Chapter 10 Flashcards
(28 cards)
The pedigree of cells in the body of a worm.
C. elegans: 959 somatic cells
Complete cell lineage-some end abruptly: become post-mitotic or undergo apoptosis.
Cell Immortalization and Tumorigenesis
Proliferative capacity of serially passaged human fibroblasts.
a.) Number of population cell doublings.
b.) Fibroblasts approximately 60 doublings.
c.) Then cell senescence
Metabolically inactive
Cannot re-enter cell cycle.
Cell-specific controls determine the number of cell generations through which a particular cell lineage can pass
Upper, growing fibroblast.
Lower, senescent cells
Large cytoplasms(fried egg appearance)
Express acidic Beta-galactosidase enzyme.
Senescent cells in vitro and in vivo
Age-dependent senescence
Number of population doublings is higher in tissues of
newborns.
Loss of Proliferative capacity depends on age, species and tissue source.
Increased age=loss of skin stem cell proliferative capacity.
Tissue regeneration decline=thinning of keratinocyte layer of skin(epidermis).
Loss of proliferative capacity with age
Possible explanations:
A.) Number of population doublings.
B.) Damage.
C.) Loss of metabolic function.
Loss of proliferative capacity with age
Embryonal stem cells are immortal.
Cancer cell lines are also immortal.
Loss of proliferative capacity
Generations of cells forming a tumor mass.
Tumor size vs. population doublings
Cancer cells need to become immortal in order to form tumors
Model of exponential growth of a tumor
Relationships between population doublings and formation of a tumor mass.
In reality, many cells of a tumor are lost to apoptosis.
Mouse mammary tumor in mouse carrying oncogenic
Wnt-1 transgene.
TUNEL, (brown spots)
Relations between population doublings and formation of tumor mass
Result is 30-40 cell generations instead of 10.
Effect of cell loss on the cell generations required to form a tumor mass
Memory is cell autonomous, ie intrinsic
Cells measure: Cumulative physiologic stress, number of generations, then “crisis”.
How do cells remember replicative history
Influence of cell culture conditions on senescence.
Oxygen tension
3% is closer to normal.
cell-physiologic stresses impose a limitation on replication
keratinocytes
on plastic:
p16(INK4A)
Less induction in presence of a feeder layer.
dependence of epithelial cells on stromal factors
human fibroblasts, in vitro
p16(INK4A)
p21(Cip1)
Also, level of p53 increases 10-40 fold
induction of tumor suppressor proteins during in vitro culture
Left, normal cells
small
few focal contacts(yl)
few actin fibers(org)
Mid, senescent cells
ectopic expression of p16(INK4A)
Right, normal cells with replicative senescence.
Normal versus senescent cells
the effect is dependent on the ability of Large T to inactivate p53 and pRb.
May be a factor for cancer cells in vivo?
Role of large T antigen in circumventing senescence
kidney tissue from rats at indicated age.
stained for Beta-galactosidase
Evidence of senescent cells in living tissues
senescent human melanocytes within a dysplastic nevi(beta-gal)
evidence of senescent cells in living cells
After replicative senescence, cells in culture will undergo “crisis”
Disintegrating cells(white, refractile)
A truly functioning counting device-telomere length
Apoptosis associated with cell population in crisis
Telomere detected by FISH.
Each chromatid with a telomere at each end.
Cells deprived of TRF2(maintains telomere structure)
End-to-end fusion.
Chromosomes with two or more centromeres.
The proliferation of cultured cells is also limited by the telomeres of their chromosomes
Human lymphocytes: telomere length decreases with population doubling.
Large TTAGGG repeats.
Immortal-just slightly longer that cells in crisis.
Shortening of telomere DNA and Cell proliferation
telomeric DNA
body of chromosomal DNA
telomeric DNA
successive cell generations
Crisis
the shortening of telomeric DNA with cell division
end-to-end fusion(occurs at G2 phase)
Breakage fusion cycles
Telomere shortening
