The Integument and Related Structures

 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.

 Skin and related structures:  1. Hair,  2. hooves,  3. paw pads  4. horns and antlers,  5. claws and dewclaws,  6. noses (muzzles)  7. skin-related glands sebaceous and sweat

 1. prevents desiccation (drying up);  2. reduces threat of injury;  3. assists in maintaining normal body temperature;  4. excretes water, salt, and organic wastes;  5. receives and conveys sensory information;  6. synthesizes vitamin D; stores nutrients

 Consists of three layers:  Epidermis  Dermis  Hypodermis

Cell types :  1. Keratinocytes – produce keratin, the tough, fibrous, waterproof protein that gives skin its resiliency and strength live cells at the basement membrane, dead at the surface (stratum corneum)  2. Melanocytes – produce melanin pigment

 3. Merkel cells – tactile sensory nerve endings found in the basal part of the epidermis  4. Langerhans cells – found in stratum spinosum;  MACROPHAGE that is specific to the epidermis ( may be involved in allergic and cell-mediated immune response in skin)

1. Stratum germinativum (basal layer):  Deepest layer  Consists of a single row of keratocytes attached to epithelial basement membrane  Merkel cells and melanocytes are also found in this layer

2. Stratum spinosum (spiny layer):  Langerhans cells found in this layer  Less active keratinocytes

3. Stratum granulosum (granular middle layer):  Composed of two to four layers of flattened, diamond-shaped keratocytes that contain lamellated granules of glycolipids  Degeneration of nucleus and organelles…the cells die in this layer  These glycolipids play a role in helping waterproof the skin and slowing water loss across the epidermis

4. Stratum lucidum (clear layer):  Found in very thick skin  Composed of a few rows of flattened dead cells

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

 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 with hair in clusters of 3 follicles per scale.  Tactile Elevations:  Tactile elevation or epidermal papilla  associated with a tactile hair (touch)  (tylotrich hairs)

1. Stratum Basale 2. Stratum Spinosum 3. Stratum Granulosum 4. Stratum Lucidum 5. Stratum Corneum 1. Living Keratinocytes, Merkel Cells, Melanocytes 2. Less active keratinocytes, Langerhans cells 3. Dying keratinocytes 4. “clear layer” dead cells 5. Dead cells

 Composed of dense irregular connective tissue  Collagen, elastic, and reticular fibers hair folliclesnerve endings glandssmooth muscleblood vessels lymphatics  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

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

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.

 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

 Pigmentation  Paw Pads  Planum Nasale  Ergots and Chestnuts  Cutaneous Pouches in Sheep

 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 (you can see it)  The more granules present, the darker the pigmentation

 Melanocyte-stimulating hormone controls dispersion of granules  Keratinocytes arrange melanin on the side of the cell with greatest amount of sun exposure  Acts to protect keratinocytes from exposure to damaging ultraviolet rays

 Thick layers of fat and connective tissue with exocrine sweat glands and lamellar corpuscles ( Lamellar corpuscles, or Pacinian corpuscles, are nerve endings in the skin responsible for sensitivity to vibration and pressure.)  Outer surface is the toughest and thickest skin in the body  Often pigmented; composed of all five epidermal layers  Stratum corneum (top layer) is thicker than all other layers combined  Conical papillae can be seen covering entire pad

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 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

 Nasal Planum is composed of polygonal plaques separated by epidermal grooves.

 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")

 Infoldings of skin  Infraorbital, interdigital, and inguinal pouches  Contain sebaceous “oil” glands  Secrete a fatty yellow substance which covers and sticks to the skin when dry

 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

 Function:  maintaining body temperature by trapping layers of air (for insulation)  Dark hair, absorbs light and can contribute to warmth  camouflage

 Parts: Hair shaft: visible above the skin Hair root: buried within the skin Hair follicle: anchors the hair extends from skin surface to the dermis (occasionally the hypodermis) .

hair bulb  Deepest part of hair follicle expands to form a hair bulb papilla  At the base of the hair bulb is a mound of dermal cells called the papilla.

Hair strands are formed as 1. epithelial cells divide, grow, and mature 2. The older cells are pushed upward 3. fill with keratin, 4. Die and move away from the papilla The dead keratinized cells (have no nucleus) form the developing hair Hair follicle consists of epidermal epithelial cells.

pore  Multiple hair strands emerge from a single epidermal opening or pore  Each strand has its own follicle and hair bulb  As many as 15 hairs may be associated with one pore  A single long hair (primary hair) surrounded by shorter secondary hairs.

 Medulla-central core  Soft keratin  Cortex  Hard keratin, thickest layer  Cuticle  Layered, hard keratin  Prevents hair from sticking and matting

 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

 Shedding-genetics and environment  Seasonal (temperature change)  Hormonal  Bitches (female dogs) will lose large amounts of hair after whelping  Called blowing the coat  Telogen effluvium

 Color comes from pigment in the cortex and medulla layers of the hair shaft: medullary 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.  Yellows and reds = pheomelanin  Brown-black = tyrosine melanin  Horses = amount and location of melanin

 Hairs may be uniformly pigmented (solid) or the pigmentation may be concentrated more at base or tip (agouti)  Gray and white hair is a result of age- melanin production decreases white hair = no pigment

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 (sheep, goats) 3. Tactile hairs  Contain numerous sensory endings : probes and feelers for the animal  Commonly known as whiskers; also mixed intermittently throughout the hair coat

 A small muscle that makes the hair “stand up” when the animal is frightened or cold arrector pili muscle  Each hair follicle has an arrector pili muscle attached to it  When the muscle contracts, due to sensory stimulation, it pulls the hair upright.  Defense mechanism for fear (sympathetic response)  Keeps insulating layers of air trapped for warmth (in humans we get goosebumps)  Responsible for sebaceous gland activity

 Sebaceous Glands  Sebum-oily (fatty acids), traps moisture, some antibacterial, antifungal properties, waterproofing  Holocrine gland  Sheep = lanolin  Also cutaneous pouches  Can clog opening of hair follicles  Whiteheads, blackheads  Pimples, pustules

 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 (ear canal)  Epithelial cells lining sebaceous gland manufacture and store sebum  Holocrine structure: epithelial cell is lost in the process of secretion

Sebum  Composed primarily of glycerides and free fatty acids (In sheep becomes lanolin)  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.

 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

Eccrine Sweat Glands : simple coiled tube  Excretory portion consists of a simple coiled tube located in the dermis or hypodermis surface of skin  Empty onto surface of skin through a long duct  Dogs: connective tissue of paw pads Apocrine Sweat Glands :  Coiled excretory portion buried in the dermis or hypodermis; single excretory duct hair follicles  Empty into hair follicles  Dogs: External ear canal

 Sweat glands (sudoriferous glands)  Eccrine sweat glands  Not connected to hair follicles  No smell associated  Human: forehead,neck, back, hands, feet  Dogs: foot pads  Apocrine sweat glands  Connected to hair follicles  Dog ears: apocrine sweat glands and sebaceous glands in dogs with long hair  Sweat - watery, electrolytes, salt  Horses sweat profusely

 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

 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 sebaceous and apocrine glands  Lined with sebaceous and apocrine glands  When an animal defecates or becomes frightened, some or all of the anal sac contents are expressed.

Claws  Hard outer coverings of the distal phalanx bones  Usually pigmented  Function in maintaining traction and serve as tools for defense and catching prey  Claws are nonretractable except in most cat species (Cheetah) “Declawing” a cat involves separating the third phalanx from the second phalanx. (Amputating)

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

 Cornified 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 (nerves and blood vessels)

foot  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.

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 “bars” that assist with blood flow

 Keratinized stratum corneum of the epidermis  In other words, the horn is made up of cornified (keratinized) epithelial cells  Emerge from the frontal bones of the skull  Horns grow continuously throughout animals life (except American Pronghorn)

 Dehorning is common, by way of cauterizing  Polled breeds are bred to be horn free

 Antlers grow and shed  Receive nourishment from the outer velvet skin  Velvet skin falls off  antlers

Dehorning