L2 - Gastrulation and Stem Cell Niche

Question 1

What sort of potency does a zygote have?

Answer 1

Totipotent

Question 2

Where does differentiation take place in the embryo?

Answer 2

Differentiation takes place in all 3 germ layers from early stages of embryogenesis
All have different derivatives
Totipotent –> pluripotent –> multipotent

Question 3

Do cells stay as multipotent cells?

Answer 3

The vast majority of cells don’t stay as multipotent cells – they differentiate

Question 4

When does embryonic organogenesis occur?

Answer 4

2 months post fertilisation

Question 5

How are adult stem cells formed?

Answer 5

Some cells set aside, undifferentiated, to contribute to individual over its lifetime

Question 6

How are gonadal germ cells formed?

Answer 6

Some cells set aside, undifferentiated, for the next generation
Specialised totipotent cells
This helps maintain the species

Question 7

Where do gametes arise and migrate to?

Answer 7

Arise from primordial germ cells

Migrate into the gonads

Question 8

In which plants and animals can somatic cells readily form new organisms?

Answer 8

Cnidarians, flatworms, tunicates

Question 9

In which animals is there an early division between somatic and germ cells?

Answer 9

Insects, vertebrates

Question 10

Where do primordial germ cells arise and migrate to?

Answer 10

1. Primordial germ cells are determined in a specific location on the edge or outside of the developing embryo
2. They migrate to the gonad and become the progenitor population for eggs and sperm

Question 11

What do you need for germ cell differentiation?

Answer 11

A totipotent cell type

A cell capable of undergoing meiosis

Question 12

Where do P cells come from?

Answer 12

C.elegans

Question 13

How are P cells formed?

Answer 13

In early cleavage and gastrulation, asymmetrical divisions produce a specialised cell - P-lineage cell
P cells produced is smaller and inherits different cytoplasmic determinants
- At posterior end

Question 14

How is the P cell made different from other cells?

Answer 14

Cell division plane is a crucial determining factor in the fate of 2 daughter cells

Question 15

In a mother cells with cytoplasmic determinant that is asymmetrically localised division along meridian creates?

Answer 15

Maintains two daughter cells, both identical to mother

Two self-renewing stem cells

Question 16

In a mother cells with cytoplasmic determinant that is asymmetrically localised division along equator creates?

Answer 16

Progenitor and a cell with different determinants

Can now differentiate

Question 17

Why are some determinants in the cytoplasm polarised?

Answer 17

They are found at one end of the cell

Question 18

Through asymmetric division P-cells inherit?

Answer 18

Specialised P-granules
Found in the cytoplasm but can get into the nucleus
Mix of proteins and RNA

Question 19

What is the role of P granules?

Answer 19

Bind to DNA of P cell and block almost all transcription
- Thus block all differentiation
In cytoplasm block translation
Some gene transcription upregulated to promote stem cell fate and cause cells to undergo meiosis

Question 20

What is the version of P cells in vertebrates?

Answer 20

Germ cells

Question 21

What is the role of germ cells in vertebrates?

Answer 21

Germ cells in all species express the translational blocker, Nanos 
This shows
- Little transcription or translation
- Thus no differentiation
- Thus a pluripotent identity
- Also they can undergo meiosis

Question 22

How are PGCs controlled?

Answer 22

PGCs are shut down transcriptionally and translationally

Question 23

How are PGCs shut down?

Answer 23

Epigenetic Silencing mechanisms include

• DNA methylation - represses gene activity
• Histone modification - histone proteins govern accessibility of gene promoters

Question 24

Where do PGCs arise from?

Answer 24

Arise from extra-embryonic mesodendodermal cells just beyond posterior part of the embryo
As the body axes form, PGCs stay in this position
- Future posterior part of hind gut

Question 25

What are the characteristics of mammalian primordial germ cells?

Answer 25

Arise from extra-embryonic mesodendodermal cells just beyond posterior part of the embryo
As the body axes form, PGCs stay in this position
- Future posterior part of hind gut

Question 26

How do germ cells migrate in the Drosophila?

Answer 26

Germ cells passively ride endoderm – up through the duct
Chemoattractive and repulsive cues drive PGCs and gonad precursor cells together and to the gonad
PGCs divide through the larval stage and differentiate at metamorphosis

Question 27

Where do PGCs attach to in the ovaries?

Answer 27

Stromal cap

Question 28

Where do PGCs attach to in the testes?

Answer 28

Hub cells

Question 29

How do germ cells migrate in the chick, mouse and human?

Answer 29

1. PGCs stay out of the embryo while the major inductive events occur
   o Body axes formation and early organogenesis
2. PGCs migrate though the hind gut along a fibronectin trail
3. Move laterally to the genital ridges which produce a chemoattractant
4. Ultimately found in the gonads which provide a specialised microenvironment that
   o Protects the PGCs in their pluripotent state
   o Supports their meiotic divisions and formation of eggs or sperm cells

Question 30

How are migrating germ cells protected from differentiating?

Answer 30

The travelling stem cell niche idea

• Support cells travel with PGCs to maintain the undifferentiated stem cell phenotype
• They secrete stem cell factor (SCF)

Question 31

What causes the formation of a teratoma?

Answer 31

Failure to migrate to protective niche/failure to make SCF causes germ cells to differentiate