Pre-implantation development

Question 1

Where does the embryo of a mammal imbed itself?

Answer 1

In the uterus

Question 2

What are the advantages of an embryo embedding itself in a uterus?

Answer 2

Gets protection from the mother and also uses the mother as a food source, waste disposal etc.

Question 3

What is the first diploid cell produces in an organisms life?

Answer 3

The zygote

Question 4

What does a diploid cell have that is different from the sex cells it is produced from?

Answer 4

It has a full set of chromosome

Question 5

What after the zygote is formed?

Answer 5

It starts travelling down the uterine tube into the uterus where it then imbeds itself

Question 6

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?

Answer 6

Cleavage: Undergoes cell division
Compaction: the cells compact together
A small ball of compact cells imbeds itself into the uterus

Question 7

What is something important that must NOT occur as the embryo moves through the uterine tube? How is this prevented?

Answer 7

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

Question 8

What does the Zona Pelucida do once it is in the uterus?

Answer 8

It then hatches onto the uterus so that the embryo comes into contact with the uterus

Question 9

During cleavage how does the size, number and total mass of cells of the embryo change?

Answer 9

The embryo gets an increasing number of smaller cells so that the total mass does not change

Question 10

What happens when the Zona Pelucida doesn’t prevent the embryo from implanting in the uterine tube?

Answer 10

Ectopic pregnancy - the embryo imbeds itself outside the uterus (e.g. uterine tube, intestinal wall…)

Question 11

At what point does a zygote become an embryo?

Answer 11

Once the zygote begins to divide

Question 12

How is the early reproductive cycle able to differentiate between species?

Answer 12

Different species undergo cleavage differently

Question 13

What is the order that cleavage increases the number of cells by?

Answer 13

From 1 cell (zygote) to 2, then 4, then 8 etc.

Question 14

What are the cells produced by cleavage called?

Answer 14

Blastomeres

Question 15

Up until the 8 cell stage how are the blastomeres packed?

Answer 15

Loosely

Question 16

What happens to the arrangement after the 8 cell stage? What is this process called?

Answer 16

The blastomere become tightly packed through a process called compaction

Question 17

When there are 16 blastomeres what does the embryo form?

Answer 17

A morula

Question 18

What are the two kinds of morula? What are the differences between them?

Answer 18

Early morula - loosely packed cells still

Late morula - more tightly packed cells

Question 19

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?

Answer 19

Blastocyst, now implants itself into the uterus wall

Question 20

At what stage does differentiation begin to occur?

Answer 20

The Late morula

Question 21

What does the differentiation do to the cell? Describe the cell structure at this point

Answer 21

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

Question 22

What are the three models of how differentiation occurs in the late morula?

Answer 22

Mosaic, positional and polarity model

Question 23

What does the mosaic model suggest is the method of differentiation inside the late morula?

Answer 23

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

Question 24

What does the positional model suggest is the method of differentiation inside the late morula?

Answer 24

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

Question 25

Which model best explains how cells differentiate in the morula? What test can support this model?

Answer 25

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

Question 26

What makes pre-implantation development important for mammalian embryogenesis?

Answer 26

The trophoblasts are needed for the embryo to implant into the uterus

Question 27

What transcription factor is expressed in the late morula? What tissue types it activate and inactive? What determines the activation of this transcription factor?

Answer 27

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)

Question 28

What does YAP do to the trophoblasts?

Answer 28

It causes the trophoblasts to begin to develop the characteristics of epithelia cells

Question 29

What important enzyme is developed by the YAP on the trophoblasts? What does it do?

Answer 29

Na+/K+ ATPase
It actively transports these ions across the epithelial membrane of the trophoblasts into the inner cell mass (transepithelia)

Question 30

What does the Na+/K+ ATPase do to the embryo?

Answer 30

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

Question 31

What does the blastocyst do?

Answer 31

Hatches onto the uterus (Zona pelucida forms break) and trophoblasts being cell to cell communication

Question 32

What does the trophoblasts form once latched onto the uterus? What kind of tissues do trophoblasts differentiate into?

Answer 32

Gives rise to the support tissue in the embryo (e.g. placenta, umbilical cord)
Differentiate into syncytial trophoblasts and cellular trophoblasts

Question 33

What do the inner cell mass form once latched onto the uterus?

Answer 33

Forms all embryonic tissue

Question 34

What is syncytial tissue?

Answer 34

Tissue where the nucleus have undergone division but cytokinesis has not occurred (e.g. multiple nucleus inside a cytoplasm)

Question 35

Where is syncytial tissue on an embryo? What is it called? Where does it come from?

Answer 35

On top of the blastocyst
It is called syncytial trophoblasts
It comes from the trophoblasts that are on the top of the embryo

Question 36

What happens to the trophoblasts that don’t turn into syncytial trophoblasts?

Answer 36

They turn into cellular trophoblasts

Question 37

What does the syncytial trophoblasts do to the uterus?

Answer 37

It creates a hole in the uterus where the embryo can imbed itself

Question 38

What kind of tissue do the inner cell mass differentiate into?

Answer 38

Hypoblasts and Epiblast

Question 39

What does this two layered disc of epiblasts and hypoblasts make the embryo? What happens at this stage?

Answer 39

A bilaminar embryo

The embryo implants into the uterus

Question 40

What tissues does the epiblast form in the foetus?

Answer 40

All embryo proper tissue

Question 41

What kind of tissues are generally derived from the syncytial trophoblasts, cellular trophoblasts and/or hypoblasts?

Answer 41

Extra-embryonic support tissues (stuff around the baby that is not needed when the baby is born)

Question 42

What model do the inner cell mass divide by?

Answer 42

Mosaic model: cells differentiate into hypoblasts or epiblasts then like cells agglomerate

Question 43

What is the name, physical description and function of the cell that the epiblasts create? What structure within the cell does this create?

Answer 43

Creates amnion cells (squamous tissue)
Separates amnion cavity from the trophoblasts
An amniotic cavity

Question 44

What does the amniotic cavity form as the embryo develops?

Answer 44

Amniotic sac

Question 45

What are hypoblasts doing all the time?

Answer 45

They are dividing

Question 46

Where are the hypoblasts growing?

Answer 46

They start to grow and then migrate around the inside wall of the cellular trophoblast

Question 47

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?

Answer 47

It is 2 cells thick

Once it is 2 cells thick it forms a yolk sac

Question 48

What does the cellular and syncytial trophoblasts eventually form?

Answer 48

It eventually will form the placenta

Question 49

In humans, how nourishing is the yolk sac? What is the function of the yolk sac in the human?

Answer 49

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

Question 50

Describe the structure of the bilaminar (two layered) embryo

Answer 50

-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