Skin

Your skin covers your whole body.

It keeps you safe from germs. It also keeps water inside your body.

Skin helps you feel the world. It can feel heat and touch.

Skin also helps you stay warm. Some animals have hair to help. Even whales have hair!

Skin can be thick or thin. The skin on your feet is very thick. The skin near your eyes is very thin.

Your skin is a wonderful shield.

Skin is the soft layer that covers the bodies of animals. It has three main jobs. It protects the body, helps control temperature, and lets us feel things.

In mammals, skin has two main parts. The first part is the epidermis. This is the outer layer. It acts as a shield. It keeps water inside and stops germs from entering. The second part is the dermis. This layer is underneath the epidermis. It is thick and stretchy. It holds things like hair and sweat glands. It also has nerve endings. These help us feel touch and heat.

Skin thickness can change. In humans, the skin near the eyes is very thin. The skin on your palms is much thicker.

Other animals have different skin. Fish and reptiles often have hard scales. Amphibians, like frogs, have skin that can let chemicals pass through. Some amphibians have special glands. These glands can let out toxins to keep them safe. Even whales have some hair on their skin!

Skin is the soft and flexible layer that covers the bodies of vertebrate animals. It acts as the first line of defense between a living thing and its environment. This layer has three main jobs: protection, regulation, and sensation. It protects the body from germs and keeps too much water from escaping. It also helps control temperature and lets animals feel the world around them.

In mammals, the skin works through several layers. The first is the epidermis, which is the outermost part. It acts as a waterproof shield to block infections. Below that is the dermis, a thick layer of connective tissue. The dermis provides strength and stretchiness to the body. It also holds hair follicles, sweat glands, and blood vessels.

Between these two layers sits a thin sheet called the basement membrane. This membrane acts like a gatekeeper for cells and molecules. The dermis is divided into two parts: the papillary region and the reticular region. The papillary region has fingerlike shapes called papillae. These shapes help the two layers of skin stay tightly connected. The deeper reticular region is much thicker and holds many important structures.

Skin thickness is not the same all over the body. In humans, the skin under the eyes is very thin at only 0.5 mm. This is why wrinkles often show up there first. However, the skin on your palms and the soles of your feet is much thicker. It can be as thick as 4 mm. This thickness helps protect the parts of the body that touch the ground or carry heavy loads.

Different animals use their skin in very different ways. Fish and reptiles often have hard scales made of beta-keratins. Amphibians, like frogs, have skin that is much more permeable. This means chemicals can pass through it easily. Some amphibians even have granular glands that can release toxins for safety. Even whales and dolphins have some hair on their skin, even if it is hard to see!

Skin is the soft, flexible outer tissue that covers the bodies of vertebrate animals. It functions as a vital interface between a living organism and its external environment. This biological barrier serves three primary roles: protection, regulation, and sensation. It acts as the first line of defense against pathogens and prevents excessive water loss. Additionally, skin aids in temperature regulation, insulation, and the production of vitamin D folates.

In mammals, the skin is a complex organ of the integumentary system. It is composed of multiple layers of ectodermal tissue. This structure guards underlying muscles, bones, ligaments, and internal organs. The skin also contributes to the formation of various extraskeletal apparatuses. These include the horns of bovids, the antlers of cervids, and the osteoderms of armadillos. Even marine mammals like whales and dolphins possess some hair on their skin.

Mammalian skin is organized into two primary layers: the epidermis and the dermis. The epidermis is the outermost layer and provides waterproofing. It acts as a barrier to infection through a stratified squamous epithelium. This layer is made of proliferating basal cells and differentiated suprabasal keratinocytes. Keratinocytes are the most abundant cells, making up 95% of the epidermis. Other important cells include Merkel cells, melanocytes, and Langerhans cells.

The epidermis contains several distinct strata, or layers. These include the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and the stratum basale. In the stratum basale, keratinocytes undergo mitosis to create new cells. As these daughter cells move upward through the strata, they undergo cell differentiation. They eventually become anucleated, meaning they lose their nuclei. During this process, they secrete keratin proteins and lipids to create mechanical strength. Finally, cells from the stratum corneum are shed through a process called desquamation.

A thin sheet of fibers called the basement membrane separates the epidermis from the dermis. This membrane controls the movement of cells and molecules between the two layers. It also acts as a reservoir for cytokines and growth factors. These molecules are released during physiological remodeling or tissue repair. The dermis lies beneath the epidermis and consists of connective tissue. It provides tensile strength and elasticity through an extracellular matrix. This matrix is composed of collagen fibrils, microfibrils, and elastic fibers embedded in hyaluronan and proteoglycans.

The dermis is structurally divided into two specific regions. The superficial papillary region contains fingerlike projections called papillae. These projections interdigitate with the epidermis to strengthen the connection between layers. The deeper, thicker reticular region contains a dense concentration of fibers. This region houses hair follicles, sweat glands, sebaceous glands, and blood vessels. It also contains mechanoreceptors, such as nociceptors and thermoreceptors, which provide the senses of touch and heat. Below the dermis lies the subcutaneous tissue, or hypodermis. This layer attaches the skin to bone and muscle and contains 50% of the body's fat.

Skin thickness varies significantly across different parts of the body. In humans, the skin under the eyes is the thinnest at only 0.5 mm. This thinness makes it one of the first areas to show wrinkles. Conversely, the skin on the palms and soles is the thickest at 4 mm. Other vertebrates utilize different skin structures for survival. Reptiles and birds have hard scales or feathers made of beta-keratins. Amphibians have highly permeable skin subject to osmosis and diffusion. For example, chemicals can diffuse through a frog's skin very quickly. Some amphibians also possess granular glands that secrete toxic compounds for defense.

Scientific study of skin has a long history. The word "skin" is a borrowing from Old Norse, referring to animal hide or fur. Originally, the term only referred to tanned animal hides. In the past, the word "hide" was commonly used for human skin. Biologists continue to find new information about ancient life through skin fossils. On 11 January 2024, researchers reported the discovery of the oldest known fossilized skin. This specimen is approximately 289 million years old and likely belonged to an ancient reptile.