Cell Regeneration and Repair

• Lecture 6 focuses on healing and repair.
• Competencies include applying knowledge of cellular physiology to describe tissue responses to injury and factors affecting repair quality.

Cell Regeneration

• Cell regeneration is controlled by cell growth, the cell cycle, stem cells, growth factors, and the extracellular matrix.

Cell Cycle Phases

• (S) DNA synthesis phase is where DNA replication occurs.
• (G2) Premitotic growth phase prepares the cell for mitosis.
• (M) Mitotic phase is when cell division occurs.
• (G1) Presynthetic growth phase is the initial growth phase.
• The cell cycle involves DNA replication and mitosis control.
• Cell numbers are altered by stem cell input rates, apoptosis, proliferation, or differentiation.

Stem Cells

• Embryonic stem cells (ES cells) are pluripotent.
• Adult stem cells in bone marrow generate multiple cell lineages.
• Induced pluripotent stem cells (iPS cells) are derived from mature cells with genes characteristic of ES cells.

Soluble Mediators

• Growth factors expand cell populations by stimulating cell division and survival.
• Most growth factors have pleiotropic effects and stimulate migration, differentiation, contractility, and specialized protein synthesis.
• Growth factors act on specific or multiple cell types, inducing proliferation by binding to specific receptors.

Extracellular Signaling Mechanisms

• Adjacent cells communicate via gap junctions.
• Autocrine signaling involves a substance acting on the cell that secretes it.
• An example is compensatory hyperplasia, such as liver regeneration.
• Paracrine signaling affects cells in the immediate vicinity and an Inflammatory cells recruitment to the infection site is an example.
• Endocrine signaling involves a regulatory substance released into the bloodstream acting on target cells at a distance.
• Hormones are an example.

Cell Surface Receptors

• Receptors with intrinsic kinase activity activate intracellular proteins like RAS and PLC-γ.
• G-protein-coupled receptors activate cyclic AMP (cAMP) and inositol-1,4,5-triphosphate (IP3).
• Receptors lacking intrinsic enzymatic activity use Janus kinases (JAKs) to activate STATs, which shuttle to the nucleus.
• Intracellular receptors require hydrophobic ligands to enter the cell and form complexes that affect gene transcription.
• RAS activation stimulates the mitogen-activated protein (MAP) kinase cascade, involved in intracellular signaling for many growth factors

Extracellular Matrix (ECM) Components

• The ECM is composed of the interstitial matrix, basement membranes, integrins, collagen, elastin, adhesive glycoproteins, and proteoglycans.
• Integrins are transmembrane glycoproteins that serve as cellular receptors for ECM components.
• Collagen provides tensile strength, while elastin provides tissue elasticity.
• Adhesive glycoproteins such as fibronectin and laminin connect cells to the ECM.
• Proteoglycans form hydrated gels and store growth factors.

Extracellular Matrix (ECM) Functions

• The ECM provides mechanical support, a substrate for cell growth, regulates cell proliferation and differentiation, and determines cell orientation.
• It also stores and presents regulatory molecules.
• An intact ECM is needed for tissue regeneration.
• If it is damaged, scar formation is favored.

Repair by Connective Tissue

• Repair by connective tissue occurs when non-dividing cells are injured or when tissue injury is severe.
• Within 24 hours of injury, fibroblasts emigrate and fibroblasts and endothelial cells proliferate.
• Angiogenesis involves the migration of EC towards the area of tissue injury.
• By 3 to 5 days, granulation tissue appears and consists of fibroblasts and capillaries in a loose ECM.
• Granulation tissue accumulates connective tissue matrix to form a scar that remodels over time.
• Angiogenesis is critical by healing injury sites, developing collateral circulations, or even allowing tumors to increase in size

Repair Process

• Vasodilation is increased by NO and vascular permeability which is increased by VEGF
• Pericytes separate
• Endothelial cells proliferate.
• Capillary tubes Remodel
• Pericytes are recruited
• EC proliferation and migration are suppressed

Migration and Proliferation of Fibroblasts

• fibroblasts are stimulated by PDGF, FGF, and TGF-β.
• Fibroblasts assume a more synthetic phenotype, increasing ECM deposition.
• Collagen synthesis is critical for wound site strength.
• Degradation of collagens and other ECM components is accomplished by matrix metalloproteinases (MMPs).
• Interstitial collagenases cleave fibrillar collagen, and gelatinases degrade amorphous collagen and fibronectin.
• Activated MMPs are inhibited by TIMPs.

Wound Healing: Repair by Scar Formation

• Tissues can be repaired by regeneration or replacement with connective tissue.
• Connective tissue deposition includes angiogenesis, fibroblast migration and proliferation, collagen synthesis, and connective tissue remodeling.
• Repair begins with granulation tissue and ends with fibrous tissue.
• Growth factors stimulate cell proliferation in repair.
• TGF-β is a potent fibrogenic agent; ECM deposition depends on the balance among fibrogenic agents, MMPs, and TIMPs.

Wound Healing

• Healing can occur by first or second intention.
• The incision causes disruption of membrane continuity of the basement
• Epithelial Regeneration dominates
• There is some ECM and scarring

Wound Healing by First Intention

• It involves initial migration of neutrophils to the incision margin towards the fibrin clot which occurs at 24 hours
• Basal cells of the epidermis begin to show increased mitotic activity and granulatioon tissue progressively invades the incision space and finally epithelial cell proliferation continues at 3-7 days
• Neovascularization peaks and fibers of collagen are present and after a week the second stage begins where collagen accumulates and fibroblast proliferation begins
• Leukocyte infilitrate is present, as well as edema and the scar comprisesa celluar connective tissues covered by an essentially normal epidermis
• The tensile strength is approximately 70 - 80% at about 3 months

Wound Healing by Second Intention

• A larger clot or scab forms at the surface of the wound
• Inflammation is more intense and much Larger amounts of tissue of granulation are formed
• Greater mass of tissue of the scar
• Involves contraction of Secondary healing
• Large Skin defects are reduced with contraction and myobribroblasts can be seen

Wound Strength

• Sutured wounds have approximately 70% of the strength of unwounded skin due to the placement of sutures.
• After suture removal, wound strength is about 10% which increases rapidly,
• Tensile strength results from collagen synthesis exceeding degradation during the first 2 months.
• The Tensile srength is 70 - 80% after roughly 3 Months but does not substantially improve

Factors Affecting Healing

• Infection is the most important cause of delayed wound healing.
• Protein and Vitamin C deficiencies inhibits collagen synthesis.
• Glucocorticoid administration results in inhibited TGF-B production and lessened fibrosis.
• Poor perfusion and mechanical variables also limit healing
• Foreign bodies limit healing
• Location of the injury hinders or aids healing

Key takeaways when healing

• Complete functional restoration occurs only in tissues made of stable and liable cells
• Even with cells that divide extensive injury results in incomplete regeneration
• Permanent cells often end up scarring with not functional compensation
• Residual scarring ends up ocurring