Microanatomy of the Integument Notes

Integument Microanatomy

Overview

  • Integument Definition: The integument consists of the skin and its epidermal derivatives.
  • Epidermal Derivatives: These include foot pads, claws, hooves, horns, various glands (including anal sacs and the uropygial gland in birds), wattles, combs, spurs, and feathers.

Skin

  • Terminology: In Latin, the skin is referred to as cutis, and in Greek, it's called derma.
  • Size: The skin is the largest organ in the body, constituting 8-16% of body weight.
Functions of the Skin
  • Protection: Acts as a barrier against physical, chemical, and biological agents.
  • Water and Electrolyte Balance: Prevents water and electrolyte loss.
  • Temperature Regulation.
  • Sensation.
  • Calcium Homeostasis.
  • Energy Storage.
  • Immune Function.

Components of the Skin

  • Epidermis: Characterized as stratified squamous keratinized epithelium.
  • Dermis.
  • Hypodermis (Subcutis): This includes:
    • Papillary layer
    • Reticular layer
    • Loose connective tissue, potentially rich in adipocytes, forming the panniculus adiposus.

Embryological Origin

  • Epidermis: Derived from the ectoderm; it is avascular and forms a thick epithelial barrier.
  • Dermis: Derived from the mesoderm; contains blood vessels and nervous tissue and supports the epidermis with connective tissue.

Skin Structure

The skin includes:

  • Hair shaft
  • Epidermis
  • Dermis (with papillary and reticular layers)
  • Subcutaneous layer
  • Hair follicle
  • Sweat pore
  • Epidermal ridge
  • Dermal papilla
  • Arrector pili muscle
  • Sebaceous (oil) gland
  • Sweat gland duct
  • Merocrine sweat gland
  • Veins and arteries
  • Areolar connective tissue
  • Sensory receptors and nerve fibers
  • Adipose connective tissue

Thick vs. Thin Skin

  • Thick Skin:
    • Features a thick epidermis.
    • Hairless.
    • Contains merocrine/eccrine sweat glands.
    • Examples: paw pads & muzzle.
  • Thin Skin:
    • Has a thin epidermis.
    • Contains hair follicles, often with arrector pili muscle.
    • Includes sebaceous glands and apocrine sweat glands.

Dermo-Epidermal Junction

  • Location: Interface between the dermis and epidermis.
  • Epidermal Pegs/Ridges: Downward projections of the deep epidermis into the dermis.
  • Dermal Papillae: Upward projections of the superficial dermis.
  • Function: Increases the surface area between layers, bringing blood supply closer to the epithelium.
  • Examples: Prominent in footpads, nasal planum, and scrotum; especially in areas subjected to traction. Normally not seen in haired skin.

Dermis Zones

  • Papillary Layer: Composed of loose connective tissue.
  • Reticular Layer: Characterized by dense irregular connective tissue containing:
    • Collagen fibers
    • Elastic fibers
    • Resident cells, transient cells, blood vessels, nerves, and lymphatics.

Cells of the Epidermis

  1. Keratinocytes: The most common cell type, making up 95% of epidermal cells.
  2. Non-Keratinocytes:
    • Langerhans cells: Involved in immunity (tissue macrophage).
    • Merkel’s cells: Function in mechanoreception.
    • Melanocytes: Provide UV protection.

Layers of the Epidermis

  • The epidermis consists of distinct layers. As cells mature, they move towards the surface, eventually dying and sloughing off.
  • From deep to superficial:
    1. Stratum Basale: Active and alive.
    2. Stratum Spinosum: Active and alive.
    3. Stratum Granulosum: Non-active, alive.
    4. Stratum Lucidum: (Not always present) deceased.
    5. Stratum Corneum: Very deceased.
Epidermal Layer Functions
  • Generate a constant supply of cells.
  • Facilitate cell adherence: Desmosomes (between cells) and hemidesmosomes (between basal cells and basal lamina).
  • Replace cytoplasm with keratin.
  • Waterproof the skin.
  • Flatten cells to create a barrier through multiple layers.

Detailed Look at Epidermal Layers

  • Stratum Basale (Basal Layer):
    • The basal layer of the epidermis, located at the epidermal-dermal junction.
    • Single layer of cuboidal to columnar epithelium.
    • Attached to the basal lamina via hemidesmosomes.
    • Actively dividing (area of mitotic activity).
    • Basal keratinocytes are functionally heterogeneous; some act as stem cells, while others anchor the epidermis.
    • Contains keratinocytes and melanocytes.
  • Stratum Spinosum:
    • Polygonal in shape.
    • Thin skin: 1-2 cells thick.
    • Thick skin: many layers thick.
    • Increased number of tonofilaments (cytokeratin) and desmosomes give a spiny appearance.
    • Cells are cohesive and resist abrasion.
    • Retains some capacity for division if needed.
  • Stratum Granulosum:
    • Layer that is 3-5 cells thick.
    • Cells begin to flatten.
    • Contain basophilic keratohyalin granules.
    • Lamellar granules (not visible via light microscopy) secrete waterproof lipid sheets, forming an “intercellular cement.”
    • No mitotic activity; it is the last living layer. The nucleus and organelles are soon to be lost.
  • Stratum Lucidum:
    • Translucent layer present only in thick skin.
    • Contains many keratin filaments and desmosomes.
    • Cellular organelles are gone – cells are fully keratinized.
    • Cytoplasm contains eleidin, a protein chemically related to keratin.
  • Stratum Corneum:
    • Outermost layer.
    • Many (up to 20) layers thick; thickness varies by location.
    • Cells consist entirely of KERATIN, a water-resistant protein.
    • No nuclei or organelles.
    • Cells continually shed at the surface.

Desmosomes vs. Hemidesmosomes

  • Hemidesmosomes: Bind cells to the basal lamina.
  • Desmosomes: Bind neighboring cells together.
  • Both connect to intermediate filaments.

Keratinization and Cornification

  • Keratinization Definition: The process by which keratinocytes differentiate, about 21 days in length in the dog.
  • Cornification Definition: Production of stratum corneum by terminal epidermal differentiation.

Non-Keratinocytes of the Epidermis

  • Langerhan’s cell: Functions in the immune system.
  • Merkel’s cells: Serve a sensory role.
  • Melanocytes: Provide protection against UV radiation.
Langerhans Cells
  • Intra-epidermal (within the epidermis) macrophages.
  • Derived from bone-marrow monocytes.
  • Role in immunity by presenting antigens to T-Cells.
  • Mediators of tolerance.
  • Not seen with routine H&E staining.
Merkel’s Cells
  • Located in the stratum basale of thick skin.
  • Act as sensory mechanoreceptors for cutaneous sensation.
  • Free nerve endings are seen at the base of these cells.
Melanocytes
  • Present in the stratum basale.
  • Neural crest origin.
  • Produce melanin pigment, which is transferred to keratinocytes of stratum basale and spinosum.
  • Protects cells from the effects of UV radiation.
Melanin
  • Melanin absorbs harmful UV-radiation.
  • Transforms the energy into harmless amounts of heat.
  • Must have tyrosinase for proper function.

Sensory Nerves of the Dermis

  • Nociceptors: Free nerve endings that reach the stratum granulosum and detect pain, itch, and temperature.
  • Encapsulated Nerve Endings:
    • Meissner’s corpuscles: Detect light pressure/touch.
    • Pacinian corpuscle: Detects deep pressure.
    • Ruffini corpuscle: Senses stretch.

Epidermal Structures/Derivatives

  • Hair follicles.
  • Sinus (tactile) hairs.
  • Sebaceous glands.
  • Apocrine (sweat) glands.
  • Merocrine/Eccrine sweat glands.
  • Mammary glands.
  • Hooves, claws/nails are modifications of the epidermis and dermis.

Functions of Hair/Feathers

  • Insulation.
  • Camouflage.
  • Social display.
  • Sense/protect.
  • Sexual dimorphism.

Anatomy of Hair

  • Hairs are flexible keratinized structures produced by a hair follicle.
  • The hair shaft is above the surface of the skin.
  • The hair root is within the follicle and ends with the bulb near/on the dermal papillae.

Hair Follicle

  • Hair follicles are continuous with the epidermis (invagination).
  • Extend into the dermis and hypodermis.
Hair Follicle Layers
  • External/Outer Root Sheath:
    • external glassy membrane = thickened basement membrane
    • external root sheath= continuous with epidermis
  • Internal Root Sheath:
    • a few layers of squamous cells
    • cuticle (overlapping keratinized cells) that interlocks with the hair cuticle
  • Hair:
    • medulla – loose cuboidal cells with areas of air
    • cortex – dense compact keratinized cells
    • cuticle – single layer of flat keratinized cells
Hair Follicle Bulb
  • Hair matrix cells cover the dermal papilla and compose most of the hair bulb.
  • Hair matrix cells are equivalent to the stratum basale and give rise to the cells that form the hair.
  • The dermal papilla contains connective tissue and blood vessels.
  • The dermal sheath is the connective tissue enclosing the inner and outer root sheath, providing blood supply peripheral to the glassy membrane.

Hair Cycle

  • Anagen: Period when hair bulb cells are mitotically active.
  • Catagen: Regressive stage when metabolic activity slows down and the base of the follicle migrates towards the surface.
  • Telogen: Resting or quiescent phase when growth stops.
  • New hair grows beneath the telogen follicle, and the old hair shaft is shed.
  • Controlled by daylight, ambient temperature, nutrition, and hormones (estrogen, testosterone, adrenal steroids, and thyroid hormone).

Types of Hair Follicles

  • (A) Primary Hair follicle:
    • Large diameter.
    • Rooted deep in dermis.
    • Has sebaceous glands, arrector pili muscle, and sweat glands.
    • Produces Primary or Guard Hair
  • (B) Secondary Hair follicle:
    • Smaller diameter.
    • Rooted nearer the surface.
    • +/-sebaceous gland; no apocrine sweat glands, no arrector pili muscle.
    • Produces Secondary or under hairs
  • Follicles can be arranged as individuals or clusters and can vary in the number of hairs per cluster, depending on the species of animals and breeds.

Compound Follicles

  • Clusters of several hair follicles.
  • The follicles merge at the level of the sebaceous gland and emerge through one external orifice.
  • Compound follicles usually have one primary follicle with an apocrine sweat gland and several secondary hair follicles.
  • Most common in carnivores.

Arrector Pili

  • Arrector pili muscle = smooth muscle attached to the follicle of primary hairs.
  • Contraction causes hair to stand up (goosebumps).
  • Plays a role in insulation.

Sinus Hairs/Whiskers

  • Tactile hairs.
  • Composed of a very large single follicle.
  • Blood-filled sinus between the inner and outer dermal root sheath.
  • Attached to skeletal muscle to allow for voluntary movement.
  • Nerve bundles penetrate the sheath.

Sebaceous Glands

  • Located in the dermis.
  • Produce sebum, a mixture of lipid and cell debris.
  • Holocrine secretion.
  • Provides antibacterial and waterproofing effects.
  • Ducts empty into a follicle.
  • Can be simple, branched, or compound glands.
Specialized Sebaceous Glands
  • Supracaudal glands – dog
  • Circumanal / Hepatoid glands
  • Submental organs/glands – cat
  • Infraorbital, inguinal, and interdigital regions of sheep
  • Horn glands – buck goat
  • Preputial glands of horses – produce smegma
  • Tarsal (Meibomian) glands – eyelids
  • Uropygial gland (the preen gland) in birds

The Uropygial/ Preen Gland

  • Strongly developed in many waterbirds such as ducks.
  • The rest of bird skin is aglandular.

Apocrine Sweat Glands

  • Secrete by apical budding/pinches.
  • Epithelial cells have apical secretory caps.
  • Simple coiled tubular glands that open into the distal hair follicle.
  • Contractile myoepithelial cells help express the product.
  • In domestic animals, located throughout most of the skin.
  • Function is mainly communication (attraction, marker).
Specialized Apocrine Sweat Glands
  • Mammary glands
  • Ciliary glands (of Moll) in eyelid, which make up tear film
  • Apocrine glands of the anal sac
  • Ceruminous glands (ear wax)

Anal Sacs

  • Supplied by perianal glands (sebaceous) and apocrine glands of the anal sac, that should be empty or be manually emptied.

Merocrine/Eccrine Sweat Glands

  • Merocrine – excrete via exocytosis.
  • Open directly onto the skin surface vs. hair follicle.
  • Minor in domestic animals except for the footpad of dogs.
  • Involved in thermoregulation and electrolyte balance; Secrete fluid onto skin surface when body temperature rises.
  • Also found on dog's nose.

Mammary Glands

  • Tubuloalveolar glands connected by ducts and separated into lobules by connective tissue septae and interstitium.
  • Specialized apocrine sweat glands.
  • Clusters of alveoli form lobules.
  • Ducts drain into sinuses.
  • Smooth muscle in the teat (sphincter).
  • Height of epithelium reflects the activity of the gland.

Hooves and Claws

  • Modification of skin.
  • Variation of the stratum corneum.
  • Supported by a highly vascular dermis.
  • Lack stratum granulosum & stratum lucidum.

Equine Hoof

  • The distal phalanx is encased in heavily keratinized epidermis (‘horn’).
  • Skin angles internally at the coronary band/groove, leading to distal growth of the stratum corneum.
  • The white line joins the sole with the wall.
Corium (Dermis) of the Equine Hoof
  • Highly vascular and innervated connective tissue (dermis).
  • Laminar corium: primary dermal laminae (500-600) interdigitate with primary epidermal laminae.
  • Papillae at coronary corium, sole corium, and distal laminae (terminal papillae).
  • The epidermis is avascular, while the corium contains nerves and a massive network of blood vessels.

Keratinized Hoof Wall Layers

  • Horn – keratin arranged in parallel microscopic tubules (like hair shaft) and intertubular horn
    • Stratum externum (tectorium):
      • Continuation of perioplic epidermis
      • “Glaze”
    • Stratum medium:
      • Majority of the wall
      • Produced from coronary epidermis
    • Stratum internum (lamellatum):
      • Primary epidermal laminae (~600)
      • INSENSITIVE LAMINAE interdigitate with primary dermal laminae
  • Stratum medium tubular and intertubular horn
  • Laminar corium with primary dermal laminae = SENSITIVE LAMINAE