The intergumentary system is an organ system that is composed of the skin and accompanying structures, such as hair, feathers, hooves, horns, nails, and scales. The primary functions of the intergumentary system are to protect the internal organs from mechanical damage and foreign invasion of pathogens, to hold the nerves responsible for processing pain, touch, pressure and temperature, to regulate body temperature, and to remove waste.
In most land vertebrates, the skin is 100% waterproof and also protects against UV radiation from the sun. The underlying layers of the skin also serve as a storage site for fats and other lipids.
The size and durability of the intergumentary system depend on the species. Elephants, for example, are known for having an extremely thick and durable skin, along with tusks and tough nails on their feet. In contrast, amphibians tend to have very thin and relatively delicate skin, as they have to absorb water through their skin.
In humans, the intergumentary is the single largest organ system, making up about 15 percent of the total body weight and having a total surface area of almost 2 m². It is estimated that an adult human has about 35 billion skin with many dying an being replaced every day. A large class of organisms has evolved to live on the skin of animals, making the intergumentary system into its own kind of microbiome.
Organs & Functions Of Intergumentary System
Skin
The skin is the main mass of the intergumentary system and composes most of its surface area. In mammals, the skin is composed of multiple layers of ectodermic tissue. All mammals have some hair on their skin, even if it may be difficult to see with the naked eye. Mammalian skin has 3 main layers called the epidermis, dermis, and hypodermis (subcutaneous tissue).
Epidermis
The epidermis is the outermost layer of skin and is the first line of defense against the external world. The epidermis is almost entirely composed of special cells called keratinocytes. Keratinocytes contain lots of keratin, a special structural protein that forms the physical barrier of the skin that keeps water and microorganisms out. Integrated into the keratinocytes are melanocytes, cells that produce the melanin that gives skin its color. Skin is flexible and durable because its constituent cells are anchored by an extracellular matrix made of lipids.
The epidermis itself is divided into 5 sub-layers, each a few layers of cells thick. The exact thickness of the epidermis differs from species to species. For humans, the epidermis has an average thickness of about 0.1 mm, being thicker on the palms and soles of the feet. Elephants, in contrast, have an epidermis that is almost 1.0 mm—10 times thicker than the human epidermis. The epidermis has almost no blood vessels, so it receives nourishment via diffused oxygen in the atmosphere.
The primary function of the epidermis is to protect the organism from threats and maintain a barrier between the organisms and the in the external world. It is not incorrect to liken the epidermis to the body’s armor. The epidermis protects against physical, chemical, and radiation damage serves as a barrier for infectious pathogens like bacteria, keeps water out of the body, regulates the amount of water in the body, and radiates heat away from the body. It also holds the cells that give skin its color.
Dermis
The dermis is the inner layer of the skin connected to the epidermis by the basement membrane. The dermis is what gives human skin its tensile strength and elasticity. It is composed mainly of collagen, a springy structural protein that is the single most abundant protein in mammal bodies. The dermis also contains blood vessels, hair follicles, sweat glands, sebaceous glands, and lymph vessels. In humans, the dermis has an average thickness of about 2 mm, being thicker on the palms and soles of the feet.
The dermis also contains the mechanoreceptor and thermoreceptor nerve cells that let us feel touch, pressure, and heat. Most mechanoreceptors work by transducing mechanical deformation into electrical signals. Merkel cells, for example, communicate touch via the release of serotonin in response to pressure and physical stress. The exact mechanism of action for thermoreceptors is not well understood, but it is thought it has something to do with certain proteins regulating the flow of ions across the cell membrane. In addition, the dermis contains hormones that are released during injury that stimulate healing and the production of new epithelial cells.
Hypodermis
The hypodermis is the lowest-most layer of the vertebral intergumentary system. Unlike the other layers, the hypodermis is not strictly a layer of cells, but a loose organization of connective tissues, fats, and nerve cells. The hypodermis contains the fibers that connect the skin to the muscles, blood vessels that supply the dermis, the roots of hair follicles, and collagen deposits. The hypodermis also stores the majority of the body’s fat deposits, in the form of adipose tissue. Subcutaneous fat acts as a cushion against mechanical stress and as an insulator and it grows or shrinks in response to the bodies nutritional state. The nerve cells in the hypodermis are mostly the same as the one in the dermis, only larger.
Hair, Feathers, Scales, Etc.
In addition to the skin, most vertebrates have accompanying structures that help protect the organism from danger, sense the environment, and stay warm.
Hair
Hair is one of the defining traits of mammals. Individual hairs are composed of filaments of keratin arranged in a three-layer structure that radiates inward. The average thickness of a human hair is about 0.017 mm and it is estimated that the average human head has about 150,000 individual human hairs, with over 5 million across the whole body. Some mammals have thick, dense coverings of hair called fur.
Hairs are rooted in the skin via hair follicles, small glandular regions located in the dermis. The dermis supplies blood to the hair follicles, which give nutrients to the hair root so it can produce the visible shaft. Technically, the visible part of the hair is “dead”; it exhibits no biological activity and is relatively inert. Attached to each hair follicle are the arrector pili, tiny muscles that cause the hair to stand on end. These muscles are responsible for goosebumps in humans.
The physical shape of each hair is determined by the shape of the follicle. This also determines what kind of hair a person has: straight, wavy, or curly. Hair is colored by two main type of melanin, eumelanin, and pheomelanin. Eumelanin gives brown and black hair its dark color, and pheomelanin is responsible for red hair. Blonde hair is the result of a lack of either pigment in hair. Gray hair results from the slowdown of pigment production that accompanies age.
The main function of hair is to preserve warmth and regulate the organism’s body temperature. Keratin conducts heat well and so can be used to trap heat to keep the skin warm. Some hairs, like the whiskers of a cat, are primarily for touch sensation, while things like hedgehogs and porcupines use their spiny hairs for protection. Hair, specifically its location and coloring, has also evolved to play a role in sexual selection in many species.
Feathers
As far as we know, only birds and other avians have feathers. Feathers are a complex intergumentary organ and aid in flight, thermal regulation, and protection from water. Bird feathers are made out a central shaft called a rachi, a base called a calamus (also called the quill) and the barbules that form the actual soft part. Like mammal hair, feathers are anchored to the skin via well-defined follicles that provide nutrients for feather growth.
Feathers come in many colors and are often a source of camouflage or mating display. Feathers are excellent insulators of heat and cold, which is why humans use them for things like coats, pillows, and blankets. Even birds themselves will use old feathers to insulate their nest. The specific shape of vaned feathers helps birds fly and a waxy coating repels liquid and keeps feathers from getting waterlogged.
Scales
Scales are the tough rigid plates attached to the skin of reptile and fish. There are many different kinds of scale, but they all serve analogous functions. The primary function of scales is to protect an organism from physical damage.
Most reptile scales are made out of variant proteins called α- and β-keratin. Scales are arranged in a flexible overlapping lattice so that they can maximize protection while still allowing for mobility. The hard smooth structure of scales repels water and keeps the organism warm. Most reptiles go through periodic stages where they will shed their outer-most layer of scales and grow new ones.
Fish scales are made out of a bunch of different inorganic substances, ranging from vitrodentine, calcium carbonate, ganoine, and isopedine. Most aquatic animal scales serve to protect the organism and reduce drag while swimming. Many fish scales also secrete a substance that prevents the growth of algae and other microorganisms.