Inflammation and Repair: Histological Features and Associated Factors

1. Histological Features of Inflammation:

Inflammation is a complex biological response to tissue injury that involves the activation of various immune cells and the release of signaling molecules, collectively known as mediators of inflammation. The primary goal of inflammation is to eliminate the inciting agent and initiate tissue repair. The main histological features of inflammation include:

• Hyperemia:

• This refers to an excessive accumulation of blood in the affected area, leading to redness and warmth.

• It is caused by vasodilation of small blood vessels (arterioles) triggered by the release of chemical mediators like prostaglandins (PGs), histamine, and nitric oxide (NO) from damaged tissue.

• The increased blood flow leads to a rush of blood into the small vessels and into the venules, resulting in hyperemia.

• Notably, hyperemia is an early response of inflammation, crucial for delivering leukocytes and immune factors to the site of injury.

• Edema:

• Edema is the accumulation of excess fluid in the interstitial space, causing swelling.

• Inflammatory exudate, the fluid responsible for edema, has a higher protein concentration than normal interstitial fluid (>3g%), with a specific gravity exceeding 1.020.

• Two primary mechanisms contribute to edema:

• Increased hydrostatic pressure: This is caused by hyperemia and obstruction of lymphatic drainage.

• Increased vascular permeability: Mediators like histamine, serotonin, prostaglandins, leukotrienes, bradykinin, fibrin, and platelet-activating factor (PAF) alter the vascular endothelial structure, increasing permeability.

• It’s important to note that edema is a natural response that helps dilute harmful agents and facilitates the delivery of leukocytes and immune factors to the injured area.

• Cellular Infiltration:

• This refers to the movement of leukocytes from the bloodstream into the injured tissue.

• Guided by chemical mediators, leukocytes undergo a three-step process:

• Margination: Leukocytes adhere to the endothelium of blood vessels.

• Diapedesis: Leukocytes migrate through the vascular endothelium.

• Chemotaxis: Leukocytes move towards the site of injury guided by chemoattractant molecules.

• Cellular infiltration is a crucial step in inflammation, facilitating the elimination of the inciting agent and initiating tissue repair.

2. Mechanism of Margination:

• Hyperemia and Increased Vascular Permeability: The enhanced blood flow and increased vascular permeability bring leukocytes closer to the vascular endothelium.

• Adhesive Molecule Interactions:

• Selectins: These are cell surface proteins that include:

• P-selectin: Translocated from Weibel-Palade bodies within endothelial cells upon exposure to inflammatory mediators.

• E-selectin: Newly synthesized on endothelial cells after exposure to cytokines like TNF-α and IL-1.

• Integrins: These are proteins on the leukocyte surface. They have low affinity for adhesion molecules on endothelial cells when inactive. During inflammation, their affinity increases.

• Immunoglobulin Superfamily (IgSF) Adhesion Molecules: These are proteins on the endothelial cell surface, including ICAM-1 (intercellular adhesion molecule-1) and VCAM-1 (vascular cell adhesion molecule-1). They interact with integrins on leukocytes.

3. Adhesion Molecules and their Roles:

• P-selectin + Sialyl-Lewis X: Facilitates leukocyte rolling along the vascular endothelium.

• E-selectin + Sialyl-Lewis X: Contributes to both rolling and adhesion of leukocytes to the vascular endothelium.

• GlyCAM-1, CD34 + L-selectin: Facilitates leukocyte rolling along the vascular endothelium.

• IgSF Adhesion Molecules (ICAM-1, VCAM-1) + Integrins: Promote leukocyte adhesion, halting rolling, and enabling diapedesis through the vascular endothelium.

Important Notes:

• Inflammation is a complex and diverse process. The histological features and mechanisms can vary depending on the type of injury, the causative agent, and the individual’s overall health status.

• While inflammation is a protective response, prolonged or uncontrolled inflammation can be detrimental to the body.

• Anti-inflammatory medications can help alleviate symptoms and limit tissue damage caused by inflammation.

Further Information:

• Chemical mediators play a significant role in inflammation.

• Different types of leukocytes participate in inflammation, including neutrophils, monocytes, and lymphocytes.

• The process of tissue repair following inflammation is complex and involves regeneration, repair, and scar formation.

This article provides a basic overview of inflammation. For further information, it is recommended to consult reliable medical sources.