Embryology 1 Flashcards
What are the 3 developmental stages of life before birth? What are their relevant timings?
Pre implantation stage - week 1 after fertilisation
Embryonic stage/ organogenesis - weeks 2-8
Fetal stage (growth and development) - weeks 9-38
List the events occuring in week 1 of development
Cleavage, morula formation, blastocyst formation, primary differentiation and hatching
What is cleavage? How and why does the overall size remain constant?
The process of the mitotic cell division of the fertilized oocyte as it travels from ovary towards the uterus
Stays same size as surrounded by tough glycoprotein coat (zona pellucida) allowing it to passage down the narrowest part of the uterine tube (isthmus)
What occurs during Morula formation? Where and when is this taking place?
Cells maximise contact with each other, forming a cluster of cells held together by tight junctions. Still all undifferentiated
Occurs on day 5 as it enters the uterus
What is the structure of a blastocyst? Why is this significant? When and where does this occur?
Hollow ball of cells: trophoblasts forms one cell thick ring outside, inner cell mass squished to one side and a fluid filled blastocyst cavity
First signs of celllular differentiation
This occurs on day 5 as embryo enters uterine cavity
How and why does the process of hatching occur?
Inner cell mass proliferates and fluid builds up in the cavity, increasing the pressure. Blastocyst eventually hatches/ bursts from zona pellucida to facilitate implantation
Implantation occurs as blastocyst begins to run out of nutrients.
Breifly describe the process of implantation:
- Blastocyst makes contact with endometrium tissue and decidualisation (changes in endometrium to prepare it for pregnancy, produces molecules and promots trophoblast cells to become invasive) occurs
- Implanting trophoblast begins to differentiate (inner layer become single layered cytotrophoblast)
- Inner cell mass forms 2 layers - the epiblast and hypoblast (together = bilaminar disc)
- Implanting sysncytiotropholast cells communicate with maternal side of placenta establishing connections to enable diffusion of oxygen, waste and nutrients via blood supply
- Syncytiotrophblast begin to produce the hormone human chorionic ganadotrophins (hCG)
What are the 4 Extra Embryonic Membranes?
Amnion, Chronion, Yolk Sac, Allantois
What is the amnion?
Continuous with epiblast of bilaminar disc
Lines the amniotic cavity which is filled with fluid to protect developing embryo.
Present until birth
What is the Chorion?
Double layered membrane formed by tophoblast and extraembryonic membrane
Forms fetal component of placenta
Lines the choronic cavity - present in in early pregancy but disappears due to expansion of amniotic cavity
What is the Yolk Sac?
Continuous with hypoblast
Important in nutrient transfer in weeks 2-3 but disappears completely by week 20
Important in blood cell formation and formation of gut
What is the Allantois?
Outgrowth of the yolk sac
Contributes to umbilical arteries and connects to fetal bladder
Define Gastrulation
The process of cell division and migration resulting in the formation of 3 germ layers forming a trilaminar embryo
What is the process of gastrulation?
- Epiblast cells rapidly proliferate and move towards the primative streak
- Cells pushed downwards and start to make new layers - invagination (controlled by fibroblast growth factor 8)
- Epiblast forms ectoderm,
- Middle cells form mesoderm
- Cells pushed to the bottom, causing hypoblast cells to be pushed aside, forming endoderm
What are the three germ layers?
Ectoderm, mesoderm, endoderm
