lecture 17 Flashcards
what do we mean when we talk about stem cells
stuff that happens in fully developed organism (has completed embryonic development)
is adult body static or dynamic
not static; in dynamic EQ; new cells are being born, differentiating, dying
stem cell
undifferentiated cell, analogous to undifferentiated embryonic cell that can become anything
what do stem cells do
continue to divide indefinitely
what happens if a cell can’t continuously divide
it’s not a stem cell
what do stem cells produce
daughter cells that either remain a stem cell or differentiate into like muscle, fat, blood etc.
what is self renewal
remains stem cell, as cell divides it’s always gonna produce at least one more stem cell, so you maintain stem cells
where are stem cells found body? specific areas or just randomly throughout
specific locations known as stem cell niches
stem cell niches
special physical and chem envs. that control, regulate stem cell division and differentiation
example of a stem cell niche
bone marrow (where white blood cells are born and differentiated)
what did people use to think before
no more cell division in an adult; all the cells needed as an adult were produced within embryo
how was this modified as time went on
maybe differentiated cells were capable of replenishing themselves, like if u hurt muscle, cells could produce more muscle cells
main assumption in 1940s
but DNA and protein synthesis would be hard to detect because it’s so rare [basically all the cells, DNA, proteins we need are already produced]
how do we know this isn’t true (how do we know adult body isn’t static)
Charles LeBlond
what did Leblond show
you can detect new DNA and protein synthesis in adults, AND stem cells are major reason why that occurs
what method did LeBlond develop to detect new protein and DNA synthesis in living tissue
radioautography
describe radioautography experiment
fed animals radioactive phosphorus followed by normal food, give it some time and cross-section animal
what were results of radioautography experiment
radioactivity can cause a rxn on photographic film to render an image –> can see a dark signal of where the radioactive phosphorous is
conclusion of radioautography experiment
radioactive phosphorus stuck around and is incorporated into cells, DNA, protein b/c we can see it on film (means that there is new DNA protein synthesis going on, otherwise it would’ve just passed through the body)
what is being detected in this radioautograph experiment, where is it being detected, what does it tell us?
radioactive phosphorus is being detected in bone marrow of leg, shows us where stem cells would be located in that tissue
was LeBlond’s results universally accepted
no, especially when results came out he got a lot of pushback
what is next discovery of LeBlodn
stem cells
how did he approach this experiment of stem cells
knew that stem cell would have a capacity for self-renewal (as it divided, exact same cell type would be left behind while differentiating cells continued) –> used a histological approach
did LeBlond use antibody staining or fluorescent stuff to detect stem cells
no
specifically how did he detect stem cells
periodic acid-schiff technique: used chemical stain to give contrast to tissues
what did he do this stem cell experiment in
took sections of mouse testes, looking for stem cells that give rise to sperm, in gonads
what were results of stem cell experiment
cell A are stem cels because as they divide they persist but also give rise to much darker stained cells
what are darker stained cells
spermatocytes, initial steps toward differentiation into sperm cell
where are stem cells
in this case by edge of tissue where they’re generating the spermatocytes
what is a general principle of stem cell and differentiation
symmetric vs. asymmetric cell division
one way stem cell differentiation can happen
asymmetric cell division
what is asymmetric stem cell division
some factor (like protein, 2nd messenger) is concentrated on one side of cell, after division the factor is gonna be found on only one daughter cell (even tho DNA divides evenly)
what does this factor do
prevents differentiation, keeps stem cell as a stem cell
what happens to daughter cells without the factor (after divisiion)
will differentiate, because it doesn’t have that factor
so how is this asymmetric division
daughter cells are different; b/c of presence of factor one of them differentiates while other remains a stem cell
what is symmetric cell division
everything is uniformly mixed in cell, when it divides it is identical –> both stem cells
how does symmetric cell division give rise to differentiation
differing chemical environment
give an example of diff. chemical env. leading to differentiation
positions are different, maybe one cell is touching another cell that has some signaling molecules (other cell is not touching this) –> environmental change to cause differentiation event
what else did leBlond discover
stem cells in the gut
how did he discover stem cells in intestine
radioactive thymidine
describe gut/intestine
in mouse intestine, microvilli is reaching up to absorb nutrients from food, lined w/ intestinal epitheliumd
describe radioactive thymidine experiment
after 8 hrs it is concentrated at bottom of microvilli (where new cells are being created)
where are new cells being created
bottom of microvillli
what happens after 72 hrs
through staining we see radioactive thymidine positive cells all along microvilli, b/c we see where the cells go not just where they are being created
explain this idea of stem cells in gut
stem cells are created at base of microvilli, as cell division continues it’s like a conveyor belt of cells just continuously pushed up to tip of vili
what happens as cells are pushed up to tip of villi
shed into lumen of gut
what are the stem cells doing to gut
constant renewal of intestinal epithelium
where are these stem cells located
stem cell crypts
how is lining of small intestine continually renewed
through cell proliferation in crypts
where are stem cells located
in crypt
what happens as the stem cells divide and differentiate
move up along crypt and epithelial villie
what happens as these stem cells move up
they are able to differentiate into diff cell types
describe stem cells at crypt base
multipotent; give rise to full range of differentiated intestinal cell types
describe pulse chase experiment for microvilli stem cells
at first, just the stem cell is blue; after 5 days and 60 days, progeny of those stem cells are also blue (extends up along microvilli)
does this microvilli pulse chase exp. have same results as radioactive thymidine
yes; shows stem cells in crypts at base of microvilli, as they differentiate progeny are pushed up along microvilliw
what ultimately happens to stem cell progeny as they are pushed up along microvilli
they are shed at tips
what are 4 major differentiated cells in intestines
absorptive cells, goblet cells, paneth cells, entereoendocrine cell
what is absorptive cell
most popular; responsible for absorbing nutrients from lumen of intestine
where do absorptive cells absorb nutrients from
lumen of intestine
what are goblet cells
secrete mucus to protect epithelium and smooth passage of food along cavity
what are paneth cells
help w/ immune functions, also in crypt they prevent stem cells from differentiating
who prevents stem cells from differentiating
paneth cells
what are enteroendocrine cells
hella rare; secrete hormones to control cell proliferation, digestion, growth
what does Wnt signaling do in the gut
maintains gut stem-cell compartment
what is critical in molecular pathways controlling differentiation of stem cells in the gut
Wnt
what is APC
turns off Wnt signaling
what happens when they experimentally knocked out APC in mouse intestines
saw formation of tumors/adenomas that are just basically out of control stem cells
what does Wnt signaling do
increases stem cell proliferation, triggers tumor formation
what does APC do
turns OFF Wnt signaling, prevents like out of control stem cell proliferation and cancer
what does loss of APC function lead to
hyperactivation of Wnt signaling
what does notch signaling do in gut
controls gut cell diversification and helsp maintain stem cell state
what does Wnt signaling lead to in crypts
expression of notch and delta
what do paneth cells express
delta
what happens when delta binds notch
notch is expressed on stem cells
what happens after delta binds notch
causes signaling that prevents stem cell from differentiating
what does that stem cell do
divides, symmetric cell division
what happens after that symmetric cell division
they differentiate
how do stem cells differentiate after symmetric cell division
lateral inhibition
what does lateral inhibition lead to
causes 2 symmetric daughter cells to become asymmetric, differentiate into diff fates
what happens to daughter cells after lateral inhibition
one daughter cell expresses delta other expresses notch
what is fate of daughter cell that expresses delta
fated to become secretory cells on the villi, secretes mucus
what is fate of daughter cell expressing notch
becomes absorptive cell
describe lateral inhibition
cells 1 and 2 both express protein X that is trying to turn off X in the other cell
one cell wins by random chance, end up with protein X in one cell and it turns off on the other cell
what happens to cause one cell to have protein X and other cell to not have it
gene regulation; X is massively transcribed in cell 1, degraded in cell 2
describe delta and notch
transmembrane proteins that bind each other
what happens when notch is activated
releases trans regulator that enters nucleus
what does notch’s transcriptional regulator do
in nucleus, causes changes to gene regulation (turn off delta expression)
what does notch do when activated
prevents cell from differentiating, actively tries to downregulate delta
why does notch prevent cell from differentiating
so it remains a stem cell (when ontch is active)
describe both stem cells in terms of notch and delta
gut stem cell divided into 2, identical; cell is expressing delta and activating notch
what are both cells trying to do
both trying to remain stem cells, both trying to turn off delta in the other one
what ends up happening in lateral inhibition
cell that has a little more delta than the other cell wins, continues to express delta while other cell turns off
what does the cell that wins do to other cell
activates notch in other cell to prevent cell specialization (remains stem cell)
what does winning stem cell do
differentiates (expresses delta, so no notch, meaning nothing preventing it from differentiating)
what happens to losing cell
remains a stem cell, turns off delta all the way; activates notch to prevent specialization
what does lateral inhibition do in terms of symmetric cell division
lateral inhibition is environmental factor that allowed one cell to differentiate and other to maintain its stemness
where are paneth cells
at base of crypt
what do paneth cells do
keeps initial stem cell a stem cell
what happens once stem cell symmetrically divides
lateral inhibition takes over
what does paneth cell express
expresses delta, keeps cells from differentiating; after division lateral inhibition takes over
what does notch activation do
maintains stemness at base of crypt (no specialization)
what are ultimate fates of losing cell (notch cell)
absorptive cells
what is ultimate fate of winning cell (delta cell)
secretory cells (goblet)
what happens if you get rid of APC
cell population expands massively
what is Wnt signaling pathway important for
controlling stem cell numbers
what happens in crypt:
bottom of crypt, Paneth cells maintain stem cell population, symmetric division, lateral inhibition (ultiately absorptive cells and goblet cells)
what is needed for lateral inhibition to work
symmetric daughter cells
what would happen if this was the only thing going on in the cell
stem cells would be depleted
what else is happening in the backgroudn
self-renewal, stem cell divides and always remains a stem cell
what does epidermal stem-cell system maintain
self-renewing waterproof barrier of dead skin cells
where can we find other stem cell populations in bodies
dermis/skin
why are we always shedding dead skin
b/c dermal stem cells are right against basement membrane, as they are displaced by further cell divisions you get movement from bottom (basement membrane) to top, which is skin surface
what happens as dermal cells move to top
differentiate and die, filled w/ keratin, hydrophobic barrier
what are stem cells of connective tissue
bone marrow stromal cells
what are mesenchymal stem cells
bone cells, fibroblasts, cartilage, fat cells, smooth muscle cells
where do mesenchymal cells reside
bone marrow
how are bones continuously remodeled
osteoclasts and osteoblasts
osteocytes
cells that can create bone
where do osteocytes come from
mesenchymal stem cells
osteoblasts
cells that can create new bone, allow it to heal
osteoclasts
remove bone, help remodel it
are muscle fibers renewed or static
constantly renewed
what do myoblasts do
fuse to form new skeletal muscle fibers
what happens to other myoblasts
not all of them fuse, some end up as satellite cells on surface of muscle fibers
what are satellite cells
stem cell population that generates muscle progenitors that can fuse & regenerate muscle fibers
what are satellite cells activated in response to
injury; create muscle progenitor cells –> help form /regenerate muscle
basically what/where are satellite cells
in muscle; stem cell population, response to injury they divide and create new muscle progenitors to make muscle
what lines all blood vessels and lymphatics
endothelial cells
what do endothelial cells do
respond to signals in environment to create new endothelial cells
is endothelial cell a stem cell
not really, but it shows how dynamic it is
what happens when we need to form a new blood vessel
endothelial cells react w/ signals in env. to change gene transc. to create new cytoskeletal structures
what do endothelial cells do in response to signals
form pseudopodial processes;
what do pseudopodial (protrusion) processes allow them to do
navigate to source of signals and create new blood vessel (like a pathfinder)
what happens if you have a region of tissue hella low in oxygen
you want to deliver oxygenated blood thru new blood vessel
describe cells/tissue w/ low oxigen
high HIF1a
what does HIF1a do
transcriptional regulator than causes cells to secrete VegF
what does VegF do
diffuses to nearest capillary, sends a signals saying it needs a blood vesel over here
what does VegF activate
activates nearest endothelial cell, causes it to form pesudopodia and form new blood vessel where it’s needed
what do tissues requiring a blood supply release
release VegF
what are required to maintain stem cell populations
self-renewal and asymmetric cell division
what do stem cells generate
diverse cell populations required to maintain gut, bone, muscles, blood vessels
