Skin (comparative anatomy)

Animal survival requires that the internal body components be separated and protected from the external environment. Most single-cell organisms are separated from the environment only by the plasma membrane (cellular membrane). In multicellular organisms, the body surface is covered by a tissue consisting of epithelial cells and connective tissue. The covering is commonly referred to as skin, but the skins of invertebrates and vertebrates have distinct differences. Invertebrates often have a single layer of surface epithelial cells, which is generally referred to as an integument. However, some invertebrates, specifically flukes and tapeworms, have a unique, living surface covering called a tegument. In this situation, the epithelial cells have fused and formed a single bag of cellular components called a syncytial epidermis. Thus, the word “skin” is often reserved specifically to describe the surface covering in vertebrates, but the word “integument” is also used.

The Structure and Physical Properties of Skin

The general structure of the skin is similar in all vertebrates, with three primary regions—the epidermis, dermis, and hypodermis. The upper region, the epidermis, comprises multiple layers of epithelial cells. All vertebrate skin has a basal layer (the stratum germinativum) consisting of mitotic cells. These mitotic cells divide to replace the cells closer to the surface as they are worn away and to heal skin wounds. In most vertebrate species, the outermost layer of cells (the stratum corneum) is dead. The dead cells are filled with a waterproofing protein called keratin, produced by keratinocytes, the major type of cells forming the epithelium. An exception can be noted in many species of fish, where the epidermis is composed entirely of living cells and the stratum corneum is absent. The epidermis will differ the most between aquatic (water-dwelling) and terrestrial (land-dwelling) organisms. Compared to mammals and reptiles, the epidermis of amphibians, birds, and fish is thinner, and the stratum corneum may only be one or two cells thick. The epidermis lacks its own blood supply (is vascular). Nourishment reaches the living cells by diffusion from the underlying dermal blood supply. No nerves are present in the epidermis.

The underlying region, the dermis, is primarily composed of connective tissue. Although epidermal and dermal thicknesses vary between groups of animals and thickness may vary along an individual’s body surface, the dermal layer is always thicker than the epidermis. The hypodermis, a layer of subcutaneous tissue immediately beneath the dermis, connects the skin with underlying tissues such as muscles and bone. In birds and mammals, in particular, this layer often contains a significant amount of fat, which provides insulation and a reserve source of energy.

The protective aspect of skin does not mean that it is entirely impenetrable or that the body is completely isolated from the environment. Materials that are fat-soluble or that disrupt cellular membranes can be absorbed across the skin surface. In some cases, beneficial chemicals cross the skin. For example, frogs, which are amphibians, actively take up oxygen and expel carbon dioxide across the skin as well as the surface of the lungs. Amphibian skin is also permeable to water, and, in fact, some species absorb amounts comparable to that obtained by drinking in other organisms. In other cases, detrimental materials such as solvents and potential environmental pollutants cross the skin. For example, acetone is present in nail polish remover, methanol is sometimes used to remove old finishes on furniture, and salts of heavy metals such as mercury or lead.

Skin, like muscle, has the properties of extensibility (stretch) and elasticity (the ability to return to the original shape after being stretched). These properties are made possible by the presence of collagen and elastic fibers as major components of the tissue comprising the dermis. When the elastic properties of the skin have been exceeded, white lines known as stretch marks appear.

Specialized Secretions and Structures Associated with Skin

Some epithelial cells produce and release protective secretions onto the external surface. Vertebrates and invertebrates both have mucous-secreting cells. On internal surfaces, such as the digestive tract, mucus protects cells from being broken down along with the food. On the external surface, mucus may trap bacteria or, as in earthworms, prevent death from desiccation (drying out). Another example of an invertebrate secretion is a covering called a cuticle. In insects, this cuticle includes a mixture of proteins that eventually harden and form the exoskeleton.

Vertebrates, including fish, birds, and humans, as well as invertebrates, and insects, secrete a group of antimicrobial (bacteria-killing) proteins called defensins (originally called magainins). Species of poison dart frogs have another type of protective secretion, which is toxic to potential predators. Some of these secretions are used on poison dart arrows.

Structures with quite varied functions are derived from skin cells, particularly epithelial cells. The feathers of birds function in flight, but they also provide insulation. Birds' beaks and claws provide a method of defense and a way to secure food. Mammalian hair is an epithelial derivative. Body hair provides protection from abrasion and sunlight and has some insulation value. In animals that have them, sweat, oil, and mammary glands are groups of specialized epithelial cells. Reptiles, for example, lack sweat glands. Light organs of deep-water fish are modified epithelial glands. The scales of reptiles, the rattles of snakes, and the claws of turtles are other examples of epithelial derivatives. Geckos can walk up walls because they have modified epidermal scales on the tips of digits which serve as suction cups.

Cells of the dermis also are the origin of specialized structures in some organisms. Although there are fewer examples, dermal derivatives include shark teeth, fish scales, and the protective armor plates of an armadillo.

Skin and Temperature Regulation

The skin is a major organ in controlling body temperature. Mammals and birds are animals that generate internal body heat (warm-blooded or endothermic). Species of reptiles, fish, and amphibians, which are cold-blooded (ectothermic), are unable to control their body temperature through internal regulators in the same way that warm-blooded animals can. Both groups of animals depend on the rich supply of blood vessels in the dermis as one mechanism for maintaining a safe body temperature. When internal body temperatures rise in endotherms, an increase in blood flow carries internal body heat to the surface, where it is lost to the environment. This process is called vasodilation. A similar increase in blood flow in ectotherms carries heat from the environment into the body and helps to warm internal organs and tissues. A decrease in blood flow will work in the opposite direction in both groups, called vasoconstriction. The hypothalamus regulates these processes in all vertebrates.

In animals with sweat glands, the evaporation of sweat secreted onto the body surface also helps to lower body temperature. Only mammals have sweat glands, but not all mammals. Humans, horses, monkeys, and apes sweat in the most traditional sense of the definition. Dogs and cats sweat a small amount through their paws, and many other mammals have sweat glands in their hands and feet, but the purpose of this sweat is grip rather than cooling. The hippopotamus secretes an oily fluid from its skin that contains red and orange pigments called hipposudoric and norhipposudoric acid. Because of the fluid's color, early explorers called it blood sweat. This fluid is not sweat or blood, but it does help cool their body by absorbing and scattering ultraviolet light from the sun. It also has antibiotic properties and is a natural sunscreen. On land, hippos produce more of this fluid to moisturize their skin and prevent dehydration. In the water, they produce less fluid but continue producing enough to waterproof their body.

One unique feature of birds is a specialized region of skin, the brood patch, located on the ventral (stomach) surface. This area is rich in blood vessels (highly vascularized) and is used to transmit heat from the female to the eggs or the hatchlings. Similarly, elephant skin contains hot spots that keep them cool, and bears have arteriovenous anastomoses (AVAs), which help regulate their temperature. AVAs are bald skin surface areas that behave like radiators and cool the animal. AVAs may occupy up to 60 percent of a bear's cardiac output in extreme weather. Giraffes also have AVAs, which are evident in their skin pattern.

Skin Coloration

Vertebrate skin contains pigment-producing cells called chromatophores. Pigment production provides many benefits to animals. Skin pigments help to limit the amount of damaging ultraviolet light or irradiation to which the deoxyribonucleic acid (DNA) in the mitotic cells and the underlying tissues are exposed. Melanocytes located in the epithelium of mammals produce a brown-black pigment called melanin. They are the only pigment-producing cells in most mammals. In addition to epithelial melanocytes, amphibians, fish, reptiles, and birds have other types of pigment-producing cells in the dermis. Examples are lipophores, which use carotene, a naturally occurring pigment in food, to synthesize yellow, orange, and red pigments, and iridophores, which use molecules called purines to synthesize iridescent pigments. The amount of pigment produced, the final location of the pigment in the cells, and the combination of cells producing it result in a range of body and feather coloration. Chromatophores account for the changes in body color that allow chameleons, flounders, and octopuses to easily blend in with different surroundings. Changes in body color also provide a means of communication between individuals. While octopuses and chameleons are well-known as animals that change color both to communicate and camouflage from predators, crustaceans, fish, lizards, and insects do so as well.

Skin color, particularly in organisms with a thin, fair surface covering like humans, is also influenced by the amount of oxygen bound to hemoglobin in the blood. Fully oxygenated hemoglobin is red, and it gives a pink coloration to the skin. Hemoglobin that is not fully oxygenated can cause the skin to appear blue or take on a purplish hue, a condition called cyanosis.

Principal Terms

Chromatophores: pigment-producing cells

Dermis: layer beneath the epidermis, primarily connective tissue but also containing nerves and blood vessels

Epidermis: surface layer of epithelial cells

Invertebrate: animal without a backbone

Mitotic Cells: cells capable of dividing and forming new cells

Vertebrate: animal with a backbone made up of individual bones called vertebrae

Bibliography

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