Early embryonic development Flashcards
What are the stages fro the oocyte being released to the blastocyst?
- released oocyte
- oocyte in tube
- oocyte penetrated by sperm
- zygote
- two-cell stage
- four-cell stage
- eight-cell stage
- morula blastocysts
Describe how ovulation occurs
- the oocyte is extruded onto the surface of the ovary
- the oocyte is actively scripted off the surface of the ovary by frimbriae of the mouth of the oviduct
- Further transport is mediated by peristaltic contractions of smooth muscle of the tubal wall assisted by cilia that line the uterine tube
What are the three stages of sperm penetration
- penetration of the corona radiata
- penetration of the zona pellucida
- fusion of the zygote and oocyte
How is polyspermy blocked?
- occurs in two phases
- fast electrical block which causes a change in the membrane potential
- slow black which is when a calcium wave causes cortical granules to fuse with the plasma membrane. Release of lysosomal enzymes alters the structure of the zona pellucid to inhibit sperm binding and penetration
What are the small cells that the zygote is divided into?
blastomeres
What are the cell divisions not accompanied by?
growth
What is the zygote surrounded by?
zona pellucida
What is the morula?
A ball of 16-32 cells formed on the 4th day
When does compaction begin?
in the 8 cell stage
How is a distinction formed between inner and peripheral cells?
When compaction and the formation of tight junctions occurs
What is the role of tight junctions?
to regulate the passage of fluid in between cells
How is the blastocyst formed?
- uptake of fluid increases the hydrostatic pressure in the zygote causing production of a fluid filled chamber called the blastocoele
- This leads to two distinct cell populations- inner cell mass (embryo blast) and the exterior trophoblast
- The zygote enzymatically bores out of the zona pellucida in a process called hatching
What are the three layers of the uterine wall?
- external perimetrium
- medial myometrium
- internal endometrium
How many days after fertilisation does the embryo implant in the endometrium?
5.5-6 days
What are the three stages that the endometrium go through?
- follicular/proliferation phase
- progestational/ secretory phase
- menstrual phase (no fertilisation) or gravid phase (fertilisation)
How does the first implantation occur?
First stage= attachment to the endometrial epithelium
- the apical surface of the endometrial endothelial cells are hormonally conditioned to express cell adhesion molecules such as integrins
- trophoblast cells of the pre-implantation blastocyte also express adhesion molecules
- attachment of the blastocyst occurs above the inner cell mass
What is the second stage of implantation?
penetration of the endometrium
Describe what happens in the second stage of implantation.
- trophoblast differentiates into an inner cell layer, cytotrophoblast and an outer multinucleate layer syncytiotrophoblast
- the synctiotrophoblast forms from fusion of many cells of the cytotropoblast, cells lose their individual cells membrane and form a large mutinucleated structure
- small projections of synctiotrophoblastt insert between the uterine cells
Describe what happens in the third stage of implantation
- the synctiotrophoblast first forms overlying the inner cell mass but eventually surrounds the whole embryo
- the synciotrophoblast is highly invasive and invades the endometrial stroma
- As the synciotrophoblast invades the endometrium the embryo starts to bed down into the uterine lining
How long does it take after fertilisation for the embryo to embed into the endometrium
12 days
What is the initial site of penetration marked by?
a bare area or a non-cellular plug (later sealed by migrating uterine endothelial cells)
In the formation of the placenta where do vacuoles start to appear?
synciotrophoblast
When the vacuoles fuse, what do they form?
large lucunae
What is the next stage after lacuna forming? At 12 days
- trophoblastic lucunae form an interconnecting network
- cells of the syncyntiotrophoblast penetrate deeper into the endometrial stroma and erode the endothelial lining of the maternal capillaries which are congested and dilated (sinusoids)
What happens 13 days after fertilisation?
- the syncytial lacunae fuse with the sinusoids and maternal blood enters the lacunar system
- cells of the cytotrophoblast proliferate and penetrate into the surrounding synctiotrophoblast forming cellular columns surrounded by syncytium- primary villi
Eventually, what will the primary villi contain?
capillaires tant link the embryonic circulatory system via the connecting stalk
What happens after 21 days?
tertiary villi are formed
What do tertiary villi contain?
a capillary network that links to the embryonic circulatory system via the umbilical artery and vein
What are tertiary villi bathed in?
maternal blood supplied by the spiral artery
What secretes hCG?
the synctiotrophoblast
What is the role of human chorionic gonadotrophin?
to maintain pregnancy by maintaining the production of progesterone
What types of pregnancy can you have, caused by abnormal implantation?
extrauterine or ectopic
What are the results of ectopic or extrauterine preganies?
Embryo usually dies around second month of gestation causing severe haemorrhaging and abdominal pain
Where do ectopic pregnancies usually occur?
ampulla
