Tissue injury + repair

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liver regeneration is not true regeneration

- resection of tissue does not cause new growth of liver

- triggers a process of compensatory hyperplasia in the remaining parts of the organ

*undergoes compensatory growth rather than replacing part of the organ that is damaged

Functional mass is restored - but not the functional mass

The pancreas also shows some compensatory growth

Tissues respond to injury in one of two ways (or a combination of both):

1. Regeneration: e.g. burn

- replacement of the diseased tissue

- by proliferation of parenchymal cells

- results in complete restoration of function

2. 'Healing' by fibrosis: scarring (cells deposit collagen)

occurs if the injured tissues are

- incapable of proliferation

- the supporting structures of the tissues are severely damaged, if basement membrane is damaged - deep wound

*cell cycle will not be initiated unless growth factors are present = mitogens

*stem cells can divide and regenerate the damaged tissues - if the stem cells are damaged they will be unable to regenerate damage

Stem cells :

Assymetric replication = one cell retains self renewing capacity and one enter differentiation

Stochastic differentiation = generate either two self renewing or 2 differentiated cells

Adult stem cells and progenitor cells reside through out the body

these stem cells reside in a specific area of each tissue called the "stem cell niche"

This niche is a particular microenvironment that fosters the growth of resident stem cells

Types of adult stem cells:

Hematopoetic (blood and immune system)

Myeloid (monocytes and white blood cells)

Lymphoid (T cells, B cells and NK cells) - in bone marrow

Mesenchymal (differentiate into cartilage, muscle, fat cells and tendons, ligaments and connective tissue)

Neural (neurons, oligodendrocytes and astrocytes)

Epithelial stem cells (most important - 60% of cells in body)

Continuously dividing = labile tissues (readily regenerate )

Quiescent = stable tissues (G0 but can go back into cell cycle) - Limited ability to regenerate (except liver)

Non-dividing = permanent tissues (cannot regenerate, injury results in scarring)

Transforming growth factor Beta (TGF beta) is the most important factor in wound healing, fibroblast migration and proliferation collagen synthesis

Extracellular matrix (ECM) - regulates growth, proliferation and movement of cell living within it

Mechanical support

Control of cell growth

Scaffolding of tissue renewal (without the ECM tissues don't have a bed to repair - cells cannot be regenerated or divide = scarring)

*If ECM is damaged, it cannot be regenerated and will scar

Skeletal muscles themselves cannot divide, but satellite cells can, replacing skeletal muscle cells

Repair: 4 sequential and overlapping processes

Haemostasis

Inflammation

Proliferation

Remodelling (strengthening of wound)

Granulation tissue = new connective tissue and microscopic blood vessels that form on the surfaces of a wound during healing process - hallmark of repair and healing

Factors that influence wound healing:

•Nutrition- vitamin C deficiency, inhibit collagen synthesis and retard healing•

•Metabolic status- Diabetes mellitus is associated with delayed healing

•Circulatory status- Inadequate blood supply, usually caused by arteriosclerosis or venous abnormalities (e.g., varicose veins) that retard venous drainage, also impairs healing

•Glucocorticoids- Well-documented anti-inflammatory eff ects, Influence various components of inflammation