Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. The Integument and Related Structures

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Learning Objectives  List the cell types that make up the epidermis and describe the function of each cell type.  List the five layers of the epidermis.  Describe the process of keratinization.  List the structures that constitute the dermis and describe the function of each.  List the structures of the hypodermis.  Describe the unique features of the paw pads and planum nasale.  Describe the parts of the hair follicle and explain how hair grows.  List and describe the three types of hair.  Describe the structure and location of sebaceous glands.  Differentiate between eccrine and apocrine sweat glands.

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Integumentary System  Skin and related structures:  Hair, hooves, horns, claws, skin-related glands  Functions: prevents desiccation; reduces threat of injury; assists in maintaining normal body temperature; excretes water, salt, and organic wastes; receives and conveys sensory information; synthesizes vitamin D; stores nutrients

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Integumentary System  Consists of three layers:  Epidermis  Dermis  Hypodermis

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Epidermis Cell types:  Keratinocytes – produce keratin, the tough, fibrous, waterproof protein that gives skin its resiliency and strength  Melanocytes – produce melanin pigment  Merkel cells – phagocytize microinvaders; macrophage specific to epidermis  Langerhans cells – found in stratum spinosum; may be involved in allergic and cell-mediated immune response in skin

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Epidermal Layers 1.Stratum germinativum (basal layer):  Deepest layer  Consists of a single row of keratocytes attached to epithelial basement membrane  Merkel cells, melanocytes, keratocytes, found in this layer

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Epidermal Layers 2.Stratum spinosum (spiny layer):  Contains several layers of cells held together by desmosomes  Langerhans cells found in this layer

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Epidermal Layers 3.Stratum granulosum (granular middle layer):  Composed of two to four layers of flattened, diamond-shaped keratocytes that contain lamellated granules of glycolipids  These glycolipids play a role in helping waterproof the skin and slowing water loss across the epidermis

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Epidermal Layers 4.Stratum lucidum (clear layer):  Found in very thick skin  Composed of a few rows of flattened dead cells  Contents of the keratogranules combine with intracellular tonofilaments to form keratin fibrils

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Epidermal Layers 5. Stratum corneum (horny outermost layer):  Composed of 20 to 30 rows of keratocyte “remnants”  Sometimes called horny or cornified cells

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Epidermis of Hairy Skin  Hairy skin usually consists of three epidermal layers rather than five (stratum basale, stratum spinosum, and stratum corneum)  The surface of hairy skin is covered in scalelike folds.  A knoblike elevation can be seen periodically  Tactile elevation or epidermal papilla  Usually associated with a tactile hair (tylotrich hairs)

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Dermis  Composed of dense irregular connective tissue  Collagen, elastic, and reticular fibers  Also includes hair follicles, nerve endings, glands, smooth muscle, blood vessels, and lymphatics  Fibroblasts, adipocytes, and macrophages also present  Two layers:  Papillary layer  Reticular layer

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Dermal Layers Papillary layer  Underneath the epithelial layer of the epidermis  Composed of loose connective tissue with loosely woven fibers and ground substance  Dermal papillae help cement the epidermis and the dermis together  Blood vessels, pain, temperature, and touch receptors also present

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Dermal Layers Reticular layer  Consists of dense irregular connective tissue  Bundles of collagen fibers from papillary layer blend into those of reticular layer  Most fibrous bundles tend to run parallel to each another.  Separations between bundles represent tension lines in skin  In areas where a great deal of bending occurs, dermal folds or flexure lines are present.

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Hypodermis  Composed of areolar tissue containing adipose, blood and lymphatic vessels, and nerves  Contains special touch receptor – the pacinian corpuscle (sensitive to heavier pressure than Meissner's corpuscle)  Fibers of hypodermis are continuous with those of dermis  Hypodermal layer permits skin to move freely over underlying bone and muscle without putting tension on skin

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Special Features of the Integument  Pigmentation  Paw Pads  Planum Nasale  Ergots and Chestnuts  Cutaneous Pouches in Sheep

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Pigmentation  Result of presence or absence of melanin granules in the extensions of melanocytes  No pigmentation if granules are concentrated around nucleus of the melanocyte  As granules move into the cellular extensions and into surrounding tissue, pigmentation becomes macroscopically apparent  The more granules present, the darker the pigmentation

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Pigmentation  Melanocyte-stimulating hormone controls dispersion of granules  Keratocytes arrange melanin on the side of the cell with greatest amount of sun exposure  Acts to protect keratocytes from exposure to damaging ultraviolet rays

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Paw Pads  Thick layers of fat and connective tissue with exocrine sweat glands and lamellar corpuscles  Outer surface is the toughest and thickest skin in the body  Often pigmented; composed of all five epidermal layers  Stratum corneum is thicker than all other layers combined  Conical papillae can be seen covering entire pad

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Planum Nasale  Top of the nose in cats, pigs, sheep, and dogs  Planum nasolabiale: the muzzle of cows and horses  Usually pigmented; aglandular except in sheep, pigs, and cows  Composed of only three epidermal layers:  Stratum germinativum, stratum spinosum, stratum corneum  Not present: stratum lucidum, stratum granulosum

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Ergots and Chestnuts  Dark horny structures found on the legs of horses, ponies, and other members of the equine family  Thought to be vestiges of carpal and tarsal pads of second and fourth digits ("splint bones")

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Cutaneous Pouches in Sheep  Infoldings of skin  Infraorbital, interdigital, and inguinal pouches  Contain fine hairs and numerous sebaceous and oil glands  Secrete a fatty yellow substance which covers and sticks to the skin when dry

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Related Structures of the Integument  Hair  Hair strands and follicles  Types of hair  Glands of the skin  Sebaceous and sweat glands  Tail glands  Anal sacs  Claws and dewclaws  Hoof  Horns

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Hair  Functions in maintaining body temperature; camouflage  Hair shaft: visible above the skin  Hair root: buried within the skin  Hair follicle: anchors the hair  Deepest part of hair follicle expands to form a hair bulb  At the base of the hair bulb is a mound of dermal cells called the papilla.  Hair strands are formed as epithelial cells mature, fill with keratin, and move away from the papilla.

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Hair Color  Melanocytes transfer melanin to the cortical and medullary cells that form the hair strand.  Different colors result from the quantity and type of melanin incorporated into the hair.  Horses produce only one type of melanin; dogs produce two.  As animals age, melanin production decreases and hair begins to turn gray.  White hair is formed when the cortex loses its pigment entirely and the medulla becomes completely filled with air.

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Types of Hair 1. Primary or guard hairs  Straight or arched; thicker and longer than secondary hairs 2. Secondary or wool-type hairs  Softer and shorter than primary hairs; wavy or bristled in the dog; predominant hair type in species with wool-type coats 3. Tactile (or “sinus”) hairs  Contain numerous sensory endings  Commonly known as whiskers; also mixed intermittently throughout the hair coat  Also called sinus hair because of the large blood sinus located in the connective tissue portion of the follicle

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Arrector Pili Muscle  Small, smooth muscle  Attached to each hair follicle  Innervated by sympathetic nervous system  Contraction of the muscle pulls the hair to an erect position

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Glands of the Skin  Sebaceous Glands  Sweat glands (sudoriferous glands)  Eccrine sweat glands  Apocrine sweat glands  Tail Glands  Anal Sacs

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Sebaceous Glands  Located in the dermis; may be simple or complex alveolar structures  Most have a single duct that empties into hair follicle; others have ducts that empty directly onto surface of skin  Epithelial cells lining sebaceous gland manufacture and store sebum  Because the epithelial cell is lost in the process of secretion, the sebaceous gland is classified as a holocrine structure.

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Sebaceous Glands Sebum  Composed primarily of glycerides and free fatty acids  Arrector pili muscle contracts and compresses sebaceous gland, forcing sebum through the duct into the hair follicle  Coats the base of the hair and surrounding skin  Helps trap moisture, keeps hair soft, pliant, and somewhat waterproof  Sebum also helps reduce the skin's risk of infection.

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Sweat Glands  Also called sudoriferous glands  Found over the entire body of most domestic species  Sweat helps cool the body through evaporation.  Two types of sweat glands: 1. Eccrine 2. Apocrine

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Sweat Glands Eccrine Sweat Glands:  Excretory portion consists of a simple coiled tube located in the dermis or hypodermis  Empty onto surface of skin through a long duct Apocrine Sweat Glands:  Coiled excretory portion buried in the dermis or hypodermis; single excretory duct  Empty into hair follicles

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Tail Glands  Oval region at the dorsal base of the tails of most dogs and cats  Contains coarse, oily hairs  Very large apocrine and sebaceous glands present  Thought to assist with recognition and identification of individual animals

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Anal Sacs  Cats and dogs have anal sacs similar to musk glands of skunks.  Located at the 5 and 7 o'clock positions relative to the anus  Connected to the lateral margin of the anus by a small single duct  Lined with sebaceous and apocrine glands  When an animal defecates or becomes frightened, some or all of the anal sac contents are expressed.

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Claws and Dewclaws Claws  Hard outer coverings of the distal digits  Usually pigmented  Function in maintaining traction and serve as tools for defense and catching prey  Claws are nonretractable except in most cat species Dewclaws  Evolutionary remnants of digits  In the dog, the dewclaw is the first digit.  In the cow, pig, and sheep, the medial and lateral dewclaws are the second and fifth digits, respectively.

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Hoof  Horny outer covering of digits of some animals  Another name for “hoof” is ungula.  Hoofed animals are called ungulates.  Hooves rest on tissue called the corium.  The corium is attached to the periosteum of the distal phalanx.  The outer hoof is a modified epithelial layer, and the corium is modified dermis.

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Hoof  The skeletal foot of the horse includes the distal part of the second phalanx, the distal sesamoid bone (navicular bone), and the entire third phalanx (coffin bone).  The coffin bone has a layer of corium, which in turn is covered by the cornified hoof.  The hoof and the corium form interdigitations called laminae.  The equine hoof is generally divided into three parts: the wall, the sole, and the frog.

Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc. Hoof The wall:  External portion of the hoof visible from the anterior, lateral, and medial views; divided into the toe, the quarters, and the heels The sole:  Plantar, or palmar, surface of the hoof; outer layers are avascular and lack innervation The frog:  Triangular horny structure located between the heels on the underside of the hoof  Divided by a central depression known as the central sulcus  Digital cushion: a thick pad of fat and fibrous tissue lies beneath the sensitive frog  Lateral cartilages extend proximally from the distal phalanx

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Horns  Epidermal in origin  Structurally similar to hair  Composed of keratin  In adults the horn is hollow and communicates directly with the frontal sinus.  The corium lies at the root of the horn and is bound to the horn process by periosteum.  The body of the horn is composed of tightly packed tubules.  The wall of the horn is thinner at the base than at the apex.