5. The Integument and Related Structures

The Integument and Related Structures

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.

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

Integumentary System Consists of three layers: Epidermis Dermis Hypodermis

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

Epidermal Layers 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

Epidermal Layers Stratum spinosum (spiny layer): Contains several layers of cells held together by desmosomes Langerhans cells found in this layer

Epidermal Layers 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

Epidermal Layers 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

Epidermal Layers 5. Stratum corneum (horny outermost layer): Composed of 20 to 30 rows of keratocyte “remnants” Sometimes called horny or cornified cells

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)

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

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

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.

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

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

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

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

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

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

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")

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

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

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.

Hair Follicle Root sheath layers: connective tissue root sheath, external root sheath, and internal root sheath Each hair strand is organized into three layers: cuticle, cortex, and medulla Root hair plexus: web of sensory nerve endings Touch receptor

Growth Cycles of Hair Anagen phase : cells are added at the base of the root, hair lengthens Catagen phase : period of transition between anagen and telogen phases Telogen phase : maximum length of hair is achieved, hair stops growing, hair follicle shortens, and hair is held in a resting phase

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.

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

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

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

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.

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.

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: Eccrine Apocrine

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

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

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.

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.

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.

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.

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

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.

5. The Integument and Related Structures

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5. The Integument and Related Structures

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