Lecture 3: Healing Flashcards
Mention and describe the three types of tissue based on their proliferative capacities.
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Labile tissue: constant proliferation to allow turnover
- Example: hematopoietic cells, surface epithelia
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Stable tissue: quiescent, but proliferation can be activated
- Example: parenchyma of most solid organs, endothelial cells, fibroblast
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Permanent tissue: no proliferation, injury is irreversible, repair leads to scar formation.
- Example: Neurons, cardiac muscle cell
Explain the mechanism of tissue regeneration in labile tissues.
Labile tissues such as the epithelial tissues, when injured, will be replaced by proliferation of residual cells and differentiation of cells derived from tissue adult stem cells.
Notes:
- Residual epithelial cells produce GFs.
- The newly generated cells migrate to fill-in the gap.
- Tissue integrity and function is restored.
Explain the characteristics of tissue regeneration in stable tissues.
In parenchymal (functional) organs, cells are capable of proliferation, but usually a limited process (except for liver).
Restoration of normal tissue architecture (regeneration) only occurs if residual tissue is structurally intact (partial liver resection). If the entire tissue is damaged, the regeneration is incomplete and is accompanied by scarring (cirrhosis).
Define interstitial matrix and basement membrane.
Interstitial matrix: synthesized by mesenchymal cells (eg. fibroblasts), forming an amorphous three-dimensional gel (collagens, fibronectin, elastin, proteoglycans…).
Basement membrane: highly organized around epithelial cells & endothelial cells
Describe the two types of healing.
- Regeneration
- occurs by the proliferation of cells that remain in tissue and survive the injury
- retain the capacity to proliferate and tissue stem cells
- e.g. skin epithelia, some parenchymal organs (liver)
- Connective tissue deposition (scar formation)
- occur by the deposition of new connective tissue (structural stability)
How is cell proliferation regulated?
- Cell-cell contact (damaged neighbouring cells, inflammatory cells)
- Cell –ECM (integrins)
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Paracrine or Autocrine factors (cytokines, hormones)
- Receptors (cell surface)
- Transcription factors (direct outcome on gene expression) - Growth factors
Mention and describe examples of major GFs responsible fro cell proliferation.
- EGF/TGF alpha (epidermal growth factor)
- enriched in macrophages, platelets, epithelial cells, tissue fluids & secretions
- mitogenic for epithelial cells and fibroblasts
- PDGF (platelet derived …)
- 3 isoforms - enriched in platelets, endothelial, smooth muscle, and tumour cells
- proliferation and migration of fibroblasts, smooth muscle cells, monocytes - also pro-inflammatory
- FGF (fibroblast …)
- large family of factors
- enriched in macrophages, T cells, endothelial cells and fibroblasts
- angiogenesis - migration during wound repair
- development of skeletal muscle and lung
- haematopoiesis
- HGF (hepatocyte …)
- enriched in fibroblasts, endothelial cells, liver non-parenchymal cells
- mitogenic for epithelial cells - liver, bile duct, lung, breast, skin
- causes cell migration
- required for survival during embryogenesis
- VEGF (vascular endothelial …)
- enriched in fibroblasts and endothelial cells
- induce blood vessel formation
- bind to receptors in endothelial cells
Recall the molecular mechanism of healing.
Mention examples of second messenger pathways.
Describe the process of transcription activation.
Transcription factors would bind DNA at specific recognition sequences in gene promoters. This would either activate or repress transcription initiation.
In repair/healing, the changes in gene expression generally lead to activation of proliferation and inhibition of differentiation.
Describe the ECM.
- Reservoir of GFs
- Acts as scaffolding
- Consists of:
- Interstitial matrix: in connective tissue, Synthesized by mesenchymal cells (e.g. fibroblasts), forming an amorphous three-dimensional gel (collagens, fibronectin, elastin, proteoglycans, hyaluronate).
- Basement membrane: highly organized to support epithelial cells, endothelial cells, and smooth muscle cells (nonfibrillar type IV collagen and laminin).
Cell interactions with ECM are critical for ___________________, ___________________, and __________________.
- Mechanical support for cell anchorage and cell migration, and maintenance of cell polarity (collagen, elastins)
- Control of cell proliferation via growth factors and integrin signalling pathways, and is a reservoir of embedded growth factors (VEGF, FGF, HGF)
- Scaffolding for tissue renewal (proteoglycan, hyaluronan)
Describe the structural fibers of ECM.
It consists of material such as elastin, fibrillin, and elastic fibres. It is found usually in blood vessels, skin, uterus, lung.
Describe the importance of the adhesive protein in ECM.
Adhesive protein, such as fibronectin, is important for signal transduction (ECM => cell interior). Actin cytoskeleton is organised into focal adhesion complexes - which activate signal transduction pathways
Describe proteoglycans in the ECM.
Proteoglycans consist of core protein + polysaccharide (glycosaminoglycans/GAGS). The polysaccharides are negatively charged, occupies a large volume, and hydrophilic. It can be found in all ECM, on the cell surface, and in biological fluids.
It regulates connective tissue structure and permeability as well as modulating cell growth and differentiation.
How is neo-angiogenesis controlled?
Other endothelial cells
- VEGFs, PDGF, NO, prostaglandins
ECM
- Integrins
- Collagen debris
Inflammatory cells
- Prostaglandins (PGs), Angiopoietins (Ang), COX, NO
- Inflammatory cytokines
GF from activated cells (eg.epithelial cells & fibroblasts)
- EGF, FGFbeta
- TGFbeta
- VEGFA
- HIF‐1alpha
Mention the factors influencing healing.
_____________ is the most important GF in the control of angiogenesis.
What is present in granulation tissue?
- Presence of neo-angiogenesis
- Reconstituted ECM
- Scaffold for healing
- migration and proliferation of fibroblasts at injury site
- deposition of ECM
- Scaffold for healing
Describes the mechanisms of fibrosis.
Describe the process of wound maturation.
The change from vascular granulation tissue to avascular scar involves changes in the composition of the ECM (mainly collagen).
It involves the balance between the proliferation of fibroblasts (ECM synthesis) and activation of matrix metalloproteinases (degrade ECM).
Describe the inflammatory phase of cutaneous wound healing.
- Haemostasis: starts with very first moments of injurt
- Platelets play a key role by aggregating - also releases fibrin. cytokines, chemokines, and hormones
- Vasocontriction - limit blood loss (pro-coagulants like prothrombin)
- Platelet aggregation and vascular leaking => clot
- Activation of platelets (degranulation) and release of chemotactic and GF (PDGF, proteases, serotonin, and histamine)
- Chemokines released by platelet activation attract inflammatory cells
- Acute inflammation
- Injured blood vessels leak transudate => oedema
- Increased porosity of blood vessels - facilitates the entry of inflammatory cells like neutrophils
- Inflammation prevents infection - damaged cells and pathogens removed form wound area
Describe the proliferation phase of cutaneous wound healing in the case of re-epithelialisation.
Starts when the new GT provides support for epithelial cells migration
- Transformation: epithelial cells become less differentiated, more SC-like (loose cell-cell contact and change shape)
- Proliferation
- Migration: newly formed cells migrate along a newly synthesized basement membrane
- Epithelial migration ceases with contact inhibition with other wound edges
- Epithelial cells differentiate (keratinisation, desquamation scales)
Describe the process of angiogenesis in the proliferation phase.
It involves endothelial cell activation and separation of pericytes from the vessel walls. The M2 macrophages and the endothelial cells secrete proteases (collagenases, MMPs) to break down existing vascular basement membrane.
Epithelial cells are then activated and attracted by GF (NO, VEGF). They then proliferate and detach from the vascular wall, degrade, and invade the ECM (guided by FGFs secreted by fibroblasts, integrins).
The sprouts extend in length until they encounter another cell (chemoattraction, Notch signalling). Then, endothelial proliferation and migration stop (PDGF, TGF-b), along with new deposition of the basement membrane and new pericytes. (Ang 1, 2)
Why is angiogenesis important in repair?
Due to the activity level of fibroblasts, inflammatory cells, and EC, all of them requires oxygen and nutrients to function. Hence, angiogenesis is imperative to sustain proliferation, migration, and differentiation.
Describe the maturation phase of the cutaneous wound healing.
In this phase, the granulation tissue is remodelled and shrinks. Cells organize their surrounding connective tissue matrix and lead to scar formation. Fibroblasts replace type III collagen into dense type I collagen. The mature orientation of collagen fibres allows tensile strength (provided by the contractile activity of myofibroblasts). FIbres are organised along the lines of tension and crosslinked - to allow strong wound.
Fibroblasts become less proliferative and eventually vascular regression leads to an avascular scar. New basement membrane also created under regenerated epithelium.
The connective tissue are also remodelled by matrix metalloproteinases (MMPs), such as collagenases (MMP-1, 2, 3), gelatinases (MMP-2, 9), and stryomelins (MMP-3, 10, 11). MMPs activation is regulated by proteases (e,g, plasmin) and inactivates by specific tissue inhibitors (TIMPs).
