Skin

Skin is the soft, flexible outer layer that covers the bodies of animals like humans and other vertebrates. It serves three important jobs: protecting the body, helping control temperature, and allowing us to feel things around us. In mammals, which include humans, skin is part of a system that also includes hair and nails, and it helps keep our muscles, bones, and inner organs safe.

Unlike the hard shells of insects or the scales of fish and reptiles, mammalian skin is made from special tissues and often has hair growing from it. Even animals that look smooth, like whales and dolphins, still have tiny hairs on their skin. Skin helps keep us warm, cool, and safe from germs, and it even helps our bodies make vitamin D when we are in the sunlight.

Scientists recently made an exciting discovery in 2024: they found the oldest known piece of skin, fossilized from about 289 million years ago. This ancient skin might have come from an early reptile, showing how important and ancient this body part truly is.

Etymology

The word "skin" originally meant the hide or fur taken from animals. It comes from an old language called Old Norse, where it was called skinn. Even farther back, the word comes from an ancient root meaning "to cut," because long ago people often cut animal hides to use for clothing.

Structure in mammals

Mammalian skin has two main layers: the epidermis and the dermis. The epidermis is the outer layer that protects the body by keeping water in and keeping harmful things out. It is made up of special cells called keratinocytes, which help form a strong barrier.

The dermis is the layer below the epidermis. It contains structures like hair follicles, sweat glands, and nerve endings that help us feel touch and temperature. The dermis also gives skin its strength and flexibility. Below the dermis is a layer called subcutaneous tissue, which connects the skin to muscles and bones and helps keep the body warm.

Structure in fish, amphibians, birds, and reptiles

Further information: Fish scale

The skin of fish and amphibians is made of live cells with very little keratin, making it often permeable. In fish, the dermis is mostly replaced by bony scales, while cartilaginous fish have tooth-like structures called denticles. Amphibians have special glands in their skin that can secrete irritating or toxic compounds for defense.

Birds and reptiles have skin similar to mammals, with a layer of dead, keratin-filled cells on the surface to reduce water loss. Unlike mammals, they do not have many skin glands, though some reptiles have pheromone-secreting cells and birds have a uropygial gland.

Main article: Reptile scales

Development

Skin structures like hair and feathers come from the outer layer of our body, called the epidermis. During early development, this layer splits into two parts. One part stays and acts like a source of new skin cells for our whole lives.

Cells in the skin work together in patterns to create features like hair. They use special signals to decide where and what to make, helping each part of the skin develop correctly.

Functions

Skin helps protect our bodies in many important ways. It acts as a shield against germs and damage, helping to keep us safe from harm. It also lets us feel things like heat, cold, touch, and pressure through special nerve endings.

Skin helps control our temperature by letting us sweat and by adjusting blood flow. It also stops too much water from leaving our bodies and stores important substances like fats and water. Some animals even breathe through their skin! Additionally, skin can help animals blend into their surroundings with colors and patterns that hide them from predators or prey.