Healing : Regeneration And Repair | Pathophysiology Notes

Injury to tissue may result in cell death and tissue destruction. Healing on the other hand is the body's response to injury in an attempt to restore normal structure and function. The process of healing involves two distinct processes:

Regeneration: When healing takes place by proliferation of parenchymal cells and usually result in complete restoration of the original tissues.

Repair: When the healing takes place by proliferation of connective tissue elements resulting in fibrosis and scarring.

1. Regeneration:

Some parenchymal cells are short lived while others have long life span. In order to maintain proper structure of tissue, these cells are under the constant regulatory control of their cell cycle. These include growth factors such as epidermal growth factor, fibroblast growth factor; platelet derived growth factor and endothelial growth factor. Depending upon their capacity to divide, cells of the body can be divided into three groups.

A. Labile cells: These cells continue to multiply throughout life under normal physiologic conditions. These include surface epithelial cells of epidermis, alimentary tract, respiratory tract, urinary tract, vagina, cervix, uterine endometrium, haemotopoietic cells of bone marrow and cells of lymph nodes and spleen.

B. Stable cells: These cells decrease or lose their ability to proliferate after adolescence but retain the capacity to multiply in response to stimuli throughout adult life. These include parenchymal cells of organs like liver, pancreas, kidneys, adrenal and thyroid. Mesenchymal cells like smooth muscle cells, fibroblasts vascular endothelium bone and cartilage cells.

C. Permanent cells: These cells lose their ability to proliferate around the time of birth. These include neuron, skeletal muscle and cardiac cells.

2. Repair:

Repair is the replacement of injured tissue by fibrous tissue. These responses take place by participation of mesenchymal cells (consisting of connective tissue, stem cells fibrocytes and histocytes). The process of repair involves:

1. Granulation tissue formation
   
2. Contraction of wounds.
   
   I. Granulation Tissue Formation:
   The term granulation tissue derives its name from slightly granular and pink appearance of the tissue. Each granule corresponds histologically to proliferation of new small blood vessels which are slightly lifted on the surface by the covering of fibroblasts and young collagen.
   
   II. Contraction of Wounds:
   The wound starts contracting after 2-3 days and the process is completed by the 14th day. During this period the wound is reduced by approximately 80% of its original size. Contracted wound results in rapid healing since lesser surface area of the injured tissue has to be replaced.
   
   In order to explain the mechanism of wound contraction a number of factors have been proposed.
   
   1. Dehydration as a result of removal of fluid by drying of wound was first suggested but without being substantiated.
      
   2. Contraction of collagen was thought to responsible for contraction but wound contraction proceeds at a stage when the collagen content of granulation tissue is very small.
      
   3. Discovery of myofibroblasts appearing in active granulation tissue has resolved the controversy. These cells have features intermediate between those of fibroblasts and smooth muscle cells. Their migration into the wound area and their active contraction decreases the size of the defect.