Skin and Appendages Notes

Skin and Its Appendages

Overview

  • The skin covers the entire body, providing protection against physical, chemical, and frictional forces.
  • It is the largest sensory organ and possesses waterproofing properties due to the presence of keratin.
  • Nails protect fingertips and aid in defense.

Composition

  • The skin comprises epithelium, connective tissue, muscles, blood vessels, and nervous tissue.
  • It is the largest organ in terms of weight and volume.

Functions

  • Protection: Against physical trauma, toxic chemicals, radiation, and sunlight.
  • Prevention of Dehydration: Maintains moisture balance.
  • Temperature Regulation: Through sweat evaporation and control of capillary network.
  • Sensory Reception: Detects touch, pain, warmth, cold, pressure, and vibration.
  • Vitamin D Synthesis: Important for calcium absorption and bone health.
  • Immunity: Mediated by Langerhans cells and lymphocytes.
  • Excretion: Via secretions from sweat, sebaceous, and apocrine glands.
  • Absorption: Certain lipid-soluble substances can be absorbed through the skin, facilitating the delivery of therapeutic agents like nicotine, steroid hormones, and sea-sickness medications via patches.

Structure

  • Epidermis: Stratified squamous keratinized epithelium with a wavy basement membrane.
  • Dermis: Irregular connective tissue containing blood vessels, nerves, lymphatics, sweat glands, arrector pili muscle, and sebaceous glands.
  • Hypodermis: Subcutaneous tissue (technically not part of the skin).

Layers of Skin

  • Epidermis
  • Dermis
  • Hypodermis (Subcutaneous tissue)

Dermis

  • Papillary Layer: Superficial layer of loose connective tissue.
  • Reticular Layer: Deeper, thicker layer made of dense irregular connective tissue.

Epidermis: Stratum Basale (Stratum Germinativum)

  • Deepest layer bordering the dermis.
  • A single layer of cuboidal or tall cuboidal cells resting on the basement membrane.
  • Contains stem cells that actively divide and give rise to cells in other layers.
  • Epidermal keratinocytes are constantly renewed, with cells shedding from the top layer and being replaced by new cells from the stratum basale.
  • Cells have less cytoplasm and closely packed nuclei, making this layer appear basophilic.
  • Keratinocytes are formed here and migrate upwards.
  • Contains melanocytes and Merkel cells.
    • Melanocytes: Located immediately above the stratum basale at the dermo-epidermal junction.
    • Merkel Cells (Merkel Disks): Sensory receptor cells that respond to continuous touch stimuli.

Epidermis: Stratum Spinosum

  • Polygonal cells joined by desmosomes, exhibiting cytoplasmic processes or spines.
  • Processes attach to adjacent cells via desmosomes.
  • Langerhans cells (modified macrophages) are present, functioning as antigen-presenting cells.
  • Langerhans cells have no cell junctions, initiate cutaneous immune responses, and transport captured antigens to deeper lymph nodes.

Epidermis: Stratum Granulosum

  • Contains flattened keratinocytes with keratohyalin granules in their cytoplasm (precursors of keratin).
  • Granules are basophilic in H&E stained sections.
  • More prominent in thick skin than in thin skin.

Epidermis: Stratum Lucidum

  • Thin layer found in thick skin.
  • Contains a few layers of flattened, densely packed cells beneath the stratum corneum.
  • Nuclei become pyknotic as cells degenerate.
  • Has a refractile appearance and may stain poorly.
  • Contains eosinophilic cells with advanced keratinization; nuclei and cytoplasmic organelles are disrupted and disappear as cells fill with keratin.

Epidermis: Stratum Corneum

  • Cells are completely filled with keratin.
  • Consists of anucleate squamous cells with keratin filaments.
  • Cellular fragmentation does not occur; cells fill with keratin and are sloughed off. The thick plasma membrane of cornified, keratinized cells is coated with an extracellular layer of lipids, forming the major water barrier in the epidermis.

Keratinization

  • Starts with the formation of keratin filaments in keratinocytes.
  • Keratin granules are formed in the stratum granulosum.
  • Filaments and granules combine to form keratin.
  • The process includes breakdown of the cell nucleus and other organelles, along with thickening of the plasma membrane.

Lipid Barrier

  • Cells of stratum spinosum and granulosum secrete granules via exocytosis into intercellular spaces between the stratum granulosum and stratum corneum.
  • Organization of intercellular lipid lamellae forms the epidermal water barrier.
  • Contains cholesterol, ceramides, and free fatty acids, providing a “Teflon-like” coating on the skin surface.
  • Destruction of the epidermal barrier over large areas (e.g., severe burns) can lead to life-threatening fluid loss.

Dermis

  • Attachment of epidermis to dermis is enhanced by increased interface between the two tissues.
  • Composed of dense irregular connective tissue providing mechanical support, strength, and thickness.
  • Derived from mesoderm.
  • Numerous fingerlike connective tissue protrusions, dermal papillae, project into the undersurface of the epidermis.
  • Similar epidermal protrusions, called epidermal ridges or rete ridges, project into the dermis.

Dermal Papillae

  • Portions of the dermis that project into the epidermis.
  • Increase contact area between epidermis and dermis, preventing detachment.
  • Contain loose connective tissue, blood vessels, and nerves.
  • The epidermis that projects into the dermis is called epidermal rete ridge.
  • At sites with increased mechanical stress, epidermal ridges are deeper, and the epithelium is thicker; dermal papillae are longer and more closely spaced.
  • Hemidesmosomes strengthen attachment of the epidermis to the underlying connective tissue.

Dermis: Papillary Layer

  • Thin layer just underneath and including dermal papillae.
  • Contains thin collagen and elastic fibers that form a network.
  • Contains blood vessels, nerves, and nerve endings concentrated in this layer and more prominent in dermal papillae.

Dermis: Reticular Layer

  • Thicker than the papillary layer.
  • Contains coarser collagen and elastic fibers.
  • Collagen and elastic fibers form regular lines of tension in the skin called Langer’s lines; incisions parallel to Langer’s lines heal with minimal scarring.
  • In the skin of the areola, penis, and scrotum, a layer of smooth muscle fibers, arranged irregularly, is seen in the deepest layer.

Hypodermis

  • Also called subcutaneous tissue.
  • Contains fat for insulation and energy storage.

Melanocytes

  • Dendritic cells scattered among the basal cells of the stratum basale.
  • Rounded cell body resides in the basal layer and extends long processes between keratinocytes of the stratum spinosum.
  • Not attached to any cell.
  • The ratio of melanocytes to keratinocytes ranges from 1:4 to 1:10 in different body parts and is constant in all races.
  • Have elongated nuclei and clear cytoplasm.
  • Produce and secrete the pigment melanin, which protects against the damaging effects of non-ionizing ultraviolet irradiation.

Nerves of Skin

  • Free Nerve Endings: Detect pain or temperature.
  • Merkel Disks: Detect continuous touch.
  • Meissner Corpuscles: Detect light touch.
  • Pacinian Corpuscles: Detect vibration and are found in the subcutaneous layer (hypodermis).
  • Free nerve endings also recognize fine touch, heat, and cold and lack Schwann cell covering.
  • Pacinian corpuscles detect pressure changes for mechanical and vibratory pressure applied to the skin surface.
  • Meissner’s corpuscles are responsible for sensitivity to light touch.

Thin Skin

  • Epidermis has 4 layers and is thin.
    • Stratum basale
    • Stratum spinosum
    • Stratum granulosum
    • Stratum corneum (thin)
  • Stratum lucidum is absent.
  • Has hair, sebaceous glands, and sweat glands and is also called hairy skin.
  • Stratum granulosum is very thin (just a single cell layer) and hard to differentiate.

Thick Skin

  • Skin is classified into thick and thin depending on the thickness of the epidermis.
  • Thick skin has a thick epidermis, an extremely thick stratum corneum, and all 5 layers are clearly seen.
  • Seen on the skin of palms and soles.
  • Has no hair follicles or sebaceous glands.
  • Has plenty of eccrine sweat glands.
  • Stratum lucidum is clearly seen.

Appendages of Skin

  • Epidermal derivatives include hair follicles and hair, sebaceous glands, sweat glands, and nails.

Accessory Structures of the Skin

  • Include glands, hair, and nails.
    • Glands: Sebaceous glands, eccrine sweat glands, and apocrine sweat glands.
      • Sebaceous Glands: Secrete into hair follicles to keep the skin soft and moist, serving as a barrier to protect the skin.
      • Eccrine Sweat Glands: Regulate body temperature and are found in both thin and thick skin.
      • Apocrine Sweat Glands: Also called sexual scent glands; found only in some special regions of thin skin, such as the axilla, nipple, and perianal and genital areas; their function in humans is not clear.

Hair

  • Found in thin skin, in the scalp, pubic region, and armpit (axilla) in adults.
Parts of Hair
  • Hair Shaft: Has a scaly surface called the cuticle.
  • Hair shaft is surrounded by an inner root sheath and outer root sheath.
  • The outer root sheath is covered by a connective tissue sheath.
  • The deep end of the hair follicle is expanded into the hair bulb, composed of a dermal papilla (hair papilla) and a hair root.
  • Hair root is composed mostly of the hair matrix, which is capable of cell division and gives rise to the hair shaft.
Hair Structure (Inside to Outside)
  • Medulla
  • Cortex
  • Cuticle
  • Inner root sheath
  • Outer root sheath
  • Connective tissue sheath
Hair Follicle
  • Produce hair and maintain its growth.
  • Extend from the epidermis to the dermis or hypodermis.
  • The basal deeper region forms a balloon-shaped structure called the hair bulb.
  • The hair bulb is made of the hair root and dermal papilla.
  • The hair root contains melanocytes and germinal matrix.
  • Cells of the germinal matrix divide and give rise to hair.
Sebaceous Glands
  • Secrete sebum that coats the hair and skin surface.
  • Develop as outgrowths of the external root sheath of the hair follicle, usually producing several glands per follicle.
  • Smooth endoplasmic reticulum forms lipid droplets.
  • Cells gradually become filled with numerous lipid droplets separated by thin strands of cytoplasm.
  • Small saccular glands open into hair follicles; in lips, corners of the mouth, margins of eyelids, and external genital organs, they open directly onto the skin.
  • Absent in thick skin.
  • The acini are lined by stratified epithelium.
  • Cells contain lipid droplets in the cytoplasm, become large, lose their nuclei, and die; the whole cytoplasm of the cell becomes sebum.
  • Secretions become less after 50 years of age.
  • Holocrine gland.
  • The duct opens in hair follicle.
  • Secrets sebum, a complex mixture of lipids.
  • Maintains stratum spinosum, stratum granulosum, nucleated keratinocytes & hair.
  • Spreads a layer of lipids over the surface of the skin and hair.
  • Acts as a natural moisturizer for the skin and natural hair conditioner for the hair.
Arrector Pili Muscle
  • Runs from a midpoint on the hair sheath to the dermal papillary layer.
  • A small bundle of smooth muscle cells.
  • Contraction pulls the hair shafts to a more erect position, producing