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Chapter 6 Integumentary System – Part 1

Integumentary System — Part 1: Skin Structure and Key Cells

  • Layers of the Skin
    • Integumentary system includes skin and its accessory structures (hair, nails, glands, sensory receptors).
    • Skin is the largest organ by weight; also called the cutaneous membrane; composed of two primary layers: epidermis (outer) and dermis (inner).
    • Subcutaneous layer (hypodermis) lies beneath dermis; insulating layer of areolar and adipose tissue; not considered part of the skin.

Epidermis

  • Epidermis features

    • Avascular, stratified squamous epithelium.
    • Deepest layer: stratum basale (basal cell layer) nourished by vessels in the dermis.
    • Basal cells divide; as they migrate outward, older cells keratinize and die.
    • Keratinization: hardening, dehydration, and keratin accumulation in epidermal cells as they migrate outward.
    • Keratin: tough, fibrous, waterproof protein stored in cells.
    • As cells reach the surface, they form the stratum corneum; outermost layer composed of tightly packed, dead cells with desmosomes; cells shed over time.
    • Thickest epidermis: on palms and soles (0.8\text{-}1.4\ \mathrm{mm}).
    • Most of body epidermis: 0.07\text{-}0.12\ \mathrm{mm} thick.
    • Functions: protect against water loss, harmful chemicals, mechanical injury, and pathogens.

  • Layers of the epidermis (Table 6.1)

    • Stratum corneum: outermost; many layers of keratinized, dead, non-nucleated cells.
    • Stratum lucidum: present only in thick skin; between corneum and granulosum; cells appear clear; nuclei/organelle visibility lost.
    • Stratum granulosum: beneath corneum; 3–5 layers of flattened granular cells with keratin fibers and shriveled nuclei.
    • Stratum spinosum: beneath granulosum; many layers with developing keratin fibers; cells begin flattening.
    • Stratum basale (basal cell layer): deepest; single row of cuboidal/columnar cells that divide; melanocytes present here.

  • Special cells of the epidermis

    • Dendritic (Langerhans) cells: in stratum spinosum; phagocytes; protect skin and underlying tissues from infection.
    • Tactile (Merkel) cells: in stratum basale; along with sensory nerve endings form tactile discs in dermis; sensory receptors for light touch.
    • Melanocytes: in stratum basale; produce pigment melanin; absorbs UV light and contributes to skin color; melanin is distributed to keratinocytes to protect DNA and other cells from UV damage.

Skin Color and Pigmentation

  • Melanin and colors

    • Eumelanin: brownish-black pigment of the epidermis.
    • Pheomelanin: reddish-yellow pigment; present in certain areas.
    • Color results from melanin amount and distribution, not from number of melanocytes (all have similar melanocyte counts).

  • Hereditary and environmental factors

    • Hereditary: genetic control of melanin production and distribution; albinism results from mutations affecting melanin genes.
    • Environmental: sun exposure, UV light, and X-rays affect color.
    • Physiological: dermal blood vessel oxygenation (pink vs cyanosis), vasodilation/vasoconstriction, dietary carotene, and jaundice.

Dermis

  • Dermis overview

    • Inner, thick layer (average 1 ext{-}2\ \mathrm{mm}).
    • Binds epidermis to underlying tissues; contains dermal papillae that interlock with epidermal ridges (fingerprints).
    • Vascularized connective tissue containing collagen and elastic fibers, muscle fibers, and nerve processes.
    • Contains hair follicles, sweat glands, sebaceous glands; sensory receptors (lamellated Pacinian and tactile Meissner).

  • Dermal layers

    • Papillary layer (superficial): areolar connective tissue; location of dermal papillae; thinner of the two layers; forms fingerprints.
    • Reticular layer (deeper): dense irregular connective tissue; thicker of the two layers.

Subcutaneous Layer (Hypodermis)

  • Not part of the skin.
  • Beneath dermis; insulating layer.
  • Composed of areolar and adipose tissue; contains blood vessels.

Clinical Connection

  • Indoor tanning and skin cancer (Clinical Application 6.1)
    • UV exposure stimulates melanocytes to produce more melanin, darkening skin.
    • Tanning beds deliver UV radiation that can overwhelm protective mechanisms.
    • Skin cancers: basal cell carcinoma and squamous cell carcinoma arise from epithelial cells; melanomas arise from melanocytes.
    • Melanomas are least common (~4\% of skin cancers) but cause ~80% of skin cancer deaths.