Integument
Skin is largest body organ and depending on the size and SA of animals, can be 20% of body weight
Functions
- Mechanical support, physical barrier, immunologic function, neurosensory function, metabolism
- Temperature regulation
 If living in hot environment, don’t want water to escape (dehydrate)
 If living in aquatic environment, don’t want water to flow into body
- Endocrine function
 Growth factors, cytokines
- Exocrine (glandular) secretions
 Regulate body temperature
 Rid waste products
- Derivatives of epidermis
 Wattles, beaks, feathers, scales, hooves, horns/antlers, claws, tactile hairs, shells
Layers of integument
- Epidermis
 Keratinized (typically) stratified squamous epithelium
 Keratinocytes are prominent cell type
 4-5 layers called “strata” (deep  superficial)
o Stratum basale (stratum germanitivum)
 Simple columnar/cuboidal cells on basement membrane (basal lamina)
 Desmosomes
- Cells attach to each other and to cells of the stratum spinosum
- Allows integument to resist stretching/tearing
 Stem cells
 Intermediate filaments
o Stratum spinosum
 “Spiny” or “prickle cell layer”
 Stratified polyhedral cells with cytoplasmic processes
 Desmosomes
- Anchored to cells of stratum basale
  Presence of intermediate filaments and lamellar granules
 Present in food pads
o Stratum granulosum
 Stratified, flattened cells parallel to epidermal-dermal junction
 Lamellar granules
-  lipid content
 Keratohyalin granules
- Precursors to filaggrin protein (key structural protein)
 This layers helps to form water-tight seal in epidermis through use of
hydrophobic lipid granules that deposit lipids into space between the strata
granulosum and corneum
o Status lucidum
 Only visible in thick, hairless skin
o Stratum corneum
 Outermost layer
 Stratified, keratinized dead cells
- Thickness varies in areas of body (thickest on foot pads)
 Intracellular lipid components
- Prevents loss and influx of fluids and exogenous substances
 Cells of epidermis
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Keratinocytes
 Form from differentiation of epithelial cells upwards from epidermis
Non-keratinocytes
 Langerhans cells
- Derived from bone marrow
- Related to monocytes/macrophages
- Antigen-presenting cells of epidermis
- Within stratified squamous epithelium of GI and reproductive tracts
 Merkel cells
- Stratum basale in both haired and non-haired skin
- Firmly attached to neighbor cells via desmosomes
- Associated with neurons
 Merkel cell-neurite complex
 Pick up neurologic stimuli
- Associated with tactile hairs
 Melantocytes
- Found in stratum basale, dermis, glands, and follicles
- Round/polygonal cells with finger-like processes
 Processes contain melanosomes that extend into stratum
spinosum and “donate” mature melanosomes (melanin) to
young keratinocytes, which phagocytose melanosomes
- Block mutagenic effects of UV light
- Albinism
 Animals lack tyrosinase, the enzyme responsible for the
formation of melanin within melanocytes
Dermis
 Dense, irregular CT layer
o Collagen, elastic, and reticular fibers mixed with ground substance
 Dominant cell type are fibroblasts
 Mast cells and macrophages also present
 Sebaceous/sweat glands, nerves, blood, and lymphatic vessels, plasma cells, adipocytes
 Divided into 2 layers
o Reticular (deep)
 Dense CT (less cells)
  Collagen and elastic fibers
o Papillary (superficial)
 Thin layer of loose CT
 Defensive cells, nerve endings also present
 Capillary network that feeds dermal and epidermal cells/structures
 Cells of Dermis
o Sensory receptor cells
 Meissner’s Corpuscle
- Within papillary layer
- Fine touch response
- Located in fingertips, lips
 Pacinian Corpuscle
- Within reticular layer and hypodermis
- Encapsulated mechanoreceptors
- Respond to vibration and pressure
Hypodermis
 “Subcutaneous” layer
 Adipose tissue, loose CT
Hair follicles
- Invaginations of epidermis into dermis
- Arrector pili muscle (smooth muscle) that inserts of CT sheath of follicle
- Types of hair follicles
 Primary/simple follicle (guard hairs)
o Follicles rooted in deep dermis
o Associated with sweat/sebaceous glands
 Secondary/compound follicle (underhair)
o Often many secondary hairs surrounding guard hair
o No glands
 Tactile hair follicle (“sinus hairs”)
o Resemble single-hair follicles
o Vascular sinus between inner and outer layers of dermal root sheath
o Skeletal muscle attached to outer layer of dermal root sheath
o Well-innervated
 Free nerve endings
 Merkel cell tactile-hair discs
 Feather follicle
o Feathers orientated like hairs in dermis and epidermis
o Follicle wall lined with strata corneum and basale
o Follicle wall surrounded by CT
o Dermal papilla give rise to well-vascularized feather pulp
- Hair
 Arise from germinal cells (matrix) of hair bulb at base of follicle
 Cells become keratinized as pushed towards surface from hair bulb
 5 layers of structure (deep  superficial)
o Medulla
o Cortex
o Cuticle
o Internal root sheath
o External root sheath
- Hair cycle
 3 phases
o Anagen (mitotically active)
o Categen (transition)
o Telogen (mitotically inactive)
 Follicle in dormant phase, which can last for a long time depending on
situation/conditions
 Seasonal changes in hair coat
o Light
o Temperature
o Hormonal influences (estrogen, androgen, thyroid, adrenal)
o Nutritional status
Glands of the integument
- Ectodermal origin
- Simple exocrine
 Connected to epidermis by single duct
 Sebaceous glands
o Associated with primary hair follicles
o Also present in some glabrous (non-hair) regions
 Teat, anus region of horses
o Secretion
 Holocrine
 Secretes oily sebum (sebocytes)
 Stimulated by androgens and testosterone
 Suppressed by corticosteroids
o Function
 Lubricates epidermis, hair
 Physical barrier
 Contains antimicrobial products (i.e. linolein acid)
 Sweat glands
o Simple, coiled tubular gland
o Secretion
 Secretes watery (99% water) substance
- Contains small amounts of protein and salts
 Apocrine
- Main type in domesticated animals
- Secretory product deposited into hair follicles in haired skin
- Secretory product deposited onto surface of epidermis in glabrous (nonhaired) skin
- Histology
 “Cytoplasmic blebbing” into lumen
- Function
 Thermoregulation
 Social behavior (pheramones)
 Merocrine
- Secretory product deposited directly onto epidermal surface
- Foot pads of carnivores, frog of horse
- Function
 Lubrication
 Thermoregulation
 Mammary glands
o Modified apocrine sweat glands
o Branched tubuloacinar glands
o Intralobular ducts
o Secretion
 Apocrine/merocrine modes
o Growth influenced by sex hormones
Epidermal derivatives
- Canine/feline claw
 Derived from dermis and epidermis extending from distal phalanx (P3)
 Dermis b/w epidermis and P3 contains vascular tissue
o Nutrients for epidermis
o Cutting into the “quick” consists of cutting into this area
 “Hard keratin” originates from epidermis of dorsal ridge (wall)
 “Soft keratin” (less compact) produced from epidermis of sole
- Equine Hoof
 Hoof wall
o Epidermis has 3 main layers
 Stratum externum (tectorium)
 Stratum medium (bulk of hoof)
- Tubular and intertubular horn produced by coronary epidermis
 Stratum internum (lamellatum)
- Formed by epidermal lamellae
- Holds horn onto hoof
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Primary epidermal lamellae and primary dermal lamellae interdigitate
in this area
 Primary epidermal lamellae
o Finger-like projections
o Keratinized and fused with inner portion of stratum medium
o Interdigitates with primary dermal lamellae
 Secondary epidermal lamellae
o Projections emerging from the primary epidermal lamellae
o Interdigitates with secondary dermal lamellae
 White line is where dead layers of hoof wall meets with laminar layer
- Non-equine hoof differences
 Lack secondary lamellae in stratum lamellatum
 NO frog
o Bulb lined by thin, soft horn
- Horns
 Permanent outgrowth of corneal process of frontal bone in skull
 Outer keratinized epidermis, dermis, thin hypodermis adjacent to periosteum
 Thick stratum corneum
o Hard tubular and intratubular horn
- Deer Antlers
 People believe that these are not epidermal derivative
 Extension of protuberance on frontal bone
 Growth
o Controlled by androgens (only in males)
o Cessation of growth and death of overlying skin (velvet) when hormone levels fall
(antlers are shed annually)
 Bone from endochondral and intermembranous ossification
Perianal Integument
- Anal glands
 Circumanal glands
o Lobulated, modified sebaceous glands located near the cutaneous regions of the anus
o Hepatoid glands (because look like liver lobes)
 Polygonally shaped, round-oval nuclei, eosinophilic (pink) cytoplasm
- Anal sac
 Keratinized, stratified squamous epithelium
 Apocrine glands nearby make substance and secrete into anal sac
- Rectum
 Non-keratinized
 Behaves and looks like a colon
 Has absorptive and secretory functions
- Anus
 Non-keratinized