Tissue Homeostasis Flashcards
What is a stem cell at the tissue level? 2 definitions:
- Cell that renews a continuously turning over tissue, needs replenishing (Blood, Skin, Germ cells)
- Cell that can renew a tissue undergoing regeneration after injury (muscle, pancreas, liver)
Why do we need stem cells?
- Tissue renewal could be considered a luxury in terms of_____ and ____
- Tissues containing cells that meet continuous insults from the environment need to have expendable cells and ____
- energy use
- evolutionary fitness
- continuous turnover
List the 3 types of tissues with high turnover rate.
- Blood
- Skin
- Gut lining (epithelium)
Why is stem cells a good idea? Why are they required? Why can’t normal cells replenish themselves
- Some differentiated cells are so highly specialised they lack a nucleus and so cannot replenish themselves (RBC)
- Some cells (in the immune system) lose DNA sequences during differentiation to generate antibodies
- So these tissues need a reservoir of pristine cells
Why is it that when growth proliferate very fast, mutation is a problem?
- if mutation gives the cell a growth advantage it will quickly generate a cancer
Why is stem cell the solution to prevent proliferative cell growth?
division of labour so jobs are separated from differentiated cell
stem cells divide vets slowly and is immortal, generates fast dividing daughters that has limited lifespan. final differentiated cell may be non-proliferative and have limited lifespan
What is the biological definition of a stem cell?
A cell that has a choice: to divide to give rise to
a) an exact copy of itself, or
b) a differentiated cell
Describe the difference between symmetrical and asymmetric division.
symmetrical = gives rise to 2 stem cells or 2 differentiated cells after division
asymmetric = gives rise of a stem cell and a differentiated cell after division
State and explain 2 ways how can asymmetry can be generated?
- From the environment (neighbouring cells, ECM and GF receives signal)
- localising components within cell (inherit set that makes it stay as a stem cell)
How were stem cells discovered? and what does high/medium/low dosage show?
- bombing of Hiroshima (sparked research on effects of radiation)
- high + medium = fatal
- low radiation = not fatal
Mouse spleen is used as an experimental site for haematopoietic cell production. Irradiated mouse with transplanted donor bone marrow had what phenotype?
Large dots resembling bacterial colonies
What is the number of spleen colonies directly proportional to? What does this show?
number of transplanted bone marrow cells. shows that bone marrow contains precursors and only small fraction turns into colonies
Linear relationship suggests each viable haematopoietic cell precursor makes a spleen colony. How to prove this?
- use marrow cells with a known chromosome marker’, and to transplant a mixture of marked and unmarked cells
- If the colonies that developed were always composed of cells with markers or cells without markers it would suggest the single-cell origin of colonies
What is the problem with this experiment? How is it fixed?
- dispersal of donor cells was not complete? need another way to uniquely label donor cells in a mixture
- This can be done by radiation itself: it induces (random) chromosomal breaks, which repair to give abnormal chromosomes that can be detected in nuclear preparations (‘chromosome spreads’)
2 more low dose radiation is incurred on irradiated mouse. Total dose of the mouse’s own haematopoietic system is ___ but dose to transplanted bone marrow induced only ______
- lethal
- limited chromosome damage
4out of 42 analysed had abnormal chromosomes. abnormality is consistent in all cells of that colony. this shows that…
all colonies were pure, not mixed
Describe the spleen colony composition.
- spleen comes from one single cell
- Spleen colonies contain many differentiated cell types
- So this was the first demonstration that single bone marrow cells can differentiate to many cell types of the haematopoietic system
Multicellularity arose ____ in plants vs animals (simultaneously/independently)
independently
Features of adult stem cells:
- Generally slow-dividing
- Maintained for the lifetime of the organism
- Restricted in their differentiation potential to the tissue they generate
Drosophila reproductive system is an example of ____ division of stem cell
asymmetric
What is a germarium?
ovaries in female drosophila divided
Describe the structure of a single germarium from top to bottom
terminal filament
cap cell
germline stem cell
cystoblast
What is a niche in terms of stem cell?
The environment of a stem cell that provides the factors needed for stem cell maintenance.
What provides the niche for stem cell in germarium?
cap cell
Cap cell provides niche for ____
____ undergo ________ _____ to produce _____ that divides 4x to produce _____ of nurse cells and oocyte
- Germline stem cells (GSC)
- GSC
- assymetric division
- cystoblast
- progenitors
Male ad female GSC are closely associated with ____ cells
somatic
Genes are important for stem cell self renewal and differentiation. e.g. Bag-of-marbles what does it do?
loss of function mutants have excessive numbers of germ cells
How does cell signalling work in switching BAM off and on?
- secreted signalling molecules from cap cells and terminal filament crosses over to germline stem cells to keep BAM switched off
- when the cells divide, the daughter cells express BAM because they are far away from the cap cells secreting the signals and moves on to differentiate
How is asymmetric division achieved at cellular level?
- they do this by orienting their cell division planes
- hub cell in male/cap cell in female - spindle is oriented so that the chromosome segregates either close or far from the hub so one stays close and one is far
What is a spectrosome and what is its function?
- a condensed organelle composed of spectrin, an actin-binding, contractile protein that normally mediates cell adhesion
- Ablating the spectrosome leads to random orientation of division planes, so it is responsible for anchoring the mitotic spindle
- The cell that inherits the spectrosome remains a GSC
- organelle in germline stem cell and sticks future germ cell to cap cell.
Give another example of stem cell oriented decision in mammals?
hair follicle
The small intestine epithelium consists of villi and crypt. What are each of the functions?
villi = absorbs nutrients
crypt = stem cells
What are the functions of each of the differentiated cell types in the gut?
- enterocyte
- goblet cell
- enteroendocrine cell
- panted cell
- enterocyte = absorbs nutrients
- goblet cell = secretes mucus
- enteroendocrine cell = secretes hormones
- paneth cell = mediates microbial immunity
CBC are cells expressing Lgr5 that sits in between paneth cells at the end of the crypt. They have been genetically marked to see what happens to their descendants. CBC cells have switched on LacZ and it is expressed in all descendants of CBC. This shows that:
CBC are the stem cells for the small intestine.
IRES is internal ribosomal entry site. what does that mean?
that allows translation of 2 proteins from one transcription
Which 2 proteins is translated from one transcript?
creERT2 and EGFP
ROsa26 gene is genetically altered to have stop codon. How?
red triangles called loxP sites
What happens in the presence of tamoxifen?
- the loxP sequences are removed to remove stop codon
- rosa26 is able to work and is able to drive LacZ positive cells which express LGR5 (activated cript enzyme)
CBC cells and paneth cells are interspersed at crypt base. Why is having paneth cells beneficial for CBC cells?
paneth cells secrete Wnt3 which increase survival of CBC cells.
This shows that ____ cells are the niche for gut stem cells
paneth
When cells are labeled at random in the gut, rosa is released by adding taxoxifen and dominant takes over. what does this show?
neutral frist model where CBC cells generally divide symmetrically
How do these CBC cells determine which ones take over?
ones that are fully surrounded by paneth cells will undergo symmetrical division and populate the whole crypt
States 3 organs with high turnover rate and 3 with low turnover rate.
high:
- haematopoietic system
- gut
- skin
low:
- lens
- brain
- heart
Outline the experiment and results of lineage tracing Lgr5+ cells after radiation in mice.
- inject tamoxifen
- Lgr5 - expressing cells heritably labelled with lacZ
- mice is either irradiated or not irradiated
- non-irradiated = Lgr5 expressing cells and descendants labelled with LacZ
- irradiated = no LacZ label
What does low dose in lineage tracing Lgr5+ cells after radiation in mice show?
low dose = no LacZ label , Lgr5 cells are killed by radiation but mouse survives!!
gut regenerates Lgr5+ cells from cells that were previously not expressing Lgr5.
Describe the switch between niche in mice after radiation
where the stem cells were supposed to be before radiation is not there anymore but some Lgr5+ positive cells must be recruited from somewhere that is not teh gut crypt - under these stressful situations where the crypt is empty (no stem cells), they switch their niche from somewhere else and populates the stem cells
Explain how cultured cells in gut regeneration from organoids works.
Lgr5+ cells can be taken out to culture and grown from single cells in presence of Wnt molecules and grow into balls that resemble crypt villus structure (containing differentiated cell and gut stem cell). grafted organoids behaved like gut cells, formed crypt villus structure that contained Lgr5+ and differentiated cells
Describe the different layers in skin epidermis:
- cornfield layers
- supra basal layers
- basal layers
- dermis
- cornfield = solid dead/differentiated skin
- supra basal = cells progress to outer surface
- basal = undifferentiated and proliferate (stem cells)
Stem cells in basal layer makes supra basal cells which are committed to _____ _____
terminal differentiation
Holoclone, meroclone and paraclone are derived from a single cell. which one could sustain long term propagation/is the stem cells and are used to treat 3rd degree burns
holoclones
Muscles are made of__ __ and don’t undergo much __ ___.
multinucleated myofibrils
cell proliferation
What does the following lead to:
- steady state
- decreased use
- increased use
– Steady-state: Undergoes some wear-and-tear through use- contraction puts stress on muscle fibres and can damage cells
– Decreased use: degeneration
– Increased use or damage: regeneration
What are the stem cells for muscle, where are they located and what do they express?
- satellite cells
- edge of muscle fibres
- Pax3
What are some organisms that have amazing regenerative powers?
- hydra
- planarians
- axolotl
Describe the mode of regeneration in hydra. e.g. when it’s cut in half how does it generate according to which half it is
- Cells sense their position in the organism and adjust gene expression pattern accordingly
- head and foot ends producing dynamic gradients of diffusible molecules (including Wnt) that specify the head and foot
In planarians, what relative regeneration takes place under these 4 scenarios:
A = cut in 2
B = cut into 3
C = cut into 3 but middle piece is too thin
D = cut into 3 middle piece too thin but second cut is delayed
A = normal worm x2
B = normal worm x3
C = normal worm x2 , middle doesn’t form worm because there is no morphogen gradient
D = normal worm x3 because time lag allows anterior posterior gradient to become established
How is the amazing axolotl limb regeneration established?
- Blastema cells re-differentiate into the cell type from which they originally de- differentiated, so blastema cells are not multipotent stem cells
What 2 signalling molecules sit on edge of limb needed for regeneration?
FGFs and SHH
CBC-‐Paneth cell relationship and villus structure differs in embryonic and adult life? if so, how?
- Paneth-‐like cells appear, 7 days after the embryonic gut is formed
– Therefore, the embryonic gut proliferates by a mechanism that does not need Paneth cells for survival
Describe the stem cells in early embryos.
Embryonic Stem (ES) cells
– Pluripotent- can make all the cell types in the body
Describe the stem cells in foetus.
– Made from partly specialised tissue cells, e.g. neural stem cells
– Multipotent or oligopotent (few)- can make several cell types
Why do embryos need SC? At 8 cell stage, embryo separates inner from outer cells. Inner becomes ___ which gives rise to ____ and outer becomes ___
- inner cell mass (ICM)
- embryonic tissue (pluripotency)
- placenta
ICM expresses ___, a ___ involved in ____. it is the source of ____
Nano
gene
pluripotent
embryonic stem cells
ES cells can be maintained in culture on a layer of ______ that secretes ______ called ____
- feeder cells
- signalling molecules
- LIF
Where is LIF secreted in the embryo? Where are its receptors?
- from outer trophoblast cells. ICM cells express receptors for LIF
Explain the phenomenon known as ‘diapause’
- the fertilisation is paused as current litter is still lactating and feeding - held in preimplantation state
- These are held in the uterus without implanting until the female weans the previous litter of pups
(3 weeks after birth)
- Embryos then implant
- This phenomenon is known as ‘diapause’ and is thought to maximise breeding performance
ICM is maintained in diapause. mice lacking ___ for ___ are unable to maintain ICM during diapause
receptors, LIF
ES cells don’t exist in the embryo in what state?
indefinitely proliferating state
Supplying cells with factors to maintain normally transient state leads to…..
arrested differentiation and unlimited proliferation
What other stages where cells can be arrested in proliferative non-differentiating state?
- ES cell
- epiblast stem cell
- neural stem cell
Describe the feature of tissue stem cells later in the embryo.
- tissue stem cells in later embryo are more restricted in ability to differentiate
What is neural stem cell? What is it made from?
made from embryo or ES cells, grow indefinitely in culture and differntiates into neural cells
What does neural stem cells differentiate into?
neurons and glial cells
What are some applications of using ES cells?
- cell therapy
- testing new pharmaceutical products on differentiated ES derived cells
Why is stem cell differentiation in vitro difficult?
- determine cell type bu adding factors
- some are receptive to those factors and others will follow the one that went the wrong way
What is iPS cells?
- adult cells can revert to phenotype almost indistinguishable to human embryonic stem cells
- induced pluripotent stem cells
