Here’s a general pathology concept that is important not only for boards but for real life: wound healing. Let’s take a look at wound healing in the skin. Whether the wound is a small kitty scratch or a huge burn, we have ways of repairing the damage and restoring function to the skin. There are two types of wound healing in the skin: healing by first intention and healing by second intention (weird names, I know, but whatever).
Healing by first intention occurs in small wounds that close easily – for example, paper cuts, or small surgical incisions in which the edges are easily approximated. In this type of healing, epithelial regeneration predominates over fibrosis. That’s a fancy way of saying that there is usually minimal scarring in this type of healing. Healing is generally fast. Here’s a summary of the timeline in most wounds healing by first intention:
By 24 hours: a clot forms, neutrophils arrive, and the epithelium begins to regenerate
By 3-7 days: macrophages arrive, granulation tissue is formed, collagen begins to bridge the incision, and the new epithelium increases in thickness.
Let’s stop right here for a moment. Granulation tissue is that stuff that forms when your body is filling in the gap between your remaining tissues. The contents of granulation tissue are 1) new, fragile blood vessels, 2) fibroblasts, and 3) a loose extracellular matrix holding it all together. The whole point of granulation tissue is to provide a place for the new structures to grow that will hold the tissue together (blood vessels and collagen). That’s it. Note: granulation tissue is not the same as a granuloma (which is a collection of macrophages) or chronic granulomatous disease (in which patients have neutrophils that don’t work right, so their macrophages are left with the job of killing bacteria, and they form little granulomas all over the place). So don’t get those terms mixed up.
Weeks later: the granulation tissue is gone, collagen has been remodeled (using little metalloproteinase enzymes like collagenase), and the epidermis is full and mature (though it lacks dermal appendages in the area of the healed wound). Eventually, a full-blown scar forms.
Healing by second intention occurs in larger wounds that have gaps between the wound margins. Examples of this type of wound are: areas of skin infarction, large burns and ulcers, and extraction sockets (where the dentist has pulled a tooth. Yes, this first- and second-intention healing applies to mucosal epithelium too!). In this type of healing, fibrosis predominates over epithelial regeneration. In other words, there’s gonna be a big scar that’s more prominent than any skin regrowth that occurs. Healing by second intention is slower. There is a lot more granulation tissue (because you have a huge gap to bridge) and more inflammation (neutrophils and macrophages coming in to clean up the dead cells and debris). Therefore, there’s a greater risk of infection and inflammation-related tissue injury. Also, the wound contracts as it heals (so you don’t have to make such a big scar). As far as a timeline goes, you can’t really make a universal timeline for second-intention healing, because it varies a lot depending on how big the wound is.
It all makes sense if you can just remember: first intention = small wounds, second intention = big wounds.
The strangely beautiful photo above, aptly titled “cat launchpad,” was taken by quinn.anya and can be found at: http://www.flickr.com/photos/quinnanya/2420314228/.
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- khash said this helped me in my microbiology/immunology final exams. thank you
- Kristine said Good analogy, Ehsam.
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