life before birth Flashcards
3 main stages
Week 1 - Preimplantation stage
Weeks 2-8 – Embryonic stage (Organogenesis)
Weeks 9-38 – Fetal stage (Growth and development)
cleavage
Mitotic divisions of the fertilised oocyte (egg)
Overall size remains the same
Allows passage down the narrowest part of the uterine tube – ISTHMUS
Surrounded by tough glycoprotein coat – ZONA PELLUCIDA to prevent premature implantation
morula formation
Around day 4 after fertilisation, cells maximise contact with each other, forming a cluster of cells held together by tight junctions
- enters the uterus
blastocyst formation
First signs of cellular differentiation
Inner cell mass – goes on to form the embryo and extraembryonic tissues
Outer cells trophoblasts – contributes to the placenta
As the embryo enters the uterine cavity, fluid enters via the zona pellucida into the spaces of the inner cell mass
A fluid filled blastocyst cavity forms
inner cell mass
forms embryo
tropoblasts
(trophectoderm)
- forms placenta structures
blastocyst cavity
fluid filled
primary differentiation and hatching
First signs of cellular differentiation – formation of the inner cell mass and trophoblast cells
Blastocyst starts to run out of nutrients, needs to implant
ICM cells undergo proliferation and the fluid builds up in the cavity, eventually resulting in the blastocyst “hatching “ from the zona pellucida to facilitate implantation
week 2- implantation
The embryo requires nutrients in order to survive, and must establish a connection with the mother which will become the placenta.
Implantation is this interaction between the embryo and the endometrial layer of the uterus.
decidualisation
When the blastocyst makes contact with the endometrium of the uterus, a process called decidualisation occurs in the stromal cells of the uterus.
It is these cells in the uterus that will form the maternal component of the placenta.
Decidualisation is a process that results in several changes in the endometrium to prepare it for pregnancy.
The process triggers the production of several molecules and promotes the trophoblast cells to become invasive.
cytotrophoblasts
The implanting trophoblast cells begins to differentiate. The cells closest to the inside of the embryo become a single layer of cells called the cytotrophoblast.
syncytiotroblast
The outer layer is much more extensive and is the invasive layer. It is called the syncytiotrophoblast
. As its name suggests, it is a syncytium of cells.
epiblast and hypoblast
The inner cell mass forms 2 layers called the epiblast and the hypoblast. Together these are known as the bilaminar disc.
end of week 2- implanting sysncytiotrophoblast cells
Towards the end of week 2, the implanting sysncytiotrophoblast cells start to communicate with the maternal side of the placenta and begin to establish a connection to enable diffusion of oxygen, waste and nutrients via the blood supply.
human chorionic gonadotrophin
By the end of week 2 of development, the syncytiotrophoblast begins to produce a hormone called human chorionic gonadotrophin (hCG).
This hormone is secreted into the urine and its presence is an early indicator of pregnancy. It is what you detect in a pregnancy test kit.
