W4L1 Tues Human development

Question 1

Fertilisation

Answer 1

The OOCYTE (i.e. egg cell) has gone through the first meiotic division (creating the 1st polar body) and is arrested at metaphase of the second meiotic division.
* A spermatazoa (sperm cell) contacts the ZONA PELLUCIDA, a thick glycoprotein layer around the oocyte.
-The sperm cell secretes proteolytic enzymes that break down the zona-pellucida allowing the sperm cell to reach the oocyte plasma membrane

Question 2

The formation of zygote

Answer 2

-The sperm and oocyte plasma membranes fuse delivering the male PRONUCLEUS into the oocyte.
-This causes a rapid release of enzymes from the egg which modify the surface so other sperm can’t enter.
-The oocyte then undergoes the second meiotic division (creating the 2nd polar body - and the female pronucleus)
-Finally, fusion of the male and female pronuclei produces the first diploid cell. This one-celled embryo is called the ZYGOTE

Question 3

Cleavage forms the morula

Answer 3

-The embryo then undergoes a series of cell divisions that subdivide the cytoplasm into smaller and smaller cells (i.e. there is no G1 or G2 cycles).
-This process is called CLEAVAGE.
-At the 8 cell stage the cells of the embryo are loosely packed and resemble a mulberry. The embryo at this stage is therefore given the name MORULA *

Question 4

Key features of epithelial cells

Answer 4

-Tight (occluding) junctions provide a barrier that prevent water and solutes from passing from one side of an epithelial layer to the other
-Adherens junctions provide strong adhesion that connects to the actin cytoskeleton and gives the epithelium mechanical strength

Question 5

morula compaction & polarisation

Answer 5

-During COMPACTION the cells start to maximise their contact with each other via the cell adhesion molecule ECadherin.
-Cells also become POLARISED along the apical- basal (i.e. outside-inside) axis. For example, microvilli become restricted to the apical surface, and key polarity proteins such as aPKC, PAR6 and Ezrin become apically localised.
-Thus the first epithelium is formed

Question 6

Creating inner and outer cells

Answer 6

-Within the morula, cell divisions can take place symmetrically along a radial axis producing two more polarised cells, or asymmetrically along a tangential axis producing a polarised outer cell, and a new unpolarised inner cell.
-These inner unpolarised cells will become the INNER CELL MASS

Question 7

Formation of the blastocyst

Answer 7

As the outer cells continue to differentiate they develop TIGHT JUNCTIONS. These, combined with a Na+ pump, serve to draw water into the morula which creates an inner fluid filled cavity.
The embryo at this stage is called the BLASTOCYST.

Question 8

Anatomy of the blastocyst

Answer 8

At this stage the cells of the blastocyst are of two types:
* the TROPHECTODERM which will give rise to the extraembryonic tissues such as the placenta.
* the INNER CELL MASS which will give rise to the embryo

Question 9

Humans hatch

Answer 9

-As the blastocyst fills with liquid and grows, it exerts pressure on the zona pellucida, which is also starting to break down and weaken. Finally it breaks through and “hatches”.
-By hatching out of the zona-pellucida the blastocyst can now make cellular contact with the epithelium of the uterine wall

Question 10

The first major cell lineage split

Answer 10

-In addition to outer cells becoming polarised they also start to express Cdx2, while the inner cells express Oct4. This represents the first example of DIFFERENTIATION
-The initial expression appears to be driven by the cell’s position. But the difference is then stabilised by the mutual repression of Cdx2 and Oct4 expression
-This sort of mutual repression is a common theme when cells go down one of two mutually exclusive lineages

Question 11

cell fate decision – epiblast or hypoblast

Answer 11

The cells of the Inner Cell Mass must now make a second choice.
-Some take on a HYPOBLAST (or primitive endoderm) cell fate and organise themselves into an epithelium that faces the blastocyst cavity.
-The others take on an EPIBLAST cell fate

Question 12

epiblast and hypoblast genetic differences

Answer 12

-there are genetic mechanisms that ensure cells adopt one of the two cell fates. ——-Cells expressing NANOG activate expression of FGF4. NANOG also represses GATA6 and FGFR2.
-The FGF4 activates the FGFR2 receptor in surrounding cells which in turn activates expression of GATA6. GATA6, in turn, represses NANOG
-Gene expression stabilises with some cells exclusively expressing NANOG (and becoming EPIBLAST cells) and some exclusively expressing GATA6 (and becoming hypoblast cells)

Question 13

Epiblast cells can be cultured as ES cells

Answer 13

the epiblast cells can be isolated and cultured indefinitely in vitro as: EMBRYONIC STEM CELLS (ES cells).
-These pluripotent cells express Nanog and Oct4, Klf4 and Sox2, and can remain in a stem cell state without differentiating.
These are the cells that are used to create mouse knockouts and can be used to derive many different types of cells

Question 14

Formation of the amniotic cavity

Answer 14

The amniotic cavity forms within the epiblast cells while the hypoblast cells extend around the inside of the trophoblast to form the primitive yolk sac

Question 15

implantation

Answer 15

-After hatching, the blastocyst implants into the wall of the uterus.

Question 16

gastrulation and the primitive streak

Answer 16

-Gastrulation is the first major embryonic event of MORPHOGENESIS, which defines the body axes:
-anterior-posterior and left-right. It begins with the formation of the PRIMITIVE STREAK, a region in which epiblast cells undergo an EMT and enter into the space between epiblast and hypoblast layers

Question 17

Model for the formation of the endoderm

Answer 17

A longstanding model for the formation of the endoderm was that the original hypoblast cells (green) were displaced by cells coming through the primitive streak (yellow). These would intercalate into the epithelium, pushing the hypoblast cells laterally and end up forming the endoderm epithelium

Question 18

Formation of the mesoderm

Answer 18

Subsequently epiblast cells coming through the primitive streak take on a mesodermal fate.
Thus, gastrulation gives rise to the three primary germ layers.

Question 19

The three primary germ layer

Answer 19

• ECTODERM (formed from epiblast cells that don’t go through the primitive streak) gives rise to skin and nervous system.
• ENDODERM gives rise to the gut and organs like the lung.
• MESODERM gives rise to muscle, heart and connective tissue

Question 20

Mouse embryo versus human embryo

Answer 20

-The mouse provides a valuable model system for early human development.
-The mouse embryo can be visualised as a human embryo which has been pushed out into a cup shape, leaving the epiblast/ectoderm on the inside, the hypoblast/endoderm on the outside, and mesoderm between

Question 21

origin of the endoderm

Answer 21

the endoderm is actually a mix of the original hypoblast cells and the epiblast cells that have come through the primitive streak

Question 22

Gastrulation

Answer 22

Cells move, en-masse, towards the primitive streak.
-There, they undergo an EPITHELIAL MESENCHYMAL TRANSITION allowing them to break free of the epiblast epithelium, move into the space between the epiblast and hypoblast, and migrate away