TISSUE RENEWAL, REGENERATION AND REPAIR

Dr. Upik A. Miskad, PhD, SpPA

REPAIR OF TISSUE

1. Regeneration Replacement of injured cells by cells of the

same type Some times no residual trace

2. Fibroplasia/ Fibrosis Replacement by connective tissue Leave permanent scar

REGENERATION OF EPITHELIAL CELLS Require the BM (basement membrane)

for complete regeneration. Involving cells migration, proliferation,

differentiation and cell-matrix interaction

Overview of healing responses after injury. Healing after acute injury can occur by regeneration that restores normal tissue structure or by repair with scar formation. Healing in chronic injury involves scar formation and fibrosis .

TISSUE PROLIFERATIVE ACTIVITY

The tissues of the body are divided into 3 groups on the basis of the proliferative activity of their cells:

1. continuously dividing (labile tissues),

2. quiescent (stable tissues), and 3. nondividing (permanent tissues).

1. CONTINUOSLY DIVIDING CELLS/ LABILE CELLS Proliferate throughout life, replacing those that

are destroyed. These tissues include

surface epithelia, such as stratified squamous epithelia of the skin, oral cavity, vagina, and cervix;

the lining mucosa of all the excretory ducts of the glands of the body (e.g., salivary glands, pancreas, biliary tract);

the columnar epithelium of the GI tract and uterus; the transitional epithelium of the urinary tract, and

cells of the bone marrow and hematopoietic tissues.

2. QUIESCENT/ STABLE CELLS have a low level of replication; can undergo rapid

division in response to stimuli and are thus capable of reconstituting the tissue of origin.

Consist of: the parenchymal cells of liver, kidneys, and pancreas mesenchymal cells such as fibroblasts and smooth

muscle, chondrocytes, and osteocytes ; vascular endothelial cells; and lymphocytes and other leukocytes.

EXAMPLE: the ability of the liver to regenerate after partial hepatectomy and after acute chemical injury.

Fibroblasts in particular can proliferate extensively, as in healing processes and fibrosis.

3. NONDIVIDING /PERMANENT CELLS

Contain cells that have left the cell cycle and cannot undergo mitotic division in postnatal life.

Include: neurons and skeletal and cardiac muscle cells

If neurons in the central nervous system are destroyed, the tissue is generally replaced by the glial cells. Recent research : limited neurogenesis from stem cells may occur in adult brains.

Cardiac muscle has very limited, if any, regenerative capacity, and a large injury to the heart muscle, as may occur in myocardial infarction, is followed by scar formation.

CONTROL OF NORMAL CELL PROLIFERATION AND TISSUE GROWTH

In adult tissues the size of cell populations is determined by the rates of cell proliferation, differentiation, and death by apoptosis .

Mechanism of regulating cells population

STEM CELLS

Research on stem cells is the core of a new field called regenerative medicine.

The challenge about cell differentiation,

The hope that stem cells may one day be used to repair damaged human tissues, such as heart, brain, liver, and skeletal muscle.

STEM CELLS Stem cells are characterized by their self-renewal

properties and by their capacity to generate differentiated cell lineages .

Two mechanism of maintenance stem cells: is achieved by two mechanisms:

(a) obligatory asymmetric replication, in which with each stem cell division, one of the daughter cells retains its self-renewing capacity while the other enters a differentiation pathway, and

(b) stochastic differentiation, in which a stem cell population is maintained by the balance between stem cell divisions that generate either two self-renewing stem cells or two cells that will differentiate.

Stem cell generation and differentiation. The zygote, formed by the union of sperm and egg, divides to form blastocysts, and the inner cell mass of the blastocyst generates the embryo. The cells of the inner cell mass, known as embryonic stem (ES) cells, maintained in culture, can be induced to differentiate into cells of multiple lineages. In the embryo, pluripotent stem cells divide, but the pool of these cells is maintained. As pluripotent cells differentiate, they give rise to cells with more restricted developmental capacity, and finally generate stem cells that are committed to specific lineages.

ADULT STEM CELLS

In adults, stem cells (often referred to as adult stem cells or somatic stem cells)

More restricted capacity to generate different cell types.

ADULT STEM CELLS

A, Skin stem cells are located in the bulge area of the hair follicle, in sebaceous glands, and in the lower layer of the epidermis. B, Small intestine stem cells located near the base of a crypt, above Paneth cells (stem cells in the small intestine may also be located at the bottom of the crypt

ADULT STEM CELLS. C, Liver stem (progenitor) cells, known as oval cells, are located in the canals of Hering (thick arrow), structures that connect bile ductules (thin arrow) with parenchymal hepatocytes (bile duct and Hering canals are stained for cytokeratin 7). D, Corneal stem cells are located in the limbus region, between the conjunctiva and the cornea.  

EMBRYONIC STEM CELLS

ES cells is a pluripotent stem cells in the inner cell mass of blastocysts in early embryonic development .•    ES cells used to study the specific signals and differentiation steps required for the development of many tissues.    •    ES cells made possible the production of knockout mice, an essential tool to study the biology of particular genes and to develop models of human disease.

ES cells may in the future be used to repopulate damaged organs. ES cells capable of differentiating into insulin-producing pancreatic cells, nerve cells, myocardial cells, or hepatocytes have been implanted in animals with experimentally produced diabetes, neurologic defects, myocardial infarction, and liver damage, respectively.

REPROGRAMMING OF DIFFERENTIATED CELLS: INDUCED PLURIPOTENT STEM CELLS

Differentiated cells of adult tissues can be reprogrammed to become pluripotent by transferring their nucleus to an enucleated oocyte. The oocytes implanted into a surrogate mother can generate cloned embryos that develop into complete animals.

This procedure, known as reproductive cloning, was successfully demonstrated in 1997.

There has been great hope that the technique of nuclear transfer to oocytes may be used for therapeutic cloning in the treatment of human diseases.

Steps involved in stem cell therapy, using embryonic stem (ES) cells or induced pluripotent stem (iPS) cells.

CELL CYCLE & REGULATION OF CELL REPLICATION

The replication of cells is stimulated by growth factors or by signaling from ECM components through integrins.

The cell cycle consists of G1 (presynthetic), S (DNA synthesis), G2 (premitotic), and M (mitotic) phases.

The cells that have not entered the cell cycle are in the G0 state.

The cell cycle has multiple controls, particularly during the transition between the G1 and S phases.

GROWTH FACTORS

The proliferation of many cell types is driven by polypeptides known as growth factors.

Growth factors, may promote cell survival, locomotion, contractility, differentiation, and angiogenesis.

All growth factors function as ligands that bind to specific receptors, which deliver signals to the target cells.

These signals stimulate the transcription of genes that may be silent in resting cells, including genes that control cell cycle entry and progression..

SIGNALING MECHANISMS IN CELL GROWTH

According to the source of the ligand and the location of its receptors

•    Autocrine signaling: Cells respond to the signaling molecules that they themselves secrete, thus establishing an autocrine loop. Tumors frequently overproduce growth factors and their receptors, thus stimulating their own proliferation through an autocrine loop.   

•    Paracrine signaling: One cell type produces the ligand, which then acts on adjacent target cells that express the appropriate receptor. The responding cells are in close proximity to the ligand-producing cell and are generally of a different type.

•    Endocrine signaling: Hormones synthesized by cells of endocrine organs act on target cells distant from their site of synthesis, being usually carried by the blood.

RECEPTORS AND SIGNAL TRANSDUCTION PATHWAYS

The binding of a ligand to its receptor triggers a series of events by which extracellular signals are transduced into the cell resulting in changes in gene expression.

Ligand: molecule that bind to the receptor.

Receptors : are generally located on the surface of the target cell but can also be found in the cytoplasm or nucleus.

Overview of the main types of cell surface receptors and their principal signal transduction pathways. Shown are receptors with intrinsic tyrosine kinase activity, seven transmembrane G protein–coupled receptors, and receptors without intrinsic tyrosine kinase activity. cAMP, cyclic adenosine monophosphate: IP3, inositol triphosphate; JAK, Janus kinase; MAP kinase, mitogen-activated protein kinase; PI3 kinase, phosphatidylinositol 3-kinase; PKB, protein kinase B, also known as Akt; PLC-γ,

phospholipase C gamma; STATs, signal transducers and activators of transcription.

RECEPTORS

1. Receptors with intrinsic tyrosine kinase activity. The ligands for receptors with tyrosine kinase activity include most growth factors such as EGF, TGF-α, HGF, PDGF, VEGF, FGF, c-KIT ligand, and insulin.

2. Receptors lacking intrinsic tyrosine kinase activity that recruit kinases. Ligands for these receptors include many cytokines, such as IL-2, IL-3, and other interleukins; interferons α, β, and γ; erythropoietin; granulocyte colony-stimulating factor; growth hormone; and prolactin.

3. G protein–coupled receptors. These receptors transmit signals into the cell through trimeric GTP-binding proteins (G proteins).

4. Steroid hormone receptors. These receptors are generally located in the nucleus and function as ligand-dependent transcription factors. The ligands diffuse through the cell membrane and bind the inactive receptors, causing their activation.

Signaling from tyrosine kinase receptors. Binding of the growth factor (ligand) causes receptor dimerization and autophosphorylation of tyrosine residues. Attachment of adapter (or bridging) proteins (e.g., GRB2 and SOS) couples the receptor to inactive RAS. Cycling of RAS between its inactive and active forms is regulated by GAP. Activated RAS interacts with and activates RAF (also known as MAP kinase kinase kinase). This kinase then phosphorylates a component of the MAP kinase signaling pathway, MEK (also known as MAP kinase kinase or MKK), which then phosphorylates ERK (MAP kinase or MK). Activated MAP kinase phosphorylates other cytoplasmic proteins and nuclear transcription factors, generating cellular responses. The phosphorylated tyrosine kinase receptor can also bind other components, such as phosphatidyl 3-kinase (PI3 kinase), which activates other signaling systems.

TRANSCRIPTION FACTORS

Many of the signal transduction systems used by growth factors transfer information to the nucleus and modulate gene transcription through the activity of transcription factors.

Transcription factors have a modular design and contain domains for DNA binding and for transcriptional regulation. The DNA-binding domain permits binding to short sequence motifs of DNA. The transactivating domain stimulates transcription of the adjacent gene.

Growth factors induce the synthesis or activity of transcription factors.

EXTRACELLULAR MATRIX (ECM) AND CELL-MATRIX INTERACTIONS

ECM is a macromolecule complex which regulates the growth, proliferation, movement, and differentiation of the cells living within it.

It is constantly remodeling, and its synthesis and degradation accompanies morphogenesis, regeneration, wound healing, chronic fibrotic processes, tumor invasion, and metastasis.

EXTRACELLULAR MATRIX ECM functions :    •    Mechanical support for cell anchorage and cell

migration, and maintenance of cell polarity    •    Control of cell growth. ECM components can

regulate cell proliferation by signaling through cellular receptors of the integrin family.   

•    Maintenance of cell differentiation. The type of ECM proteins can affect the degree of differentiation of the cells in the tissue, also acting largely via cell surface integrins.   

•    Scaffolding for tissue renewal. The maintenance of normal tissue structure requires a basement membrane or stromal scaffold.

  •    Establishment of tissue microenvironments. Basement membrane acts as a boundary between epithelium and underlying connective tissue and also forms part of the filtration apparatus in the kidney.   

•    Storage and presentation of regulatory molecules. For example, growth factors like FGF and HGF are secreted and stored in the ECM in some tissues. This allows the rapid deployment of growth factors after local injury, or during regeneration.

ECM

The ECM is composed of three groups of macromolecules: fibrous structural proteins, such as

collagens and elastins that provide tensile strength and recoil;

adhesive glycoproteins that connect the matrix elements to one another and to cells; and

proteoglycans and hyaluronan that provide resilience (pegas) and lubrication.

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ECM

Two basic forms of ECM: 1. interstitial matrix2. basement membranes.

The interstitial matrix is found in spaces between epithelial, endothelial, and smooth muscle cells, as well as in connective tissue. It consists mostly of fibrillar and nonfibrillar collagen, elastin, fibronectin, proteoglycans, and hyaluronan.

Basement membranes are closely associated with cell surfaces, and consist of nonfibrillar collagen (mostly type IV), laminin, heparin sulfate, and proteoglycans

COLLAGEN

COLLAGEN Collagen is the most common protein

providing the extracellular framework for all multicellular organisms.

Currently, 27 different types of collagens encoded by 41 genes dispersed on at least 14 chromosomes are known.

Each collagen is composed of three chains that form a trimer in the shape of a triple helix.

ELASTIN, FIBRILLIN, AND ELASTIC FIBERS

Tissues such as blood vessels, skin, uterus, and lung require elasticity for their function.

Proteins of the collagen family provide tensile strength, but the ability of these tissues to expand and recoil (compliance) depends on the elastic fibers. These fibers can stretch and then return to their original size after release of the tension. Morphologically, elastic fibers consist of a central core made of elastin, surrounded by a peripheral network of microfibrils.

Substantial amounts of elastin are found in the walls of large blood vessels, such as the aorta, and in the uterus, skin, and ligaments.

CELL ADHESION PROTEINS

Most adhesion proteins, also called CAMs (cell adhesion molecules), can be classified into four main families:

1. immunoglobulin family CAMs, c2. adherins, 3. integrins, and 4. selectins.

CELL ADHESION PROTEINS

Integrins bind to ECM proteins such as fibronectin, laminin, and osteopontin providing a connection between cells and ECM, and also to adhesive proteins in other cells, establishing cell-to-cell contact.

Fibronectin is a large protein that binds to many molecules, such as collagen, fibrin, proteoglycans, and cell surface receptors. It consists of two glycoprotein chains, held together by disulfide bonds.

Laminin is the most abundant glycoprotein in the basement membrane and has binding domains for both ECM and cell surface receptors. In the basement membrane, polymers of laminin and collagen type IV form tightly bound networks.

THANKS FOR ATTENTION