Pre-implantation development Flashcards
Where does the embryo of a mammal imbed itself?
In the uterus
What are the advantages of an embryo embedding itself in a uterus?
Gets protection from the mother and also uses the mother as a food source, waste disposal etc.
What is the first diploid cell produces in an organisms life?
The zygote
What does a diploid cell have that is different from the sex cells it is produced from?
It has a full set of chromosome
What after the zygote is formed?
It starts travelling down the uterine tube into the uterus where it then imbeds itself
What does the embryo do as it travels through the uterine tube? What is the end result of this once the zygote reaches the uterus?
Cleavage: Undergoes cell division
Compaction: the cells compact together
A small ball of compact cells imbeds itself into the uterus
What is something important that must NOT occur as the embryo moves through the uterine tube? How is this prevented?
It must not imbed itself into the uterine tube
The embryo is constantly surrounded by a Zona Pelucida and prevents the embryo from being imbedded in the uterine tube
What does the Zona Pelucida do once it is in the uterus?
It then hatches onto the uterus so that the embryo comes into contact with the uterus
During cleavage how does the size, number and total mass of cells of the embryo change?
The embryo gets an increasing number of smaller cells so that the total mass does not change
What happens when the Zona Pelucida doesn’t prevent the embryo from implanting in the uterine tube?
Ectopic pregnancy - the embryo imbeds itself outside the uterus (e.g. uterine tube, intestinal wall…)
At what point does a zygote become an embryo?
Once the zygote begins to divide
How is the early reproductive cycle able to differentiate between species?
Different species undergo cleavage differently
What is the order that cleavage increases the number of cells by?
From 1 cell (zygote) to 2, then 4, then 8 etc.
What are the cells produced by cleavage called?
Blastomeres
Up until the 8 cell stage how are the blastomeres packed?
Loosely
What happens to the arrangement after the 8 cell stage? What is this process called?
The blastomere become tightly packed through a process called compaction
When there are 16 blastomeres what does the embryo form?
A morula
What are the two kinds of morula? What are the differences between them?
Early morula - loosely packed cells still
Late morula - more tightly packed cells
If you were to cut an embryo in half and see that inside it was a fluid filled cavity, what stage of the embryo development is this? What does this cell do now?
Blastocyst, now implants itself into the uterus wall
At what stage does differentiation begin to occur?
The Late morula
What does the differentiation do to the cell? Describe the cell structure at this point
Creates two types of tissue: Trophoblasts (cells on the outside surface of the morula) and Inner cell mass (cells on the inside), this is surrounded by Zone Pelucida
What are the three models of how differentiation occurs in the late morula?
Mosaic, positional and polarity model
What does the mosaic model suggest is the method of differentiation inside the late morula?
Cells differentiate into one or the other tissue randomly and the cells that turn into trophoblasts go to the outside, the inner cell mass migrate into the inside, no organisation on how the cells differentiate only organised once differentiation has occurred
What does the positional model suggest is the method of differentiation inside the late morula?
The position of the cells inside the morula determines whether the cells turn into trophoblasts (only half of the cell is in contact with other cells, other half with zona pelucida) or inner cell mass (all of cell surface is in contact with other cells) – differentiation determined by amount of cell to cell contact
Which model best explains how cells differentiate in the morula? What test can support this model?
The positional model
If you remove the cells on the outside of the morula (the trophoblasts) the the next cells inside immediately convert from inner cell mass into trophoblasts
What makes pre-implantation development important for mammalian embryogenesis?
The trophoblasts are needed for the embryo to implant into the uterus
What transcription factor is expressed in the late morula? What tissue types it activate and inactive? What determines the activation of this transcription factor?
YAP
It active in trophoblasts and inactive in inner cell mass
The activity of this factor is determined by cell to cell contact (more cell to cell contact = inhibition)
What does YAP do to the trophoblasts?
It causes the trophoblasts to begin to develop the characteristics of epithelia cells
What important enzyme is developed by the YAP on the trophoblasts? What does it do?
Na+/K+ ATPase
It actively transports these ions across the epithelial membrane of the trophoblasts into the inner cell mass (transepithelia)
What does the Na+/K+ ATPase do to the embryo?
It increase the concentration of Na+ and K+ which then creates an osmotic gradient attracting water into embryo
this create microlumens (little pockets of water) until they form cavities (by process called cavitation) which are filled with liquid
This pushes the inner cell mass to one part of the embryo forming a single large cavity called a blastocyst cavity and the embryo is called a blastocyst
What does the blastocyst do?
Hatches onto the uterus (Zona pelucida forms break) and trophoblasts being cell to cell communication
What does the trophoblasts form once latched onto the uterus? What kind of tissues do trophoblasts differentiate into?
Gives rise to the support tissue in the embryo (e.g. placenta, umbilical cord)
Differentiate into syncytial trophoblasts and cellular trophoblasts
What do the inner cell mass form once latched onto the uterus?
Forms all embryonic tissue
What is syncytial tissue?
Tissue where the nucleus have undergone division but cytokinesis has not occurred (e.g. multiple nucleus inside a cytoplasm)
Where is syncytial tissue on an embryo? What is it called? Where does it come from?
On top of the blastocyst
It is called syncytial trophoblasts
It comes from the trophoblasts that are on the top of the embryo
What happens to the trophoblasts that don’t turn into syncytial trophoblasts?
They turn into cellular trophoblasts
What does the syncytial trophoblasts do to the uterus?
It creates a hole in the uterus where the embryo can imbed itself
What kind of tissue do the inner cell mass differentiate into?
Hypoblasts and Epiblast
What does this two layered disc of epiblasts and hypoblasts make the embryo? What happens at this stage?
A bilaminar embryo
The embryo implants into the uterus
What tissues does the epiblast form in the foetus?
All embryo proper tissue
What kind of tissues are generally derived from the syncytial trophoblasts, cellular trophoblasts and/or hypoblasts?
Extra-embryonic support tissues (stuff around the baby that is not needed when the baby is born)
What model do the inner cell mass divide by?
Mosaic model: cells differentiate into hypoblasts or epiblasts then like cells agglomerate
What is the name, physical description and function of the cell that the epiblasts create? What structure within the cell does this create?
Creates amnion cells (squamous tissue)
Separates amnion cavity from the trophoblasts
An amniotic cavity
What does the amniotic cavity form as the embryo develops?
Amniotic sac
What are hypoblasts doing all the time?
They are dividing
Where are the hypoblasts growing?
They start to grow and then migrate around the inside wall of the cellular trophoblast
Once the hypoblasts grow around the inside of the cellular trophoblasts how thick is the cell wall of the embryo? What forms inside the embryo?
It is 2 cells thick
Once it is 2 cells thick it forms a yolk sac
What does the cellular and syncytial trophoblasts eventually form?
It eventually will form the placenta
In humans, how nourishing is the yolk sac? What is the function of the yolk sac in the human?
It is no very nourishing as it is relatively small and empty.
It nourishes the embryo in the second and third week of development, provides blood cells and gives rise to germ line that will migrate into the gonads
Describe the structure of the bilaminar (two layered) embryo
-A layer of epiblasts on top of a layer of hypoblasts
-An amniotic cavity above the epiblasts and a yolk sac underneath the hypoblasts
-This is all surrounded by a layer of cellular trophoblasts
-The cellular trophoblasts with the amniotic cavity is lined with amnion cells
the cellular trophoblasts with the yolk sac is lined with hypoblast
-The top top of the embryo is covered with syncytial trophoblasts
