Repair of Cells Flashcards
Regeneration
implies complete reconstitution. Tissues with high
proliferative capacity (e.g., hematopoietic system, gastrointestinal epithelium, etc.) can continuously renew themselves and
regenerate after injury, provided their stem cells are not
destroyed and provided they have an intact connective tissue
scaffolding
Repair
may restore some normal structure, and therefore function,
but may also leave some deficits. Healing in this setting involves
some combination of regeneration and scar formation (fibrosis).
The relative contribution of the two processes depends on the
capacity of the injured tissue to regenerate, the extent of injury
(i.e., how much matrix is damaged), and the extent of fibrosis
driven by the mediators of chronic inflammation.
Control of Normal Cell Proliferation
and Tissue Growth
• Increased cell proliferation can be accomplished by shortening
the cell cycle or by recruiting quiescent cells into the cell cycle.
• Increased baseline cell numbers may reflect increased proliferation, decreased cell death, or decreased differentiation.
• Differentiated cells that cannot proliferate are called terminally
differentiated cells. In some tissues, differentiated cells are not
replaced (e.g., cardiac myocytes); in others, they die but are continuously replaced by new cells generated from stem cells (e.g.,
skin epithelium).
• Cell proliferation can involve physiologic (e.g., hormonal) or
pathologic stimuli (e.g., injury, mechanical forces, or cell death).
• Proliferation and differentiation are controlled by soluble and/or
contact-mediated signals, and these signals may be stimulatory
or inhibitory.
Cell proliferation can involve physiologic or
pathologic stimuli
hormonal, injury, mechanical forces, or cell death
• Continuously dividing (labile) cell
proliferate throughout life,
replacing those that are destroyed (e.g., surface epithelia and
marrow hematopoietic cells). Typically, mature cells derive from
stem cells (see later discussion) with unlimited capacity to proliferate. The progeny of mature cells have the capacity to differentiate into several cell types.
Quiescent (stable) cells
normally involved in low-level replication but are capable of rapid division in response to stimuli
Quiescent (stable) cells are found in
liver, kidney, fibroblasts, smooth muscle, and endothelial cells
Nondividing (permanent) cells
cannot undergo division in postnatal life. Destruction
of such cells typically leads to either glial proliferation (brain) or
scar (heart), although limited re-population from a small group
of stem cells has been demonstrated
Nondividing (permanent) cells example
Neurons, Cardio myocyte
Mature skeletal muscle
does not divide but has regenerative capacity through the differentiation of
intrinsic satellite cells.
Stem Cells
Stem cells are characterized by their self-renewal capacity and by
their capacity to generate differentiated lineages.
• Asymmetric replication
With each cell division, one cell retains
self-renewing property while the other enters a differentiation
pathway
Stochastic differentiation:
The stem cell pool is maintained by
balancing divisions that generate two stem cells with those that
create two cells that differentiate.
Embryonic Stem Cells
Isolated from the inner cell mass of normal blastocysts, embryonic
stem (ES) cells are pluripotent
Pleuripotent Embryonic stem cell
that is, they have the capacity to
generate all cell lineages
Multipotent Stem cells
pluripotent cells can give rise to
multipotent stem cells, which have more restricted developmental
potential and eventually produce differentiated cells that form
adult tissue
ES cells can be maintained in vitro
undifferentiated
cell lines or induced to differentiate along a variety of cell lineages
ES cells lineages
• ES cells identify signals required for normal tissue differentiation.
• ES cells generate animals congenitally deficient in specific genes
(knockouts) by inactivating or deleting a gene in an ES cell and
then incorporating the modified ES cell into a developing blastocyst. Similarly, replacement of a wild-type gene with a specific
mutation (knock-in) can be performed. The power of the methodology has been further expanded by the ability to express gene
deficiencies in only selected cell or tissue types, and by the ability
to turn genes “on” and “off” at will in adult animals (conditional
gene deficiency).
• ES cells potentially repopulate damaged organs.
knockouts
ES cells generate animals congenitally deficient in specific genes by inactivating or deleting a gene in an ES cell and
then incorporating the modified ES cell into a developing blastocyst.
knock-in
replacement of a wild-type gene with a specific mutation
conditional
gene deficiency
ability
to turn genes “on” and “off” at will in adult animals
Induced Pluripotent Stem Cells
Functionally similar to ES cells, induced pluripotent stem (iPS)
cells have been generated by “reprogramming” adult differentiated
cells through transduction of genes encoding ES cell transcription
factors
How are Induced and Generating Pluripotent Stem cells differentiated
This is to be distinguished from generating pluripotent
stem cells from adult differentiated cells by transferring their nucleus to an enucleated oocyte
Such nuclear transfer techniques are
inefficient, and the resulting stem cells do not have high fidelity for
gene expression, probably due to vagaries in histone demethylation
in the transferred nuclei.
The iPS cells, on the other hand, faithfully
generate cells of all three germ layers and can be genetically
manipulated. This suggests that they may become a source of
patient-specific stem cells that can not only repair damaged tissues
but also potentially replace congenitally defective cells
Adult (Somatic) Stem Cells
Adult (somatic) stem cells have been identified in many mature
tissues (e.g., bone marrow, gastrointestinal tract, skin, liver, pancreas, and adipose tissue). Typically they have a more limited
capacity to differentiate.
ASCs are located in a niche
bulge area of hair follicles
Somatic stem cells give rise to rapidly proliferating
transit
amplifying cells
Transit Amplifying cells lose the capacity
asymmetric division and
become progenitor cells with a limited developmental potential.
Somatic stem cells are typically responsible for generating the
mature cells of the organ in which they reside, thereby
maintaining normal tissue homeostasis; they also have variable
potential to differentiate more broadly and to repopulate tissues
following injury
Transdifferentiation
cell differentiates
from one type to another;
developmental plasticity
the capacity to transdifferentiate into multiple lineages
hematopoietic stem cells (HSCs) that would normally only contribute to blood cell elements are capable of transdifferentiating
in vitro into
neurons, cardiomyocytes, hepatocytes, and other
adult cell lineages
Bone marrow contains pluripotent HSCs
regenerating
all blood cell elements, as well as multipotential marrow stromal
cells (MSCs) capable of differentiating into bone, cartilage, fat, muscle, or endothelium, depending on the tissue to which they migrate.
Liver stem cells
residing in the canals of Hering (junction between
hepatocytes and the biliary system) give rise to bipotent
progenitors called oval cells
Liver stem cells are active only if
direct hepatocyte proliferation is not possible (e.g., in fulminant hepatic failure
The brain contains which stem cells
neural stem cells
neural stem cells
capable of generating
neurons, astrocytes, and oligodendroglial cells, although the extent to which these are integrated into neural circuits is unclear.
neural stem cells are found in
the dentate gyrus of the hippocampus and the
subventricular zone.
Skin stem cells occur in the
hair follicle bulge (contributing to
follicular lineages), sebaceous glands
epidermal interfollicular regions have a turnover period of
4 weeks
Bulge stem cells can
replenish epidermis
after wounding, but they do not participate in normal epidermal
homeostasis
Small intestinal crypt epithelium
monoclonal, deriving from a single stem cell located immediately above the Paneth cells (3 to 5 day
turnover);
represents differentiated
epithelium derived from multiple crypts.
intestinal villus epithelium
skeletal and cardiac
myocytes
cannot proliferate
Regeneration of injured skeletal
muscle is accomplished by
satellite cells, a stem
cell pool in adult muscle
Limbal stem cells in the cornea
between the epithelium of
the cornea and conjunctiva) maintain the outermost layers of
the corneal epithelium
Cell proliferation is stimulated by a combination of soluble growth factors and extracellular matrix (ECM) signals transmitted via
integrins
The cell cycle comprises
G1 (pre-synthetic), S (DNA synthesis),
G2 (pre-mitotic), and M (mitotic) phases; quiescent cells are in
a physiologic state called G0
Liver Regeneration
Liver regeneration occurs by two major mechanisms: proliferation
of remaining hepatocytes and repopulation from progenitor cells.
Proliferation of hepatocytes following partial hepatectomy
Resection of
up to 90% of the liver can be corrected by residual hepatocyte
proliferation triggered by cytokines and polypeptide growth
factors
• In the priming phase, cytokines
such as IL-6 (from Kupffer
cells), make remaining hepatocytes competent to respond to
growth factor signals.
IL-6 formed in the
kupffer cells
In the second phase of regeneration
Hepatocyte Growth Factor and TGF-a, acts on Primed hepatocytes to stimulate entry into cell cycle
The wave of hepatocyte replication is followed by replication of
nonparenchymal cells
Kupffer cells, endothelial cells, and stellate cells
In the termination phase
hepatocytes return to quiescence
antiproliferative cytokines of
TGF-β
Liver regeneration from progenitor cells
When hepatocyte
proliferative capacity is impaired (chronic liver injury or
inflammation), liver progenitor cells (in specialized niches called
canals of Hering) contribute to repopulation
Repair by Connective Tissue Deposition
Macrophages (mostly M2 type) play a central role in repair by
clearing offending agents and dead tissue, providing growth factors for cellular proliferation, and secreting cytokines that stimulate fibroblast proliferation and connective tissue synthesis and deposition. Repair begins within 24 hours of injury; by 3 to 5 days, granulation tissue is apparent
Steps
Angiogenesis
Granulation Tissue
Connective tissue remodelling
Granulation tissue
hrough the migration and proliferation
of fibroblasts and deposition of loose connective tissue, combined
with the new vessels and interspersed leukocytes. The amount of
granulation tissue depends on the size of the tissue deficit created
by the wound and the intensity of inflammation.
Connective tissue remodeling.
The amount of connective tissue
progressively increases in the granulation tissue, forming a scar
that can remodel over time.
Angiogenesis
formation of new blood vessels; these are leaky
(accounting for edema in healing wounds) because of incomplete
interendothelial junctions and because VEGF increases vascular
permeability.
Process of angiogenesis
• Vasodilation in response to NO and increased permeability in
response to VEGF
• Separation of pericytes from the vessel wall and basement
membrane breakdown allowing vessel sprouting
• Migration of endothelial cells toward the area of tissue injury
• Proliferation of endothelial cells
• Remodeling into capillary tubes
• Recruitment of periendothelial cells (pericytes for small
capillaries and smooth muscle cells for larger vessels)
• Suppression of endothelial proliferation and migration, and
redeposition of the basement membrane
• VEGF (mostly VEGF-A
timulates both migration and
proliferation of endothelial cells; fibroblast growth factors (FGFs),
mainly FGF-2, also stimulate endothelial cell proliferation, as well
as promoting macrophage, epithelial, and fibroblast migration.
(Ang 1 and Ang 2
ive the structural
maturation of new vessels by recruiting pericytes and SM cells
Ang 1
interacts with a tyrosine kinase receptor on endothelial cells
called Tie2. PDGF and TGF-β also participate in the stabilization
Notch signaling
regulates the sprouting and branching of new
vessels, ensuring the proper spacing to effectively supply healing
tissues.
ECM proteins contribute through interactions with integrin
receptors in endothelial cells and by providing
mechanical
scaffold
Matrix metalloproteinases (MMPs)
degrade ECM to permit
remodeling and extension of the vascular tube
Deposition of Connective Tissue
Deposition of connective tissue occurs through fibroblast migration
and proliferation, followed by ECM deposition; PDGF, FGF-2, and
TGF-β (mostly from M2 macrophages) all contribute.
most important: it stimulates fibroblast migration and
proliferation, increased synthesis of collagen and fibronectin, and
decreased degradation of ECM by inhibiting MMPs.
TGF-β
Healing by First Intention
Healing by first intention (or primary union) occurs when injury
involves only the epithelial layer; repair is mainly by epithelial
regeneration. In a clean, uninfected surgical incision approximated
by surgical sutures there is only focal disruption of the basement
membrane and relatively minimal cell death:
Wounding activates coagulation pathways
clot (containing
fibrin, fibronectin, and complement proteins) stops the bleeding
and acts as a scaffold for migrating cells. As dehydration occurs,
a scab is formed.
• Within 24 hours
neutrophils arrive at the incision margin,
releasing proteolytic enzymes that begin to clear the debris.
Within 24 to 48 hours, epithelial cells from both edges have
migrated and proliferated along the dermis, depositing basement
membrane components as they progress.
By day 3
neutrophils have been largely replaced by macrophages,
and granulation tissue progressively invades the incision space,
with collagen fibers evident at the incision margins.
By day 5
neovascularization reaches its peak with the ongoing
migration of fibroblasts, which are producing ECM proteins. The
epidermis recovers its normal thickness as differentiation of
surface cells yield a mature epidermal architecture with surface keratinization
During the second week
there is continued collagen accumulation
and fibroblast proliferation, but leukocyte infiltrate, edema, and
vascularity is diminished
By 4 weeks
the scar is well-formed with few inflammatory cells
Healing by Second Intention
Healing by second intention (or secondary union) happens when tissue loss is more extensive (e.g., large wounds, abscesses,
ulceration, and ischemic necrosis [infarction]); repair involves a
combination of regeneration and scarring. The inflammatory
reaction is more intense, and there is abundant granulation tissue,
with subsequent increased ECM accumulation and formation of a
large scar, followed by myofibroblast wound contraction
In wounds causing large tissue deficits
inflammation is more
intense because large tissue defects have a greater volume of
necrotic debris, exudate, and fibrin that must be removed.
Wound contraction generally occurs in large surface wounds;
within 6 weeks, large skin defects can be contracted to 5% to 10%
of their original size
Wound Strength
Carefully sutured wounds have approximately 70% of the strength
of normal skin; after suture removal at 1 week, wound strength is
approximately 10% of that of unwounded skin
Tensile strength
increases through collagen synthesis during the first 2
months of healing, and at later times from structural modifications
of collagen fibers (cross-linking, increased fiber size)
Wound
strength reaches approximately 70% to 80% of normal by
3 months
• Deficient scar formation
Inadequate granulation tissue or collagen
deposition and remodeling can lead to either wound dehiscence or
ulceration.
• Excessive repair
Excessive granulation tissue (proud flesh) can
protrude above the surrounding skin and block reepithelialization
Excessive collagen accumulation forms a
raised
hypertrophic scar
rogression beyond the original area of
injury without subsequent regression is termed a
keloid
Formation of contractures
Although wound contraction is a normal
part of healing, an exaggerated process is designated a
contracture
It will cause wound deformity
producing hand
claw deformities or limit joint mobility
EGF
Mitogenic for keratinocytes and fibroblasts; stimulates keratinocyte migration and granulation tissue formation
TGF-a
Similar to EGF; stimulates replication of hepatocytes and most epithelial cells
HB-EGF
Keratinocyte replication
HGF
Enhances proliferation of hepatocytes, epithelial cells, and endothelial cells; increases cell motility,
keratinocyte replication
