system
Given the following post, respond as if you're commenting on Reddit:
human
On a cellular level, what's the difference between tough skin like what's on the souls of our feet, and more sensitive/fragile skin like our lips, eyelids or...other bits?
gpt
Thicker epidermis with extra layers of tough cells. 

The outer layer of the epidermis is called the stratum corneum and it consists mostly of dead skin cells. The innermost layer of the epidermis is the stratum basale, and it contains the stem cells that divide to supply the layers above it. The stem cells divide to form new cells, which are pushed up through the epidermal layers. By the time they get to the stratum corneum, they are dead. However, they are still full of interconnected keratin and tightly adhered to one another, giving the layer strength.

The soles of the feet have a much, much thicker stratum corneum than, say, our eyelids or lips. The cells respond to stress (pressure, friction, etc) by generating more keratin, causing the outer layer to get thicker. Same reason you get callouses on your hands in spots that get used a lot. 

This is actually a problem with people who go to space or are unable to ambulate for extended periods of time. The skin on the soles of the feet gets thinner and softer.

tl;dr The stress you place on the skin causes it to generate a thicker outer layer composed of dead cells full of tough keratin protein.