Niches

Question 1

What is a stem cell niche?

Answer 1

stem cell niche is the in vivo microenvironment where stem cells both reside and receive stimuli that determine their fate. Therefore the niche should not be considered simply a physical location from stem cells rather as the place where extrinsic signals interact and integrate to influence stem cell behaviour

Question 2

Is a niche dynamic or static?

Answer 2

stem cell niche is not some quite place where the cells are protected or sth like that. It is more useful to think of the niche as the control tower where the whole operation is taking place

Question 3

Is the niche protective?

Answer 3

there is some element of protection provided by the niche as they are usually situated in places deep within the organisms (like bones)

Question 4

How do daughter cells arise in classic developmental biology?

Answer 4

• in classic developmental biology daughter cells that differentiate along distinct lineages arise by one of two mechanisms:
  
  • uneven partition of the cell fate determinants of the mother cell (intracellular signalling)
  • orientation of the plane division in which the daughter cells are placed in two different environments leading to differentiation (extracellular signals)

Question 5

What regulates the balance of differentiation and self renewal?

Answer 5

in reality the balance between self renewal and differentiation is regulated by the balance of extracellular and intracellular signals known today as a stem cell niche

Question 6

When did the concept of a stem cell niche arise?

Answer 6

stem cell niche concept arose when stem cells such as hematopetic stem cells lost their abilities yo self renew when placed in a different environment

Question 7

How is the number of stem cells maintained?

Answer 7

if you don’t have space in the niche your daughter cells are pushed out to differentiate → the number of stem cells is limited by the availability of niches

Question 8

What role does adhesion play in the stem cell niche?

Answer 8

adhesion of stem cells to either the underlying basement membrane or the support cells plays an important role in anchoring stem cells within the niche

Question 9

What were some hypothesis described by Schoefield 1978?

Answer 9

• stem cells must exist in the association with other cels and those other cells determine the behaviour of the stem cells
• the cells within the tissue prevent the maturation of the stem cell
• if the environment of the progeny of the stem cell is different than the stem cell only after the first generation there will be differences between the stem cell and the progeny

Question 10

What are the criteria that identify the stem cell niche according to Scadden 2010?

Answer 10

• the niche can exist even in the absence of the stem cells
• niche cells can be abscent and replaced by ECM
• cell-cell and cell-matrix contacts are essential
• blood vessels are always present in the vicinity
• neuronal inputs have increasingly been described

Question 11

What experiment lead to the the identification of Germ line stem cells?

Answer 11

Lineage tracing by clonal marking analysis has led to the identification of GSCs in both the male and the female germ lines in vivo, within their normal environment

Question 12

What experiment allows for analysis of specific genes in stem cell maintnance?

Answer 12

Clonal analysis also allows the generation of mutant GSCs in an otherwise wild-type animal, allowing the analysis of a specific gene’s function on stem cell maintenance, self-renewal, and survival.

Question 13

How do GSCs divide?

Answer 13

both male and female GSCs normally divide with invariant asymmetry, producing precisely one daughter stem cell and one daughter cell that will initiate differentiation.

Question 14

Describe the cell adhesion significance in the context of drosophila germ cells

Answer 14

In both the ovary and the testis, GSCs are in intimate contact with surrounding support cells that provide critical self- renewal signals, maintenance signals, or both, thereby constituting a stem cell niche. Oriented division of stem cells is important for placing one daughter cell within the niche while displacing the other daughter cell destined to ini- tiate differentiation outside of the germ-line stem cell niche.

Question 15

What were the first niches described?

Answer 15

the first niches were described in the drosophila gonads

Question 16

What do the male and female GSCs have in common?

Answer 16

the systems have some things in common - in bothe the stem cells are directly in contact with other cells (cap cells and Hub cells)

Question 17

Which daughter cell differentiates in drosophila GSCs in females?

Answer 17

When a female GSC divides, the daughter cell that lies closer to the terminal filament and cap cells retains stem cell identity; the daughter cell that is displaced away from the cap cells initiates differentiation as a cystoblast.

Question 18

What are the ahesion molecules responsible for adhesion in drosophila germ line niches?

Answer 18

Immunofluorescence analysis revealed that the Drosophila E-cadherin homolog Shotgun (Shg) and β-catenin homolog Armadillo (Arm) are highly concentrated at the interface between the GSCs and the cap cells in females
and between the GSCs and the hub cells in the male.

Question 19

What does the removal of shg do in female drosophila?

Answer 19

Removal of shg activity from the germ-line using clonal analysis resulted in failure of female GSCs to be efficiently recruited into their niches in the developing ovary.

Question 20

What is the significance of the drosophila studies?

Answer 20

the male and female germ lines of Drosophila have provided a genetic system in which to test the principles and investigate the basic underlying mechanisms of the stem cell niche theory.

Question 21

What are the themes arising from the drosophila studies?

Answer 21

• Several themes arising from the analysis of Drosophila
  male and female GSCs offer potential paradigms for analysis of stem cell
  niches in mammalian systems.
  
  • stem cells are usually adjascent to support cells that secrete factors required for maintaining stem cell identity
  • cell-cell adhesion between stem cells and the niche is required for stem cell maintnance

Question 22

What is dermal papilla important for?

Answer 22

dermal papilla is very important for the hair folicle growth and for the formation during embryonic develompent

Question 23

What is one of the simplest niches in a mammal?

Answer 23

Skeletal muscle

Question 24

Give an example of the tissue properties controlling the stem cell niche

Answer 24

stem cell niches are also controlled by simple properties of the tissues, for example in the intestinal crypt the position of the cells in the groove determines whether the cell will differentiate or not

Question 25

What happens to the hair folicle after embryonic development?

Answer 25

after formation during the embryonic development the lower portion of the hair folicle cycles through periods of growth, regression and quiescence

Question 26

Describe the hair folicle niche

Answer 26

Using multiple strategies, a stem cell niche for the mammalian epidermis has been located along the upper portion of the hair follicle in a region called the bulge. Specifically, the bulge is located along the outer root sheath, which is contiguous with the interfollicular epidermis

Question 27

What is the role of Beta 1 integrin in the hair folicle niche

Answer 27

Both in the human and mouse epidermis, β1 integrin expression is enriched in cells within the bulge region of the outer root sheath. Targeted disrup- tion of the β1 integrin gene in the outer root sheath cells did not disrupt the first hair cycle; however, proliferation of matrix cells was severely impaired, resulting in progressive hair loss and dramatic hair follicle abnormalities.

Question 28

Describe other pathways that can affect the hair folicle niche

Answer 28

• both the Sonic Hedgehog (Shh) and the Wnt/β-catenin sign- aling pathways have been shown to affect some aspects of cell proliferation and differentiation in the epidermis and epidermal appendages
• Shh signal- ing appears to specify hair follicle placement and growth during embryogen- esis, as well as postnatal follicle regeneration

Question 29

How many cells leave the intestinal crypt everyday?

Answer 29

Intestinal cells leave the crypt at a rate of 200–300 cells/day and migrate onto ciliated villi that protrude into the gut lumen

Question 30

What are transient amplyfing cells in the intestinal niche?

Answer 30

• Within a crypt, approx-imately 4–5 stem cells generate transit-amplifying cells, which are capable of up to six transit divisions.
• Migration of these transit-amplifying cells out of the pro- liferative zone is required for the onset of differentiation
• The stem cells may be maintained at the base of crypts, embedded in the intestinal wall, for protection from toxins passing through the gut lumen.

Question 31

What is the significance of Wnt in the intestine?

Answer 31

• Wnt signaling is involved in controlling the proliferation and differentiation of intestinal epithelial cells.
• in humans mutations in the APC gene (a negative regulator of Wnt signalling) are linked to the development of colorectal cancers
• Also, constitutively active nuclear complexes of Tcf4-β-catenin are found in APC −/− colon carcinoma cell lines or in cell lines that have a stable form of β-catenin, suggesting that hyperactivation of Tcf4 may contribute to cellular transformation. Loss of the Tcf4 transcription factor, which is expressed in the intestinal epithelium, leads to the depletion of stem cells and the failure to maintain the proliferative compartments in the intervillus pockets of the neonatal small intestine.

Question 32

Where is beta catenin found in the intestinal niche and waht is the significance of it?

Answer 32

• If crypts serve as a niche to support the self-renewal of intestinal stem cells, this would imply that cells near the intestinal stem cells may be the source of a secreted self-renewal signal.
• Nuclear β-catenin is found only in the cells at the base of the crypts within the adult mouse small intestine, suggesting activation of the Wnt pathway in these cells
• mesenchymal cells underlying the crypt epithelium could be a source for a secreted Wnt ligand that could act as a paracrine signal to direct the proliferation of stem cells, progenitor cells, or both in the intestinal epithelium.

Question 33

Where to HSCs reside in the bone?

Answer 33

HSCs reside along the inner surface of the bone and differentiating cells migrate towards the celter of the bone marrow cavity

Question 34

What is CXCL12?

Answer 34

CXCL12 and CXCR4 is a general magnet for cells so sometimes we end up having cells in the bone marrow that we don’t really want like metastatic breast cancer cells

Question 35

What is the role of CXCL12?

Answer 35

• during embryogenesis it directs the migration of hematopoetic cells from the liver into the bone marroe
• also chemotactic for mesenchymal stem cells and is expressed in the area of inflamatory bone destruction
• in adulthood it plays an important role in angiogenesis by rectuiting endothelial progenitor cells from the bone marrow through the CXCR4 dependant mechanism

Question 36

What is CXCR4?

Answer 36

• CXCR4 - receptor of CXC12
  
  • important in hemtopoetic cell homing to the bone marrow
  • plays a role in hematopoetic stem cell quiescence
  • The CXCR4 receptor upon binding its ligands triggers multiple signaling pathways that orchestrate cell migration, hematopoiesis and cell homing, and retention in the bone marrow

Question 37

What is the role of osteoblasts and what is the evidence behind it?

Answer 37

• osteoblasts are very important for the hematopoetic cells
• scientists took mice with overactive osteoblasts → they had more bone and as was later discoverted also more hematopoetic stem cells, which later led to the conclusion that a hematopoetic niche needs to at least consist of a stem cell sitting on a osteoblast
• osteoblasts secrete an elevated level of the Notch ligand Jagged-1, suggesting that activation of the Notch signalling transduction pathway in HSCs may support HSC proliferation

Question 38

The role of N-cadherin in HSC niche

Answer 38

Furthermore, the cell adhesion molecule N-cadherin, which is expressed by the spindle-shaped N-cadherin+ CD45− osteoblasts, may be responsible for holding HSCs within the niche and close to self-renewal and survival signals.

Question 39

The role of Wnt in HSCs

Answer 39

Studies have shown that signaling through the canonical Wnt pathway can direct HSC self-renewal in vitro and in vivo. Wnt is a secreted growth factor that binds to members of the Frizzled (Fz) family of cell surface receptors.

Question 40

The role of beta catenin in the HSC niche?

Answer 40

• The β-catenin molecule serves as a positive regulator of the pathway
• Reya et al. (2003) have demonstrated that transduction of HSCs with a retro- virus encoding a constitutively active β-catenin molecule results in self-renewal and expansion of HSCs in culture for at least four weeks and in some cases as long as 1–2 months under conditions in which control HSCs do not survive in culture beyond 48 hours.