C

The Integumentary System - Study Notes

The Integumentary System

Structure and Components

  • The Integumentary System consists of:

    • Skin - the largest organ in the body

    • Hair

    • Nails

    • Sweat Glands

    • Sebaceous (Oil) Glands

    • Subcutaneous Tissue (Hypodermis)

Structure of Skin

  • The skin is comprised of two distinct regions:

    • Epidermis

    • Superficial region composed of epithelial tissue

    • Avascular (without blood vessels)

    • Dermis

    • Underlies the epidermis

    • Composed of fibrous connective tissue

    • Vascular, which allows diffusion of nutrients to the epidermis

    • Hypodermis

    • Subcutaneous layer that is deep to the skin

    • Not considered part of the skin itself

    • Mostly consists of adipose tissue that absorbs shock and insulates

    • Anchors skin to underlying structures (predominantly muscles)

Fascia

  • Fascia is a thin sheath of fibrous tissue enclosing a muscle or other organ.

THE EPIDERMIS

  • The Epidermis includes several cell types:

    • Keratinocytes

    • Primarily responsible for forming the protective outer layer

    • Melanocytes

    • Produce melanin, contributing to skin color and protection against UV radiation

    • Dendritic (Langerhans) Cells

    • Macrophages that patrol deep in the epidermis and activate the immune system

    • Tactile (Merkel) Cells

    • Sensory receptors that sense touch

Layers of the Epidermis

  1. Stratum Basale (Basal Layer)

    • Deepest layer of the epidermis

    • Firmly attached to the dermis

    • Actively undergoing mitosis (cell division)

    • Takes 25-45 days to reach the skin surface

    • Contains 10-25% melanocytes

  2. Stratum Spinosum (Prickly Layer)

    • Composed of 8-10 cell layers

    • Contains keratinocytes that appear spiky due to desmosomes and keratin filaments

    • Dendritic cells are also present in this layer for immune surveillance

  3. Stratum Granulosum (Granular Layer)

    • Consists of 4-6 cell layers that transition toward keratinization

    • The nuclei and organelles begin to disintegrate

    • Keratinization begins; keratohyaline granules accumulate

    • Contains lamellar granules, a water-resistant glycolipid that slows water loss from the skin

    • Cells above this layer die due to distance from dermal capillaries

  4. Stratum Lucidum (Clear Layer)

    • Found only in thick skin (e.g., palms and soles)

    • Thin, translucent band with 2-3 rows of dead keratinocytes

  5. Stratum Corneum

    • Most superficial layer consisting of 20-30 layers of dead cells

    • Composed of flat membranous sacs filled with keratin along with glycolipids in the extracellular space

    • Apoptosis occurs here; refers to the programmed cell death and shedding of dead skin cells

    • Humans can shed roughly 50,000 cells every minute through this process.

Special Features

  • The Epidermis participates in apoptosis, where dead cells slough off as dandruff and dander.

The Dermis

  • Dermis consists of two layers:

    • Papillary Layer

    • Superficial layer composed of areolar connective tissue

    • Contains loose fibers that allow phagocytes to patrol for microorganisms

    • Contains dermal papillae - fingerlike projections that enhance grip and contribute to fingerprints

    • Houses free nerve endings for pain and Meissner's corpuscles for touch

    • Reticular Layer

    • Makes up ~80% of the dermal thickness

    • Consists of dense irregular connective tissue with elastic and collagen fibers

    • Contains the cutaneous plexus, a network of blood vessels that supply nutrients to the skin

    • Cleavage lines (tension lines) in this layer are made by collagen fibers running parallel to the skin's surface; important for surgical incision healing

Skin Color

  • Skin color is influenced by three pigments:

    1. Melanin

    • Only pigment produced in the skin; synthesized by melanocytes.

    • Sun exposure stimulates melanin production; forms range from yellowish to brownish-black

    • Freckles and pigmented moles represent localized accumulations of melanin.

    1. Carotene

    • Yellow-orange pigment, especially visible in palms and soles; can be converted to vitamin A.

    1. Hemoglobin

    • Contributes to the pinkish hue of fair skin; visible through thinner skin layers in Caucasians.

Epidermal Derivatives

  • The integumentary system's derivatives include:

    • Hair

    • Nails

    • Sweat Glands

    • Sebaceous Glands

Hair

  • Composed of dead keratinized cells; lacks hair on palms, soles, and other designated areas.

  • Functions of hair:

    • Warns about insects on the skin

    • Guards against physical trauma, heat loss, and UV radiation

Structure of Hair

  • Hair consists of:

    • Shaft: The extended, visible part above the skin.

    • Root: The portion embedded beneath the skin.

    • Medulla: Central part with large cells and air spaces.

    • Cortex: Several flat layers surrounding the medulla.

    • Cuticle: Thin outer layer made of overlapping cells.

Hair Growth

  • Vellus hair: Fine, pale body hair; generally less noticeable.

  • Terminal hair: Coarse and long hair found on the scalp, eyebrows, axillary, and pubic areas post-puberty.

  • Average hair growth rate is 2.25 mm per week, with approximately 90 scalp hairs lost daily.

Nails

  • Scale-like epidermis modifications containing hard keratin, acting as a protective cover for the distal surfaces of fingers and toes.

  • The nail matrix is responsible for nail growth, while various features such as the eponychium (cuticle) and lunule are present.

Sweat Glands

  • Sudoriferous glands (sweat glands) found on all skin surfaces, with about 3 million present in a single person. Two main types include:

    • Eccrine (Merocrine) Sweat Glands:

    • Most prevalent; involved in thermoregulation and secrete sweat that is primarily water and electrolytes.

    • Apocrine Sweat Glands:

    • Found in axillary and anogenital areas, producing milky sweat rich in fats and proteins.

Functions of Skin

  • Primary roles include:

    • Protection: Acts as a barrier against pathogens and includes:

    1. Chemical Barrier: E.g., sebum, sweat, and melanin.

    2. Physical Barrier: Composed of keratinized cells that restrict permeability.

    3. Biological Barrier: Immune cells like dendritic cells and macrophages.

    • Regulation of Body Temperature: Maintains thermoregulation through sweat production.

    • Sensation: Contains receptors for touch, pain, temperature, and pressure.

    • Metabolic Functions: Synthesizes vitamin D and other compounds.

    • Blood Reservoir: Dermis can hold large volumes of blood.

    • Excretion of Wastes: Through sweat production.