Microanatomy of the Integument Study Notes

Overview and Definitions of the Integumentary System

Integument Definition: The integument is defined as the skin plus all epidermal derivatives.
Epidermal Derivatives: These include specialized structures such as: * Foot pads and claws. * Hooves and horns. * Glands, specifically including anal sacs. * Uropygial glands (found in birds). * Wattles, combs, spurs, and feathers.
Terminology: * Latin Prefix/Suffix: Cutis. * Greek Prefix/Suffix: Dérma.
Vital Statistics: The skin is the largest organ in the body, accounting for approximately $8-16\%$ of the total body weight.
Primary Functions: * Protection: Serves as a physical, chemical, and biological barrier. * Regulation: Prevents the loss of water and electrolytes; involved in temperature regulation. * Sensation: Houses receptors for external stimuli. * Metabolism/Storage: Involved in calcium homeostasis, energy storage, and immune function.

Structural Components and Embryological Origin

Epidermis: * Classification: Stratified squamous keratinized epithelium. * Origin: Ectoderm derived. * Characteristics: Avascular; serves as a thick barrier.
Dermis: * Classification: Connective tissue supporting the epidermis. * Layers: Divided into the papillary layer and the reticular layer. * Origin: Mesoderm derived. * Characteristics: Contains blood vessels and nervous tissue.
Hypodermis (Subcutis): * Composition: Loose connective tissue, often rich in adipocytes. * Terminology: When rich in fat, it is referred to as the panniculus adiposus.

Categorization of Skin: Thick vs. Thin

Thick Skin: * Epidermis: Characterized by a thick epidermal layer. * Hair Status: Hairless. * Glands: Contains merocrine/eccrine sweat glands. * Examples: Paw pads and the muzzle (e.g., Bovine muzzle).
Thin Skin: * Epidermis: Relatively thin. * Hair Status: Contains hair follicles, frequently associated with arrector pili muscles. * Glands: Contains sebaceous glands and apocrine sweat glands. * Example: Canine haired skin.

The Dermo-Epidermal Junction (DEJ)

Structure: This is the interface where the dermis and epidermis meet, featuring specific interlocking projections: * Epidermal Pegs/Ridges: Downward projections of the deep epidermis into the dermis. * Dermal Papillae: Upward projections of the superficial dermis into the epidermis.
Purpose: The interlocking increases surface area, strengthening the bond between layers and bringing the blood supply closer to the avascular epithelium.
Locations: Most prominent in areas subjected to traction, such as footpads, the nasal planum, and the scrotum. These are not normally seen in haired skin.

Histology of the Dermis

Papillary Zone: Located superficially; composed of loose connective tissue.
Reticular Zone: Located deeper; composed of dense irregular connective tissue.
Compositional Elements: * Fibers: Collagen and elastic fibers. * Cells: Resident cells and transient cells. * Vascular/Neural: Contains blood vessels, nerves, and lymphatics.

Specific Layers and Cells of the Epidermis

Keratinocytes: The most common cell type, representing $95\%$ of the total epidermal cells.
Non-Keratinocytes: * Langerhans Cells: Bone-marrow derived monocytes; intra-epidermal macrophages presenting antigens to T-cells for immunity. They are not visible on routine H&E stains. * Merkel’s Cells: Mechanoreceptors for cutaneous sensation, located in the stratum basale of thick skin. * Melanocytes: Neural crest origin cells located in the stratum basale. They produce melanin pigment transferred to keratinocytes to protect against UV radiation via the enzyme tyrosinase.
Stratigraphic Order (Deep to Superficial): 1. Stratum Basale: A single layer of cuboidal to columnar epithelium attached to the basal lamina via hemidesmosomes. It is the site of active mitosis (stem cells). 2. Stratum Spinosum: Polygonal cells ( $1-2$ cells thick in thin skin; many layers in thick skin). Spiny appearance due to desmosomes and tonofilaments (cytokeratin). Retains some mitotic capacity. 3. Stratum Granulosum: $3-5$ cells thick; cells flatten and become mitotically inactive. Contains basophilic keratohyalin granules and lamellar granules (lipid sheets for waterproofing). 4. Stratum Lucidum: Present only in thick skin; translucent layer where organelles are lost. Contains eleidin, a protein related to keratin. 5. Stratum Corneum: The outermost layer, up to $20$ layers thick. Composed of dead cells filled entirely with keratin; no nuclei or organelles.

Definitions of Keratinization and Cornification

Keratinization: The process by which keratinocytes differentiate. In the dog, this process lasts approximately $21\,\text{days}$ .
Cornification: The specific production of the stratum corneum through terminal epidermal differentiation.

Sensory Innervation of the Dermis

Nociceptors: Free nerve endings extending to the stratum granulosum. Detect pain, itch, and temperature.
Meissner’s Corpuscles: Detect light pressure and touch.
Pacinian Corpuscles: Detect deep pressure and vibration (prominent on feline limbs associated with sinus follicles).
Ruffini Corpuscles: Detect stretch.

Anatomy and Physiology of Hair

Anatomy: * Hair Shaft: Part above the skin surface. * Hair Root: Part within the follicle, ending in the hair bulb. * Hair Bulb: Surrounds the dermal papilla which contains blood vessels.
Follicle Layers: * External Root Sheath: Continuous with the epidermis; separated by an external glassy membrane. * Internal Root Sheath: Includes a cuticle that interlocks with the hair cuticle. * Hair Structure: Consists of the medulla (loose cuboidal cells/air), cortex (dense keratin), and cuticle (flat keratinized cells).
Hair Cycle: 1. Anagen: Active mitotic phase of hair bulb cells. 2. Catagen: Regressive stage; metabolic activity slows; follicle base migrates superficially. 3. Telogen: Resting phase; growth stops and the hair is eventually shed. * Controls: Growth is regulated by daylight, temperature, nutrition, and hormones (estrogen, testosterone, adrenal steroids, thyroid hormone).
Follicle Types: * Primary (Guard) Hair: Large diameter, rooted deep, associated with sebaceous glands, arrector pili muscles, and sweat glands. * Secondary (Under) Hair: Smaller diameter, rooted shallower, no arrector pili or apocrine sweat glands. * Compound Follicles: Multiple follicles merging at the sebaceous gland level to exit one orifice. Typical in carnivores.
Sinus (Tactile) Hairs/Whiskers: Large single follicles with a blood-filled sinus between dermal root sheaths. Attached to skeletal muscle for voluntary movement.

Glandular Structures

Sebaceous Glands: Dermal glands producing sebum (lipid/debris) via holocrine secretion for waterproofing and antibacterial action. Specialized types include Meibomian glands (eyelids) and Uropygial glands (birds).
Apocrine Sweat Glands: Simple coiled tubular glands secreting via apical budding (secretory caps). Found throughout domestic animal skin; used for communication. Specialized types include mammary glands and glands of the anal sac.
Merocrine/Eccrine Sweat Glands: Excrete via exocytosis directly onto the skin. In domestic animals, these are minor and primarily found on dog footpads for thermoregulation.
Mammary Glands: Tubuloalveolar glands. Milk-secreting epithelial cells and myoepithelial cells for contraction. Ducts include lactiferous, teat, and papillary ducts.

The Equine Hoof and Claws

General: Specialized modifications of the stratum corneum lacking stratum granulosum and lucidum, supported by a highly vascular dermis (corium).
Corium (Dermis): Highly vascular and innervated. Includes the coronary, perioplic, laminar, sole, and frog corium.
Hoof Wall Layers: * Stratum Externum (Tectorium): The "glaze" or continuation of perioplic epidermis. * Stratum Medium: Majority of the wall; contains tubular and intertubular horn. * Stratum Internum (Lamellatum): Known as the insensitive laminae; contains approximately $500-600$ primary epidermal laminae that interdigitate with the sensitive laminar dermis (corium).