Repair of Skin

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    Because our skin is the part of us that confronts our world directly, it is also quite likely to be damaged frequently.  It has to have a method for repair.  Because it is made up of epidermis and dermis, both highly regenerative tissues, it is quite capable of much repair... but there is a limit.

Recovery after damage in general:

Think about the epidermis for starters...

    What part of the epidermis allows for regeneration?  I hope you are now thinking about the stratum basale.  All the cell division to replenish the epidermis occurs in the stratum basale.  And this epidermal layer is also the one that picks up the most nutrients from the underlying dermal blood vessels.  I have drawn an extremely simplified schematic of epidermis in the figure below in part A.   It is so simplified that not all the layers are included, and I just wrote "etc." to remind you that more layers are needed on top.  I also only drew the nuclei in the cells of the stratum basale, so that this layer would stand out.

    If you are cut very lightly, so that just the epidermis is affected (and then you don't even bleed), the cells in the stratum basale can easily divide and repair the damage.  This is shown in part B of the figure below.  In this figure, the stratum basale is unaffected, so it can just keep dividing and recreate the layers above it to return the epidermis to the "normal" condition shown in part A.

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    However, if the cut is deeper, and cuts through the entire epidermis, the repair is more difficult.  First of all, you will bleed and a scab will form in the cut.  Secondly, if the two ends of the cut are at all separate, as shown in part C of the above figure, the stratum basale has to divide to re-grow and fill up the space between the ends.  After this is complete, regrowth of the entire epidermis is simple-- all that is needed is more cell division of the cells in the stratum basale.

    Now, you should understand why, when you have a large cut, it is necessary to get stitches, to sew the two ends back together.  After stitches, the stratum basale doesn't have to grow too much laterally before it encounters the other side of the cut... then it can focus on re-growing the more apical layers of the epidermis so that you will heal fully and faster.  Also, the stratum basale can only reform completely if it has something on which to grow!  It has to be able to find good dermis to sit on and to make a new basement membrane with.  If the ends are sewn back together, the dermis will recover faster as well.

Now think about the dermis...

    The dermis is also highly regenerative.  Why?    Because its fibroblasts are numerous and can re-secrete the matrix fibers as well as divide to make more fibroblasts.  Fibroblasts are also quite mobile, and can move into the damaged area so that they will re-establish the dermis where it needs it.   Macrophages can also crawl around and clean up the scab material, so that it gets cleared away as it is no longer needed to maintain the continuity of the skin.

Burns:

    I would like you to consider burns separately, because they tend to be less discrete than cuts.  A cut can be localized to a certain place, but burns can be much more extensive across the skin area.  The healing process from a burn, as far as the epidermis and dermis regeneration goes, is similar.  But if you receive large burns, your recovery and treatment will differ.

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First-degree burns

    These burns are the easiest from which to recover.  These are burns that only affect the epidermis.  That means that the dermis remains intact and can support the regeneration of the epidermis.  It may happen that the skin will get inflamed (due to responses in the underlying dermis) during this time... the inflammation is to encourage clean-up cells (macrophages) and nutrients to reach the damaged area.

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Second-degree burns

    In these burns, the dermis is also affected, at least partially.   But, with a second-degree burn, most (or at least some) skin accessory organs typically remain OK.  Remember, these accessory organs (like glands and hairs) are built from invaginations of the epidermis, creating the epithelium of the glands or hair follicles.  If these organs survive the burn, the epithelial cells that remain will begin to divide to regenerate a new epidermis.

    In order to understand this, you have to picture a larger area of skin, maybe a few centimeters in diameter, where all of the epidermis is gone and much of the dermis is harmed.  This damaged area of skin is depicted in the animated image below.  You don't want to sew the ends of the burn back together by stitching them, because there really aren't any "ends" to sew... the burn is more spread out.  How can the epidermis re-establish itself across this wide space?

    The accessory organs are still in that space-- imagine them as "seeds" for outgrowth of new epidermis.  They way this would work is in this animation.  You'll see the areas where accessory organs are diagrammed as small pink dots (little spots of epithelium that remain).  As these re-grow, the new epithelial tissue, now epidermis, is slowly reestablished across the burn area.

burnregen.gif (89225 bytes)

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Third-degree burns

    Third-degree burns are the worst.  In these, even the accessory organs are destroyed.  How can we recover from this?  Skin grafts are about the best way... but they cannot really be done across one's entire body, so a very widespread third-degree burn is not usually something that a person can recover from.

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Skin Grafts

    This is a surgical procedure.  Pieces of skin are taken from elsewhere on your body and put into the location of the burn.  Since these pieces were cut out of somewhere else on your body, the remaining skin at the healthy site must be stitched back together-- this makes that area of your body extremely tight-- it lacks the normal amount of surface area.  The tight area will produce more skin, slowly, and eventually the tightness will go away.  The healthy skin that was cut out is now the skin graft, and it is placed over the burned area.  If this skin graft takes to this new location, without infection occurring, it will grow out and fix the burned area up.

Besides cell division, what else is going on in skin during regeneration?

    Figure 6.14 in your book is excellent in showing the steps skin takes to recovery.  Any cut or burn that gets into the dermis leads to bleeding.  Blood that flows outside of a blood vessel will clot (as you will learn in more detail next semester) and form a scab.  The scab prevents further blood loss and inhibits infections from entering the body.  As cells in your skin divide and attempt to repair the damage, the clot will have to be removed.  Macrophages invade and remove scab material as is needed to enable tissue recovery.  Follow this process in this figure in your book!

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.