Lecture 13

Question 1

What are the major developmental events that occur during the first few weeks of human development:

Answer 1

Fertilisation
Cleavage of the zygote
Formation of morula and blastocyst
Blastocyst implantation gastrulation embryonic folding (a very basic understanding only)

Question 2

What are some of the developmental events in weeks 2 - 4

Answer 2

Trophoblast development and embryonic disk
Gastrulation
Structures derived from the three primary germ layers

Question 3

What is embryogensis

Answer 3

production of an embryo from a zygote (first 8 weeks of development), applicable to all species

Question 4

What are the purposes of embryogensis

Answer 4

Patterning – cells acquire identity in space and time
The major axis are defined – anterior(head)/posterior(legs) and dorsal(back)/ventral(front)
Three germ layers are defined
Rudiments of the major organs

Question 5

Describe the process of fertilisation

Answer 5

Sperm and oocyte come together and their pronuclei fuse
Importantly, gametes formed via meiosis therefore haploid
When the sperm reaches the egg, Meiosis II hasn’t been completed, it only occurs at the point of fertilisation. Importantly, the egg will divide via unequal cell division - 3 smaller cells and one large egg form. The sperm will fuse with the large egg - as this means that the cell will have what it needs in the cytoplasm for development.

Question 6

What does the sperm have to pass through

Answer 6

• Somatic/ovarian cells called the corona radiata cells (produced when the egg was released from the ovaries) which surround the egg
• Then it must pass through the zona pellucida (fibrous matt surrounding egg which the sperm releases enzymes for in order to be able to burrow/pass through)
• Then must fuse with the plasma membrane of the secondary oocyte
• Finally through cytoplasm to fuse with the oocyte nucleus.

Question 7

What are the comparative sizes of the gametes

Answer 7

Sperm is much smaller than the egg. Sperm just contains a head for the DNA and sac of enzymes that allow for it to pass through the egg, and a flagella tail for swimming to the egg. In most organisms, the egg contains what is required for early development to occur. In humans the egg is about 1/10th of a millimetre - about the width of a human hair

Question 8

What does the egg prevent

Answer 8

Typically only 200 sperm reach the oocyte and mechanisms are there to ensure only one sperm is used - there are mechanisms that control polyspermy

Question 9

Describe cleavage

Answer 9

Once fertilisation occurs, the zygote - now called a embryo - starts to divide. The
Cells divide but don’t grow, therefore they get smaller. We need the smaller cells to produce more intricate/complex shapes for the embryo
No increase in cell volume or mass of embryo during first couple of days
Polar bodies exist at start but likely don’t divide, cells that are produced via the cleavage of egg are called blastomeres
The first cleavage (events) is completed about 30 hours after fertilisation

Question 10

Describe the formation of the morula

Answer 10

During the first 4 days, the cells keep dividing but are covered in the zona pellucida, and by day 4 the morula is formed - loosely packed cells covered in the zona pellucida. Cells still all have equal differentiation potential

Importantly, all of this occurs within the female reproductive tract and the eggs is moving through slowly

Question 11

Formation of the blastocyst

Answer 11

On day 5 the cells become more tightly packed together - adhesion increases - and the zona pellucida disappears. The first epithelial layer is produced and water/fluid flows into the hollow cavity - called the blastocyst cavity. This structure is called the blastocyst - a hollow ball of cells. The blastocyst cavity is important to allow for space for cells to rearrange
Day 5 is also when cellular differentiation starts to take place and there are tow different cells types with diff differentiation potentials:
- Trophoblast cells are on the outside of the
blastocyst which go on to produce extraembryonic
tissues
- Inner cell mass/Embryoblast cells are on the inside
to form embryo itself
Blastula = early form of blastocyst

Question 12

Explain how the importance of the eggs being released

Answer 12

The egg is released once a month from ovaries due to enzymes into the fallopian tube, wafted along by cilia and fluid movement, sperm enter via uterus and fertilisation occurs within the fallopian tube (about 12-24 hours after ovulation). If two eggs are released from the ovaries non-identical twins are formed, if a single egg splits divides into two, identical twins are formed

Question 13

Describe implantation

Answer 13

At about day 6, the blastocyst that is produced implants on the uterus wall, this is needed for gas and nutrient exchange.
If implantation occurred in the uterine tube placenta would not occur. The zona pellucida is what ensures that the zygote doesn’t implant to the fallopian tube. There is some absorption of nutrients from uterine fluid but it is not what is going to keep the embryo alive and placenta cannot be formed in the uterine tube.
Part of implantation is the formation of the placenta (very important) AND the formation of the bilaminar disk
It’s when pregnancy occurs

Question 14

Describe the formation of the placenta

Answer 14

Occurs after implantation
First indication of placenta forming is development of the trophoblast cells:
- Cytotrophoblasts - the inner trophoblasts cells
- Syncytiotrophoblasts - the trophoblast cells that are in between the cytotrophoblasts and the mothers tissue.
These cells go on to form the chorionic villi of the foetal placenta (what allows for nutrient and gas exchange to foetus)
They secrete enzymes to allow embryo to burrow into uterine wall and for the uterine wall to start to grow its form of the placenta back
Also secrete human chorionic gonadotropin, the hormone detected in pregnancy test kits. The role of it is to keep the uterine wall intact (stops menstruation) to allow placenta to form
As placenta is forming also see development of bilaminar embryonic disk; inner cell mass/myoblasts turn into two different tissue types that will give rise to embryo itself:
Hypoblasts (yellow) - give rise to endodermal tissue
Epiblasts (blue) - give rise to ectodermal layer
Next process that occurs is gastrulation (end of second week) - formation of three germ layers - takes place

Question 14

Describe the formation of the bilaminar disk

Answer 14

Occurs after implantation (roughly during formation of placenta)
As placenta is forming (during implantation) also see development of bilaminar embryonic disk; inner cell mass/myoblasts turn into two different tissue types that will give rise to embryo itself:
Hypoblasts (yellow) - give rise to endodermal tissue
Epiblasts (blue) - give rise to ectodermal layer

Question 15

Describe gastrulation

Answer 15

By the end of the second week, the bilaminar disc would have separated from trophoblasts and will be connected to cytotrophoblast cells by a connecting stalk on the posterior side, and around the bilaminar disk/developing embryo/what was the intra cell mass are two sacks.
One of the sacks (below) is called the yolk sac, which has a role to provide nutrients and other functions. The other is the amnion. Both sacs are extraembryonic tissues. The bilaminar disks - the cells that connect the two sacs is the only thing that forms the embryo
The beginning of gastrulation occurs when a primitive streak on the dorsal side of the ectodermal layer of the bilaminar disk is produced. In this process the ectodermal cells are rising up and moving in to displace endodermal tissue underneath to produce mesoderm tissue. This produces the trilaminar disc - tightly packed endodermal and ectodermal layer and loosely packed mesodermal layer
All occurs at the end of the second week (about 16 days post fertilisation)
Once 3 germ layers have been established the cells in the layers can talk to each other and early organogenesis can take place
First tissue to be formed is in the mesoderm layer and is called the Noto cord - thick rod of cell (develops around 22-24 post fertilisation). It begins to instructs what the surrounding tissue should do, defines where the future backbone is going to develop

Question 16

Describe embryonic folding

Answer 16

22 days post fertilisation, the 3 layered disk that is kinda 2D folds in two ways:
The ectodermal layer of the folds around mesoderm and endodermal layers - there are head and tail folds
Simultaneously, there are lateral folds of the ectoderm which form a tube (of ectodermal cells), inside of which is the gut and inbetween is the mesoderm. Neural and notochord formed, aorta and midgut is produced in the end
Now the amnion completely surround ght embryo, it protects it phsyically, buffers it from heat and keeps the shape
By 8 weeks most key organs defined

Question 17

Describe cell differnentation potential

Answer 17

Once a cell has been differentiate, it has limited division potential
Stem cells are cells that can replace themselves and can generate differentiated
Can define stem cells as a cell that divide to form one daughter that goes onto differentiate and one daughter that trains its stem cell properties - has unlimited capacity to do this
We need them for growth, renewal, and repair
Stem cells live in a stem cell niche and receive signals
Not all stem cells are equal, as soon as differentiation starts to take place, they become limited in their differentiation potential
Zygotes (fertilised egg) are totipotent, capable of giving rise to all cell types of body and extra embryonic tissues
Epiblasts/inner cell mass cells are Pluripotent - capable of giving rise to all cell types of body (embryo), but cant give rise to extraembryonic tissue (trophoblasts are also pluripotent but opposite is true)
Multipotent cell - cells within organisms that are only capable of giving rise to all cell types of a particular tissue or organ
Nullipotent (or unipotent) - not capable of giving rise to other cell type (e.g. keratinocyte skin cell)

Question 18

Desribe the stem cells in skin and their significance

Answer 18

Keratinocyte stem cells (in their niche) are found at the bottom of the epidermal layer of thes skin (just above the basal lamina of the basement membrane). This means they are protected from external forces
These cells can renew indefinitely, but only give rise to other keratinocytes. As they divide they give rise to another stem cell and a more differentiated keratinocyte.
The keratinocyte cells produced from the stem cells stack up in layers, and all are full of keratinocyte proteins. Just above the stem - when relatively new - the cells become cobbled and round
As they age even more they become flattened out and lose their nuclei - die. This allows for protection.
These stem cells are important for constant renewal (regeneration takes about 2-4 weeks)

Question 19

How can skin stem cells be used to treat conditions

Answer 19

There is a fragile skin condition called epidermolysis bullosa, those that have it have no functioning proteins for connecting the dermis to the epidermis
Usually gene that is broken is collagen 7
In order to produce long term differences using gene editing, need to edit and retain stem cells
They take cells from patients and use CRISPR cas9 to fix the gene, then can grow sheets of skin to hopefully graft them on patients
Using patients own cells so no graft vs host issues
Immunohistochemistry - use antibodies to find presence of proteins

When analysing results:
We can define keratinocytes by the proteins they have
One protein that identifies keratinocyte stem cells is FOX M1 (found via immunohistochemistry)