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# Skin Cell Regeneration

**Does it take 28 days to regenerate all layers in the epidermis? Or just the top layer?**

Primarily, all layers of the epidermis except the stratum basale (the lowermost layer of the epidermis). 

The stem cells in the basal layer divide into two. In general, the new cell differentiates and becomes a skin cell (keratinocyte), while the original cells stay a stem cell (so that it can continue to create new skin cells.

The keratinocyte moves up and ages through the layers until it gets to the outermost layer. This takes about a month in most healthy adults.

This is a highly complex process which is regulated by many factors and controls.

You can learn more about this process here: [__Epidermis and Its Renewal by Stem Cells.__](https://www.ncbi.nlm.nih.gov/books/NBK26865/)

**Is this turnaround the same in all parts of the body (e.g. face vs knees vs palms)?**

We don’t have a definitive answer in humans for this question yet. A [__2016__](https://pubmed.ncbi.nlm.nih.gov/27657513/) paper looking at skin turnover in mice found that there may actually be a difference in the time and rate of the turnover area of thin vs thicker skin. This suggests that there are likely to be differences in humans as well, and [__new techniques__](https://www.mdpi.com/2079-9284/4/4/47) for examining this in humans are in development.

**What if you had a scar or operation? Does it heal in the same period?**

Wound healing [__time__](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2903966/) depends on the size, immune system, age, hormones, stress, obesity, and so on.

For example, one paper found that most [__ulcers__](https://bmjopen.bmj.com/content/3/8/e003091) took 63 days to heal, but a paper cut only takes a few days to heal. For surgical wounds, it could be a few weeks to a few months.

Scars are very different. [__Scars__](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5792238/) are mostly unorganised collagen. Because they are not made up of keratinocytes (the skin cells that go through the shedding process), they don’t go through regeneration. 

The skin cells just move to the side of the scar. And you’ll be learning more about wound healing later in your certification.