Microanatomy of the Integument Study Notes

Overview of the Integumentary System

Definition: The integument is defined as the skin plus its epidermal derivatives.
Epidermal Derivatives: * Foot pads and claws. * Hooves and horns. * Glands, including anal sacs. * Uropygial gland (found in birds). * Wattles, combs, spurs, and feathers.
Nomenclature: * Latin prefix/suffix: Cutis. * Greek prefix/suffix: Derma.
Vital Statistics: The skin is the largest organ in the body, comprising $8-16\%$ of total body weight.

Functions of the Skin

Protection: Serves as a physical, chemical, and biological barrier.
Homeostasis: Prevents the loss of water and electrolytes.
Thermoregulation: Regulation of body temperature.
Sensation: Detects environmental stimuli.
Metabolism: Involved in Calcium ( $Ca^{2+}$ ) homeostasis and energy storage (adipocytes).
Immunology: Performs essential immune functions.

Structural Components and Embryology

Epidermis: * Tissue Type: Stratified squamous keratinized epithelium. * Embryological Origin: Ectoderm. * Characteristics: Avascular and functions as a thick barrier.
Dermis: * Layers: Divided into the Papillary layer (superficial) and Reticular layer (deep). * Embryological Origin: Mesoderm. * Characteristics: Contains blood vessels, nervous tissue, and connective tissue; provides support to the epidermis.
Hypodermis (Subcutis): * Tissue Type: Loose connective tissue. * Feature: May be rich in adipocytes, forming the panniculus adiposus.

The Dermo-Epidermal Junction

Interface: The boundary where the dermis and epidermis meet.
Epidermal Pegs/Ridges: Downward projections of the deep epidermis into the dermis.
Dermal Papillae: Upward projections of the superficial dermis.
Functional Significance: * Increases surface area between the two layers. * Brings the blood supply closer to the avascular epithelium. * Commonly found in areas subjected to traction: footpads, nasal planum, and scrotum. * Note: These are not normally seen in haired skin.

Detailed Anatomy of the Dermis

Papillary Zone: Composed of loose connective tissue. (Mnemonic: "Papa's belt is loose").
Reticular Zone: Composed of dense irregular connective tissue.
Constituent Elements: * Fibers: Collagen and elastic fibers. * Cells: Both resident and transient cells. * Systems: Blood vessels, nerves, and lymphatics.

Cellular Composition of the Epidermis

Keratinocytes: The most common cell type, representing $95\%$ of the total epidermal cell population.
Non-Keratinocytes: * Langerhans Cells: Tissue macrophages involved in immunity; antigen-presenting to T-cells. Derived from bone-marrow monocytes. Not visible with routine H&E staining. * Merkel’s Cells: Act as mechanoreceptors for sensory perception. (Mnemonic: "Merker is a mechanic"). Located in the stratum basale of thick skin; associated with free nerve endings. (Mnemonic: "Merkel got marked on floor"). * Melanocytes: Provide UV protection. Origin is the neural crest. Located in the stratum basale. Produce melanin pigment and transfer it to keratinocytes in the basale and spinosum layers. Requires the enzyme tyrosinase for proper function.

Layers of the Epidermis

Cells mature from deep to superficial, eventually dying and sloughing off at the surface.

Stratum Basale: * Basal layer at the dermo-epidermal junction. * Single layer of cuboidal to columnar epithelium. * Attached to the basal lamina via hemidesmosomes. * Area of high mitotic activity (mitotically active). * Contains stem cells and anchoring keratinocytes.
Stratum Spinosum: * Cells are polygonal in shape. * Thickness: 1-2 cells thick in thin skin; many layers thick in thick skin. * Characterized by increased tonofilaments (cytokeratin) and desmosomes, giving a "spiny" appearance. * Capable of mitosis/cell division if needed.
Stratum Granulosum: * Layer is 3-5 cells thick. * Cells begin to flatten and contain basophilic keratohyalin granules. * Lamellar granules: Secreted to form "intercellular cement" (waterproof lipid sheets). Visible only via electron microscopy. * Mitotically inactive; the last living layer before nuclei and organelles are lost.
Stratum Lucidum: * A translucent layer present only in thick skin. * Cells are fully keratinized (organelles gone). * Cytoplasm contains eleidin, a protein related to keratin. (Mnemonic: "Not always present but when present brings teleidin").
Stratum Corneum: * Outermost layer, up to 20 layers thick. * Cells consist entirely of keratin (water-resistant protein). * No nuclei or organelles; cells are continually shed.

Thick vs. Thin Skin

Thick Skin: * Thick epidermis, hairless. * Contains merocrine/eccrine sweat glands. * Examples: Paw pads, muzzle (bovine muzzle).
Thin Skin: * Thin epidermis. * Contains hair follicles, arrector pili muscles, sebaceous glands, and apocrine sweat glands. (Mnemonic: "I think my hair is thinning").

Processes of the Epidermis

Keratinization: The process by which keratinocytes differentiate. In dogs, this takes approximately $21\text{ days}$ .
Cornification: The production of the stratum corneum through terminal epidermal differentiation.
Cell Adhesion: * Desmosomes: Bind neighboring cells together. * Hemidesmosomes: Bind basal cells to the basal lamina. * Both connect to intermediate filaments.

Sensory Nerves of the Dermis

Nociceptors: Free nerve endings reaching the stratum granulosum; detect pain, itch, and temperature.
Encapsulated Nerve Endings: * Meissner’s Corpuscles: Detect light pressure and touch. (Mnemonic: "Messi fitenight"). * Pacinian Corpuscle: Detects deep pressure and vibration. (Mnemonic: "Pacinian/Pressure"). * Ruffini Corpuscle: Senses stretch. (Mnemonic: "Rufus dog").

Anatomy and Physiology of Hair

Functions: Insulation, camouflage, social display, sensing/protection, and sexual dimorphism.
Structure: * Hair Shaft: Part above the skin surface. * Hair Root: Part within the follicle, ending in the hair bulb on the dermal papilla. * Follicle: Continuous with the epidermis (invagination).
Follicle Layers: * External Root Sheath: Includes the glassy membrane (thickened basement membrane). * Internal Root Sheath: Includes the cuticle which interlocks with the hair cuticle. * Hair Core: Medulla (loose cuboidal cells, air spaces), Cortex (dense keratinized cells), and Cuticle (flat keratinized cells).
Hair Cycle (Mnemonic: "Anne is antive tell ten to gusting stoat"): * Anagen: Active mitotic phase in hair bulb cells. * Catagen: Regressive stage; metabolic activity slows; follicle migrates surface-ward. * Telogen: Resting phase; growth stops. Old hair is shed as new hair grows beneath it. * Controls: Regulated by daylight, temperature, nutrition, and hormones (estrogen, testosterone, thyroid, adrenal steroids).

Types of Hair Follicles

Primary (A): Large diameter, deep in dermis. Associated with sebaceous glands, arrector pili, and sweat glands (Guard hairs).
Secondary (B): Smaller diameter, shallow. No apocrine glands or arrector pili (Under hairs).
Compound Follicles: Clusters where follicles merge at the level of the sebaceous gland and exit one orifice. Common in carnivores. (Mnemonic: "Sellotape").
Arrector Pili: Smooth muscle attached to primary follicles; causes "goosebumps" and aids insulation.
Sinus Hairs (Whiskers/Tactile Hairs): * Very large single follicles. * Contain a blood-filled sinus between dermal root sheaths. * Attached to skeletal muscle for voluntary movement. * Often accompanied by Pacinian corpuscles on the limbs.

Cutaneous Glands

Sebaceous Glands: * Dermal location; produce sebum (lipid + debris). * Holocrine secretion: The whole cell is secreted. * Functions: Antibacterial and waterproofing. * Specialized Sebaceous Glands: Supracaudal (dog), circumanal/hepatoid, submental (cat), preputial (horse - smegma), Tarsal/Meibomian (eyelids), Uropygial (birds).
Apocrine Sweat Glands: * Mechanism: Apical budding/pinching (secretory caps). * Structure: Simple coiled tubular glands emptying into hair follicles. * Help from myoepithelial cells for expression. * Specialized Apocrine Glands: Mammary glands, Ciliary glands (Moll), Anal sac glands, Ceruminous glands (ear wax).
Merocrine/Eccrine Sweat Glands: * Mechanism: Exocytosis. * Open directly onto skin (not follicles). * Found mainly on dog footpads; used for thermoregulation.

Mammary Glands

Structure: Tubuloalveolar glands separated into lobules by connective tissue septae.
Lining: Clusters of alveoli drain into ducts and then into sinuses.
Epithelium: Height varies with activity. Duct/Sinus epithelium is usually cuboidal; Teat canal is stratified squamous.

Specializations: Hooves and Claws

Modifications: Variations of the stratum corneum supported by vascular dermis. They lack the stratum granulosum and stratum lucidum.
Equine Hoof Layers: * Stratum Externum (Tectorium): The "glaze." * Stratum Medium: The majority of the wall; produces horn tubules ( $t$ ) and intertubular horn ( $ih$ ). * Stratum Internum (Lamellatum): The insensitive laminae ( $pe$ ).
Corium (Dermis): * Highly vascular and innervated. * Laminar Corium: Contains sensitive laminae ( $lc$ ). * Primary dermal laminae ( $500-600$ ) interdigitate with epidermal laminae. * White Line: Joins the sole with the wall.