Anatomy and Physiology of Animals/The Skin

The skin is the first of the eleven body systems to be described. Each chapter from now on will cover one body system.

The skin, sometimes known as the Integumentary System is, in fact, the largest organ of the body. It has a complex structure, being composed of many different tissues. It performs many functions that are important in maintaining homeostasis in the body. Probably the most important of these functions is the control of body temperature. The skin also protects the body from physical damage and bacterial invasion. The skin has an array of sense organs which sense the external environment, and also cells which can make vitamin D in sunlight.

The skin is one of the first systems affected when an animal becomes sick so it is important for anyone working with animals to have a sound knowledge of the structure and functioning of the skin so they can quickly recognize signs of disease.

Objectives

After completing this section, you should know:

the general structure of the skin
the function of the keratin deposited in the epidermis
the structure and function of keratin skin structures including calluses, scales, nails, claws, hoofs and horns
that antlers are not made either of keratin or in the epidermis
the structure of hairs
the structure of the different types of feathers and the function of preening
the general structure and function of sweat, scent, preen and mammary glands
the basic functions of the skin in sensing stimuli, temperature control and production of vitamin D
the mechanisms by which the skin regulates body temperature

The Skin

The skin comes in all kinds of textures and forms. There is the dry warty skin of toads and crocodiles, the wet slimy skin of fish and frogs, the hard shell of tortoises and the soft supple skin of snakes and humans. Mammalian skin is covered with hair, that of birds with feathers, and fish and reptiles have scales. Pigment in the skin, hairs or feathers can make the outer surface almost any color of the rainbow.

   Skin is one of the largest organs of the body, making up 6-8% of the total body weight. It consists of two distinct layers. The top layer is called the epidermis and under that is the dermis

The epidermis is the layer that bubbles up when we have a blister and as we know from this experience, it has no blood or nerves in it. The cells at the base of the epidermis continually divide and push the cells above them upwards. As these cells move up they die and become the dry flaky scales that fall off the skin surface. The cells in the epidermis die because a special protein called keratin is deposited in them. Keratin is an extremely important substance for it makes the skin waterproof. Without it, land vertebrates like reptiles, birds, and mammals would, like frogs, be able to survive only in damp places.

Skin Structures Made Of Keratin

Claws, Nails and Hoofs

Reptiles, birds, and mammals all have nails or claws on the ends of their toes. They protect the end of the toe and may be used for grasping, grooming, digging or in defense. They are continually worn away and grow continuously from a growth layer at their base (see diagram 5.2).

File:Anatomy and physiology of animals Carnivores claw.jpg

Diagram 5.2 - A carnivore’s claw

Hoofs are found in sheep, cows, horses etc. otherwise known as ungulate mammals. These are animals that have lost toes in the process of evolution and walk on the “nails” of the remaining toes. The hoof is a cylinder of horny material that surrounds and protects the tip of the toe (see diagram 5.3).

Diagram 5.3 - A horse’s hoof

Horns And Antlers

True horns are made of keratin and are found in sheep, goats, and cattle. They are never branched and, once grown, are never shed. They consist of a core of bone arising in the dermis of the skin and are fused with the skull. The horn itself forms as a hollow cone-shaped sheath around the bone (see diagram 5.4).

Diagram 5.4 - A horn

The antlers of male deer have quite a different structure. They are not formed in the epidermis and do not consist of keratin but are entire of bone. They are shed each year and are often branched, especially in older animals. When growing they are covered in the skin called velvet that forms the bone. Later the velvet is shed to leave the bony antler. The velvet is often removed artificially to be sold in Asia as a traditional medicine (see diagram 5.5).

Diagram 5.5 - A deer antler

Other animals have projections on their heads that are not true horns either. The horns on the head of giraffes are made of bone covered with skin and hair, and the ‘horn’ of a rhinoceros is made of modified and fused hair-like structures.

Hair

Hair is also made of keratin and develops in the epidermis. It covers the body of most mammals where it acts as an insulator and helps to regulate the temperature of the body (see below). The color in hairs is formed from the same pigment, melanin that colors the skin. Coat color may help camouflage animals and sometimes acts to attract the opposite sex.

Diagram 5.6 - A hair

Hairs lie in a follicle and grow from a root that is well supplied with blood vessels. The hair itself consists of layers of dead keratin-containing cells and usually lies at a slant in the skin. A small bundle of smooth muscle fibers (the hair erector muscle) is attached to the side of each hair and when this contracts the hair stands on end. This increases the insulating power of the coat and is also used by some animals to make them seem larger when confronted by a foe or a competitor(see diagram 5.6).

The whiskers of cats and the spines of hedgehogs are examples of special types of hairs.

Feathers

The lightness and stiffness of keratin is also a key to bird flight. In the form of feathers, it provides the large airfoils necessary for flapping and gliding flight. In another form, the light fluffy down feathers, also made of keratin, are some of the best natural insulators known. This superior insulation is necessary to help maintain the high body temperatures of birds.

Diagram 5.7 - A Contour Feather

Contour feathers are large feathers that cover the body, wings, and tail. They have an expanded vane that provides the smooth, continuous surface that is required for effective flight. This surface is formed by barbs that extend out from the central shaft. If you look carefully at a feather you can see that on either side of each barb are thousands of barbules that lock together by a complex system of hooks and notches. if this arrangement becomes disrupted, the bird uses its beak to draw the barbs and barbules together again in an action known as preening (see diagram 5.7).

Diagram 5.8 - A Down Feather

Diagram 5.9 - A Pin Feather

Down feathers are the only feathers covering a chick and form the main insulation layer under the contour feathers of the adult. They have no shaft but consist of a spray of simple, slender branches (see diagram 5.8).

Pin feathers have a slender hair-like shaft often with a tiny tuft of barbs on the end. They are found between the other feathers and help tell a bird how its feathers are lying (see diagram 5.9).

Skin Glands

Glands are organs that produce and secrete fluids. They are usually divided into two groups depending upon whether or not they have channels or ducts to carry their products away. Glands with ducts are called exocrine glands and include the glands found in the skin as well as the glands that produce digestive enzymes in the gut. Endocrine glands have no ducts and release their products (hormones) directly into the bloodstream. The pituitary and adrenal glands are examples of endocrine glands.

Most vertebrates have exocrine glands in the skin that produce a variety of secretions. The slime on the skin of fish and frogs is mucus produced by skin glands and some fish and frogs also produce poison from modified glands. In fact, the skin glands of some frogs produce the most poisonous chemicals known. Reptiles and birds have a dry skin with few glands. The preen gland, situated near the base of the bird’s tail, produces oil to help keep the feathers in good condition. Mammals have an array of different skin glands. These include the wax producing, sweat, sebaceous and mammary glands.

Wax producing glands are found in the ears.

Sebaceous glands secrete an oily secretion into the hair follicle. This secretion, known as sebum, keeps the hair supple and helps prevent the growth of bacteria (see diagram 5.6).

Sweat glands consist of a coiled tube and a duct leading onto the skin surface. Their appearance when examined under the microscope inspired one of the first scientists to observe them to call them “fairies’ intestines” (see diagram 5.1). Sweat contains salt and waste products like urea and the evaporation of sweat on the skin surface is one of the major mechanisms for cooling the body of many mammals. Horses can sweat up to 30 liters of fluid a day during active exercise, but cats and dogs have few sweat glands and must cool themselves by panting. The scent in the sweat of many animals is used to mark territory or attract the opposite sex.

Mammary glands are only present in mammals. They are thought to be modified sweat glands and are present in both sexes but are rarely active in males (see diagram 5.10). The number of glands varies from species to species. They open to the surface in well-developed nipples. Milk contains proteins, sugars, fats and salts, although the exact composition varies from one species to another.

Diagram 5.10 - A Mammary Gland

The Skin And Sun

A moderate amount of UV in sunlight is necessary for the skin to form vitamin D. This vitamin prevents bone disorders like rickets to which animals reared indoors are susceptible. Excessive exposure to the UV in sunlight can be damaging and the pigment melanin, deposited in cells at the base of the epidermis, helps to protect the underlying layers of the skin from this damage. Melanin also colors the skin and variations in the amount of melanin produce colors from pale yellow to black.

Sunburn And Skin Cancer

Excess exposure to the sun can cause sunburn. This is common in humans, but light skinned animals like cats and pigs can also be sunburned, especially on the ears. Skin cancer can also result from excessive exposure to the sun. As holes in the ozone layer increase exposure to the sun’s UV rays so too does the rate of skin cancer in humans and animals.

The Dermis

The underlying layer of the skin, known as the dermis, is much thicker but much more uniform in structure than the epidermis (see diagram 5.1). It is composed of loose connective tissue with a felted mass of collagen and elastic fibres. It is this part of the skin of cattle and pigs etc. that becomes commercial leather when treated, The dermis is well supplied with blood vessels, so cuts and burns that penetrate down into the dermis will bleed or cause serious fluid loss. There are also numerous nerve endings and touch receptors in the dermis because, of course, the skin is sensitive to touch pain and temperature.

When looking at a section of the skin under the microscope you can see hair follicles, sweat, and sebaceous glands dipping down into the dermis. However, these structures do not originate in the dermis but are derived from the epidermis.

In the lower levels of the dermis is a layer of fat or adipose tissue (see diagram 5.1). This acts as an energy store and is an excellent insulator especially in mammals like whales with little hair.

The Skin And Temperature Regulation

Vertebrates can be divided into two groups depending on whether or not they control their internal temperature. Amphibia (frogs) and reptiles are said to be”cold blooded” (poikilothermic) because their body temperature approximately follows that of the environment. Birds and mammals are said to be warm blooded (homoiothermic) because they can maintain a roughly constant body temperature despite changes in the temperature of the environment.

Heat is produced by the biochemical reactions of the body (especially in the liver) and by muscle contraction. Most of the heat loss from the body occurs via the skin. It is therefore not surprising that many of the mechanisms for controlling the temperature of the body operate here.

Reduction Of Heat Loss

When an animal is in a cold environment and needs to reduce heat loss the erector muscles contract causing the hair or feathers to rise up and increase the layer of insulating air trapped by them.

Diagram 5.11a) Hair muscle relaxed...............Diagram 5.11b) Hair muscle contracted

Heat loss from the skin surface can also be reduced by the contraction of the abundant blood vessels that lie in the dermis. This takes blood flow to deeper levels, so reducing heat loss and causing pale skin (see diagram 5.12a).

Diagram 5.12a) Reduction of heat loss by skin

Shivering caused by twitching muscles produces heat that also helps raise the body temperature.

Increase Of Heat Loss

There are two main mechanisms used by animals to increase the amount of heat lost through the skin when they are in a hot environment or high levels of activity are increasing internal heat production. The first is the expansion of the blood vessels in the dermis so blood flows near the skin surface and heat loss to the environment can take place. The second is by the production of sweat from the sweat glands (see diagram 5.12b). The evaporation of this liquid on the skin surface produces a cooling effect.

The mechanisms for regulating body temperature are under the control of a small region of the brain called the hypothalamus. This acts like a thermostat.

Diagram 5.12b) - Increase of heat loss by skin

Heat Loss And Body Size

The amount of heat that can be lost from the surface of the body is related to the area of skin an animal has in relation to the total volume of its body.

Small animals like mice have a very large skin area compared to their total volume. This means they tend to lose large amounts of heat and have difficulty keeping warm in cold weather. They may need to keep active just to maintain their body temperature or may hibernate to avoid the problem.

Large animals like elephants have the opposite problem. They have only a relatively small skin area in relation to their total volume and may have trouble keeping cool. This is one reason that these large animals tend to have sparse coverings of hair.

Summary

Skin consists of two layers: the thin epidermis and under it the thicker dermis.
The Epidermis is formed by the division of base cells that push those above them towards the surface where they die and are shed.
Keratin, a protein, is deposited in the epidermal cells. It makes skin waterproof.
Various skin structures formed in the epidermis are made of keratin. These include claws, nails, hoofs, horn, hair, and feathers.
Various Exocrine Glands (with ducts) formed in the epidermis include sweat, sebaceous, and mammary glands.
Melanin deposited in cells at the base of the epidermis protects deeper cells from the harmful effects of the sun.
The Dermis is composed of loose connective tissue and is well supplied with blood.
Beneath the dermis is insulating adipose tissue.
Body Temperature is controlled by sweat, hair erection, dilation, and contraction of dermal capillaries and shivering.