1. Chapter 6 Integument

2. Introduction The integument, or skin, is a composite organ. – It is composed of the epidermis, dermis, and basement membrane. The integument is one of the largest organs of the body, making up about 15% of the body weight. – The epidermis produces hair, baleen, feathers, claws, nails, horns, beaks, and some scales. – The dermis gives rise to dermal bones and osteoderms of reptiles – Collectively they form the teeth, scales, and denticles of fishes.

3. As the critical border between the organism and its environment, the integument has a variety of specialized functions. – It forms part of the exoskeleton and thickens to resist mechanical injury and establishes a barrier to prevent the entry of pathogens. – The integument helps hold the shape of an organism and regulates osmotic movements. – It holds feathers for locomotion, hair for insulation, and horns for defense. – Skin pigments also aid in protection from UV radiation and display bright color for courtship.

4. Dermis The most conspicuous part of the dermis is the fibrous connective tissue composed mainly of collagen. – Collagen fibers may be woven into distinct layers called piles. – Alternating layer of piles give shape to the skin and prevent it from sagging. In aquatic vertebrates allow the skin, aligned in orderly piles forming a stratum compactum to stretch when pulled at oblique angles. – This flexibility allows it to accommodate to lateral bending, but resists distortion in body shape. – Because it does not wrinkle it allows water to flow smoothly without turbulence over the body

5. In terrestrial vertebrates, the stratum compactum is less obvious because movement depends more on the limbs than on body movements. – Consequently, collagen fibers are present in terrestrial vertebrates, but are much less regularly ordered and do not form piles.

6. Epidermis The epidermis, outer layer of the skin, in many vertebrates, produces a thick layer of mucus to moisten and protect the skin. – In fishes it provides some protection from bacteria and lessens resistance to laminar flow – In amphibians it aids in similar functions and helps prevent desiccation. In terrestrial vertebrates it often forms a keratinized, or cornified layer, called the stratum corneum. – All cells in this layer are dead cells. – Their formation various proteins accumulate and form keratin in a process called keratinization. – This layer helps to reduce water loss in dry terrestrial environments.

7. In areas of the body where friction is common, such as the soles of feet or palms of hands, a thick protective layer, or callus, forms to protect from mechanical damage. Finally, scales form within the integument of many aquatic and terrestrial vertebrates. – Scales are basically folds in the integument. – If dermal contributions predominate, as in dermal bone, the fold is termed a dermal scale. – An epidermal fold is termed an epidermal scale.

8. Fishes With few exceptions the skin of fishes is non- keratinized and covered instead with mucus. Unlike terrestrial animals, the epidermis of fishes is alive and active on the body surface. – The mucus layer forms from individual cells in the epidermis with contributions from multicellular glands. – The mucus coat, or cuticle, resists penetration by infectious bacteria and contributes to laminar flow of water. – It also makes fish slippery to predators and includes chemicals that are repugnant, alarming, or toxic to predators. Fish dermis often gives rise to dermal bone and dermal scales.

9. Chondrichthyes In cartilaginous fishes, dermal bone is absent, but the surface denticles, termed placoid scales persist. – These scales are what give the skin of sharks and rays their rough feel. The dermis is composed of fibrous connective tissue, especially elastic collagen fibers, whose regular arrangement forms a fabric-like wrap in the dermis. – This gives the skin strength and prevents wrinkling during swimming.

10. Placoid scales themselves form in the dermis, but project through the dermis to reach the surface. – A cap of enamel forms the tip, dentin lies beneath, and a pulp cavity resides within. Chromatophores occur in the lower part of the epidermis and upper part of the dermis.

11. Bony Fishes The epidermis includes a basal layer of cells above which are the stratified epidermal cells. – Within these layered cells are secretory club cells that produce the mucus cuticle, or slime The dermis of bony fishes is subdivided into a superficial layer of lose connective tissue and a deeper layer of dense fibrous connective tissue. – Chromatophores are found in the dermis. – The most important structural component of the dermis is the scale.

12. Based on their appearance several types of scales are recognized among bony fishes. The cosmoid scales, seen in primitive fishes, sit upon a double layer of bone. In cosmoid scales, there is a thick, well- developed layer of dentin (cosmine) beneath a thin layer of enamel. The ganoid scale is distinguished by a surface of thick enamel (ganoin) without an underlying layer of dentin.

13. The teleost scale lacks enamel, dentin and a vascularized bony layer. Two kinds of teleost scale are recognized. – The cycloid scale, composed of concentric rings, and the, – Ctenoid scale with of projections along its posterior margin

14. Tetrapods Although keratinization occurs in fishes, among terrestrial vertebrates it becomes a major feature of the integument. Extensive keratinization produces a prominent outer cornified layer, the stratum corneum, that resists mechanical abrasion. – The stratum is also mixed with lipids to increase the resistance to water loss in the dry terrestrial environment. Multicellular glands, which are more prevalent than in fishes, reside within the dermis and reach the surface through common ducts that pierce the cornified layer.

15. Amphibians Amphibians are unique, in that during the course of their lives they metamorphose from aquatic to a terrestrial form. In most modern amphibians, the skin is also specialized as a respiratory surface. – Some species rely entirely on cutaneous respiration

16. Frogs and salamanders lack all traces of dermal scales. In these vertebrates the dermis is composed of fibrous connective tissues and a stratum that affords some protection from abrasion as well as protecting from loss of moisture. Generally the skin of frogs and salamanders includes two types of multicellular glands, mucus and poison glands that are located in the dermis. Chromatophores may occasionally be found in the epidermis, but most reside in the dermis. Capillaries, which reside in the dermis of most vertebrates, extend into the epidermis of amphibians, and air in cutaneous respiration.

17. Reptiles The skin of reptiles reflects their grater commitment to terrestrial life. – Keratinization is much more extensive, and skin glands are fewer than in amphibians. Scales are present, but are fundamentally different that those of fishes. – The reptilian scale usually lacks the bony under support or any significant structural support of the dermis. – Instead, it is a fold of the epidermal surface, forming an epidermal scale. – Large, plate-like scales are referred to a scutes and may be modified into crests, spines, or hornlike processes.

18. The dermis of reptiles is composed of fibrous connective tissues. The epidermis is divided into three layers, the stratum basale, stratum granulosam, and stratum corneum. – This changes in reptiles that shed large portions of cornified skin. – Shedding of the cornified layer, termed molting or ecdysis, results in removal of extensive sections of the superficial epidermis. Integumentary glands of reptiles are restricted to certain areas of the body – Most are thought to play a role in courtship or to discourage predation.

19. Birds In birds the epidermis comprises the stratum basale and stratum corneum. – Between which is a transitional layer of cells that transform into the keratinized layer of the corneum. Bird skin has few glands. – The major gland, or uropygial gland, located at the base of the tail secretes lipids that is smeared onto the feather to repel water. – The other gland, in marine birds, is a salt gland which excretes excess salt ingested with food.

20. Feathers distinguish birds from all other vertebrates. – Feathers are structurally elaborate and come in a variety of forms. – All feathers are nonvascular and nonnervous outgrowths of the skin. Generally the modern bird feather is built from a tubular central shaft, the rachis, which carries on either side a vane, or series of barbs with interlocking connections called barbules. – The rachis continues proximally as the barbless calamus, or quill, which anchors the feather to the body and is moved by dermal muscles.

21. In modern birds, feathers are of many types, serving many functions. Flight feathers are long and the vanes asymmetrical around the stiffened rachis. Contour feathers cover the body and usually have symmetrical vanes. Down feathers lack a distinctive rachis and non-interlocking barbs extend from the calamus as a fluffy feather important in insulation.

22. Old feathers are shed, or molted, and the beginning of a new feather soon grows, the feather filament soon grows out of the follicle as a consequence of cellular proliferation at the follicular base. – The feather filament continues to grow out from the follicle accompanies by the highly vascularized dermal core, which extends through the follicle above the surrounding integument. Dermal muscles, connected in a network of muscles, act to erect the feather. The patterning zone determines the spacing between feather parts. – This spacing allows adjacent barbs and barbules to separate as they unfurl. – Preening of the unfurling feather encourages the overlap and interlocking of barbules a the mature feather takes final shape.

23. Function of Feathers There are several types of feather. 1.Contour feathers aerodynamically shape the surface of the bird. 2.Down feathers lie close to the skin as thermal insulation. 3.Filoplumes are often specialized for display 4.Flight feathers of the wings are a type of contour feather. These feathers have some value as insulation, however their,main function is stabilization in flight.

24. Chromatophores occur within the epidermis, and their pigments are carried into the feathers to give them color. Light refraction off the feather barbs also play a role in pigmentation of feathers and adds iridescence.

25. Evolution of Feathers In modern birds it is easy to see that feathers assist in flight. But, they likely had a different function in the past. One theory is that early feathers played a role in surface insulation. – Thus, protecting ectothermic birds from excessive heat or endothermic birds in trapping heat generated from within. Another theory is that birds were initially used to aid in gliding and that they later functioned to assist in flight. Regardless of their original purpose, feathers were modified reptilian scales (or at least derived from a similar process)

26. Mammals As in other vertebrates, the two main layers of mammalian skin are the epidermis and dermis, which join and interface through the basement membrane. Beneath this lies the hypodermis that is made of connective tissues and fat.

27. Epidermis In mammals the epidermis may be locally specialized as hair, nails, or glands. Epithelial cells of the epidermis are keratinocytes and belong to keratinizing systems that form the dead, superficial cornified layer of the skin. – Surface keratinized cells are continually exfoliated and replaced by cells arising from the deepest layer of the epidermis. The pattern of keratinization is most obvious in areas that are most often in contact with their surroundings (soles of feet, pads of paws).

28. Chromatophores can be found all over the body surface. – They secrete granules of pigment called melanin. Skin color results from the combination of the yellow stratum corneum, the red underlying blood vessels, and the dark pigmented granules secreted by the chromatophores.

29. Dermis The mammalian dermis is double layered. – The outer papillary layer and the inner reticular layer. Blood vessels, nerves, and smooth muscle occupy the dermis but do not reach into the epidermis. The mammalian dermis produces dermal bones, but these contribute to the skull and pectoral girdle. – They rarely form scales, except in certain situations where there is secondary development of dermal bones in the integument. (Armadillos)

30. Blood vessels and nerves enter the dermis; hair follicles and glands project inward from the epidermis. The dermis is usually composed of irregularly arranged fibrous connective tissue that is mixd with elastic fibers. – This gives it the ability to stretch and retun to its original shape. – As the skin ages it loses this elasticity and begins to sag.

31. Hair Hairs are slender, keratinous filaments. – The base of the hair is the living root. – Its remaining length makes up the nonliving shaft. The outer surface of the shaft often forms a scaly cuticle, beneath it is cortex and at the core is the medulla.

32. Hair shafts grow from a living follicle, which goes through a cycle of activity in three stages; 1.Growth: active proliferation of cells in the hair papilla. 2.Degeneration: hair producing cells become inactive and die. 3.Resting: lasts weeks to months Eventually stem cells in the follicle resume the growth phase and the old hair shaft falls out. The cycle is intrinsic and does not seem to change with cuing of the hair shaft. Chromatophores in the hair follicle contribute to hair color. – As we age hairs become gray, because special stem cells within the follicle begin to die.

33. A thick covering of hair, fur or pelage, is generally composed of guard hairs and underfur. – Guard hairs are large coarse hairs that are the most abundant type of hair on the outer surface. – The underfur is usually much finer and shorter. Both types of fur function largely as insulators. Some hairs are specialized. – Sensitive nerves are associated with the roots of whiskers, vibrissae, located around the snout of many mammals. Such sensory functions may have been the firs use of fur, prior to evolving into a thermal insulator.

34. Glands Principally there are 3 types of integumentary glands; sebaceous, eccrine, and apocrine. – Scent, sweat, and mammary glands are derived from them. 1.The sebaceous glands produce an oily secretion, sebum, that is released onto the hair follicles in order to condition and waterproof fur. 2.Eccrine glands produce thin, watery fluids, are not associated with hair follicles, and begin to function before puberty. – In most mammals they are associated with the soles of the feet and hands, tails, and any other surface in contact with abrasive surfaces. 3.Apocrine glands produce a viscous, lipid-containing fluid, are associated with hair follicles, begin to function at puberty and are used primarily for chemical signaling.

35. Mammary glands produce milk, a watery mixture of fats, carbohydrates, and proteins that nourish the young Ectodermal mammary ridges, within which the glands form, are located along the ventrolateral surface of the embryo – The number of mammary glands varies among species. With few exceptions mammary glands become functional only in females. – Lactation, release of milk, can occur with or without the development of breasts or nipples.

36. Nails, Claws, and Hooves Nails are plates of tightly compacted, cornified epithelial cells on the surface of fingers and toes; thus, they are products of the keratinizing system of the skin. Nails protect the tis of digits from inadvertent mechanical injury and help stabilize the skin so that a secure grip can be maintained. – Only primates have nails, all other vertebrates have either claws or hooves. Claws are laterally compressed, keratinized projections from the tips of digits Hooves are enlarged, keratinized plates on the tips of the ungulate digit.

37. Horns and Antlers Mammals, dinosaurs, and some extinct turtles are the only vertebrates to possess true horns or antlers. Both antler and horn are composed of a mixture of skin and underlying bone. – In horns, the integument produces a tough, cornified sheath that fits over the bony core that is never branched. – In antlers, the overlying skin, velvet, apparently shapes and provides a vascular supply to the growing bone.

38. True antlers are only found in members of the Cervidae family. – Typically only males possess antlers, which are branched and shed annually. – Under hormonal control and tend to appear for mating season. True horns are found among members fo the family Bovidae. – Commonly horns occur in both genders, are retained year round, and continue to grow throughout life. – The horn is unbranched and formed by a bony core and a keratinized sheath.

40. The horns of giraffe and rhino are different still; In giraffe horns form from separate cartilaginous processes that ossify, fuse to the top of the skull, and remain covered with living, non-cornified skin. The rhinoceros horn does not contain a bony core, and is a exclusive product of the epidermis. – It forms from compacted keratinous fibers.

41. Baleen The integument within the mouths of mysticete whales forms plates of baleen that act as strainers to extract krill from the water gulped into the mouth.

42. Fin