Zoology Notes: 009 Chapter 5

Chapter 5. The Integumentary System Every living organism is separated from its environment by a covering, or integument. This covering is considered as the largest organ that composes an organism’s body. It serves as protection from foreign matter and gives communication with the environment. It also enables an organism to live in a particular environment. The integument or skin is not merely a protective wrapping; it has many varied functions. The skin is elastic, rugged, and under ordinary conditions, self-generating. It is almost entirely waterproof, providing an efficient, closely regulated thermal barrier and participates in the dissipation of water and in the temperature-regulating functions of the body. In most forms it is tough and pliable, is impervious to water (land forms), and is resistant to most germs. It protects underlying cells from the rays of the sun and prevents excessive water loss. In warm-blooded animals, it is vitally concerned with the regulation of body heat. The skin contains the receptors of many senses. It has excretory and, in some forms, respiratory functions as well. The Skin Layers The skin consists of two layers: a superficial covering of stratified squamous epithelium, the epidermis, and a deeper layer of dense irregular connective tissue, the dermis (also known as the corium). This general arrangement of two layers of the skin is found throughout the body, including

areas of modified epidermal structures such as hairs, horns, and hooves. The Epidermis. This is the outer, relatively thin layer of the skin that is composed of closely packed cells with little intercellular material. It serves as the barrier against chemicals, radiation, or microbes. It is composed of stratified squamous epithelium that in most areas can be divided into different sub-layers. The stratum corneum forms the outermost layer of the epidermis and consists of dead cells completely filled with protein called keratin. These keratinized cells are constantly in the process of flaking off the surface of the skin in the form of dandruff. This layer’s thickness is greatest in areas subjected to considerable wear, such as ordinary calluses and in the footpads of dogs. The stratum lucidum occurs only in non-hairy skin regions. It has a translucent appearance and lies between the s. corneum and s. granulosum. The stratum granulosum is not always present and contains granules; however these granules do not contribute to skin color. This layer is active in keratinization. The stratum spinosum is also known as the spinous, or prickle-cell layer. Its cells have a spiny appearance due to the numerous desmosomes on their surface. The stratum basale (germinativum) is made up of columnar cells that undergo frequent mitosis to renew the layers that lie above. The epidermis will regenerate as long as this layer remains intact. Melanin, the principal pigment of the skin, is formed in

Fig. 5.1. Layers of the skin.

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this layer by cells called melanocytes. Darkly pigmented skin does not contain a greater number of melanocytes, but the melanocytes are more active. The Dermis. The dermis makes up the bulk of the skin and provides physical protection. It is composed of an association of fibers, mainly collagen, with materials known as glycosaminoglycans, which are capable of maintaining the turgidity of the skin. The dermis also contains nerves, arteries, capillaries, and lymphatics of the skin. The papillary layer forms projections into corresponding depressions in the overlying epidermis. The reticular layer is the lower layer between the papillary layer and the subcutaneous tissue. The hypodermis attaches the dermis to underlying structures. The Hypodermis. The hypodermis is composed of loose connective tissue that separates the dermis from underlying structures such as bone and deep fascia. It is important because it permits movement of the skin without tearing. It also permits a layer of fat to be interposed between the skin and deeper structures. Skin Pigmentation The color of the skin is often due to special pigment, melanin, which may be in the form of granules scattered through the layers of the epidermis (mammals) and to special pigment cells, chromatophores that are found chiefly in the dermis. Absence of pigment in the skin, albinism, which may be partial or total, can be dangerous because it makes the animal more susceptible to the effects of sunlight. Derivatives of the Skin The skin is variously modified in different vertebrates to form the socalled skin derivatives. These include the bony and horny structures, such as scales, claws, nails, horns, and antlers, in addition to glands, hairs and feathers. Hairs. The skin of most mammals is covered by hairs. Hairs are cornified epidermal products which serve mainly for insulation. It is replaced periodically by molting of the old and growth of new coverings. Each hair strand is composed of three parts: the cuticle, cortex and medulla. The cuticle, the outermost portion, contains several layers of overlapping, scale like cells. The cortex, or principal portion of the hair Fig. 5.2. Hair and glands. consists of elongated cells united to make flattened fibers. The central axis of the hair, known as the medulla, is composed of many sided cells with air spaces between them. The visible portion of the hair is called the shaft while the cells embedded in the skin form the root. Enclosing the root is the hair follicle, a tubular invagination of the epidermis surrounded by a

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sheath of connective tissue. At its base, the follicle expands into a bulb, into which dermal tissue, the papilla of the hair projects from below. The papilla contains blood vessels which provide nutrition for the hair, and nerve endings. When erector pili muscles contract, the skin assumes a so-called “goose flesh” appearance where hair is sparse and results in a certain degree of “hair standing on end” where the hair is prominent. Feathers. Birds are covered by feathers. These are dry, non-living, cornified products of the epidermis that insulate the body, provide streamlined exterior contours of bodily form, and make the broad surfaces of the wings and tail used for flight. Scales. Fish scales are bony dermal plates that are covered with live epidermis bearing a superficial layer of dead cells that are constantly being replaced. Amphibians have moist naked skins without scales. The superficial layer of their epidermis contains keratin, which is replaced when lost. Reptiles have horny scales (of epidermal origin) that prevent loss of water. Fig. 5.3. Hair, l.s. and c.s. These scales are also found on the legs and feet of birds and on tails of certain mammals. In crocodiles and turtles, the scales form horny plates that overlie the bony dermal plates of the back and belly. Claws, Nails, and Hooves. Nails are outgrowths from the epidermis that are made up of hardened skin cells. There are two parts of the nail: the matrix (skin below the nail) and the lunula (white crescent shaped spot that contain smaller cells and less blood). All claws, nails and hooves are keratinized epidermal structures. A claw is shaped to cover the sides, top, and tip of a terminal joint. A nail is flattened and covers the dorsal surface of the distal phalange. A hoof extends across the end of the digit and covers the plantar surface also.

Fig. 5.4. Nails showing the different components.

Glands. The glands of the skin are chiefly of two kinds, sudoriferous and sebaceous. The sudoriferous or sweat glands consist of a tube, the lower, secretory part of which is coiled in the deep part of the corium or in the subcutis to form a round or oval ball. The sebaceous glands are in great part associated with the hairs, in the follicles into which they open. Glands According to the Modes of Secretion Merocrine (Eccrine) Glands – mode of secretion is one in which the product is released as small secretory granules. The secretory product is discharged from the cell without disruption of the cell membrane (exocytosis). Apocrine Glands – secretory product is discharged along with a rim of cytoplasm and plasmalemma. Holocrine Glands – the entire cell is extruded and constitutes the secretory product. Glands According to Secretion 31

Sudoriferous (Sweat) glands – can be found over the entire bodies; these are coiled tubes of epidermal origin, though they lie in the dermis. Each consists of a secretory portion and an excretory duct. These glands are found in most parts of the skin except the lips and the glans penis. Most are not associated with hair follicles. They are most numerous in the palms and soles. Sweating leads to a loss of heat in the body owing to the fact that heat is required to evaporate water in the sweat, thus sweating helps to lower the body temperature. Two types of sweat glands can be found: • Merocrine or Eccrine glands – not associated with hair follicles and found in most parts of the body; numerous in the palms and soles. • Apocrine types – connected to hair follicles and reach deeply into the subcutaneous layer of the skin. They are found in certain regions of the body such as the armpits, anogenital area, navel and nipples. Sebaceous glands – are generally classified as holocrine glands, because the secretory products are produced by disintegration of epithelial cells within the glands. The glands consist of a series of lobes, or acini, each with a duct running toward the main sebaceous duct. They are usually attached to hair follicles and pour their secretion, sebum, into the follicular canal. Sebum, the oily substance, is primarily responsible for lubrication of the surface of the skin. When the cells disintegrate, sebum is secreted along the hair shaft onto the surface of the skin, providing a cosmetic gloss. Functions of the Skin Sensation. Located in the skin are specific receptors sensitive to the four basic sensations of pain, touch, temperature and pressure. Protection. The skin forms an elastic, resistant covering that protects man from his complex environment. It prevents the passage of harmful physical and chemical agents and inhibits excessive loss of water and electrolytes. Thermoregulation. Heat is lost from the body by conduction, convection, radiation, and evaporation. These processes are regulated by nervous and chemical activation of the sweat glands and by dilation and constriction of the cutaneous vessels. As the body needs to dissipate heat, blood vessels of the skin dilate allowing more blood to come to the surface with a resulting heat loss. Secretion. The skin plays a part in the secretory functions in the body. Sebum secreted by sebaceous glands has antifungal and antibacterial properties and helps maintain the texture of the skin.

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