Contents 1 Introduction Structure and function of the skin Wound definition and implications 2 Phases and processes during healing Inflammatory phase

Transcription

1 Mathematical Models for Wound Healing Events Part 1: Biological background E. Javierre 1, F. J. Vermolen 2, P. Moreo 1,3, J. M. García-Aznar 1,3, M. Doblaré 1,3 1 Centro de Investigación Biomédica en Red en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN) Zaragoza, Spain etelvina.javierre@unizar.es 2 Group of Numerical Analysis, Delft Institute of Applied Mathematics Delft University of Technology, The Netherlands 3 Group of Structural Mechanics and Material Modelling Aragón Institute of Engineering Research (I3A) University of Zaragoza, Spain

2 Contents 1 Introduction Structure and function of the skin Wound definition and implications 2 Phases and processes during healing Inflammatory phase Proliferative phase Remodelling phase 3 Types of wounds Epidermal wound healing Dermal wounds Chronic wounds

3 Introduction Structure and function of the skin The skin forms the external covering of the body and is its largest organ (15-20% of the total mass). Layers of the skin: - epidermis: composed of keratinized stratified squamous epithelium - dermis: composed of a dense connective tissue that imparts mechanical support, strength, and thickness to the skin. Functions of the skin: - mechanical, permeability and ultraviolet barrier - external environment sensor - body temperature and water loss regulation - endocrine secretion of hormones, cytokines and growth factors - immunologic information to appropriate effector cells in the lymphatic tissue

4 Wound: definition and implications Introduction Wound: (from Mediline Plus) a a physical injury to the body consisting of a laceration or breaking of the skin or mucous membrane b an opening made in the skin or a membrane of the body incidental to a surgical operation or procedure The existance of a wound compromises organ integrity... decreasing the mechanical strength... putting the immunologic system at risk... impairing organ function Patients with impaired or unsuccessful wound healing often have a poor life quality, aesthetic scarring and in some cases even emotional distress. USA, 2003: 3.4 million of patients with chronic wounds $8000M (Clark et al. 2007)

5 Inflammatory phase Phases and processes during healing 1. Vasoconstriction and hemostasis Ruptured cell membranes release inflammatory factors (thromboxanes, postraglandins) which trigger the vasoconstriction of injured blood and lymphatic vessels preventing blood loss. Wounding activates the coagulation cascade, triggered by platelets-derived inflammatory factors and glycoproteins that cause platelets to aggregate. The blood clot serves as a provisional matrix formed of fibrin, fibronectin, vitronectin and thrombospondin with embedded platelets and various blood cells that provides a scaffold for cellular migration. From: Singer and Clark 1999 Shai and Maibach 2005

6 Inflammatory phase Phases and processes during healing 2. Vasodilatation, increased permeability Following vasoconstriction, histamine and postraglandis induce vascular dilatation and permeability increase, which facilitates the ingress of growth factors and white blood cells into the wound. The injury may become swollen due to plasma leaking into the surrounding tissue. 3. Phagocytosis Chemically attracted to the wound, monocytes enter the wound through the blood vessels walls where they mature into macrophages. From: Singer and Clark 1999 Shai and Maibach 2005 White blood cells (neutrophils, leukocytes) and macrophages act against pathogenic organisms, debride the necrotic tissue and secrete other growth factors that further activate the wound healing process.

7 Proliferative phase Phases and processes during healing 1. Angiogenesis Imperative to successful healing as supports cell function (i.e. cellular migration and proliferation, collagen synthesis, etc) with nutrients and oxygen. Hypoxia stimulates macrophages and platelets to secrete certain angiogenic factors. Endothelial cells are attracted to the wound by fibronectin and angiogenic factors. Endothelial cells migration and proliferation results into sprout tips branching from existing vasculature into the wound site. Parallel degradation of the blod clot (fibrinolysis) is necessary. From Singer and Clark 1999

8 Proliferative phase Phases and processes during healing 2. Fibroplasia and Granulation tissue formation Fibroblasts invasion occurs in parallel to angiogenesis. In a first stage, fibroblasts migrate from the sides into the blood clot adhering to fibronectin. After a few days, fibroblasts start synthesizing collagen (type III) to which they adhere for further migration, which results in some sort of structural alignment that gradually enables skin integrity to be restored. The granulation tissue consists of inflammatory cells, endothelial cells, collagen, fibroblasts, immature collagen (type III) and myofibroblasts and presents a reddish and granular appearance. From Singer and Clark 1999 As fibroblasts proliferate and produce collagen, proteoglycans, glycoproteins, elastin, fibronectin, the granulation tissue is replaced with a provisional extracellular matrix

9 Proliferative phase Phases and processes during healing 3. Wound contraction Early fibroblasts migration into the wound increases the mechanical stress at the wound edge that in action with cytokines induce differentiation into myofibroblasts. Myofibroblasts are smooth muscle cells rich of actin filaments that induce the contractile forces on the wound edge towards its center. From Singer and Clark 1999 Tomasek et al Wound contraction does not involve the formation of new tissue but the centripetal movement of healthy tissue surrounding the wound to achieve minimal scarring.

10 Proliferative phase Phases and processes during healing 4. Re-epithelialization Shortly after wounding, epidermal cells undergo phenotypic alteration that includes dissolution of intracellular desmosomes and formation of peripheral cytoplasmatic filaments. The free-edge effect and the absence of intracellular adhesions induce the lateral movement of epidermal cells. The migrating epidermal cells dissect the wound, separating desiccated eschar from viable tissue. Epidermal proliferation at the wound margin is required to sustain migration. Local release of growth factors and increased expression of growth factor receptors stimulate epidermal migration and proliferation. From Singer and Clark 1999 Degradation of the provisional extracellular matrix is required to permit epidermal cells migration. This degradation depends on the production of collagenase and the activation of plasmin, both mediated by epidermal cells.

11 (Scar) Remodelling phase Phases and processes during healing Continuous process of dynamic equilibrium between the lysis of the early synthesized collagen type III and the synthesis and subsequent alignment of collagen type I (more stable) effectively increasing the wound tensile strength.

12 Time perspective Phases and processes during healing

13 Classification of wounds Types of wounds 1 Epidermal wounds a In adults b In embryos 2 Full thickness or dermal wounds 3 Chronic or non-healing wounds a Hypertrophic scars b Keloids c Pressure ulcers

14 Epidermal wound healing in adults Types of wounds Epidermal wounds are superficial wounds that do not affect (or barely affect) the underlaying dermis. Example: a blister. Epithelial cells at the wound margin undergo phenotypic alteration that gives them the ability to move via finger-like lamellipodia: rolling mechanism (mammals) sliding mechanism (amphibian) Mattila and Lappeleinen 2008 Absence of contact inhibition and the change in cell shape stimulate the mitotic activity. increase of the mitotic activity in a band behind the wound margin maximal mitotic activity at the wound margin (15 x normal mitotic rate)

15 Mathematical models of epidermal wound healing Sherrat & Murray 1991 Model of epidermal cells migration with a mitosis-controlling chemical n t = D 2 n + s(c)n 2 n kn n 0 c t = Dc 2 c + f(n) λc Types of wounds activator inhibitor Predicted re-epithelialization in good agreement with experimental results on circular wounds in rabbit ears Travelling wave solutions as an estimate of the healing rate Investigate the effect of the mitosis-controlling chemical on the wound shape during healing

16 Mathematical models of epidermal wound healing Extensions to Sherratt and Murray 1991: (corneal wound healing) Dale et al. 1994: chemical enhancement of epithelium cells diffusion Types of wounds Gaffney et al. 1999: distinguish between active and quiescent cells, obtain a better approximation of experimental mitotic rate Adam : mitogenic chemical with a discontinuous switch mechanism c t = D 2 c + P 1 Ωal λc healing starts if and only if c θ at the wound edge analyze conditions for healing initiation critical size defect planar, circular and spherical wounds Extensions to Adam 1999: Vermolen et al. 2006: general wound morphologies; investigate mitogenic concentration at the wound edge Javierre et al. 2008: temporal evolution of the wound; study the effect of wound morphology on the healing process; investigate conditions for incomplete healing

17 Embryonic wound healing Types of wounds Embryonic wound healing do not heal by lamellipodial crawling but instead by circumferential tension at the wound edge. Quick reorganization of the actin filaments of the cells at the wound margin results in the formation of a pulling actin cable. Experimental observations: fast healing scarless (perfect healing) mechanically driven, low concentrations of growth factors Grasso et al. 2007

18 Mathematical models of embryonic wound healing Types of wounds Sherratt 1993: formation of the actin cable " # G`Eε +Γ ui + τgi {z } {z} λgi {z} = 0 elastic stress contraction stress attachment to underlying substratum G initial density of intracellular actin filaments

19 Dermal wound healing Types of wounds A dermal wound compromises the integrity of the dermis Lost tissue is regenerated through a sequence of partly overlapping sequential biological processes that require highly orchestrated interactions between cells: macrophages endothelial cells, fibroblasts, myofibroblasts,... chemicals: PDGF, VEGF, TGF-β,... proteins: collagen, fibrin,... Dynamic synthesis and degradation of the ECM

20 Types of wounds Chronic or non-healing wounds Fibroplasia related excessive connective tissue deposition - Hypertrophic scars: after trauma remains within the wound confines may regress in time - Keloids: genetic predisposition extends beyond the original wound rarely regress require surgical intervention Olsen et al. 1996: wound contraction model predicts formation of keloids due to unbalanced chemical kinetics