Lecture 48 Flashcards
What is ovulation, fertilisation and where does the fertilised egg end up?
Ovulation occurs on day 14 of the 28 day cycle and only survives 24 hours, the oocyte and surrounding cells are swept into the uterine tube. Fertilisation is when a male and female (haploid) gamete fuse to form a diploid cell, this usuually occurs in the ampulla of the uterine tube, this occurs via insemination from sperm deposited in the vagina.. The fertilised egg will implant within the endometrium of the uterus.
What is capacitation?
Sperm undergo capacitation in the female tract, this increases motility, makes the acrosome membrane fragile and lasts about 6-8 hours.
What are the phases of fertilisation of the ovum? How many sperm?
Only one sperm can penetrate the ovum, they race through the female tract towards the ovum and multiple will attempt to penetrate the three outer layers. The three phases are: 1. the penetration of the corona radiata
- the penetration of the zona pellucida and
- penetration of the oocyte cell membrane.
What occurs one the sperm enters the oocyte?
upon the sperm entering the oocyte the zona pellucida becomes impenetrable and meiosis II completes, leading to the ovum being produced (and a polar body forming to remove the extra sister chromatid). The female pronucleus then forms and the spematozoon nucleus moves close to the female pronucleus and the tail breaks down. The pronuclei fuse to form a single diploid zygote (if XX will be female, if XY will be male). Cleavage occurs once the pronuclei fuse (cell division). The fertilised egg will continue to divide and travel to the uterine cavity.
What is cleavage? What structures does it lead to?
Cleavage is mitotic cell division of the fertilised egg, 2 cells to 4 4 cells to 8 etc. This is upto 16 cells by about 3 days (known as the morula, the inner mass becomes the epiblast, the outer mass becomes the trophoblast). The cells are each known as blastomeres now. By days 4-5 it becomes fluid filled and becomes known as the blastocyst.
How does implantation occur and what does it do to prevent losing the embryo?
Implantation occurs after the blastocyst breaks out of the zona pellucida, it then moves to the endometrium on roughly days 6-7 and implantation begins, by day 10 the blastocyst should be completely implanted. For this to happen the outer (trophoblast) layer forms two layers, the inner is the cytotrophoblast and the outer is the synctiotrophoblast. These outer cells will invade the endometrium (invasion, synctiotrophoblast, the inner cell mass will be on the endometrium side, the blastocyst cavity will be on the other side.) which will envelop the blastocyst and the trophoblast then secretes human chorionic gonadotropin (hCG) which helps maintain the corpus luteum (and hence progesterone, therefore no shedding of endometrium) (DON’T MENSTRUATE I HAVE A KID).
What is ectopic pregnancy?
Ectopic pregnancy is implantation outside of the uterine cavity, this is rarely viable and can lead to internal haemorrhages.
What has occured for cells by week 2?
in week two of development there are two layers of trophoblast, two layers of inner cell mass (epiblast and hypoblast) and two cavities (amniotic cavity (surrounds and protects developing embryo, surrounded by epiblast) and yolk sac (primordial germ cells arise near here and is first sire of blood cell formation, surrounded by hypoblast).
What occurs by week 4?
By week 4 the placenta permits diffusion via maternal fetal blood, the embryo grows out from the wall of the uterus and is connected to the placenta bia the umbilical cord, bblood vessels carry oxygenated blood to the fetus (fetal vein) and deoxygenated blood away (fetal artery), the fetal and maternal blood do not mix. It is made up of both maternal tissue and fetal tissue (the chorion, trophoblast components). Exchange occurs across the chorionic villi (contains blood vessels carrying fetal blood and covered in maternal blood).
Once the placenta is assembled (by week 4) the embryo grows and pushes out from the wall.
