Fertilization and Implantation

00:01 In this lecture on germ cell development, we're going to follow the process of fertilization and implantation into the uterine lining.

00:09 Now after ovulation has occurred, the oocyte does not immediately enter the oviduct but is in actually in the peritoneal cavity for a limited period of time.

00:18 Generally, the oviduct's fimbriae, the finger-like extensions of its end will move towards the portion of the ovary that is released in the egg and sweep the follicular fluid and the oocyte into the oviduct.

00:32 There we'll move into the infundibulum and eventually to the ampulla which is where most fertilization takes place.

00:39 On the other end, approximately 1% of the spermatozoa that are entering into the vagina can actually make it to the cervix and then to the uterus, but thereafter, contractions of the uterus and the oviduct will propel them towards the ampulla where fertilization most commonly is going to occur.

00:58 But as the egg is released from the ovary, the corpus luteum will form and allow a pregnancy be maintained should fertilization happen.

01:08 Now after the spermatozoa have been in the uterine environment for a couple of hours, they become capacitated and the acrosome reaction occurs which allows the cap on the spermatozoa, the acrosome, to maneuver through the dense cloud of cells around the secondary oocyte so as the cumulus oophorous cells have turned into the corona radiata, they've maintained and protected the egg, but now the spermatozoa are able to migrate through and reach the zona pellucida, that glycoprotein coating that's deep to the corona radiata and just on the outside of the egg.

01:44 As a single spermatozoa migrates and makes its way through that zona pellucida it will fuse with the cytoplasm of the oocyte and it will move into the oocyte and at that point fertilization has occurred.

01:58 And at this time, the egg does everything in its power to prevent more than one sperm from entering the egg, so there's an immediate reaction where cortical granules in the cytoplasm of the egg release and make the zona pellucida absolutely impermeable to further sperm.

02:15 Polyspermy will result in multiple problems such as aneuploidy, too many copies of the chromosomes present, so the egg does everything in its power to prevent that from happening.

02:26 Once the spermatozoa has entered the oocyte, it completes the process of meiosis two and moves through metaphase into the stage where it has a mature haploid nucleus that can combine with the haploid nucleus that was brought into it by the spermatozoa.

02:43 The male and female pronuclei within the cytoplasm of the egg will replicate their DNA and then fuse.

02:52 This fused nucleus will then serve as the first nucleus of this brand new and unique individual and the chromosomes will associate and begin the process of getting ready for cell division and will then proceed through mitosis as we've seen before, so through metaphase, anaphase, telophase, producing instead of one gigantic zygote, a multiple celled morula.

03:18 The first division creates a 2-celled zygote after roughly 30 hours and we move on to the 4-celled stage after roughly 40 hours.

03:27 In this process, we don't actually enlarge as an organism but rather the cytoplasm that's in existence within the egg gets subdivided more and more finally.

03:37 The cells in the new zygote compact bringing them a little bit further away from the zona pellucida and protecting them from the outside environment.

03:46 And after three days, we have roughly 16 cells taking up the space that was taken up by a single egg just a bit earlier.

03:53 At this point, we have a group of cells that are on the inside separated from the zona pellucida called the inner cell mass and the group on the outside, conveniently called the outer cell mass.

04:04 After four days, the zona pellucida has degraded to the point where it allows fluid from outside to permeate into the morula.

04:12 This creates a cavity known as the blastocyst cavity and at this point we call the entire assembly a blastocyt.

04:19 The blastocyt have a group of cells on the inside that segregate off to one pole with the cavity intervening on the other side.

04:29 These groups of cells are called the embryoblast and will give rise to all the tissues and all the structures of the embryo, and thereafter the fetus.

04:38 The cells on the outside are called the trophoblast and they will become the supporting tissues for the developing embryo and fetus such as the placenta.

04:47 After about four days, the fertilized egg and then zygote, then morula, then blastocyst has migrated into the uterus and is ready to implant.

05:00 At this point it starts producing human chorionic gonadotropin, hCG, which is going to keep progesterone levels high and maintain the follicular region of the uterus.

05:13 So it's gonna keep the uterine lining thick and able to support the growing embryo and eventually, enough progesterone will be produced by the placenta that we will no longer need the corpus luteum in the egg to do that job.

05:26 Because human chorionic gonadotropin is the first new hormonal signal signaling that we have an implanted fetus, it is what you use in pregnancy tests so pregnancy tests generally measure the levels of human chorionic gonadotropin in the urine.

05:41 Eventually, the placenta will take over and we have a cell sustaining embryo and fetus that will maintain its environment and allow itself to develop further up until the 9th month period when delivery occurs.