Early Embryonic Development 1Dr. JohnsonWeek 6, Friday 9/18/14

Lesson Objectives1. Review events of ovulation and fertilization2. Describe timing of early development3. Understand totipotency of early blastomeres4. Understand in vitro fertilization5. Describe normal implantation6. Understand clinical presentation of abnormal implantation

PART 1: Ovulation

A. Overview1. This event occurs mid-cycle of menstruation2. During this event, the secondary oocyte, the zona pellucida, the corona radiata and mass of mucus known as the liquor folliculi are ALL released3. After being released, this collection is set off into the ciliated epithelium of the uterine tubes and swept into the lumen of the female reproductive tract..the mucus is sooo important in that the cilia can use that to push this collection down the tubes

4. What am I looking at? This shows the preovulatory follicle that houses a primary oocyte, stuck in meiosis I, at metaphase. The next image is the ovulatory event when the secondary oocyte is released with the corona radiata, zona pullicida, and mucus. At this point the secondary oocyte is stuck in Meiosis II awaiting fertilization . IN the third stage, the follicular cells remaining will transition into luteal cells that will form the corpus luteum. There is mitotic division increasing the cell amount of the corpus luteum and the inside of the follicle is filled with fibrin. 5. The important things to know here are that the LH surge produced by the pituitary gland is what causes the follicle to rupture, it triggers the completion and division of meiosis I with polar body 1 production and the ovulatory event to occur. The corpus luteum that is left behind serves as an endocrine organ and will release progesterone to prepare the lining of the uterus by increasing vascularization of the endothelium.

6. This image is showing the uterine tube and ovary in better detail. Look at how the fimbriae of the fallopian tubes are finger like projections that will catch the released oocytehowever, note that the uterine tube is open to the peritoneal cavity, if the oocyte does not properly travel down the tube toward the uterus it can float off into the abd cavity. The tube works via peristalsis to push the oocyte down toward the uterus. Uterine infections can also cause peritoneal infections

*This shows the ovulated secondary oocyte, There are a lot of follicular cells attached to the outside of the zona pellucida

B. Corpus Luteum1. The remnants of the mature follicle becomes the corpus luteum2. Granulosa and thecal cells differentiate(luteinize) forming apparatus for steroid biosynthesis 3. The corpus luteum will release progesterone, which is a hormone that supports implantation which would occur during the second half of the ovulatory cycle. This hormone affects the glands in the uterine lining, making them secretory and the glands start to secrete trophic and stimulatory factors

PART 2: Fertilization

A. Consequences1. Triggers meiosis II in secondary oocyte which results in the production of a second polar body and an ovum2. There various steps that contribute to fertilization: block to polyspermy, reestablish polyploidy, initiate developmental program, determine genetic sex

3. You can see that the sperm attach to the ovum externally and fight to enter the cell. Once a sperm successfully reaches the cell, it will trigger the completion of meiosis II. The sperm will then release its nucleus into the ovum and the cell will be officially diploidy at this point. This diploid cell will then undergo cell division resulting in the two-cell stage

4. The texbooks will say that the two cell stage is succeeded by even numbered cell divisions, however in real life it is asynchronous. The zona pullicida remains intact because the cells are not yet attached to each other. So after about 1 day following fertilization the two cell stage will be reached, at 40 hours that 4 cell stage will be reached and after 4 days will be the morula stage which is more than 16 cells(maybe 50 cells). The morula has not yet reached the uterine lining. Note the cells of the embryo are called blastomeres.

5. This image on the left is showing the sperm attached to the growing and dividing cell. The image on the right shows about the 20 cell stage

B. Compaction1. Individual blastomeres have changes in cell surface chemistry and they now adhere strongly to one another2. This creates the outer and inner population of cells3. Both populations continue to divide, producing the trophoblast and embryoblast(inner cell mass). The trophoblast will develop into the placenta and fetal membranes and will be on the polar end of the developing embryo4. When cells on the outside of the morula change structure and become tightly adherent, this is the first obvious differentiation of the embryo

5. The tight junctions between the epithelial cells that surround the cells hold the cells together. The image on the right is post compaction and the left is the precompaction morula.

C. Early Blastomeres are Totipotent1. Removal of early blastomeres still results in formation of complete fetus, thus blastomere biopsy possible for prenatal genetic diagnosis2. Dyes injected into one of four blastomeres end up in both trophoblast and embryoblast3. Therefore, early blastomeres are totipotent. This means they are not differentiated in the morula stage of 4 cells so they are NOT predestined to form a certain part of the body. 4. For the sake of nomenclature, the blastomeres are the cells that divide within the morula

D. In Vitro Fertilization1. You have a patient with scarring and bloackage of uterine tubes but otherwise healthy2. Woman can be hormonally primed to multiple ovulatory events in single cycle and secondary oocytes can be collected3. An egg and sperm are mixed in culture dish in the lab4. After fertilization, several early embryos are collected and transferred into the uterus via canula5. Woman can then become pregnant and deliver a normal baby6. Risks include multiple pregnancies, there is a slight possible increase of birth defects, but this is minimal and may be confounded by genetics of parents who require IVF7. Other options include artificial insemination, egg donation, surrogate mother, screening for genetic defects, sex selection

8. An embryo will not be able to implant without the dissolution of the zona pellucida, and this will happen after about 5 days after fertilization. So after this time, the blastocyst will release hydrolytic enzymes that will degrade the zona and the embryo can then implant. In the pic you can see the inner cell mass which will develop into the fetus and the trophoblast cells on the opposite end will form the placenta. Note the zona consists of carbohydrates. The degradation of the zona should occur once the blastocyst has reached the uterus to prevent improper implantation

PART 3: Implantation

A. Implantation1. Blastocyst hatches from zona pellucida2. Blastocyst attaches at pole containing inner cell mass3. Outer trophoblastic cells proliferate and fuse to form outer syncytium4. Results in formatio of inner cytotrophoblast CTB and superficial syncytiotrophoblast STB. The CTB are one cell one nucleus layer, however the STB is a syncytium so it consists of fused cells. The STB burrows into maternal tissue and release enzymes that degrade maternal tissue.5. The blastocyst will always attach on the upper part of the posterior uterus that way the placenta will be up top and there is a lot of space for the growing uterus underneath

B. More Implantation 1.STB is an invasive motile layer, it will break down endometrial epithelium and degrades the stroma2. Maternal blood vessels open, bathing the STB3. STB continues to invade and expand, forming irregular surface with lacunae(surface irregularities). 4. The syncytiotrophoblasts form lacunae which will help to expose more of the blastocyst to maternal blood supply. At this point the embryo is doubling in size and volume each day or so. It is completely dependent on maternal blood supply for oxygen and nutrients-this is why lacunae are necessary

*Eventually the structure will be completely embedded into the maternal tissue (endometrium). The inner cell mass forms 2 layers: an epiblast and a hypoblast. The epiblast is increasing in number of cells and thickening. The hypoblast is also increasing cell number. The inner cell mass is what actually becomes the embryo

PART 4: Implantation ComplicationsA. Bilaminar Disc Stage1. The inner cell mass forms two layers-epiblast and hypoblast2. Trophoblast forms invasive layer of cells for implantation3. These two layers the epiblast and hypoblast are sandwiched together and will be contained within the chorion shell, which is beginning to form the placenta.B. Abnormal Implantation1. An abnormal implantation is one that occurs in any site other than the upper, posterior portion of the uterine body2. An ectopic implantation(pregnancy) in any abnormal implantation where development occurs outside of the uterine lumen3. About 95% of all abnormal implantations are ectopic implantations but the two are not synonymous 4. Placenta previa most common type of abnormal nonectopic implantation

* #5 will cause placenta previa, #4 is interstitial implantation(ectopic), #3 is tubal pregnancy(ectopic).

C. Ectopic Pregnancy1. Defined as an implantation where development of conceptus occurs outside the uterine lumen2. About 95% of ectopic pregnancies occur in the uterine tubes, but tubal pregnancy and ectopic pregnancy are not synonymous3. Rest of ectopics occur at scattered sites, ex includes mesentery, rectouterine pouch(of Douglas), ovary kidney, diaphragm, etc*Embryo can happily implant in the tube and eventually blow the wall causing massive bleeding into the peritoneum: emergent surgery neededD. More on Abdominal Pregnancy

E. Ectopic Pregnancy is Common1. About 90,000 cases/year in US2. About 16/100 pregnancies3. About 30 cases/year at GWUMC EDF. Predisposing Factors1. Prior pelvic surgery2. Past history of IUD use3. Pelvic Inflammatory Disease PID, Chlamydia trachomatis infections, syphilis, gonorrhea G. Signs and Symptoms of Rupture1. Reproductively competent, sexually active, sudden onset of unilateral lower quadrant pain2. Distended firm abdomen with rebound tenderness3. Normal temp, tachycardia, low BP, low Hct, +B-hCGH. Treatment1. Early ultrasound when pregnant establishes implantation site and can rule out or diagnose ectopic pregnancy2. Unruptured- methotrexate(chemo drug that can be used to kill the embryo) or surgical removal and repair3. Ruptured- urgent surgical removal and repair

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