Tissue Repair, Renewal and Regeneration
1) Background
a) Healing Process of Tissues (after injury to cells, from series of damaging events)
i) Regeneration (complete restitution of lost or damaged tissue)
ii) Repair (May restore original structure, can cause structural derangements)
b) Healthy Tissues
i) Healing (Regeneration/Repair) - occurs after any insult that causes tissue destruction, essential for the survival
of the organism
2) Regeneration
a) Proliferation of cells and tissues – replace lost structures
i) Growth of amputated limb (amphibians)
ii) Mammalian Whole Organs and Complex Tissues
(1) Rarely – after injury, Applied to liver growth after
partial resection or necrosis (compensatory growth
rather than true regeneration)
iii) Hematopoietic System, Skin, GI Tract
(1) High Proliferative capacity, renew themselves
continuously, regenerate after injury
3) Repair
a) Combination of regeneration/scar formation (deposition of
collagen)
b) Contribution of regeneration/scarring depends on
i) Ability of tissue to regenerate, extent of the injury
(1) Ex: Superficial Skin Wound – heals through the regeneration of the surface
epithelium
c) Chronic Inflammation
i) Accompanies persistent injury, stimulates scar formation
(1) Local Production of growth factors/cytokines promote fibroblast
proliferation and collagen synthesis
d) Fibrosis
i) Extensive deposition of collagen
ii) ECM – components are essential for wound healing (provide framework for cell
migration, maintain correct cell polarity for the re-assembly of multilayer
structures, participate in angiogenesis – form new blood vessels)
(1) Fibroblasts, Macrophages, Others (produce growth factors, cytokines, and
chemokines – critical for regeneration and repair)
4) Normal Cell Proliferation
a) Adult Tissues
i) Size of cell populations (det. by rate of cell proliferation, differentiation and
death)
ii) Increased cell numbers may result (increased proliferation, decreased cell death)
b) Apoptosis
i) Process required for tissue homeostasis, induced by a variety of pathological stimuli
c) Terminally differentiated cells
i) Differentiated cells incapable of replication
ii) Impact of differentiation
(1) Depends on tissues under which it occurs (differentiated cells are not replaced, differentiated cells die but
are continuously replaced by new cells generated from stem cells)
5) Cell Proliferation (Stim. by physiological/pathological conditions)
a) Physiological Proliferation
i) Proliferation of endometrial cells under estrogen stimulation during menstrual cycle
ii) Thyroid-stimulating hormone-mediated replication of cells of the thyroid that enlarges the gland
iii) Stimuli may become excessive, creating pathological conditions
b) Pathologic Proliferation
i) Nodular prostatic hyperplasia (Dihydrotestosterone stimulation)
ii) Nodular goiters in the thyroid (Increased serum levels of thyroid-stimulating hormone)
c) Control of Cell Proliferation
i) Controlled by signals from microenvironment
(1) Stimulate or inhibit proliferation
(2) Excess of stimulators or deficiency of inhibitors (leads to net growth – in the case of cancer uncontrolled)
6) Tissue Proliferative Activity
a) Tissues of Body (divided into three groups)
i) Continuously dividing (labile tissues)
(1) Cells proliferate throughout life – replace destroyed cells
(2) Surface epithelia
(a) Skin, Oral Cavity, Vagina, Cervix
(b) Lining Mucosa of excretory glands of body - salivary glands, pancreas, biliary tract)
(c) Columnar epithelium of GI tract/uterus, Transitional epithelium of urinary tract, Cells of BM and
hematopoietic tissues
(d) Mature Cells are derived from adult stem cells – tremendous capacity for proliferation
ii) Quiescent (stable tissues)
(1) Low level of replication
(2) Cells from these tissues undergo rapid division in response to stimuli, capable of reconstituting the tissue of
origin
(3) Examples
(a) Parenchymal cells of liver, kidney, and pancreas
(b) Mesenchymal cells (fibroblasts and SM)
(c) Vascular Endthelial cells, Lymphocytes and other leukocytes
(d) Fibroblasts, endothelial cells, SM cells, Chondrocytes and osteocytes (Quiescent in adult mammals,
proliferate in response to injury, fibroblasts proliferate extensively)
(e) Ability of Liver to regenerate (Partial hepatectomy, Acute chemical injury
iii) Non-dividing (permanent tissues)
(1) Examples
(a) Neurons in the CNS (Destruction of cells – replaced by the proliferation of the CNS supportive elements
: glial cells)
(b) Mature Skeletal Muscle (Cells don’t divide)
(i) Regenerative Capacity – through the differentiation of satellite cells (attached to the endomysial
sheaths)
(c) Cardiac Muscle
(i) Very Limited regenerative capacity
(ii) Large injury to heart muscle (MI) – followed by scar formation
7) Stem Cells
a) Characterization
i) Self-renewal properties, capacity to generate differentiated cell lineages
b) Need to be maintained during life
i) Achieved by two mechanisms
(1) Obligatory Asymmetric Replication (With each stem cell division one of the daughter cells retain its selfrenewing capacity while the other enters differentiation pathway)
(2) Stochastic Differentiation (Maintained by balance between stem cell divisions that generate either two selfrenewing stem cells OR two that differentiate)
c) Embryonic Stem Cells
i)
Pluripotent
(1) Generate all tissues of the body
(2) Give rise to Multipotent Stem Cells (More restricted developmental potential, eventually produce
differentiated cells (three embryonic layers)
ii) Inner Cell mass of Blastocysts (early embryonic development)
(1) Contains pluripotent stem cells
(2) Cell isolated from blastocysts (maintained in culture as undifferentiated cells lines, Induced to differentiate
into specific lineages – Heart and Liver)
d) Adult Stem Cells (Somatic Stem Cells)
i) Restricted capacity to generate different cell types (identified in many tissues)
ii) Reside in special microenvironments
(1) Niches
(a) Composed of mesenchymal, endothelial and other cell types
(b) Generate or transmit stimuli that regulate stem cell self-renewal and the generation of progeny cells
iii) Reprogramming of Differentiated Cells
(1) Differentiated cells of adult tissues can be reprogrammed to pluripotent
(a) Transfer nucleus to enucleated oocyte  Implanted into a surrogate
mother can generate cloned embryos that develop into complete
animals (reproductive cloning successfully demonstrated in 1997)
(i) Generate ES cells – kept in culture then induced to differentiate into
various cell types
(ii) “In principle” can be transplanted to repopulate damaged organ
iv) Location – Tissues
(1) Continuously divide: BM, skin, lining of GI
(2) May be present in organs: Liver, Pancreas, and adipose tissue (don’t actively
produce differentiated cell lineages)
v) Transit Amplifying Cells
(1) Rapidly dividing cells – generated by somatic stem cells
(2) Lose capacity of self-perpetuation
(3) Give rise to cells w. restricted developmental potential (progenitor cells)
vi) Transdifferentiation (change in differentiation of a cell from one type to
another)
vii) Developmental Plasticity (Capacity of cell to transdifferentiate into diverse lineage)
Bone
Marrow
Liver
Brain
Skin
Intestinal
Epithelium
Skeletal
Muscle
Cornea
-Contains hematopoietic stem cells, contain stromal cells (AKA multipotent stromal cells, mesenchymal
stems cells or MSCs)
-Hematopoietic Stem Cells (Generate all blood cell lineages, reconstitute BM after depletion – caused by
disease or irradiation)
-Hematopoietic Stem Cells (Used for treatment of hematologic disease, collected directly from BM,
Umbilical Cord Blood, Peripheral blood of individuals receiving cytokines)
-Marrow Stromal Cells (MSCs) :
*Multipotent, potentially important therapeutic applications (Generate chondrocytes, osteoblasts,
adipocytes, myoblasts and endothelial cell precursors – depends on the tissues they migrate to)
*Migrate to injured tissues, generate stromal cells or other lineages, don’t participate in normal
tissue homeostasis
-Contains stem cells/progenitor cells in canals of Hering (Junction between biliary ductular system and
parenchymal hepatocytes, Gives rise to a population of precursor cells)
- Oval Cells: Bipotential progenitors, capable of differentiating into hepatocytes and biliary cells
*Function as a secondary/reserve compartment, activated only when hepatocyte proliferation is
blocked
*Proliferation/Differentiation : Fulminant hepatic failure, Liver tumorigenesis, Chronic hepatitis and
advanced liver cirrhosis
-Neural Stem Cells (NSCs) – Occur in the brain, Neural Precursor Cells, Capable of generating neurons,
astrocytes and oligodendrocytes
-Identified in the subventricular zone, or dentate gyrus of the hippocampus
-Human epidermis has high turnover rate (~4 weeks)
-Stem cells – located in three areas of the epidermis
*Hair Follicle Bulge (niche for stem cells that produce cells of hair follicle
*Interfollicular Areas of Surface Epidermis (Stem cells are scattered individually through epidermis, divide
infrequently, generate transit amplifying cells)
*Sebaceous Glands
-Small Intestine
*Crypts : Monoclonal Structures, Derived from single stem cells, Regenerate the crypt (3-5days)
*Villus : Differentiated compartment, contains cells from multiple crypts
-Myocytes don’t divide (even after injury)
-Grow/Regeneration of injured skeletal muscle – occurs by replication of satellite cells (located beneath
myocyte basal lamina, constitute reserve pool of stem cells, generate differentiated myocytes after injury)
-Transparency of Cornea (Integrity of the outermost corneal epithelium) – maintained by limbal stem cells
(LSCs), located at the junction between the epithelium of the cornea and the conjunctiva
8) Cell Cycle
a) Replication of cells – stimulated by growth factors, stimulated by signaling from ECM components (integrins)
b) Each Cell Cycle Phase – dependent on proper activation, dependent on completion of the previous one, cycle stops
at a place at which an essential gene function is deficient
c) Cell Cycle – multiple controls/redundancies, particularly during transition between G1 and S phases
i) Go  G1 (Transcriptional activation of large set of genes – various proto-oncogenes, genes required for
ribosome synthesis and protein translation)
(1) G1 Cells – progress through the cycle, reach critical stage at G1/S transition (restriction point-rate limiting
step for replication),past the restriction point normal cells are irreversibly committed to DNA rep
ii) Progression through cell cycle (esp G1/S) tightly regulated by cyclins (proteins) and cyclin-dep kinases (enzyme)
(1) Cyclin-CDK complexes, tightly reg by CDK inhibitors, Some GFs shut off production of these inhibitors
iii) Surveillance Mechanisms (geared at sensing DNA/chromosome damage), quality controls – checkpoints ensure
DNA damaged cells don’t replicate
Cell Cycle Checkpoints
G1/S
Monitor integrity of DNA before replication
G2/M
Checks DNA after rep, monitor if cell can safely enter mitosis
-Cell senses DNA damage – checkpoint activation delays the cell cycle (triggers DNA repair mechanism)
-DNA damage too severe (cells eliminated by apoptosis, enter non-replicative state called senescence)
-Checkpoint defects allows cells with DNA strand breaks/chromosome abnormalities to divide  Produce mutations in
daughter cells that may lead to neoplasia
9) Growth Factors
a) Proliferation of many cell types – driven by polypeptides
b) Restricted or multiple cell targets – promote cell survival,
locomotion, contractility, differentiation, and angiogenesis
c) Functions as ligands that bind to specific receptors (deliver signals
to target cells – stimulate transcription of genes that may be silent
in resting cells)
Growth Factors
Epidermal Growth
Factor (EGF)
Transforming
Growth Factor (TGFα)
Hepatocyte Growth
Factor (HGF)
Platelet-Derived
Growth Factor
(PDGF)
Vascular Endothelial
Growth Factor
(VEGF)
Fibroblast Growth
Factor (FGF)
Transforming
Growth Factor (TGFβ)
Cytokines
-Receptor: EGFR
-Mitogenic for variety of epithelial cells, hepatocytes, and fibroblasts – widely distributed in tissue
secretions and fluids
-Receptor: EGFR
-Originally extracted from sarcoma virus-transformed cells
-Action: Iinvolved in epithelial cell proliferation in embryos/adults, malignant transformation of
normal cells to cancer
-Homology with EGF, binds to EGFR, shares biological activity of EGF
-Produced by Fibroblast, most mesenchymal cells, endothelial cells and liver nonparenchymal cells
-Identical to a previously identified GF – isolated from fibroblasts (scatter factor)
-Action: Mitogenic Effects: Hepatocytes/Most Epithelial Cells, Promotes cell scattering/migration
 enhances survival of hepatocytes
-Family of closely related proteins (two chains each
-Three isoforms : AA, AB, BB (secreted as biologically active molecules)
-Produced By: Activated macrophages, endothelial cells, SM cells, many tumor cells
-Action: Migration/Proliferation of fibroblasts, SM cells and monocytes (areas of inflammation and
healing skin wounds)
-Action: Potent inducer of blood vessel formation in early development, central role in growth of
new blood vessels in adults, promote angiogenesis in chronic inflammation, healing of wounds
and in tumors
-More than 20 members
-Action: Wound healing responses (re-epithelialization of skin wounds), hematopoiesis,
angiogenesis, development (Skeletal/Cardiac Muscles, Lung maturation, specification of the liver
from endodermal cells)
-Produced by: Platelets, Endothelial Cells, Lymphocytes, Macrophages
-Action: Potent fibrogenic agent (stim. fibroblast chemotaxis, enhances production of collagen,
fibronectin and proteoglycan,Inhibit collagen degradation- dec. matrix proteases, inc. protease
inhibitor activities)
-Development of fibrosis in variety of chronic inflamm. responses (Lunges, Liver, Kidney)
-Imp. Functions: Mediators of Inflamm. and Immune Response
-TNF/IL-1 : Participate in wound healing
TNF/IL-6L Involved in initiation of liver regeneration
10) Signaling Mechanisms
a) Receptor-Mediated Signal Transduction
i) Activated by binding: Ligands, GFs, Cytokines to specific receptors
b) Three Modes of Signaling
i) Based on source of ligand/location of its receptor (Autocrine, Paracrine, Endocrine)
Autocrine
-Cells responding to signaling molecules that THEY secrete, establishes an autocrine loop
-Autocrine growth regulation: Plays role in liver regeneration, proliferation of Ag-stim. lymphocytes
-Tumors overproduce GF’s and receptors to stim. their proliferation
Paracrine
-One cell produces ligand  receptors on adjacent (close proximity) target cells
-Paracrine Stimulation: Common in CT repair of healing wounds (Macrophage produces facto – growth
effect on adjacent cell – fibroblast)
*Necessary for hepatocyte replication during liver generation, notch effect in embryonic
development, wound healing and renewing tissues
Endocrine
-Hormones synthesized by cells of endocrine organs (act on target cells distant from synthesis site,
carried by blood, GFs may also circulate & act on distant sites: HGF)
-Several Cytokines: Associated w. systemic aspects of inflammation (Act as endocrine agents)
11) Receptor Types
a) Properties of major types of receptors
i) Importance: How they deliver signals to cell interior (pertinent to understanding of normal unregulated
(neoplastic) cell growth)
Intrinsic
-Most GFs (EGF, TGF-A, HGF, PDGF, VEGF, FGF, c-KIT ligand and Insulin)
Tyrosine Kinase
-Receptors Belonging to the Family (Extracellular ligand-binding domain, Transmembrane region,
Activity
Cytoplasmic tail that has intrinsic tyrosine kinase activity)
-Binding Induces: Dimerization of receptor, Tyrosine phosphorylation, Activation of receptor tyrosine
kinase (active kinase phosphorylates: activates downstream effector molecules, which mediate effects of
receptor engagement with a ligand)
-Recruit Kinases, ligands include many cytokines (IL2, IL3, Interferons A/B/G, Erythropoietin, GH,
Prolactin)
Lacking Intrinsic
-Transmit extracellular signals to the nucleus (Activates members of JAK family of proteins, link the
Tyrosine Kinase
receptors and activate cytoplasmic transcription factors) : STATS (directly shuttle into nucleaus and
Activity
activate gene transcription
G Protein-Transmit signals into cell through trimeric GTP-binding proteins
coupled
-Contains seven transmembrane alpha helices
Receptors
-Largest family of plasma membrane receptors (nonodorant G protein-coupled receptors – account for
about 1% of the human genome)
-Ligands signaling through this type of receptor (Chemokines, Vasopressin, Serotonin, Histamine, NE/E,
Calcionin, glucagon, PTH, Corticotropin, rhodopsin) ----- large number of drugs target these receptors
Steroid
Hormone
Receptors
-Located in nucleus, function as ligand dependent transcription factors (Ligands diffuse through cell
membrane, bind the inactive receptors – cause their activation, activated receptor binds to specific DNA
sequences) ---- bind to other transcription factors
- Ligands : Thyroid Hormone, Vitamin D, Retinoids
-Receptors: Peroxisome proliferator-activated receptors (nuclear receptors involved in broad range of
responses – adipognesis, inflammation, artherosclerosis)
12) Transcription Factors
a) Transfer of info to the nucleus (modulate
gene transcription, through action of these
factors)
b) Transcription of factors that regulate cell
proliferation (products of several growthpromoting genes : cMYC, cJUN AND
products of cell cycle-inhibiting genes : p53)
c) Contain domains for DNA binding and
transcriptional regulation
13) Mechanism of Tissue and Organ Regeneration
a) Characteristics
i) Can’t regenerate whole tissues/organs in mammals (Absence of blastema formation- source of cells for
regeneration, rapid fibroproliferative response after wounding)
b) Wnt/B-Catenin
i) Highly conserved pathway, participates in regeneration of planaria flatworms, fin & heart regeneration in zebra
fish, blastem and patterning formation in limb regeneration in newts
ii) Mammals: Modulates stem cell function (intestinal epithelium, BM, muscle), participates in liver regeneration
after patial hepatectomy, stimulates oval cell proliferation after liver injury
c) Compensatory Growth – even this process not “truly” regeneration
i) Resection of tissue – doesn’t cause new growth, merely compensatory hyperplasia of remaining organ
ii) Ex: Liver, Kidney, Pancreas, Adrenal Glands, Thyroid, Lungs (very young animals)
Compensatory Growth of Organs
-New nephrons can’t be generated in adults, growth of contralateral kidney (after unilateral nephrectomy),
involves nephron hypertrophy and replication of proximal tubule cells
Pancreas -Limited capacity to regenerate exocrine componenets and islets
-Regeneration of pancreatic beta cells (B-cell replication, transifferentiation of ductal cells, differentiation of
putative stem cells)
Liver
-Resection of ~60% of liver in living donor, doubling of liver remnant in ~1month
-Portions of liver remaining after partial hepatectomy – intact “mini liver”, rapidly expands and reaches mass
of original liver (WITHOUT regrowth of resected lobes)
-Restitution of functional mass rather than reconstituition of the original
-Almost ALL hepatocytes replicate during liver regeneration
-Hepatocytes : Quiescent Cells (Several hours to enter cell cycle, progress G1, Reach S-phase of DNA rep)
-Wave of hepatocyte rep. (Synchronized, Followed by synchronous rep of nonparenchymal cells – Kupffer
cells, endothelial cells and stellate cells) : Triggered by combined actions of cytokines/polypeptide GFs, except
Autocrine activity of TGF-A
-Two major restriction points for hepatocyte rep: Go/G1, and G1/S
*Quiescent Hepatocytes: become competent to enter cell cycle (mediated by the cytokines TNF/IL6 and
components of complement system), Stimulated by HGF, TGF-A, and HB-EGF, primed hepatocytes enter cell
cycle and undergo DNA replication ---- NE, Serotonin, Insulin, Thyroid and GH help facilitate entry of
hepatocytes into cell cycle
*Individual Hepatocytes: Replicate once or twice during regeneration, return to quiescence in strictly reg.
sequence of events
*Intrahepatic Stem or progenitor cells – don’t play role in compensatory growth
Kidney
14) Extracellular Matrix / Cell-Matrix Interactions
a) Tissue Repair & Regeneration
i) Depends On: Activity of Soluble Factors, Interactions between cells and components of ECM (regulates growth,
proliferation, movement and differentiation of cells)
ii) Functions (ECM)
(1) Mechanical Support: Cell anchorage, migration and maintenance of cell polarity
(2) Control of Cell Growth: Regulate cell proliferation by signaling through cellular receptors of integrin family
(3) Maintenance of Cell Differentiation: Type of ECM proteins, affect degree of differentiation of cells in the
tissues
(4) Scaffolding for Tissue Renewal: Maintain normal tissue structure (requires basement membrane), Integrity
of basement membrane (stroma of parenchymal cells) – critical for organized regeneration of tissues
(5) Establishment of Tissue Microenvironments: Basement Membrane (Boundary between epithelium and
underlying CT, forms part of filtration environment in kidney)
(6) Storage and Presentation of Regulatory Molecules: GFs FGF/HGF are secreted and stored in ECM in some
tissues, allows rapid deployment of GFs after local injury or during regeneration
iii) Three Groups of Macromolecules
(1) Fibrous Structural Proteins: Collagens and elastins (provide tensile strength and recoil)
(2) Adhesive Glycoproteins: Connect matrix elements to one another and to cells
(3) Proteoglycan/Hyaluronan: Provide resilience and lubrication
iv) Two Basic Forms of ECM
(1) Interstitial Matrix:
(a) Found in spaces between epithelial, endothelial, SM cells, as well as CT
(b) Consists mostly of fibrillar/non-fibrillar collagen, elastin, fibronectin, proteoglycan, hyaluronan
(2) Basement Membrane
(a) Closely associated with cell surfaces
(b) Consist of nonfibrillar collagen (mostly type IV), laminin, heparin sulfate, and proteoglycans
v) Fibrous Structural Proteins
(1) Collagen
(a) Characteristics
(i) Most common protein in animal world (provides extracellular framework)
(ii) No collagen would mean human would be reduced to clump of cells
(iii) Each collagen composed of three chains (triple helix shape)
(b) Types
(i) I, II, III, V, and XI
(ii) Fibrillar collagens, triple-helical domain, proteins found in extracellular fibrillar structures
(iii) Type IV: Long, interrupted triple helical domains (form sheets) – main components of basement
membrane, along with laminin
(c) Collagen Fibril Formation
(i) Oxidation of lysine and hydroxylysine residues by extracellular enzyme lysyl oxidase (cross-linking
between chains of adjacent molecules – major contributor to tensile strength of collagen)
(d) Vitamin C
(i) Required for hydroxylation of procollagen, requirement that explains the inadequate wound
healing in scurvy
(e) Genetic Defects in collagen production (Inherited syndromes – Ehlers-Danlos syndrome and
osteogenesis imperfect)
(2) Elastin, Fibrillin, Elastic Fibers
(a) Blood Vessels, Skin, Uterus, Lungs (require elasticity for their function)
(b) Morphologically (consist of central core made of elastin – surrounded by peripheral network of
microfibrils)
(c) Substantial Amounts of Elastin (Found in walls of large blood vessels – aorta, uterus, skin, ligaments)
(d) Fibrillin
(i) Scaffolding for deposition of elastin and assembly of elastic fibers, influence the availability of
active TGF-B in the ECM
(ii) Inherited Defects in Fibrillin: Marfan Syndrome (form abnormal elastic fibers), changes in
cardiovascular system (aortic dissection) and the skeleton
(3) Cell Adhesion Proteins
(a) AKA CAMs (Cell Adhesion Molecules)
(i) Function as transmembrane receptors – sometimes stored in cytoplasm
(ii) Can bind to similar or diff molecules in other cells – interaction between the same cells, different
cell types
(b) Classified into four main families: Immunoglobulin family CAMs, Cadherins, Integrins, Selectins
(i) Integrins : Bind to ECM proteins such as fibronectin, laminin and osteopontin (provides a
connection between cells and ECM and adhesive proteins in other cells)
(ii) Cadherins:
1. Link cell surface with cytoskeleton, binding actin to intermediate filaments (Linkages:
Mechanism of transmission of mechanical force, activation of intracellular signal transduction
pathway)
2. Connect plasma membrane of adjacent cells, two types of cell junction (Zonula Adherens,
Desmosomes)
3. Diminished function of E-Cadherin: Contributes to certain forms of breast/gastric cancer
(iii) ECM Proteins
1. Fibronectin
a. Binds to many molecules,consists of 2 glycoprotein chains, held together by disulfide bonds
b. Messenger RNA has two splice forms, tissue fibronectin and plasma fibronectin (plasma stabilize blood clot that fill gaps created by wounds, substratum for ECM deposition and
formation of provisional matrix during wound healing)
2. Laminin
a. Most abundant glycoprotein in basement membrane, binding domains for ECM & cell
surface proteins, mediates attachment of cells to CT substrates
(4) Other Secreted Adhesion Molecules
(a) SPARC (Secreted Protein Acidic and Rich in Cysteine)
(i) “Osteonectin” – contributes to tissue remodeling in response to injury (angiogenesis inhibitor)
(b) Thrombospondins
(i) Similar to SPARC, Inhibit Angiogenesis
(c) Osteopontin (OPN)
(i) Regulates Calcification, mediator of leukocyte migration involved in inflammation, vascular
remodeling and fibrosis in various organs
(d) Tenascin Family
(i) Involved in morphogenesis and cell adhesion
vi) Glycosaminoglycans (GAGs)
(1) Makes up 1/3 of component in ECM (consist of long repeating polymers of specific disaccharides), linked to
core protein
(2) Forming molecules called proteoglycans
(a) Ground substance (organize ECM) – regulate CT structure and permeability
(b) Intergral membrane proteins (act as modulators) – Inflammation, Immune Responses, Cell Growth &
Differentiation, Binding of other proteins, Acitvation of GFs/Chemokines
(3) Four Structurally Distinct Families
(a) Heparan Sulfate, Chondoitin/Dermmatan Sulfate, Keratan Sulfate, Hyaluronan (HA)
(b) Hyaluronan (HA)
(i) Found in ECM of many tissues
(ii) Abundant In: Heart Valves, Skin, Skeletal Tissues, Synovial Fluid, Vitreous of the Eye, Umbilical Cord
(iii) Forms viscous hydrated gel (gives CT the ability to resist compression forces)
(iv) Provide resilience and lubrication to CT (notably for cartilage in joints)
(v) Concentration increases in Inflammatory Diseases (Rheumatoid arthritis, scleroderma, psoriasis
and osteoarthritis)
(vi) Hyaluronidases
1. Enzymes that fragment hyaluronan
15) REPAIR by Connective Tissue – replacement of nonregenerated parenchymal cells with CT
a) Severe or Persistent Tissue Injury – damage to parenchymal/stromal cells (leads to situation where repair can’t be
accomplished by parenchymal regeneration alone)
b) Timeline
i) Tissue repair begins within 24 hours of injury (Stim. emigration of fibroblasts, Induction of fibroblasts &
endothelium)
(1) Wounding – rapid activation of coagulation pathway (forms blood clot on wound surface) – entrapped
RBCs, fibrin, fibronectin and complement components (clot serves to stop bleeding and as a scaffold for
migrating cells – attracted by GFs, cytokines and chemokines released into area)
(2) Release of VEGF (increased vessel permeability and edema)
(3) Dehydration occurs at external surface of clot (forms scab that covers wound) – Neutrophils appear at the
margins of the incision (use scaffold provided by the fibrin clot to infiltrate in, release proteolytic enzymes
that clean out debris and invading bacteria)
ii) Epithelial Cells (24-48hrs) move from the wound edge along the cut margins of the dermis, depositing
basement membrane components as they move (fuse at midline creating continuous epithelial layer beneath
scab that closes the wound)
iii) Neutrophils – replaced by macrophages by 48-96 hours (macrophages are key cellular constituents of tissue
repair, promoting angiogenesis and ECM deposition)
iv) Specialized Type of Tissue Appears at 3-5 days
(1) Fibroblast and Vascular Endothelial Cells (Proliferate in first 24-72 hours, Form specialized type of tissue
“GRANULATION TISSUE- HALLMARK OF TISSUE REPAIR”)
(2) Characteristic of Healing = “Granulation Tissue” (Pink, Soft – like under a scab, characterized by fibroblast
proliferation and new thin walled delicate capillaries)
v) Granulation Tissue
(1) New Vessels are Leaky – Allow the passage of plasma proteins and fluid into the extravascular space, new
granulation tissue is often edematous
(2) Progressively invades the incision space
(3) Amount of Granulation Tissue Formed depends on size of tissue
deficit created by wound, intensity of inflammation (more
prominent in healing by secondary union)
(4) 5-7 days Granulation Tissue fills the wound area
(neovascularization is maximal)
vi) Leukocytic infiltrate, edema and increased vascularity decrease
during the second week (blanching begins – increased accumulation
of collagen within wound area and regression of vascular channels)
vii) Original granulation tissue scaffolding is converted to a pale,
avascular scar (by end of month scar is made up of acellular CT
devoid of inflammatory infiltrate – covered by intact epidermis)
c) Four Components of Process
i) Angiogenesis
(1) Two processes
(a) Vasculogenesis: Assembly of primitive vascular network (from angioblast)
(b) Angiogenesis/Neovascuilarization: Pre-existing blood vessels send out capillary sprouts
(2) Critical process in healing at sites of injury, development of collateral circulation at ischemic sites (stim.
following MI or Artherosclerosis), Allows tumors to grow
(3) Vasodilation: Response to NO, VEGF (Induced increased permeability of the preexisting vessel)
(4) Proteolytic degradation of the basement membrane of parent vessel: Matrix Metalloproteinases (MMPs),
Disruption of cell-cell contact between endothelial cells by plasminogen activator
(5) Migration of endothelial cells (toward angiogenic stimulus)
(6) Proliferation of endothelial cells (just behind leading front of migrating cells)
(7) Maturation of endothelial cells (includes inhibition of growth/remodeling of capillary tubes)
(8) Recruitment (Pariendotlial cells, pericytes and vascular SM cells to form mature vessel)
(9) Induced by many factors: bFGF (basic FGF), VEGF (vascular endothelial growth factor)
ii) Migration/Proliferation of Fibroblasts
(1) Migration of fibroblasts to site of injury (Driven by chemokines – TNF, PDGF, TFG-B, FGF)
(2) Proliferation triggered by multiple GFs (PDGF, TGF-B, FGF, Cytokines IL1, TNF) – macrophages main source
for these factors
(3) TGF-B = most important fibrogenic agent (produced by most cells in granulation tissue, cause fibroblast
migration & proliferation, increased synthesis of collagen and fibronectin and decreased degradation of
ECM by metalloproteinases)
iii) Deposition of ECM
(1) Collagen fibers are present at margins of incision (at first vertically oriented, don’t bridge the incision)
(a) 24-48hrs epithelial cells move along margins of dermis (depositing basement membrane components
as they move – fuse in midline, produce thin continuous epithelial layer that closes the wound)
(2) Full Epithelialization of wound surface – much slower in healing by secondary union (gap to be bridged is
much greater – subsequent epithelial cell proliferation thickens epidermal layer)
(3) Macrophages – Stimulate fibroblasts (signal through chemokine receptor CXCR3 – promotes skin reepithelialization)
(4) At same time of epithelialization – collagen fibrils become more abundant, begin to bridge the incision
(5) Provisional matrix containing fibrin, plasma fibronectin and type III collagen is formed (replaced by matrix
primarily composed of type I collagen)
iv) Remodeling (maturation and reorganization of fibrous tissue)
(1) Excessive deposition of collagen and other ECM components, deposition of collagen in chronic diseases
(2) Recovery of Tensile Strength
(a) Fibrillar collagens (mostly type I collagen) – form a major portion of the CT in repair sites, essential for
the development of strength in healing wounds
(b) Net collagen accumulation (depends not only on increased collagen synthesis but also dec.
degradation)
(c) Length of Time to achieve maximal strength : Sutures removed from an incisional wound (end of first
week 10%, strength increases rapidly over 4 weeks, slows at 3rd month, reaches plateau at 70-80%)
(d) Lower Tensile Strength (healed wounded area may persist for life)
(e) Occurs through: excess of collagen synthesis over collagen degradation during 1st two months of
healing, structural modifications of collagen fibers after collagen synthesis ceases
16) Cutaneous Wound Healing
a) Three Phases: Phases Overlap (separation is somewhat arbitrary)
i) Inflammation (Initial injury causes platelet adhesion and aggregation, formation of a clot in the surface of the
wound)
ii) Proliferation (Formation of granulation tissue, proliferation and migration of CT cells and re-epithelialization of
the wound surface)
iii) Maturation (Involves ECM deposition, tissue remodeling and wound contraction)
b) Simplest type of cutaneous wound repair (healing of clean, uninfected surgical incision), approximated by surgical
sutures, referred to as healing by primary union or by first intention
c) Types of Wounds
i) Incision
(1) Death of limited number of epithelial and CT cells, disruption of basement membrane continuity, reepithelialization to close the wound (occurs with formation of relatively thin scar)
ii) Excisional Wounds
(1) Repair process is more complicated, create large defects on skin surface (extensive loss of cells and tissue)
d) Healing of these Wounds
i) More intense inflammatory reaction, formation of abundant granulation tissue, extensive collagen deposition,
Lead to formation of substantial scar (Generally contracts, healing by secondary union/second intention)
e) Factors Influencing Wound Healing
i) Systemic Factors (Nutrition – protein deficiency, esp. Vitamin C deficiency ; Metabolic Status – Diabetes
associated with delayed healing)
ii) Circulatory Status (Modulate Wound Healing, Inadequate blood supply)
iii) Hormones (Glucocortocoids – well documented anti-inflammatory, influence various components of
inflammation, agents also inhibit collagen synthesis)
iv) Infection (Results in persistent tissue injury and inflammation)
v) Mechanical Factors (Early motion of wounds, can delay healing, compressing blood vessels and separating
edges of the wound)
vi) Foreign Bodies (unnecessary sutures or fragments of steel, glass or even bone impedes healing)
vii) Size, Location, Type of Wound (Richly Vascularized areas, such as the face heal faster than poorly vascularized
AND small incisions heal faster with less scar formation than large wounds or blunt trauma wounds)
f) Complications in Wound Healing
i) Arise from abnormalities (three categories)
(1) Deficient Scar Formation
(a) Wound dehiscence (Rupture of wound – most common after abdominal surgery, due to increased
abdominal pressure --- vomiting, coughing, etc.)
(b) Ulceration (Inadequate vascularization during healing, areas devoid of sensation)
(2) Excessive Formation of the Repair Components
(a) Give rise to hypertrophic scars and keloids (accumulation of excessive amounts of collagen) – develop
after thermal or traumatic injury (involves deep layers of the dermis)
(b) Keloid – individual predisposition, more common in African americans
(c) Exuberant Granulation – Deviation in wound healing, form excessive amounts of granulation tissue,
protrudes above level of surrounding skin, blocks re-epithelialization, must be removed by cautery or
surgical excision
(3) Formation of Contractures
(a) Occurs in large surface wounds
(b) Contraction helps close wound by decreasing gap between dermal edges (reducing wound SA) – imp
feature in healing by secondary union
(c) Replacement of granulation tissue with a scar (involves changes in the composition of the ECM)
(d) Particularly prone to develop on palms, soles and anterior aspect of thorax (commonly seen after
serious burns and can compromise movement of joints)
ii) Fibrosis
(1) Denote the excessive deposition of collagen and other ECM components in a tissue, deposition of collagen
in chronic diseases