Healing, Regeneration, and Tissue Repair

Comprehensive vocabulary flashcards covering the concepts of cellular proliferation, phases of granulation tissue, angiogenesis, primary and secondary intention wound healing, complications of repair, and stages of bone fracture healing.

Healing

The physiological process by which the body replaces destroyed tissue with living tissue.

Regeneration

The renewal of lost tissue in which destroyed or lost cells are replaced by tissues or parenchymal cells that are identical in type.

Repair (Healing by Scarring)

The orderly process by which lost tissue is replaced by granulation tissue that progressively matures into fibrous scar tissue, occurring when specialized cells cannot divide or when the stromal framework is destroyed.

Labile Cells

Continuously cycling cells that undergo constant turnover through the programmed division of stem cells, such as surface epithelial cells of the skin, gastrointestinal tract, and urinary tract, as well as lymphoid and haematopoietic cells.

Stable Cells (Quiescent Cells)

Cells in the $G_0$ phase of the cell cycle that exhibit low normal rates of replication but retain the ability to re-enter the cell cycle and divide rapidly in response to injury (e.g., hepatocytes, renal tubular epithelium, fibroblasts, osteoblasts, and vascular endothelial cells).

Permanent Cells

Terminally differentiated, nondividing cells that have permanently exited the cell cycle and cannot undergo mitotic division to replace lost tissue (e.g., adult neurons, cardiac myocytes, skeletal muscle cells, and cells of the lens).

Stem Cell Niche

A specialized, protective tissue microenvironment in which adult tissue stem cells reside to undergo asymmetric division and maintain lifelong physiologic turnover and repair.

Granulation Tissue

A pink, soft, granular tissue formed during repair, histologically characterized by proliferating fibroblasts, newly formed leaky capillary loops (angiogenesis), a loose extracellular matrix, and interspersed inflammatory cells.

Phase of Demolition

The initial clearing phase of tissue repair during which autolytic enzymes from dead cells and proteolytic enzymes from disintegrating neutrophils break down necrotic tissue, while infiltrating macrophages phagocytose and digest the debris.

Fibronectin

An extracellular matrix glycoprotein secreted by fibroblasts that binds to fibrin, provides structural scaffolding for the rebuilding matrix, and serves as a chemotactic recruiter for macrophages and additional fibroblasts.

Angiogenesis (Neovascularization)

The development and budding of new blood vessels from preexisting vascular channels or through the homing and mobilization of bone marrow-derived endothelial progenitor cells ($\text{EPCs}$).

Vascular Endothelial Growth Factor (VEGF)

The key polypeptide growth factor that triggers angiogenesis by stimulating endothelial cell migration and proliferation and enhancing vascular permeability.

Hypoxia-Inducible Factor (HIF)

A transcription factor stabilized by low oxygen tension (hypoxia) that moves into the cell nucleus to act as a genetic switch, activating transcription and synthesis of $\text{VEGF}$.

Tip Cells

Specialized endothelial cells situated at the leading edge of a sprouting capillary branch that extend filopodia and migrate directional up a $\text{VEGF}$ chemokine gradient.

Stalk Cells

Proliferating endothelial cells trailing directly behind the migratory tip cell that multiply to elongate the capillary sprout and hollow out to form a vessel lumen.

Vascular Involution

The programmed regression and apoptosis of excess capillary networks during late-stage wound healing, converting vascularized granulation tissue into a pale, avascular fibrous scar.

Wound Contraction

The mechanical reduction of an open wound defect by approximately $70\% \text{ to } 80\%$ of its original surface area, significantly decreasing the total amount of tissue replacement required.

Myofibroblasts

Differentiated mesenchymal cells possessing contractile properties intermediate between fibroblasts and smooth muscle cells that migrate into wounds $2 \text{ to } 3$ days post-injury to pull the edges together.

Healing by First Intention (Primary Union)

Healing of a clean, uninfected surgical incision whose margins are closely approximated by sutures or staples, characterized by minimal tissue loss, minimal granulation tissue, and a thin, linear scar.

Healing by Second Intention (Secondary Union)

Healing of extensive wounds with wide deficits, pronounced tissue necrosis, or bacterial contamination, requiring abundant granulation tissue filling, intense inflammation, and prolonged wound contraction.

Transforming Growth Factor-beta (TGF-beta)

A cytokine that acts as the principal stimulant for fibroblast proliferation and collagen synthesis while concurrently serving as a growth inhibitor for epithelial and endothelial cells.

Wound Dehiscence

The spontaneous mechanical rupture or bursting open of a previously closed surgical wound, seen in $0.5\% \text{ to } 5\%$ of abdominal operations as a result of poor collagen cross-linking, infection, or elevated intra-abdominal pressure.

Incisional Hernia

A bulging defect through a weakened surgical abdominal scar caused by deficient extracellular matrix cross-linking and incomplete healing, permitting protrusion of intra-abdominal viscera.

Keloid

An exuberant, raised hyperplastic scar that grows beyond the original boundaries of the initial injury, continues progressing past 6 months, and frequently recurs following surgical excision.

Hypertrophic Scar

An elevated scar composed of excessive collagen deposition that remains confined to the borders of the original injury site, stabilizes within 6 months, and does not recur after surgical excision.

Contracture (Cicatrisation)

An exaggerated, pathological reduction in the size of a wound that leads to severe tissue distortion, limitation of joint motion, or stricture and stenosis in hollow organs such as the esophagus or intestine.

Proud Flesh

The excessive accumulation of exuberant granulation tissue that protrudes above the level of the surrounding skin, mechanically hindering re-epithelialization.

Implantation (Epidermoid) Cyst

A late complication of wound healing caused by the mechanical displacement of epidermal fragments into deeper dermal or subcutaneous tissues, where they persist and proliferate.

Woven Bone

Immature, non-lamellar bone characterized by haphazardly arranged collagen bundles and osteocytes, decreased calcium mineral content, and reduced osseomucin.

Lamellar Bone

Mature adult bone composed of parallel sheets of collagen or concentric lamellae organized systematically around vascular channels into Haversian systems.

Provisional Callus

The temporary bridge of unorganized granulation tissue, cartilage, and woven bone that unites fractured bone fragments before osteoclasts and osteoblasts remodel it into definitive lamellar bone.

Myositis Ossificans

An abnormal bone formation process that occurs when a traumatic fracture haematoma extends into adjacent torn skeletal muscle, undergoes organization, and pathologically ossifies.