29 lecture 1 is inter introduction to development and fertilization there are a lot of steps between fertilization and birth you have fertilization then the fertilized egg has to implant and the placenta develops the fetus develops then gestation labor and parturition or birth uh the first two months after fertilization is embryonic development and from week nine until birth is fetal development and the developing human is a fetus so we have the gestation period that's fertilization of birth it's approximately 40 weeks between 38 and 40 weeks prenatal period is period before birth and the neonatal period is after after they're born first month and a half after they're born during the embryological development the first two months this is when all the organs develop this is the most crucial time in terms of making sure that the mother doesn't have any drugs in her system or anything like drinking smoking things like that that might affect a baby because this is when the organs develop after that they just grow the problem is most women don't realize they're pregnant at first and they may you know be having alcohol and maybe they're taking prescription drugs for something and the the embryo is exposed to that a very vulnerable time for it um developmental anatomy is studying these these stages embryology is fertilization to the fetal period and obstetrics deals with pregnancy labor and the neonatal period so we're going to be looking uh first at the process of fertilization we talked about ovulation already but fertilization has to occur within 12 to 24 hours after ovulation sperm are viable for for 24 to 48 hours following ejaculation so you have a small time window of time during which fertilization has to occur and then the divisions called cleavage produce a solid ball of cells called the morula and then that is going to the cells in there will differentiate into a blastocyst and then it implants and that happens about six days after fertilization so during fertilization you have two haploid gametes the sperm in the egg they merge together and you end up with a single diploid cell it normally happens in the uterine too when it's about a third of the way down the tube to the uterus the oocyte usually dies within 24 hours so it's important that fertilization occurs within those first 12 to 24 hours following ovulation the movement of the oocyte through the uterine tube is through peristaltic contractions as well as the cilia and then the sperm swim up the uterus and into the uterine tube by undulating motions of the flagella and also muscular contractions of the uterus the changes that sperm have to undergo in the female reproductive tract to allow them to fertilize an egg that's called capacitation this is the final maturation step to fertilize an egg the sperm has to penetrate through the outermost layer called the corona radiata and then a layer called the zona pellucida and then the cell membrane there is a sperm receptor in the zona pellucid flucida called the zp3 receptor it's a glycoprotein and it binds to membrane proteins in the head of the sperm and triggers what's called an acrosomal reaction remember the acrosome is a little cap on the top of the head of the sperm and it's filled with enzymes these enzymes then digest a path through the zona pellucida and only one sperm is allowed to to get through the barrier and reach the plasma membrane of the oocyte so the first things that happen before fertilization can occur are that the oocyte has to be transported towards the uterus through peristalsis the movement of cilia and it's going to release chemical attractants um to draw the sperm towards it the sperm swim towards it using their flagellum and this is where the prostaglandins and come in that are released in the semen they help to stimulate uterine contractions and that helps also to propel the sperm capacitation as i said happens within the female reproductive tract it the acrosomal membrane becomes more fragile the fusion of sperm with the the egg is called syngamy and polyspermy or fertilization of the oocyte by multiple sperm is prevented by specific chemical changes that prevent a second sperm from entering the oocyte so here we have a and secondary oocyte and these are the granulosa cells that make what's called the corona radiata which is the outermost layer these are remnants from the follicle we have the zona pellucida next which is a clear layer and then we have the cytoplasmic membrane of the oocyte and remember that the secondary oocyte arrests in metaphase of meiosis ii so the first thing the sperm cell is going to do is try to burrow its way through the granulosa cells to get to the zona pellucida then when it hits the zonoplasty it binds to zp3 receptor or sperm receptor and the acrosomal reaction occurs the enzymes are released from the head of the sperm and it digests its way through the zona pellucida once it it enters it gets through the plasma membrane and enters the secondary oocyte it the oocyte is triggered to complete meiosis and then the dna in the sperm called the pro-nucleus and the dna and the oocyte also pro-nucleus fuse together and that makes a zygote or fertilized egg now when the sperm fuses with the cell membrane of the oocyte it gets through the corona radiata burrows through it it digests it binds to zp3 receptor and the acrosomal reaction happens and enzymes are released and it digests its way through the zona pellucida and gets to the cell membrane one to three seconds after it does the membrane depolarizes and other cells can't fuse with it this is called the fast block to polyspermy the next thing that happens is in a result of that depolarization it triggers the intracellular release of calcium and the calcium causes exocytosis of molecules that harden the zona pellucida and that's the slow block so it's kind of like first it puts up an electric fence and then it builds a wall this calcium is similar to what happens um when a a an action potential is traveling down to the end of a neuron and causes depolarization and that triggers the opening of calcium channels and that is what causes the exocytosis of the neurotransmitters is the same type of thing that's happening except in here the calcium is intracellular rather than extracellular so the sperm enters it triggers the oocyte to complete meiosis ii gets rid of its second polar body and then the sperm loses its tail and just becomes a male pronucleus then the male and female haploid pronuclei fuse together and that's the actual fertilization now the fertilized ovum has 23 pairs of chromosomes it's 2n and it's called the zygote but it still has the zona pellucida around it twins happen um in two different ways dizygotic twins are fraternal twins and they may or may not look alike they look as similar as any other two siblings sometimes in families brothers and sisters have strong resemblance with one another sometimes they don't the olsen twins are fraternal even though people think they're identical they're not they're fraternal twins but they have a great deal of similarity in their look this comes from the independent release of two oocytes that were fertilized by two separate sperm identical twins are monozygotic meaning they came from one fertilized egg and there are genetically identical they're always going to be the same sex and they separate after fertilization if it takes too long for them to separate or if it doesn't completely separate then we have conjoined twins these are twins that share some of their body structures the goals for lecture one are lists and describe the stages of development from fertilization to birth draw a second dairy oocyte label the following layers the zona pellucida the corona radiata the cytoplasmic membrane describe the process of fertilization including the following terms capacitation acrosomal reaction polyspermy fast block to polyspermy slobot polyspermicing me from nucleus fertilization zygote monozygotic twins digotic twins and conjoined twins