Gamete Transfer & Fertilization PDF

CHAPTER 9 Gamete Transfer and Fertilization Fertilization Fertilization (conception) is the fusion of a sperm and egg. Fertilization restores the diploid compliment of genetic material. https://www.biologyonline.com/tutorials/human-reproduction-and-fertilization In order for fertilization to oc...

CHAPTER 9 Gamete Transfer and Fertilization Fertilization Fertilization (conception) is the fusion of a sperm and egg. Fertilization restores the diploid compliment of genetic material. https://www.biologyonline.com/tutorials/human-reproduction-and-fertilization In order for fertilization to occur, the gametes from both individuals must travel towards one another and undergo physical/biochemical changes. Semen Release Sex accessory gland secretions mix with the sperm to create semen (seminal fluid). This fluid is continually being replenished. There are three stages to semen release. 1. Before orgasm: small amount, bulbourethral gland secretions 2. During orgasm: majority release, seminal vesicle/prostate secretions 3. After orgasm: small amount, seminal vesicle secretions https://www.sciencedirect.com/topics/agricultural-and-biologicalsciences/sperm-transport Contents of Seminal Plasma Many different components make up the seminal plasma, secretions from the sex accessory glands. Components include solvents, mucus, enzymes, energy sources, zinc, prostaglandins, and ATP. Human Reproductive Biology, 4th Ed. (2014) pg. 160 Sperm Number and Structure A single ejaculation has an average of 182 million sperm. An individual sperm is typically 60 70µm long. The head region contains the haploid nucleus and the acrosome. The acrosome contains many enzymes vital for fertilization. Midpiece contains mitochondria to generate energy, mainly for flagellar motion of tail. Human Reproductive Biology, 4th Ed. (2014) pg. 161 Vaginal Sperm Shortly after entry into the vaginal canal the semen thickens which helps retain the semen in the vagina. https://www.researchgate.net/figure/nteraction-between-enzyme-model-with-a-band-g-chain-of-fibrinogen-pink-licorice_fig1_343701262 After 20 minutes the semen will liquefy again. The presence of semen increases the vaginal pH to 7.2. Orgasm of the individual with the vagina causes rhythmic contractions and negative pressure in the vagina moving sperm towards the cervix/uterus. Cervical Sperm The cervix has many narrow fold and crypts. Much mucus (mucin fibers) is also found in this area. Liquefication of the mucus in this area around ovulation allows sperm to pass into the uterus. Normal sperm are able to move into the cervix, but sperm with abnormal tails/weak swimming can’t. Only about 1 million sperm make it through the cervix. https://rep.bioscientifica.com/configurable/content/journals$002frep$002f158$002f1$002fREP18-0595.xml?t:ac=journals%24002frep%24002f158%24002f1%24002fREP-18-0595.xml Uterine Sperm The uterus does not contain a lot of fluid and sperm essentially “climb” up the uterus. Climbing is assisted by uterine contractions and cilia action. Contractions are induced by oxytocin. https://sitn.hms.harvard.edu/tag/macrophages/ Sperm movement into the uterus triggers an immune response and many macrophages make their way into the uterus. Only a few thousand sperm reach the uterotubal junction, of these many pick the wrong side and only a few hundred ever reach the waiting egg. Transport into the Oviduct Internal environment is less inhospitable, but thick mucus slows movement. Acts as a temporary storage area for the deposited sperm. Many sperm will swim past the ovulated egg and out the infundibulum of the oviduct. Those that meet the egg do so at the ampullary-isthmic junction. One ovulation is complete the infundibulum engulfs the ovary and rhythmic contractions help propel the egg and sperm back towards the uterus. Capacitation and Hyperactivation Sperm are not able to fertilize an egg at the time of ejaculation. This ability is gained through the process called capacitation. Capacitation involved the removal and/or modification of molecules which generally act to stabilize the plasma membrane in the head region. Destabilization allows for the acrosome reaction later on which is fundamental to fertilization. Capacitation also triggers hyperactivation – increased activity of the tail and swimming process in sperm. Timing of Fertilization Sperm can survive in the reproductive tract of XX individuals for a maximum of six days. An egg is only viable for 24-48 hours after ovulation. This means the highest chance of fertilization comes in the five days before ovulation, the day of ovulation, and to a lesser degree the day after ovulation. Most pregnancies occur from sperm that were already in the body prior to ovulation. Process of Fertilization Several steps are involved in the fertilization of an egg by a sperm. These steps can be broken down into: 1. Penetration of the cumulus oophorus 2. Penetration through the zona pellucida 3. Attachment to egg plasma membrane 4. Cortical reaction 5. Completion of meiosis II 6. Pronuclei generation 1. Move through the Cumulus Oophorus The cumulus oophorus is the layer of follicle cells which surrounds the ovulated secondary oocyte. Sperm produce hyaluronidase which breaks down hyaluronic acid. https://en.wikipedia.org/wiki/Hyaluronidase Hyaluronic acid holds together the cells of the cumulus oophorus. This allows the sperm to reach the inner layer of the zona pellucida. 2. Move through the Zona Pellucida The zona pellucida is composed of three different glycoproteins. Receptors on the sperm plasma membrane recognize one of these glycoproteins. Binding of the sperm to this layer triggers the acrosome reaction. AN influx of calcium ions and increasing pH and cAMP levels leads to exocytosis of the acrosomal vesicle. Human Reproductive Biology, 4th Ed. (2014) pg. 167 Small openings in the acrosome release hydrolytic enzymes which degrade the zona pellucida. 3. Attach to Egg Plasma Membrane Once through the zona pellucida the sperm enters the perivitelline space. As the sperm approaches the plasma membrane is aligns parallel to it. The posterior part of the sperm fuses with the egg. Fusion allows passage of sperm pronucleus (and centriole), midpiece, and much of the tail. Figure 9.5 - Human Reproductive Biology, 4th Ed. (2014) pg. 168 4. Cortical Reaction Only one may enter. To prevent other sperm from entering the egg, enzymes are release to alter the glycoprotein receptors in the zona pellucida. These enzymes are released from cortical granules which are triggered to fuse and open with an abrupt into in cytoplasic free calcium. This release is termed the cortical reaction and represents one process in a series of events known as egg activation. 5. Completion of Meiosis II As calcium rises after fertilization, the egg is triggered to complete the second half of meiosis II. This creates the second polar body and removes the extra copies of DNA from potentially mingling with the sperm DNA. Figure 9.6 - Human Reproductive Biology, 4th Ed. (2014) pg. 168 6. Pronuclei Generation After entering the egg, the nuclear membrane of the sperm disintegrates. The exposed sperm DNA decondenses and a new membrane then forms to envelope the sperm pronucleus. The egg and sperm pronuclei migrate towards one another, replicate their DNA, and fuse to create the zygote (syngamy). Figure 9.6 - Human Reproductive Biology, 4th Ed. (2014) pg. 168 Sex Ratio For every four haploid spermatozoa produced, two contain a Y chromosome and two contain an X chromsome. All eggs contain X chromosomes. Human Reproductive Biology, 4th Ed. (2014) pg. 169 Primary sex ratio (early aborted embryos) is 120:100 (not expected 100:100). Secondary sex ratio is 105:100 (births). Multiple Embryos Twins naturally occur in one of every 80-90 pregnancies. Two ova and two sperm = fraternal twins (dizygotic). Much more common. https://www.genome.gov/genetics-glossary/identical-twins One ova and one sperm spontaneously splitting = identical twins (monozygotic). Larger numbers of multiples increases the likelihood of multiple zygote origins. Parthenogenesis Parthenogenesis is the development of a new zygote without a fertilization event. This is a new organism forming from an egg not penetrated by sperm. Commonly occurs in many other organisms (insects, fish, some amphibians, reptiles, and domestic turkeys. Does not occur in humans. https://www.researchgate.net/figure/The-cyclically-parthenogenetic-life-cycle-ofDaphnia-During-favourable-conditions_fig1_227225101 Chromosomal Aberrations Occurs from errors in meiosis or fertilization. Most (>90%) will spontaneously abort within the first trimester. Androgenesis – only chromosomes from sperm Gynogenesis – only chromosomes from egg Triploidy – 3 copies of each chromosome. Represents less than 1% of human embryos. Aneuploidy – incorrect number of chromosomes (extra or missing) Chromosomal translocation/deletion – extra or missing pieces of chromosomes.