Tissue Renewal, Regeneration, and Repair

1. Tissue Renewal, Regeneration, and Repair Lisa Stevens, D.O.

2. Background • Injury to cells---series of damaging events---initiation of healing process • Regeneration • Complete restitution of lost or damaged tissue • Repair • May restore some original structures • Can cause structural derangements • Healthy tissues • Healing (regeneration/repair) • Occurs after any insult that causes tissue destruction • Essential for the survival of the organism

4. Regeneration • Proliferation of cells and tissues to replace lost structures • Growth of an amputated limb in amphibians • Mammalian whole organs and complex tissues • Rarely regenerate after injury • Applied to liver growth after partial resection or necrosis • Compensatory growth rather than true regeneration

5. Regeneration • Hematopoietic system, skin, GI tract • High proliferative capacity • Renew themselves continuously • Regenerate after injury

6. Repair • Combination of regeneration and scar formation • Deposition of collagen • Contribution of regeneration and scarring • Ability of the tissue to regenerate • Extent of the injury • Example • Superficial skin wound • Heals through the regeneration of the surface epithelium

8. Repair • Chronic inflammation • Accompanies persistent injury • Stimulates scar formation • Local production of growth factors and cytokines • Promote fibroblast proliferation and collagen synthesis

9. Fibrosis • Extensive deposition of collagen • Extracellular matrix (ECM) • Components are essential for wound healing • Provide the framework for cell migration • Maintain the correct cell polarity for the re-assembly of multilayer structures • Participate in angiogenesis (formation of new blood vessels)

10. Fibrosis • Extracellular matrix (ECM) • Fibroblasts, macrophages, and others • Produce growth factors, cytokines, and chemokines • Critical for regeneration and repair

11. Normal Cell Proliferation • Adult tissues • Size of cell populations • Determined by rate of cell proliferation, differentiation, and death • Increased cell numbers may result • Increased proliferation • Decreased cell death • Apoptosis • Physiologic process required for tissue homeostasis • Induced by a variety of pathologic stimuli

13. Normal Cell Proliferation • Terminally differentiated cells • Differentiated cells incapable of replication • Impact of differentiation • Depends on the tissue under which it occurs • Differentiated cells are not replaced • Differentiated cells die but are continuously replaced by new cells generated from stem cells

14. Cell Proliferation • Stimulated by physiologic and pathologic conditions • Physiologic proliferation • Proliferation of endometrial cells under estrogen stimulation during the menstrual cycle • Thyroid-stimulating hormone-mediated replication of cells of the thyroid that enlarges the gland • Stimuli may become excessive, creating pathologic conditions

15. Cell Proliferation • Stimulated by physiologic and pathologic conditions • Pathologic proliferation • Nodular prostatic hyperplasia • Dihydrotestosterone stimulation • Nodular goiters in the thyroid • Increased serum levels of thyroid-stimulating hormone

16. Cell Proliferation • Controlled by signals from the microenvironment • Stimulate or inhibit proliferation • Excess of stimulators or a deficiency of inhibitors • Leads to net growth and, in the case of cancer, uncontrolled growth

17. Tissue Proliferative Activity • Tissues of the body • Divided into three groups • Basis of the proliferative activity of their cells • Continuously dividing (labile tissues) • Quiescent (stable tissues) • Nondividing (permanent tissues)

18. Tissue Proliferative Activity • Continuously dividing tissues (labile tissues) • Cells proliferate throughout life • Replaces destroyed cells • Surface epithelia • Stratified squamous epithelia of the skin, oral cavity, vagina, and cervix • Lining mucosa of all the excretory ducts of the glands of the body • Salivary glands, pancreas, biliary tract

19. Tissue Proliferative Activity • Continuously dividing tissues (labile tissues) • Surface epithelia, cont’d • Columnar epithelium of the GI tract and uterus • Transitional epithelium of the urinary tract • Cells of the bone marrow and hematopoietic tissues • Mature cells are derived from adult stem cells • Tremendous capacity to proliferate

20. Tissue Proliferative Activity • Quiescent tissues (stabile tissues) • Low level of replication • Cells from these tissues • Undergo rapid division in response to stimuli • Capable of reconstituting the tissue of origin • Parenchymal cells of liver, kidneys, and pancreas • Mesenchymal cells • Fibroblasts and smooth muscle

21. Tissue Proliferative Activity • Quiescent tissues (stabile tissues) • Vascular endothelial cells • Lymphocytes and other leukocytes • Example • Ability of liver to regenerate • Partial hepatectomy • Acute chemical injury

22. Tissue Proliferative Activity • Quiescent tissues (stabile tissues) • Fibroblasts, endothelial cells, smooth muscle cells, chondrocytes, and osteocytes • Quiescent in adult mammals • Proliferate in response to injury • Fibroblasts proliferate extensively

23. Tissue Proliferative Activity • Nondividing tissues • Contain cells that have left the cell cycle • Cannot undergo mitotic division in postnatal life • Neurons • Skeletal muscle cells • Cardiac muscle cells

24. Tissue Proliferative Activity • Nondividing tissues • Neurons in the central nervous system (CNS) • Destruction of cells • Replaced by the proliferation of the CNS-supportive elements • Glial cells

25. Tissue Proliferative Activity • Nondividing tissues • Mature skeletal muscle • Cells do not divide • Regenerative capacity • Through the differentiation of the satellite cells • Attached to the endomysial sheaths • Cardiac muscle • Very limited regenerative capacity • Large injury to the heart muscle • Myocardial infarction • Followed by scar formation

26. Stem Cells • Characterized by: • Self-renewal properties • Capacity to generate differentiated cell lineages • Need to be maintained during the life of the organism • Achieved by two mechanisms • Obligatory asymmetric replication • With each stem cell division, one of the daughter cells retains its self-renewing capacity while the other enters a differentiation pathway

27. Stem Cells • Need to be maintained during the life of the organism • Achieved by two mechanisms • Stochastic differentiation • Stem cell population • Maintained by the balance between stem cell divisions that generate either two self-renewing stem cells or two cells that will differentiate

29. Stem Cells • Embryonic stem cells (ES cells) • Pluripotent • Generate all tissues of the body • Give rise to multipotent stem cells • More restricted developmental potential • Eventually produce differentiated cells • Three embryonic layers

30. Stem Cells • Adult stem cells (somatic stem cells) • Restricted capacity to generate different cell types • Identified in many tissues • Reside in special microenvironments • Niches • Composed of mesenchymal, endothelial, and other cell types • Niche cells generate or transmit stimuli that regulate stem cell self-renewal and the generation of progeny cells

32. Embryonic Stem Cells • Inner cell mass of blastocysts in early embryonic development • Contains pluripotent stem cells (ES cells) • Cells isolated from blastocysts • Maintained in culture as undifferentiated cell lines • Induced to differentiate into specific lineages • Heart and liver cells

33. Embryonic Stem Cells • ES cells may in the future be used to repopulate damaged organs • Effectiveness of these procedures in animals • Under intense study • Much debate about the ethical issues associated with the derivation of ES cells from human blastocytes

34. Reprogramming of Differentiated Cells • Induced Pluripotent Stem Cells • Differentiated cells of adult tissues can be reprogrammed to become pluripotent • Transferring their nucleus to an enucleated oocyte • Oocytes implanted into a surrogate mother can generate cloned embryos that develop into complete animals • Reproductive cloning • Successfully demonstrated in 1997 by the cloning of Dolly the sheep

35. Reprogramming of Differentiated Cells • Great hope that the technique of nuclear transfer to oocytes may be used for therapeutic cloning in the treatment of human diseases • Nucleus of a skin fibroblast from a patient • Introduced into an enucleated human oocyte • Generate ES cells, which are kept in culture, and then induced to differentiate into various cell types

36. Reprogramming of Differentiated Cells • In principle, these cells can then be transplanted into the patient to repopulate damaged organs • Therapeutic as well as reproductive cloning are inefficient and often inaccurate • Deficiency in histonemethylation in reprogrammed ES cells • Results in improper gene expression

38. Adult Stem Cells • Adult organism • Stem cells are present in tissues • Continuously divide • Bone marrow, skin, and the lining of the GI tract • Stem cells may also be present in organs • Liver, pancreas, and adipose tissue • Do not actively produce differentiated cell lineages

39. Adult Stem Cells • Transit amplifying cells • Rapidly dividing cells generated by somatic stem cells • Lose the capacity of self-perpetuation • Give rise to cells with restricted developmental potential • Progenitor cells

40. Adult Stem Cells • Transdifferentiation • Change in the differentiation of a cell from one type to another • Developmental plasticity • Capacity of a cell to transdifferentiate into diverse lineages

41. Stem Cells in Tissue Homeostasis • Stem cells • Bone marrow • Skin • Gut • Liver • Brain • Muscle • Cornea

42. Bone Marrow • Contains hematopoietic stem cells (HSCs) • Contains stromal cells • AKA multipotentstromal cells, mesenchymal stem cells or MSCs • Hematopoietic Stem Cells • Generate all of the blood cell lineages • Reconstitute the bone marrow after depletion • Caused by disease or irradiation

43. Bone Marrow • Hematopoietic Stem Cells • Widely used for the treatment of hematologic diseases • Collected directly from: • Bone marrow • Umbilical cord blood • Peripheral blood of individuals receiving cytokines • Granulocyte-macrophage colony-stimulating factor, which mobilize HSCs

44. Bone Marrow • Marrow Stromal Cells (MSCs) • Multipotent • Potentially important therapeutic applications • Generate chondrocytes, osteoblasts, adipocytes, myoblasts, and endothelial cell precursors • Depends on the tissue to which they migrate • Migrate to injured tissues • Generate stromal cells or other cell lineages • Do not participate in normal tissue homeostasis

45. Liver • Contains stem cells/progenitor cells in the canals of Hering • Junction between the biliaryductular system and parenchymalhepatocytes • Give rise to a population of precursor cells • Oval cells • Bipotential progenitors • Capable of differentiating into hepatocytes and biliary cells

46. Liver • Oval cells • Function as a secondary or reserve compartment • Activated only when hepatocyte proliferation is blocked • Proliferation and differentiation • Fulminant hepatic failure • Liver tumorigenesis • Chronic hepatitis and advanced liver cirrhosis

47. Brain • Neurogenesis from neural stem cells (NSCs) • Occurs in the brain of adult rodents and humans • AKA neural precursor cells • Capable of generating neurons, astrocytes, and oligodendrocytes • Identified in two areas of adult brains • Subventricular zone (SVZ) • Dentate gyrus of the hippocampus

48. Skin • Human epidermis has a high turnover rate • About 4 weeks • Stem cells are located in three different areas of the epidermis • Hair follicle bulge • Constitutes a niche for stem cells that produce all of the cell lineages of the hair follicle

49. Skin • Stem cells are located in three different areas of the epidermis • Interfollicular areas of the surface epidermis • Stem cells are scattered individually in the epidermis and are not contained in niches • Divide infrequently • Generate transit amplifying cells • Generate the differentiated epidermis • Sebaceous glands