repair revision l General pathology for dental student

Repair of tissue and healing of bone fracture  l General pathology revision for dental student ; describe definition of repair and types of cells with examples

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REPAIR OF TISSUE

• Repair is the replacement of damaged tissue by new healthy one.

Factors Affecting Repair:

Local Factors:

1. Severity of tissue damage.
2. Type of damaged cells: Labile, stable or permanent cells.
3. Blood supply : Poor blood supply and ischemia delays repair.
4. Infection and presence of foreign bodies: Delay repair.

General Factors:

1. Age: Repair is more rapid and adequate in young age.
2. Protein deficiency: Specially of sulphur containing amino acids delay repair.
3. Vitamin deficiency: Vitamin C deficiency causes defective formation of collagen and osteoid tissue. Vitamin D deficiency causes failure of calcification of osteoid tissue.
4. Zinc deficiency: Zinc is necessary for collagen synthesis.
5. Glucocorticosteroids and cytotoxic drugs: Delay repair.
6. Diabetes mellitus: Increases susceptibility to infection, so delays repair.

Types of Repair:

1. Regeneration.
2. Healing by fibrosis.

REGENERATION

• The replacement of damaged cells by new cells of the same kind. This is done by proliferation of the surrounding living cells. Cells of the body are divided according to the power of regeneration into three groups:

Labile Cells:

• These cells proliferate continuously throughout life to replace ageing cells. These cells have a good power of regeneration. The group includes:
• Surface epithelium:e.g.

1. Stratified squamous epithelium of the skin, oral cavity, pharynx.
2. Larynx, esophagus, cervix uteri and vagina.
3. Columnar epithelium of the alimentary tract, respiratory tract and endometrium.
4. Transitional epithelium of the urinary tract.

• Haemopoietic and lymphoid tissue: The red bone marrow replaces the old red cells continuously. In hemorrhage and red cell hemolysis rapid growth of the red marrow occurs.

Stable Cells:

• Do not proliferate under normal conditions, but proliferate when there is a need. The group includes:

1. Parenchymatous stable cells: Liver, pancreas, salivary glands, endocrine glands and renal tubules.
2. Mesenchymal stable cells: Fibroblasts, chondroblasts and osteoblasts.

• Regeneration occurs with small damage to stable cells, while repair with fibrosis occurs in large damage as these cells have a moderate power of regeneration.

Permanent Cells:

• Cannot proliferate at all. The group includes:

1. Muscle cells: Damaged muscle cells are replaced by fibrous tissue deposited by the fibroblasts (fibrosis).
2. Nerve cells: Damaged nerve cells are replaced by glial fibers deposited by the astrocytes (gliosis).

 

EXAMPLES OF REGENERATION

Regeneration of Skin:

• The epidermis is labile cells which regenerate easily. The damaged dermis heals by granulation tissue and fibrosis.

Regeneration of Liver Cells:

• When the damage to the liver cells is mild and the fibrous framework of the lobules is preserved, regeneration occurs and the lobular pattern is preserved.

• When the damage to the liver cells is severe and the fibrous framework of the lobules is destroyed, the liver cells regenerate in the form of new nodules of liver tissue called regeneration nodules as in liver cirrhosis.

Repair of Bone Fracture:

• For bony union to occur adequate immobilization is needed. The stages of healing are:

• Hematoma formation: With fracture some vessels of bone and soft tissue are torn off. The bleeding leads to the formation of a hematoma.

• Traumatic inflammation: The mechanical injury causing the fracture produces a mild inflammatory reaction in the bony ends.

• A scanty inflammatory exudate collects. The products of inflammation are rapidly removed by macrophages and the bone fragments by osteoclasts.

• Formation of granulation tissue: The hematoma is then invaded by capillary loops and mesenchymal cells derived from the periosteum and the endosteum of the cancellous bone. The soft granulation tissue formed is called "soft callus".

• Woven bone and cartilage formation: The mesenchymal cells start bone and cartilage formation in the soft callus which become more hard and is called "provisional callus". Complete formation of the provisional callus tabes about 3-4 weeks.
• Formation of woven bone: The mesenchymal osteoblasts form both collagen fibers and the ground substance osseomucin in which they are embedded. The tissue produced is called osteoid. The collagen bundles are irregularly disposed with no attempt at lamellar structure. The osteoid undergoes calcification helped by the enzyme alkaline phosphatase released by the osteoblasts, and is then called woven bone.
• Formation of cartilage: The mesenchymal osteoblasts behave as chondroblasts and form cartilage. The cartilage matrix undergoes calcification which causes degeneration of the cartilage cells.

• Formation of lamellar bone: The dead calcified cartilage disintegrates and the woven bone is removed by osteoclasts. The area gets invaded by blood vessels and osteoblasts which deposit down osteoid in which collagen bundles are arranged in orderly lamellar fashion. The osteoid calcifies to form lamellar bone.

• Remodelling: During lamellar bone formation the external and internal callus are removed by osteoclasts. The intermediate callus undergoes remodelling by osteoblastic and osteoclastic activity. The remodelling process is directed by the muscle and weight-bearing stresses imposcd on the bone.

• Bone marrow regeneration.

Bony union may fail to occur due to:

1. Faulty immobilization which leads to fibrous union.
2. Infection at the site of fracture.
3. Impairment of the blood supply.
4. Interposition of soft tissue between the bony ends.
5. Nutritional disturbance of the body and old age.