Xeroderma pigmentosum


Introduction to the human skin

The skin is one of the largest organs and constitutes 16%1 of the human body weight. It weighs around 5kgs and covers an area of about 2 square meters. It is one of the tissues in our body that duplicates the most and the fastest. The skin has three main functions: protection, regulation and sensation.

The primary function of the skin is to act as a barrier. The skin provides protection from mechanical impacts, pressure, micro-organisms, radiation and chemicals. Skin also regulates several aspects of the body such as temperature via sweat and hair, changes in peripheral circulation and fluid balance. Finally, the skin contains an extensive network of nerve cells that detect and relay changes in the environment. There are separate receptors for heat, cold, touch, and pain.2

The skin is comprised of three main layers:

  • The epidermis: Its main job is to protect the body acting as a barrier and help control body temperature. It is made up of four types of cells: keratinocytes (which comprised the 90% of the epidermis), melanocytes, Langerhans cells and Merkel cells.
  • The dermis is much thicker than the epidermis. It contains hair roots, blood vessels, lymph vessels, glands, and nerve endings. Blood and lymph vessels in the dermis bring nutrients to the dermis and epidermis. Glands make fluids the body needs, and the connective tissue (which builds an extracellular matrix) holds all these structures in place and allows the skin to stretch.
  • The hypodermis is the subcutaneous tissue, which means “below the skin”. It is mostly made of fat and connective tissue. It connects the skin to muscles and bones and it also saves body heat, stores energy, and absorbs shock to protect the body from injury.
Human skin
Scheme of the skin components and layers. Adapted from Wikimedia

Skin is rich in cell types that have the potential to grow cancer if exposed to repeated ultraviolet trauma, such as excessive sun exposure.

1Leider, M. On the Weight of the Skin. J. Invest. Dermatol. 12, 187–191 (1949).
2Tortora, G. J. & Grabowski, S. R. Principles of anatomy and physiology. (HarperCollinsCollege, 1993).
Last modified
21 October 2019