We probably all fell for that joke as kids, but all joking aside, some of the terms used in medicine can be tricky to understand. Medicine is a foreign language that must be learned as if you were learning Spanish or French. Dermatology seems to be a branch of medicine that has unfamiliar or confusing terminology. I would like to address different skin conditions that can manifest in the feet and how they vary. But before we discuss them, I think it would be beneficial to review the components of skin.
The outermost layer of the skin is called the epidermis (0.04 mm thick). There is no blood supply to it and there are no nerves within it. This layer protects your muscles and internal organs from the external environment, and can be considered part of the immune system. The epidermis can be broken down into 5 different layers. One of these layers is responsible for cell division, which replaces the dead skin that falls off. A different layer contains melanocytes which are responsible for skin color. Another layer contains cells that are “sown” tightly together to prevent passage of foreign elements. Yet another layer secretes a fatty substance that acts a barrier to water and other harmful substances. This is a very robust barrier that is tough to cross, which is why you don’t do not swell up with water when you go swimming!
Maybe you have heard that anything you put on your skin (lotions, make-up, etc.) gets into your blood. But when we remember that there are no arteries in the epidermis, that a layer of cells are tightly packed together, and that a substance must pass the fatty barrier to reach our blood, you can be assured that almost nothing gets across. Only harsh chemicals in highly concentrated amounts should cause you to worry.
The middle layer of the skin is called the dermis (0.5 mm thick). This layer contains blood vessels, nerves, glands, and hair follicles. Collagen and elastin give structural strength to the dermis. Depending on the area of the body, the skin will have different glands that secrete either a smelly substance (armpits, groin), a watery substance (palms of hands, soles of feet), or an oily substance (causes acne).
The last layer of the skin is called the hypodermis or subcutaneous tissue. This layer is made up of fat cells and allows the skin to slide without friction over the underlying muscle and bone.
Each of these layers (or structures within a layer) can be struck with disease. We will discuss common foot dermatologic conditions in our next series of posts.