Healing is the body response to injury in an attempt to restore normal structure and function. It involve 2 process-Regeneration or Repair.
-In Regeneration when healing take place by proliferation of parenchymal cell and result in complete restoration of original tissues.
– In Repair when healing take place by proliferation of connective tissue element resulting in fibrosis or scarring.
1) Regeneration– Some parenchymal cells are short lived while others have longer life span, in order to maintain proper structure of tissue these cells are the under constant regulatory control of their cell cycle. Depending upon their capacity to divide the cell of body can be –
a) Labile cell- These cell continue to multiply through out life under normal physiological condition, these include surface epithelial cell of epidermis, respiratory tract, urinary tract, vagina, cervix, uterine endometrium etc.
b) Stable cell- These cell decreases or lose their ability to proliferate after adolescence but retain their capacity to multiply in response to stimuli through out adult life. This include parenchymal cell of organ like liver, pancreas, kidney or mesenchymal cell like smooth muscle cell, fibroblast and vascular endothelium.
c) Permanent cell- These cell lose their ability to proliferate around the time of birth, these include neurons of nervous system, skeletal muscle and cardiac muscle cell.
2) Repair– Is the replacement by mesenchymal cell consist of fibrous tissue, 2 process is involved in this is- Granulation tissue formation, Contraction of wound.
a) Granulation tissue formation-The term granulation tissue derives from slightly granular and pink appearance of tissue, three phases are observed in this-
-phase of inflammation- Following trauma there is acute inflammatory response with exudation of neutrophils, monocytes appears within 24 hrs.
-phase of clearance- Combination of proteolytic enzyme from neutrophils and autolytic enzyme from dead tissue cell and phagocytic activity of necrotic tissue.
-phase of in growth of granulation tissue- (i) Angiogenesis (neovascularisation)-Formation of new blood vessels at the site of injury takes place by proliferation of endothelial cell from the margin of severed blood vessel. Initially proliferated endothelial cell are solid, but within in lumen is developed and start carrying blood, initially it is more leaky and causes edematous appearance. Vascular endothelial growth factor, platelet derived growth factor, basic fibroblast growth factor, all are associated with cellular proliferation.
(ii) fibrogenesis- The new fibroblast originate and also collagen fibre begin to appear by about 6th day, as maturation proceed more of collagen is formed, which result in formation of inactive looking scar.
b) Contraction of wound-The wound start contracting after 2-3 days and the process is completed by the 14th day, during this period the wound is reduced by 80% of its original size by-
-dehydration as a result of removal of fluid by drying of wound.
-contraction of collagenes.
-myofibroblast appearing in granulation tissue which have the features of intermediate between fibroblast and smooth muscle cell. Their active contraction decreases the size of wound as it contain actin and myosin.