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Here’s a general pathology concept that is important not only for boards but for real life: skin wound healing. 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

This type of healing occurs in relatively 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 pretty fast, which makes sense, given that these wounds are pretty small and the edges are close together. Here’s a summary of the typical timeline for healing by first intention:

0-24 hours: a clot forms, neutrophils arrive, and the epithelium begins to regenerate
By 3-7 days: macrophages have already arrived (and started cleaning up the mess the neutrophils made), granulation tissue is formed, tiny little collagen fibers begin to show up, 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 (these are the cells that produce collagen!), and 3) a loose extracellular matrix holding it all together.

The whole point of granulation tissue is to provide the necessary components for healing and scar formation. As the fibroblasts produce collagen over the days and weeks to come, granulation tissue slowly recedes, and eventually it disappears entirely.

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: granulation tissue is gone, collagen has been laid down and 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). Basically, a nice scar has formed!

Healing by second intention

This type of healing occurs in wounds that have gaps between their margins. In general, these wounds are larger than first-intention wounds. Examples of this type of wound include: burns, deep lacerations, 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 (scarring) predominates over epithelial regeneration. Compared to first-intention healing, there are some important differences:

  • Healing is slower (and it’s not really possible to make a universal timeline for second-intention healing, because the timing varies a lot depending on how big the wound is).
  • There’s more granulation tissue (because you have a huge gap to bridge).
  • There’s more inflammation (more dead cells and debris means more neutrophils and macrophages!).
  • There’s a greater risk of infection (as you’d expect with a big open wound!) and inflammation-related tissue injury.
  • The wound contracts as it heals (so you don’t have to make such a big scar).

But all this stuff is really logical, if you think about it. You just need to remember which term goes with which type of wound. So just remember:

  • First intention = smaller wounds with closeable edges
  • Second intention = larger wounds with far-apart edges