Why Tissue Repair

Hello everyone, it's Lenny again with another medical topic, this time I will go over tissue repair which something amazing that occurs in our body yet we barely give it any attention. Some of us might notice the changes in before and after yet not the process of that, and since it's an amazing precise process, I chose it to be as my main topic for today.

Overview

Tissue repair is a regenerative or repair (or both of them together) process that occurs after inflammation and through replacement of damaged tissue, due to various reasons like chemical or physical agents, with either a native tissue which is known as regeneration, or with fibrous tissue known as repair (usually leaving a scar after).

Regeneration

In the case of regeneration, tissues are classified based on their regenerative capacity into: labile, stable, and
permanent.

Liable tissues have stem cells and they can go into the cell cycle in order to regenerate the tissue like in Bone marrow (hematopoietic stem cells).

Stable tissues enter the cell cycle when necessary. For instance, hyperplasia in liver.

permanent tissue can't regenerate like in myocardial cells or neurons.

Repair

Repair usually occurs when the cells doesn't have the regeneration ability or when the cell injury is bad enough that stem cells are lost. Repairing process usually leaves a scar behind due to replacement of native tissue with connective tissue and to be more specific with fibrous tissue. The formation of scar isn't enough to perform the ability that the tissue used to do yet it provides enough structural stability.

Scar formation

The formation of scar involve the formation of new blood vessels in a process known as angiogenesis. Granulation tissue proliferating fibroblasts, deposition of loose connective tissue. And the formation of the granulation tissue is the initial phase of repair.

The scar occurs through the replacement of type 3 collagen with type 1 collagen since type 1 collagen has more strength.

Mechanism Of Tissue Repair

It all starts with the inflammation which might last for few days before it's followed by proliferation. The proliferation phase is about rebuilding with new tissue made up of collagen and extracellular matrix, and actually this process involves the angiogenesis, going a little bit over Angiogenesis ( which is the formation new blood vessels from existing blood vessels) angiogenesis is critical in healing at sites of injury and this is important for the granulation tissue to be formed and healthy and have enough oxygen and nutritious. And the last part of proliferation is the formation of epithelial cells of the surface again.

Now we want to have a tissue with specific shape so we must go over the maturation or remodeling. The outcome of the repair process is a balance between synthesis and degradation of ECM proteins, so the degradation of collagen ( As I mentioned before from type 3 to type 1 replacement to allow more strength) and other extra cellular matrix components is required and it's accomplished by a family of matrix metalloproteinases (MMPs).

Finally

After going through a very complicated process we will have either complete regeneration and function of the tissue overtime, or we end up with a scar that varies in levels depending on the severity of the injury and this scar is actually important to allow the stability of the tissue structure and keeping it intact too. Any failure in the healing process might lead chronic wounds.

Note: I tried to simplify most of the information here because going over all the process in details especially about growth factors might make the article very hard to digest for many which isn't something I want to happen.

References

Book: fundamentals of pathology Pathoma + Local professor PP slides.
Website: Reference