Early Embryo Development Flashcards
When is fertilisation age measured from?
From the time of fertilisation, assumed to be more than one day from the last ovulation
What other name is fertilisation date known as?
Conceptualisation date
How practical is fertilisation date?
Not very, its also difficult to know the time exactly so its not used very commonly
When is gestational age measured from?
From the time of the beginning of the last period
How is gestational age determined?
Fertilisation date + 14 days if known
Early obstetric ultrasound
How practical is gestational date?
More practical, it is used often clinically
What is carnegie age?
23 stages of embryo development based on embryo features not time
What window does carnegie age cover?
0-60 days fertilisation age (in humans)
What are the 3 major stages of time in embryo foetal development?
Embryogenic stage
Embryonic stage
Foetal stage
How many days post fertilisation is the embryogenic stage?
14-16 days
How many days post fertilisation is the embryonic stage?
16- 50 days
How many days post fertilisation is the foetal stage?
50-270 days or 8-38 weeks
What happens in the embryogenic stage?
Establishment of the early embryo from the fertilised oocyte
What are the 2 populations of cells in the embryogenic stage and what do they do?
Pluripotent embryonic cells (contribute to fetus) Extraembryonic cells (contribute to the support structures eg placenta)
What do pluripotent embryonic cells contribute to?
The foetus
What so extraembryonic cells contribute to?
Support structures eg placenta
What happens in the embryonic stage?
Establishment of the germ layers and differentiation of tissue types
Establishment of the body plan
What is present when the foetal stage is reached?
Major organ systems
What stages does the first trimester include?
Embryogenic and embryonic
What stages does the second trimester include?
Foetal stage
When does the transition from embryo to foetus occur?
At the end of the first trimester
How many cells is an ovulate oocyte?
1
How many cells is a zygote?
1
How many cells is a cleavage stage embryo
2-8
How many cells is a morula?
16
How many cells is a blastocyst?
200-300
When is the zona pellucida present?
From ovulated oocyte until blastocyst
Does the embryo transcribe its own genes?
No, it is dependant on materal mRNAs and proteins to get through the first few divisions until it has 4/8 cells. Failure in the mother to synthesise and store these mRNAs during oogenesis can impair development of the embryo
What happens at the maternal to zygotic transition?
Transcription of embryonic genes begins due to zygotic genome activation
Increased protein synthesis
Organelles mature eg mitochondria, Golgi
At what cell stage does compaction occur?
When the zygote is around 8 cells
Describe the process of compaction
Outer cells become pressed against zona pellucida
Change from spherical to wedge-shaped
Outer cells connect to each other through tight gap junctions and desmosomes
Forms barrier to diffusion between inner and outer embryo
Outer cells become polarized
What are the cell types in the blastocyst? Describe them
Inner cell mass are pluripotent embryonic cells that will contribute to the final organism
Trophoectoderm are extra-embryonic cells that contribute to the extraembryonic structures that support development
What is the zona pellucida?
Hard protein shell inhibiting polyspermy and protects early embryo
What is the blastoceal?
Fluid-filled cavity formed osmotically by trophoblast pumping Na+ ions into cavity and then water following osmotically
What is hatching?
Degredation and weakening of the zona pellucida to allow the blastocyst to escape it
Why is hatching required?
To allow the blastocyst to implant into the uterus
How is hatching achieved?
Enzymatic digestion of the zona pellucida and cellular contractions to allow the blastocyst to escape the seal
What does the morula split into? Describe what each component contributes to
Inner cell mass which contributes to embryonic cells
Trophectoderm which contributes to extra embryonic cells
What is formed when trophoblast cells fuse
Syncitiotrophoblast
What ability does the syncitiotrophoblast have?
It can invade and destroy local maternal cells in the endometrium, the destruction of capillaries allows the embryo to be bathed in blood which is an important interface