Fundamentals Block 4: Regeneration and Repair

What is tissue repair?

the general process by which damaged tissue attempts to return to its original state
→ initiated when inflammation begins

What are the two processes that dictate tissue repair?

Repair can be done in two methods
→ Regeneration - replacement of lost cells by proliferation of surviving parenchymal cells or stem cells
→ Fibrosis - replacement of damaged tissue with fibrous connective tissue (scar formation)

How does the body determine which of the two methods of tissue repair it will choose to undergo at the site of injury?

The method of repair is dependent on two factors
→ Proliferative capacity of parenchymal cells
whether or not the cells are permanent, labile, or stable

→ Extent of Injury
minor injuries in labile and stable cells will typically heal by regeneration whereas severe injuries will have some degree of fibrosis
chronic injury/inflammation will always lead to fibrosis

What are the two sources of cells for regeneration?

1) Some parenchymal cells are capable of re-entering the cell-cycle and work in proliferation
→ this can be done in tissues like the hepatocytes (liver)

2) Stem cells
→ typically located in the basal tissue layer

What is the stimuli for regeneration

Paracrine signaling via growth factors that can be secreted by inflammatory cells, parenchymal cells, and stromal cells

What is TGF-alpha

Growth factor responsible for stimulating epithelial cells and hepatocytes during repair

What is TGF-beta

Growth factor responsible for stimulating fibroblasts, inhibits acute inflammation

What is EGF

Growth factor responsible for stimulating many cell types as well as promoting formation of granulation tissue

What is PDGF

Growth factor responsible for stimulating endothelium, smooth, muscle, and fibroblasts

What is FGF

Growth factor responsible for stimulating fibroblasts and triggering angiogenesis

What is VEGF

Growth factor responsible for stimulating endothelium and causing angiogenesis

What do macrophages do in tissue repair?

Macrophages trigger the tissue repair process through secretion of cytokines and polypeptide growth factors

What are the three different types of cells in tissue repair?

Labile
→ continuously being lost and replaced by maturated from stem cells
→ requires pool of stem cells

Stable
→ cells are in G0 and have minimal proliferative activity. Capable of diving in response to injury or loss of tissue mass

Permanent
→ Always in G0, terminally differentiated and nonproliferative

What are the three phases of tissue regeneration at the liver?

Priming
→ cytokines like IL-6 produced by kupffer cells (resident macrophages) act on hepatocytes to make the parenchymal cells able to receive growth factor signaling

Growth
→ Growth factors like HGF and TGF-a will act on primed-hepatocytes to stimulate cell metabolism and trigger them to enter from G0 → G1
→ proliferation of hepatocytes is followed by proliferation of nonparenchymal cells like macrophages and endothelial cells

Termination
→ hepatocytes return to quiescence

What are the steps of deposition/fibrosis?

Scar Formation follows three steps:
1) Hemostasis - seal injured vessels

2)Inflammation:
→ complement, chemokines, and other inflammatory mediators will recruit neutrophils and monocytes/macrophages

3) cell proliferation
→ angiogenesis
→ fibroblasts - deposits type III collagen
→ myofibroblasts - contract the wound

4) Remodeling
→ replacement of type III collagen with type I collagen via collagenases
→ needs zinc as a cofactor

What are the three cell types that are proliferated during deposition/fibrosis? What do each of them do?

Epithelial Cells
→ produce growth factors and cover up the wound

Endothelial Cells/Pericytes
→ form new blood vessels via angiogenesis

Fibroblasts
→ proliferate and migrate to site of injury and lay down collagen and fibronectin and ECM in order to form the granulation tissue

What is Granulation Tissue?

The pink granular tissue that forms before scar tissue
→ Migration and proliferation of fibroblasts and deposition of loose connective tissue, together with proliferation of fibroblasts and new thin-walled, delicate capillaries in a loose extracellular matrix, often with admixed inflammatory cells, mainly macrophages
→   Eventually, the granulation tissue is remodeled (matrix metalloproteinases) and replaced by mature, well-compacted collagen.

What is Healing by first or primary intention

First: incision of the skin closed with surgical sutures
→ wound sealed by blood clot (fibrin plug)
→ neutrophils and macrophages arrive and small granulation tissue is formed
→ surface epithelial cells regenerate and close the incision gap

What is healing by second intention

Second Intention is typical in more severe/deep injuries of the skin, where the edges cannot be closed
→ → wound sealed by blood clot (fibrin plug)
→ neutrophils and macrophages arrive and large and persistent granulation tissue is formed
→ surface epithelial cells regenerate and close the incision gap

the skin wound will contract because action of myofibroblasts in the connective tissue, causing the wound to shrink

What is Angiogenesis? When does it occur

New blood vessel development from existing vessels
→ healing at sites of injury
→ development of collateral circulation during ischemia
→ allows tumors to grow beyond constraints of blood supply

What are the seven steps of angiogenesis?

• Vasodilation in response to nitric oxide (NO) and increased permeability induced by vascular endothelial growth factor (VEGF)

• Separation of pericytes from the abluminal surface and breakdown of the basement membrane to allow formation of a vessel sprout

• Migration of endothelial cells toward the area of tissue injury (leading tip cell)

• Proliferation of endothelial cells just behind the leading front (tip) of migrating cells

• Remodeling into capillary tubes

• Recruitment of periendothelial cells (pericytes for small capillaries and smooth muscle cells for larger vessels) to form the mature vessel

• Suppression of endothelial proliferation and deposition of the basement membrane

What are the seven impairments to wound healing?

→ Mechanical factors - leads to dehiscence or separation of wound edges
→ Nutrition deficiencies
→ Infection - prolonging inflammation causing progressive tissue injury
→ Corticosteroids
→ Ischemia
→ Chronic Inflammation
→ Diabetes

How can Diabetes impair wound healing? (3)

Healing is impaired due to effects of non-enzymatic glycosylation...

Advanced atherosclerosis leads to peripheral vascular disease and lack of perfusion to tissues

Diabetics are more susceptible to infections because of decreased neutrophil function and impaired cytokine production by macrophages

What can excessive wound healing result in?

gives rise to hypertrophic scars and scars that grow beyond margins of injury and fails to regress keloid
→ can lead to functional compromise

What are the four types of collagen and their location

Type I - bone, tendons ligaments

Type II - cartilage

Type III - granulation tissue

Type IV - basement membrane of epithelium

Describe the collagen synthesis process (6)

mRNA is translated into preprocollagen and then moved into the lumen of the rough ER
→ preprocollagen will undergo hydroxylation of proline and lysine residues using Vitamin C as a cofactor
→ glycosylation
→ procollagen is made following triple helix formation
→ procollagen is secreted and is cleaved at C and N terminus to make tropocollagen
→ crosslinking of tropocollagen makes collagen

What is a Hypertrophic scar?

excess production of scar tissue (type I collagen) that is localized to the wound

What is a Keloid

excess production of scar tissue (type III collagen) that is WAY out of proportion to the injury
→ seen genetically more commonly in African Americans