectly suited for their job. Mitosis replaces germinal cells only when the girl’s an unborn. The two divisions of meiosis result in 1 ovum. Theaverage38-weekpregnancyinhumanscanbe positionedonagraphshowingthecorrelationbetween animalsizeandthedevelopmentoftheyoungatbirthfor otheranimals Mammals either give birth to babies which aren’t far developed, have their eyes closed, no hair, can’t defend themselves or can’t look for food on their own. Or they give birth to babies which are further developed and have hair, can look for food on their own and even can defend themselves a bit against predators. Mostly big mammals use the second strategy. It is correlated with long times of pregnancy. The longer the women is pregnant, the more developed the baby will be at birth. By comparing the duration of pregnancies of different mammals, one can tell how far developed the baby will be at birth. Annotationofdiagramsofseminiferoustubuleandovaryto showthestagesofgametogenesis Annotationofamaturespermandeggtoindicatefunctions Sperm Acrosome: Release enzymes when binding with follicle so the sperm can get through the follicle. Haploid nucleus: Contains the 23 single chromosomes. Tell the sperm what to do (swim etc.) Mitochondria: Produced ATP such that the sperm is able to swim. Flagellum: Used to swim. Ovum Follicle cells: Protect the secondary oocyte. Zona pellucida: Makes sperm’s acrosome release digestive enzymes which enable them to pull though the layers. After the first sperm entered, the zona pellucida hardens, trapping any other sperms which couldn’t get inside. New coming sperms can’t attach to the zona pellucida anymore. Cytoplasm: Containing yolk and all organelles typical for body cells Plasma membrane: Pulls sperm inside and makes that no other sperm can enter (change of characteristics of egg). Numerous cortical granules: Tell de zona pellucida to harden and keep other sperm outside. Haploid nucleus: Contain the 23 chromosomes