anatomy
poorva
TissueGrowthRegenerationRepair-2.pptxTissue Growth, Regeneration & Repair
Srujana Rayalam DVM, PhD
Dept. of Pharmaceutical Sciences
PCOM-GA campus
PHAR113G Anatomy, Physiology & Pathophysiology I
8/24/2020 1:51 PM
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Learning Objectives
Describe how tissue repair differs from regeneration
Explain the differences between continuously dividing, quiescent & non-dividing tissues
Describe the major classes of adult stem cells in different tissues & their functions
Describe the different phases of the cell cycle
Describe the checkpoints in the cell cycle & understand the role of cyclins
Compare and contrast the differences between autocrine, paracrine & endocrine actions
Describe the principle types of signal transduction pathways
Describe the structure & functions of the extracellular matrix
Describe various cell adhesion molecules
Describe the phases of wound healing
Contrast healing by primary & secondary union and describe the mechanisms leading to development of fibrosis
Cell vs. Tissue
Injury to tissue triggers either regeneration or repair
Regeneration - proliferation of cells and tissues to replace lost structures
Predominates in continuously proliferating tissues
Requires intact extracellular matrix
Repair - combination of regeneration and scar formation by the deposition of collagen
Some structure and function lost
Occurs in tissues with limited ability to proliferate
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Mechanisms of Tissue Repair
Robbins & Cotran Pathologic Basis of Disease
5
Regeneration vs. Repair
Cell and Tissue Regeneration
The regeneration of injured cells and tissues involves cell proliferation, which is driven by growth factors and is critically dependent on the integrity of the extracellular matrix, and by the development of mature cells from stem cells.
Tissue Proliferative Activity
Based on the intrinsic proliferative activity, tissues are divided into three groups
Continuously dividing (labile tissues)
Epithelial tissue
Stratified squamous - skin, oral cavity, vagina, and cervix
Pseudostratified columnar epithelium – respiratory tract
Cuboidal epithelium - the lining mucosa of all the excretory ducts of the glands of the body
Transitional epithelium - urinary tract
Columnar epithelium - GI tract and uterus
Cells of the bone marrow and hematopoietic tissues
Tissues contain abundance of stem cells
Tissue Proliferative Activity…cont’d
Quiescent/stable tissues
low level of replication but divide rapidly in response to stimuli
parenchymal cells of liver, kidneys, and pancreas
fibroblasts and smooth muscle
vascular endothelial cells
Non-dividing or permanent tissues
cells that cannot undergo mitotic division
neurons
skeletal muscle
satellite cells
cardiac muscle
Stem cell generation and differentiation
Unlimited potential for self-renewal and capable of generating differentiated cell lineages
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Embryonic vs Adult Stem Cells
ES cells derived from inner cell mass of blastocyst
AS cells found in small numbers in specific stem-cell niches. Stem-cell niche is an area of a tissue that provides microenvironment for stem cells to be in a self-renewable state.
ES cells are pluripotent
Most AS cells are multipotent
Adult Stem Cell Niches
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Hair: 3 locations
Epidermis
Hair follicle
Sebacious gland
Liver: Oval cells
Can generate into hepatocytes and biliary cells
Intestine:
Stem cells in the crypts
Cornea: Stem cells in the limbus
Bone marrow: Hematopoietic stem cells and marrow stromal stem cells
Stem cell Applications: Regenerative Medicine
Differentiated progeny of ES or AS cells can be used to repopulate damaged tissues or even to construct entire replacement organs.
Induced pluripotent stem cells (iPSc) are a type of pluripotent stem cells artificially derived from a non-pluripotent cell like an adult skin cell
iPScs are not totipotent and do not involve the destruction of an embryo
Other applications:
Help increase our understanding of how diseases occur
Drug development:
Test new drugs for safety and effectiveness
Identify teratogens
Somatic Cell Nuclear Transfer Technology (FYI)
The nucleus of a donated egg is removed and replaced with the nucleus of a mature, "somatic cell" like a skin cell
The resulting cell is totipotent and can potentially develop into specialized cells that are useful for treating severe illnesses
Cell Cycle: Control of Cell Proliferation
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Mitosis
Gap 1 (G1) : Cells enter cell cycle
Longest phase and most variable in length; Proliferation occurs when stimulated by environmental signals
Gap 0 (G0): Quiescence maintained; continued basal metabolism but cells do not replicate
S Phase (DNA synthesis phase): Duplication of chromosomes
G2 (pre-mitotic phase): Further growth in cell size
M (mitotic phase): Cell division
G1/S checkpoint: ensures integrity of DNA before replication
DNA damage – delays progression to S phase
Irreparable damage – apoptosis or senescence
G2/M checkpoint: ensures fidelity of DNA replication
DNA damage – delays progression to M phase
Irreparable damage – apoptosis
Regulation of Cell Cycle
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Progression through the G1/S and G2/M transition, is tightly regulated by proteins called cyclins and associated enzymes called cyclin-dependent kinases (CDKs)
CDK 4/6 inhibitor for breast cancer
Pfizer - 2014
Regulation of Cell Cycle
Enforcing the cell cycle checkpoints is the job of CDK inhibitors (CDKIs); they accomplish this by modulating CDK-cyclin complex activity.
For example, one family of CDKIs—composed of three proteins called p21 (CDKN1A), p27 (CDKN1B), and p57 (CDKN1C)—broadly inhibits multiple CDKs.
Defective CDKI checkpoint proteins allow cells with damaged DNA to divide, resulting in mutated daughter cells with the potential of developing into malignant tumors.
CDKIs have applications in cancer treatment
Growth Factors and Regulation of Cell Proliferation
General Patterns of Intercellular Signaling
Autocrine signaling - release of factor that stimulates same cell e.g. tumor cells
Paracrine signaling - stimulation of nearby cell; common in connective tissue repair of healing wounds
Endocrine signaling - stimulation of distant cell; pituitary hormones, cytokines
Robbins & Cotran Pathologic Basis of Disease (8th edition)
19
Main types of cell surface receptors and their principal signal transduction pathways
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Signaling from tyrosine kinase receptors: Insulin & EGF
Transmembrane receptors
Signaling initiated by dimerization
Phosphorylation of tyrosine residues on the receptor
↑ transcription of specific genes
Posner & Laporte et al., 2010
Receptors lacking intrinsic tyrosine kinase activity: Growth Hormone
Kaabi, 2012
Transmit extracellular signals to the nucleus by activating Janus kinases (JAK).
The JAKs link the receptors with and activate cytoplasmic transcription factors called STATs (signal transducers and activation of transcription), which directly shuttle into the nucleus and activate gene transcription.
G protein–coupled receptors
Largest family of plasma membrane receptors - accounting for about 1% of the human genome
Enormous number of common pharmaceutical drugs targets GPCRs
Ligand Gated Ion Channels
Opens or closes in response to concentration of signaling ligands
Receptor protein is part of an ion channel protein complex
Ligands
GABA receptor
Cl
Acetylcholine R
Na or Ca
http://www.creative-biostructure.com
Nuclear Receptors
Ligands diffuse through the cell membrane and bind the inactive receptors either in the cytosol or nucleus, causing their activation Activated receptor, upon binding to specific DNA sequences, modulates gene expression
Two Major Types:
Steroid (Type I)
Typically non-DNA bound in the inactive state
Translocates to the nucleus upon ligand binding
glucocorticoid receptor
Non-Steroid (Type II)
Typically DNA bound in the inactive state
thyroid hormone receptor
Nuclear Hormone Activation
Steroid hormone receptors: Estrogen receptor, Androgen receptor, Vitamin D receptor
Non-steroid hormone receptors: retinoid x receptor, liver x receptor, farnesoid x receptor, PPAR receptors
The Extra Cellular Matrix (ECM)
The ECM regulates the growth, proliferation, movement, and differentiation of the cells
Functions:
Mechanical support for cell anchorage and cell migration
Maintenance of cell polarity
Control of cell growth
Maintenance of cell differentiation
Scaffolding for tissue renewal
Establishment of tissue microenvironments
Storage and presentation of regulatory molecules
27
ECM…cont’d
ECM is composed of three groups of macromolecules:
Fibrous structural proteins - collagens and elastins
Adhesive glycoproteins – fibronectin and laminin
Proteoglycans and hyaluronan
Basic forms of ECM:
Interstitial matrix - found in spaces between cells and consists mostly of fibrillar and nonfibrillar (type IV) collagen, elastin and proteoglycans
Basement membrane - closely associated with cell surfaces, and consist of nonfibrillar collagen, laminin and proteoglycans
Main Components of ECM
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Collagen
Synthesized from fibroblasts
Several types of collagen
Type 1 : Comprises 80% of total collagen in skin and muscle tissue
Composed of 3 chains that form a triple helix
Cross linking of collagen molecules gives it strength
Cross-linking is Vitamin C dependent process
Vitamin C deficiency (Scurvey) - skeletal abnormalities and bleed easily
Types I, II, III, V and XI - fibrillar collagens (triple-helical domain is uninterrupted for more than 1000 residues)
Type IV – basement membrane collagen (long but interrupted triple-helical domains and form sheets instead of fibrils)
Most abundant protein in the body
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Types of Collagen
| FIBRILLAR COLLAGENS | |
| I | Ubiquitous in hard and soft tissues |
| II | Cartilage, intervertebral disk, vitreous |
| III | Hollow organs, soft tissues |
| V | Soft tissues, blood vessels |
| BASEMENT MEMBRANE COLLAGENS | |
| IV | Basement membranes |
Osteogenesis Imperfecta
Inherited disorder of type I collagen
Characterized by
Below average height (short stature)
Blue tint to the whites of eyes (blue sclera)
Multiple bone fractures
Early hearing loss (deafness)
Because type I collagen is also found in ligaments, persons with OI often have loose joints (hypermobility) and flat feet
Symptoms of more severe forms of OI may include:
Bowed legs and arms, Scoliosis (S-curve spine), low bone mass, bone fragility, and fractures
Elastin (Elastic fibers)
Very elastic and capable of reversibly stretching to twice their length
Composed primarily of desmosine and isodesmosine (special amino acids)
Synthesized from fibroblasts
Secreted as a precursor molecule (tropoelastin)
Cross linked after secretion
Location : Blood vessels, Ligaments, Lungs and Skin
Adhesion Molecules
Multiadhesive ECM glycoproteins
Fibronectin – binds to integrins
Laminin – present in basement membrane
Cell adhesion molecules (CAMs)
Transmembrane proteins with structural & signaling roles
Provide link between extracellular matrix & cytoskeleton inside cell
Four types of CAMs:
Immunoglobulin family CAMs
Cadherins
Integrins
Selectins
34
IgG Superfamily CAMs
Includes intercellular adhesion molecules (ICAMs)
Bind to integrins on leukocytes and mediate their flattening onto the blood vessel wall with their subsequent extravasation into the surrounding tissue.
Selectins
Selectins: expressed on leukocytes - L-selectin
endothelium - E-selectin
both platelets and endothelium - P-selectin
Ligands for selectins – sialylated oligosaccharides bound to mucin-like glycoprotein backbones
Selectins – weak rolling interaction
Integrins – firm adhesion
Integrins
Superfamily of surface proteins present on a wide variety of cells
Involved in the adhesion of cells to other cells or to specific components of the ECM
Extracellular segment binds to specific proteins in ECM; intracellular domain binds to proteins of the cytoskeleton, such as actin and vinculin
Help integrate responses of cells like movement and phagocytosis, to changes in the environment
Cadherins
Calcium-dependent adhesion molecules
Transmembrane proteins that act as both ligands and receptors
Responsible for the selective cell-cell adhesion and play a fundamental role in normal development and maintaining the integrity of multicellular structures.
Proteoglycans and Non-proteoglycans
Proteoglycans are glycosaminoglycans (GAGS) attached to ECM proteins
GAGs make up the third type of component in the ECM
Distinct families of GAGs: heparan sulfate, chondroitin/dermatan sulfate and keratan sulfate
Hyaluronan (HA), a non-proteoglycan, helps provide resilience and lubrication to many types of connective tissue, notably for the cartilage in joints
Intra-articular Hyaluronic Acid Injections for Knee Osteoarthritis
Healing of Injury by Repair Cutaneous Wound Healing
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Inflammatory Phase
Clot formation (hemostasis) and Chemotaxis
Provides scaffold for cell migration to injury site
Exposed collagen activates clotting cascade and inflammatory phase
Neutrophils and macrophages attracted to the site
40
Proliferative Phase
Epithelialization, Angiogenesis and Provisional Matrix Formation
Begins when wound is covered by epithelium
Production of collagen is hallmark
7 days to 6 weeks
Work horse of wound repair - Fibroblast
Produce Granulation Tissue:
Collagen (type III) and elastin
Glycosaminoglycans
Fibronectin
Tissue fibroblasts become myofibroblasts induced by TGF-b1
41
Epithelialization
Basal epithelial cells at the wound margin flatten (mobilize) and migrate into the open wound
Basal cells at margin multiply (mitosis) in horizontal direction
Basal cells behind margin undergo vertical growth (differentiation)
42
Angiogenesis
Vasodilation – Nitric oxide and VEGF
Proteolytic degradation of the basement membrane - matrix metalloproteinases (MMPs)
Migration of endothelial cells
Proliferation of endothelial cells
Maturation of endothelial cells
Recruitment of periendothelial cells
Angiogenesis from pre-existing vessels
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Angiogenesis…cont’d
EPCs - recruited from the bone marrow
Express markers of hematopoietic stem cells as well as VEGFR-2, and vascular endothelial–cadherin (VE-cadherin)
Re-endothelization of vascular implants and the neovascularization of ischemic organs
Angiogenesis from Endothelial Precursor Cells (EPCs)
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Modulating VEGF/VEGFR Signaling
Steward. Horizons in Cancer Therapeutics. 2004;5(2):11-21
tyrosine kinase inhibitors
VEGF – A key to angiogenesis
Vascular endothelial growth factor - One of the most important angiogenic factor
Tyrosine kinase receptor
VEGF also causes vascular permeability, which leads to vessel leakiness – characteristic feature of tumor blood vessels
45
New capillary formation in response to wounding
Maturation Phase
Replacement of granulation tissue by scar tissue
Balance between collagen synthesis and degradation by matrix metalloproteinases (MMPs) - an important feature of tissue repair
Type III collagen is replaced by type I collagen
Disorganized collagen fibers are rearranged, cross-linked, and aligned along tension lines
Tensile strength of the wound gradually increases
Wound Contraction
Actual contraction with pulling of edges toward center making wounds smaller
Myofibroblast: contractile properties
Surrounding skin stretched, thinned
Original dermal thickness maintained
No hair follicles, sweat glands
Extensive remodeling of scar occurs over 1-2 months
48
Healing by primary vs. secondary union
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Healing by ‘primary union’ or by ‘first intention’
Secondary Union
Injuries of different extent show distinct features of wound healing
Healing by ‘secondary union’ or by ‘second intention’
Fibrosis
Robbins & Cotran Pathologic Basis of Disease (8th edition)
Repair, regeneration and fibrosis: an overview
Robbins & Cotran Pathologic Basis of Disease (8th edition)
51
Summary
Tissue repair vs. regeneration
Continuously dividing, quiescent & non-dividing tissues
Adult stem cell niches & their functions
Cell cycle, check points and cyclins
Autocrine, paracrine & endocrine actions
Signal transduction pathways
Components of extracellular matrix
Cell adhesion molecules
Phases of wound healing
Healing by primary vs. secondary union
