Obstetrics and Gynaecology
Contributing Authors:
Linus Kutup, Eric Hoffmeister, Benjamin Kersch, Niclas Samirae, Tobias Verdegem, Alexander Wolff, Lara Afaneh, Joana Strzlkowski, Nadine Fernandez, Katharina Weitzel
1. Fertilization, Implantation, and Development of the fertilized egg.
Fertilization describes the fusion of a haploid spermatozoa with a haploid oocyte-> a diploid cell result (ZYGOTE). This zygote starts dividing by mitosis (spermatozoa and oocyte were ‘generated’ by meiosis!!)
– Fertilization usually takes place in the uterine tube -> After oocyte has been released from ovary (ovulation), it travels along the uterine tube (facilitated by cilia on the uterine tube wall). The spermatozoa must travel all the way from the vagina, through the cervix and through the uterus until the reach the uterine tube.
– Cervical mucus ‘catches’ weak spermatozoa and nourishes strong spermatozoa -> helps the strong spermatozoa to survive up to 7 days.
– Uterus helps the spermatozoa by contracting from bottom to top
– 500-800 spermatozoa (of initial 3-5 million) reach the uterine tubes and on their journey, they undergo further maturation (called capazitation)
– approx. 200 spermatozoa manage to come close to the oocyte in the uterine tube
– Important facts for further understanding:
a) The oocyte possesses 2 surrounding layers:
– inner layer: Zona pellucida
– outer layer: Corona radiate
b) The oocyte constantly releases Fertilisin
c) The head of a spermatozoa is surrounded by a layer called acrosome
– As several spermatozoa come very close to the oocyte, the fertilisin degrades the acrosomes -> now, the heads of the spermatozoa release enzymes (Hyaluronidase, Neuroamidase, Acrosine) -> These enzymes help the spermatozoa to penetrate the corona radiate of oocyte -> then, acrosine helps the fastest spermatozoa to penetrate the inner layer (zona pellucida) as well -> this fastest spermatozoa enters the cytosol and immediately the zona pellucida undergoes structural changes so that Acrosine can no longer destroy it and thus no other spermatozoa can enter!!!
-> Fertilization takes place approx. 12 hours after ovulation
-> The end result fertilization is the diploid ZYGOTE
Implantation:
a) Preimplantation:
– The zygote undergoes mitotic divisions as it keeps traveling towards the uterine cavity
– approx. after 3 days the lumen of the uterus is reached, and zygote consists of 32 cells -> is called MORULA now
– The cells of the morula keep dividing and the result is the BLASTOCYST
– The blastocyst travels along the uterine endometrium (decidua) and develops specific cell layers (day 5-6):
– outermost cell layer: Trophoblasts
– fluid filled Blastocyst cavity
– innermost -> Embryoblast, which consists of Endoderm and Ectoderm
b) Nidation = Implantation:
– takes about 5 days
– occurs after 7 days!
– For the past 7 days, the fertilized egg has released mediators, that stimulated the endometrium to undergo decidual changes (-> increased vascularization, round cell shape, etc) -> preparation for implantation and further growth
– At the beginning (before placenta develops) the blastocyst is nourished by nutrients in the ECM of the decidua (histotrophic phase)
– the Blastocyst usually implants into the UPPER AND POSTERIOR PART OF UTERUS-> the zona pellucida dissolves and the blastocyst sticks to endometrium (decidua) -> this is called apposition and the pole of the embryo always points toward the endometrium
– The trophoblast (outermost layer of blastocyst) develops into 2 distinct cell layers:
– outer: syncytiotrophoblasts
– inner: Cytotrophoblasts
– The trophoblast with its 2 cell layers grows and grows and ‘digs’ into the endometrium until the complete blastocyst is inside the endometrium and surrounded by it:
– decidua capsularis -> part of endometrium that surrounds most of the blastocyst
– decidua basilaris -> surrounds only the lower part of blastocyst
-> The implantation can cause bleeding which can be mistaken for menstruation!!!!
Further development:
– Inside the trophoblast (outer layer of blastocyst) cavities form -> Lacunae
– Enzymes released from the trophoblasts injure and damage maternal vessels in the endometrium -> maternal vessels rupture -> mother’s blood runs into the Lacunae -> placenta starts to form!!!!
– Now that the blastocyst is in contact with mother’s blood, hCG from syncytiotrophoblast can be found in mother’s blood and urine -> positive pregnancy test (approximately 10-12 days after fertilization)
– Now 3 cavities develop inside the blastocyst:
– The blastocyst cavity (from earlier) develops into yolk sac (Dottersack) -> Below embryoblast
– Above, the embryoblast develops into the amniotic cavity
– chorionic cavity
-> The trophoblast is now called CHORION
– Now, the 2 layers of Embryoblast develop into 3 layers:
– Ectoderm: gives rise to skin and nervous system, …
– Mesoderm: gives rise to bones, muscles, vessels….
– Ectoderm: gives rise to internal organs
???? THAT’S IT
Why does the mother’s immune system not attack the growing embryo?
-high immunotolerance of uterus!
– Trophoblasts (outermost cells of embryo) have low antigenicity
– increased synthesis of glucocorticoids decreases strength of maternal immune system
Important numbers and terms:
a) Post- conceptionem (p.c.) -> after fertilization
b) post – menstruationem (p.m.) -> every day after the last day of previous menstruation -> usually a pregnancy is calculated from this point, even though fertilization took place actually 14 days later (ovulation occurs on day 14 of menstruation, and fertilization occurs shortly after). So, a woman has her last day of menstruation of previous cycle, then 14 days later ovulation and fertilization take place). And then, another 14 days later the woman realises that her menstruation is not starting because of pregnancy!)
Birth takes place approximately 268 days after fertilization. But since we calculate the duration of pregnancy from the last day of the last menstruation (which was approximately 14 days before menstruation), we say that a pregnancy takes 280 days (282 is more exact but seems like 280 is better to remember)
280 days are 40 weeks which are 10 month -> Yes, boys and girls, pregnancy takes 10 months!!!!!!
If you want to calculate, when a woman is supposed to deliver her baby, you take the last day of her last menstrual bleeding and add 280 days to it!!!!!!
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