Wound Repair, Tissue Renewal, Regeneration and Repair Flashcards
how is regeneration achieved?
- proliferation of remaining mature cells 2. adult stem cells may regenerate lost tissue
what is regeneration?
complete restoration of lost or damaged tissues
what is repair?
combination of regeneration and scar formation
what does the contribution of regeneration and scar formation in repair depend on?
the ability of the type of injured tissue to regenerate and the extent of the injury
what are the cell categories according to their proliferative activity?
- labile tissues 2. stable tissues 3. permanent tissues
what are labile tissues?
continuously dividing tissues that are continuously being lost and replaced by proliferating mature cells and maturing tissue stem cells (short G0) – usually contain stem cells which have unlimited capacity to proliferate
what are stable tissues?
quiescent tissues that are in a G0 stage of the cell cycle but with growth factor signals at the site of injury, the residual cells will proliferate and new cells will develop from tissue stem cells. there is a limited ability to regenerate (except liver). low level of replication.
what are permanent tissues?
terminally differentiated and nonproliferative cells – have left cell cycle forever and CANNOT undergo mitosis postnatally
what tissues are labile?
epidermis of skin, GI, hematopoietic cells
what tissues are stable tissues?
limited ability to regenerate (except for liver) 1. parenchymal cells of liver, pancreas, kidney 2. fibroblasts, chondroblasts, osteocytes, smooth muscle 3. vascular endothelial cells
what tissues are permanent tissues?
neurons, mature skeletal muscle cells (may regenerate through satellite cells), cardiac muscle
what is G1 stage?
presynthetic phase
what is S stage?
cell actively synthesizing DNA
what is G2 stage?
postsynthetic premitotic stage
what is G0 stage?
intermitotic stage, resting cell can go into G1 stage then S, then G2 then M
what are stem cells characterized by?
- self renewal 2. asymmetric division (one daughter cell differentiates into mature cell, other remains stem cell)
2 main categories of stem cells
- embryonic stem cells (pluripotent) 2. somatic/adult stem cells (restricted in cell types)
where are adult stem cells located?
special microenvironments (niches) 1. isthmus of gastric glands 2. crypt cells in intestines 3. bulge area of hair follicle 4. limbus of cornea 5. canals of hering in liver
where are adult stem cells present?
continuously dividing tissue like bone marrow, skin, lining of GI tract, others
more examples of labile tissues
- surface epithelium like skin, oral cavity, vagina, cervix 2. columnar epithelium of GI tract and uterus 3. lining mucosa of excretory ducts (salivary glands, pancreas, biliary tract) 4. transitional epithelium of urinary tract 5. bone marrow
stem cell niches in the skin hair follicle bulge but also ?
interfollicular stem cells also scattered throughout surface epidermis and sebaceous gland
what are stem cell in skin hair follicle bulges involved in?
regeneration of hair follicle cells as well as surface epidermis of skin after injury
where are crypt cells and what do they do
stem cells in the crypts of intestines for regeneration of gut epithelium
2 mechanisms that liver can regenerate
- proliferation of surviving hepatocytes after injury triggered by cytokines and GF 2. liver stem cells (oval cells) in niches (canals of hering)
what is required for liver regeneration?
reticulin framework must be intact for regeneration
what 3 conditions are required for healing by regeneration
- tissue is composed of labile or stable cells 2. area of injury must contain some surviving (viable) cells that are capable of undergoing cell division 3. connective tissue framework must be intact to serve as scaffolding
explain stem cell niches in the brain
mature neurons do not undergo cell division but neural stem cells are present in the brain of adult humans but it is unclear if newly generated neurons are integrated into neural circuits
what can neural stem cells do?
neural precursor cells are capable of generating neurons, astrocytes and oligodendrocytes
explain stem cell niches in skeletal muscle
does not divide after injury, but satellite cells are a reserve pool of stem cells located beneath myocyte lamina with the ability to generate differentiated myocytes after injury
when does healing by scar formation occur?
in injured tissues incapable of regeneration due to: 1. tissue consists of permanent non dividing cells 2. no surviving tissue cells remain 3. connective tissue framework is destroyed
what does connective tissue repair involve?
ECM
what is part of the ECM?
- interstitial matrix 2. basement membrane
what is part of the interstitial matrix
collagen, elastin, fibronectin (adhesive glycoprotein)
what is part of the basement membrane/
collagen type IV, laminin (adhesive glycoprotein)
what part of cell membrane binds to ECM proteins? (the adhesive glycoproteins)
integrins
6 functions of the extracellular matrix
- mechanical support 2. control of cell growth 3. maintenance of cell differentiation (integrins) 4. scaffolding for tissue renewal 5. establishment of tissue microenvironments 6. storage and presentation of regulatory molecules
how does ECM control cell growth
can regulate cell proliferation by signaling through cellular integrin receptors
how is ECM used for scaffolding for tissue renewal
must be uninjured for regeneration, otherwise scar will occur
how does ECM establish tissue microenvironments/
the basement membrane forms a boundary between the epithelium and underlying connective tissue ex. glomerular basement membrane
how does ECM work for storage and presentation of regulatory molecules?
some growth factors are secreted and stored in the ECM, allowing for rapid deployment after injury
what are the main components of the ECM
- fibrous structural proteins like collagen and elastins (tensile strength and recoil) 2. adhesive glycoproteins that connect the matrix elements to one another and cells (fibronectin, laminin) 3. proteoglycans and hyaluronan provide resilience and lubrication
how is collagen made?
procollagen is secreted from the cell and cleaved by protease to form basic unit of fibrils
what is fibril formation associated with?
oxidation of lysine and hydroxylysine residue by lysyl oxidase, results in cross linking with adjacent molecules
how is vitamin C involved with collagen formation?
activates prolyl and lysyl hydroxylases which hydroxylate procollagen.
what happens in vitamin C deficiency?
inadequate hydroxylated procollagen cannot acquire a stable helical configuration, results in scurvy
what is scurvy
vitamin C deficiency, poor healing and bleeding tendency due to weak vascular basement membrane, children will have skeletal deformities
where is type I collagen
ubiquitous in hard and soft tissues
where is type II collagen
cartilage, intervertebral disk, vitreous fluid
where is type III collagen
hollow organs, soft tissues
where is type IV collagen
basement membranes
where is type V collagen
soft tissues and blood vessels
where is type VI collagen?
microfibrils
where is type VII collagen?
anchoring fibrils at dermal epidermal junctions
where is type IX collagen?
cartilage, intervertebral disks, vitreous fluid
where is type XVII collagen?
transmembrane collagen in epidermal cells
where is type XV and XVIII collagen?
endostatin forming collagens, endothelial cells
3 phases of cutaneous wound healing?
- inflammation -initial injury has clot formation with platelet adhesion and aggregation 2. proliferation - formation of granulation tissue (prolif of fibroblasts and vascular endothelial cells) 3. maturation and reorganization - ECM deposition, remodeling, wound contraction
what occurs in acute inflammation with wound healing
- activation of coagulation pathways with blood clot form on wound surface 2. growth factors, cytokines, and chemokines are released 3. neutrophils appear within 24hours
what causes a scab?
dehydration of surface clot
what does the clot do?
stops bleeding and acts as scaffold for cells migrating into the wound
what is one cause of edema in wound healing?
VEGF causes edema due to increased vascular permeability
what shows up 24hours of injury?
neutrophils that release proteolytic enzymes to clear debris
what do polypeptide messenger molecules do?
bind to specific membrane receptors on target cells to stimulate transcription of genes that may be silent in resting cells
how do GF promote cell proliferation?
recruit G0 cells into the cell cycle (mitogenic)
what else go GF do in wound healing?
influence other cell functions like cell differentiation, chemotaxis, angiogenesis
what is granulation tissue?
pink, soft granular appearance on surface of wounds
how much granulation tissue is formed?
depends on size of defect and amount of inflammation
what forms granulation tissue.
proliferation of fibroblasts and vascular endothelial cells in the first 24-72 hours
when is granulation tissue maximal?
5-7 days, apparent at 3-5 days
what is the histology of granulation tissue?
blood vessel formation, fibroblast proliferation, edema, macrophages mostly, other inflammatory cells
when is collagen synthesis by fibroblasts apparent?
days 3-5, continues for several weeks
describe what new vessels are like in angiogenesis?
are immature and have leaky interendothelial junctions, allows exudate to form, and makes granuation tissue edematous
where do new blood vessels originate
in part from budding of preexisting blood vessels that are within the area of repair
when growth factors are involved in angiogenesis?
VEGF and FGF
what does notch signaling do in angiogenesis
regulates sprouting/branching of new blood vessels
how does ECM help in angiogenesis
provides scaffolding for vessels and interacts with endothelial cells integin receptors
what cell replaces neutrophils in 48-96hours
macrophages
what is M1
classically activated macrophages that secrete mediators of inflammation to clear debris, fibrin and other foreign material
what is M2
alternatively activated macrophages that promote angiogenesis, activate fibroblasts, and stimulate collagen synthesis
how do macrophages assist in debridement and removal of injured tissue and debris
phagocytosis, collagenase, elastase
how do macrophages have antimicrobial activity
secrete nitric acid and ROS
how do macrophages help with chemotaxis and proliferation of fibroblasts and keratinocytes
secrete PDGF, TGFbeta, TNF, IL-1, KGF-7
how do macrophages help with angiogenesis
secrete VEGF, FGF-2, and PDGF
how do macrophages help with deposition and remodeling of ECM
secrete TGFbeta, PDGF, TNF, OPN IL-1, collagenase, and MMPs
what is the most important fibrogenic agent?
TGFbeta
what does TGFbeta do
causes: 1. fibroblast migration and proliferation 2. increased synthesis of collagen and fibronectin 3. decreased degradation of ECM by MMP
what does the provisional matrix have at first?
fibrin, plasma, fibronectin and type III collagen
what replaces the type III collagen as wound healing progresses?
type I collagen
when does epithelialization occur in cutaneous wounds?
24-48hours
what is epithelialization
cells move from the wound’s edge along the cut margins of the dermis and deposit basement membrane as they move, then epithelial cells meet in the midline beneath the scab.
what do macrophages do in epithelialization
stimulate fibroblasts to produce FGF-7 (keratinocyte GF) and IL-6, enhancing keratinocyte migration and proliferation along with other growth factors
what is the most important cytokine for the synthesis and deposition of connective tissue proteins?
TGFbeta
what is tissue remodeling, what does it involve
replacement of granulation tissue with scar tissue, involves both synthesis and degradation of ECM components by MMP
what are MMPs dependent on
zinc
what does a mature scar look like
- amount of collagen and ECM proteins increases 2. number of fibroblasts and vascularity decreases
what does a fully mature scar have
practically avascular, has very few cells, no dermal appendages (hair), and has dense collagen
what does granulation tissue have that fully mature scars do not
numerous blood vessels, edema and loose ECM containing occasional inflammatory cells, minimal mature collagen
how long does granulation tissue/healing
3-7days
what is healing by first intention
cutaneous wound healing with wounds with opposing edges like healing of clean, uninfected surgical incision or knife wound where the edges are nicely approximated (sutures, staples)
what is healing by second intention
cutaneous wound healing with wounds that have separated edges, have more extensive loss of cells and tissue.
how does a scalpel blade impact wounds
injures a limited number of epidermal cells, injures the underlying connective tissue
what occurs within 24 hours of healing by first intention?
incisional space immediately fills with clotted blood. cellular evidence of acute inflammation appears within 24hours == neutrophils
when does chronic inflammatory response replace acute inflammatory response?
by day 3 of wound healing macrophages
when does granulation tissue begin forming
3 days
why does granulation tissue begin formin
growth factors secreted by platelets, macrophages, and endothelial cells
what do GF incude?
regeneration of basal cells of the epidermis
when has granulation tissue filled the incisional space>
day 5
what occurs in the 2nd week of healing by first intention
collagen accumulates, leukocytes, edema and increased vascularity begin to disappear
what occurs by the end of the 1st month during healing by first intention
the connective tissue scar is formed and the epidermis is fully regenerated
when does tensile strength of the scar plateau?
3 months
what is wound contraction unique to?
healing by second intention
what is different about healing by second intention
- inflammation reaction is moire intense
- much larger amount of granulation tissue
- wound contraction by myofibroblasts
- substantial scar formation
what causes wound contraction?
myofibroblasts
describe wound strength when sutures are removed after 1 week
wound strength is 10% of unwounded skin
what systemic factor retards wound healing
diabetes
what occurs by 3 months of wound healing
tensile strength plateaus to 70-80% may never regain 100%
local factors of decreased wound healing
blood supply
denervation
local infection
foreign body
hematoma
mechanical stress
necrotic tissue
protection
surgical techniques
type of tissue
systemic factors that slow wound healing
age
anemia
drugs
genetic disorders
malnutrition
obesity
systemic infection
diabetes
temp
trauma
uremia
vitamin C deficiency
trace metal deficiency
what are hypertrophic scars
raised, red, shiny scars that do not extend beyond the original wound site
what causes hypertrophic scars
excess fibroblast activity and collagen deposition
what are keloids
scar grows beyond initial wound site, could grow indefinitely and aggressively and produce tumor like scars
what causes keloids
excess production of type III collagen
what could be a predisposition in keloids
more common in African Americans, commonly affects earlobes, face, neck, sternum, forearms
