C

The Integumentary System

Integumentary System Overview

  • The integumentary system consists of:
    • Skin: The largest organ in the body.
    • Hair.
    • Nails.
    • Sweat glands.
    • Sebaceous (oil) glands.
    • Subcutaneous tissue.

Structure of Skin

  • Skin consists of two distinct regions:
    • Epidermis:
    • Superficial region.
    • Consists of epithelial tissue.
    • Avascular (without blood vessels).
    • Dermis:
    • Lies beneath the epidermis.
    • Composed of fibrous connective tissue.
    • Vascular (contains blood vessels).
    • Diffusion of nutrients occurs from the dermis to the epidermis.

Hypodermis (Subcutaneous Layer)

  • Located deep to the skin.
  • Not considered a part of the skin.
  • Mostly composed of adipose tissue, which:
    • Absorbs shock.
    • Provides insulation.
    • Anchors skin to underlying structures (primarily muscles) and fascia (a thin sheath of fibrous tissue enclosing a muscle or organ).

Epidermis Structure and Cells

  • Dendritic (Langerhans) cells:
    • Function as macrophages that patrol the deep epidermis.
    • They are key activators of the immune system.
  • Tactile (Merkel) cells:
    • Sensory receptors that sense touch.

Layers of Epidermis

  1. Stratum Basale (Basal Layer):

    • Deepest of all epidermal layers.
    • Firmly attached to the dermis.
    • Actively undergoes mitosis (cell division).
    • Takes about 25-45 days for cells to migrate to the surface and die.
    • Comprises 10-25% melanocytes.

  2. Stratum Spinosum (Prickly Layer):

    • Contains 8-10 layers of cells.
    • Web-like structure that allows resistance to tension and pulling forces.
    • Looks spiky due to keratinocytes.
    • Contains melanosomes and dendritic cells that patrol the area.

  3. Stratum Granulosum (Granular Layer):

    • Contains four to six cells thick.
    • Cell appearance changes; cells flatten.
    • Nuclei and organelles disintegrate as keratinization begins.
    • Keratohyaline granules accumulate and contribute to keratin fiber formation.
    • Cells above this layer begin to die due to distance from dermal capillaries.

  4. Stratum Lucidum (Clear Layer):

    • Found only in thick skin (e.g., palms and soles).
    • Composed of a thin, translucent band of 2-3 rows of clear, flat, dead keratinocytes.
    • Situated above the stratum granulosum.

  5. Stratum Corneum:

    • Most superficial layer composed of 20-30 layers of dead cells.
    • Cells are essentially flat membranous sacs filled with keratin and glycolipids in the extracellular space.
  • Cell Life Cycle in Epidermis:
    • Cells undergo controlled cell death (apoptosis);
    • Dead cells are sloughed off as dandruff and dander.
    • Humans can shed approximately 50,000 cells every minute.

Dermis Structure

  • Composed of two layers:

    1. Papillary Layer:
    • Superficial layer made of areolar connective tissue with loose fibers for patrolling microorganisms.
    • Contains dermal papillae that project into the epidermis, which house capillary loops and touch receptors (Meissner's corpuscles).
    • In thick skin, dermal papillae form friction ridges that enhance grip and contribute to fingerprint patterns.
    1. Reticular Layer:
    • Accounts for about 80% of dermal thickness and consists of dense irregular connective tissue.
    • Contains many elastic fibers for stretch-recoil properties and collagen fibers for strength and hydration through a network of blood vessels called the cutaneous plexus.
    • Contains flexure lines at joints due to dermal folds.
    • Cleavage Lines:
      • Formed by parallel collagen fibers.
      • Important for surgical incisions as they heal better when made parallel to cleavage lines.

Skin Color

  • Three pigments contribute to skin color:
    1. Melanin:
    • The only pigment produced in the skin by melanocytes; has varieties ranging from reddish-yellow to brownish-black.
    • Sun exposure increases melanin production, which serves to protect skin DNA from UV radiation damage.
    • Local accumulations of melanin appear as freckles or pigmented moles.
    1. Carotene:
    • Yellow to orange pigment most evidently observed in palms and soles; can be converted to vitamin A.
    1. Hemoglobin:
    • Provides a pinkish hue in fair skin; skin transparency affects its visibility.

Epidermal Derivatives

  • Hair, Nails, Sweat Glands, and Sebaceous (oil) glands) are classified as epidermal derivatives:

Hair

  • Consists of dead keratinized cells; absent on palms, soles, lips, nipples, and parts of external genitalia.
  • Functions of hair include:
    • Alerts to insects crawling on the skin.
    • Provides protection against physical trauma.
    • Reduces heat loss.
    • Shields the skin from sunlight.
  • Structure of Hair:
    • Hair consists of:
    • Shaft: Extends above the scalp (completed keratinization).
    • Root: Area within the scalp (keratinization ongoing).
    • Medulla: Central core of large cells and air spaces.
    • Cortex: Several layers of flattened cells surrounding the medulla.
    • Cuticle: Outermost layer of overlapping cells.
  • Hair color is determined by the presence of different types of melanins produced in hair follicles.
  • Types of Hair:
    • Vellus Hair: Fine, pale hair of children and women.
    • Terminal Hair: Coarse, long hair found on scalp and eyebrows, appearing at puberty in certain areas.

Nails

  • Scale-like modifications of epidermis containing hard keratin.
  • Protect the dorsal surfaces of fingers and toes.
  • Components of Nails:
    • Nail matrix: Area responsible for nail growth.
    • Nail folds: Overlapping skin borders of the nail.
    • Eponychium: Also called cuticle.
    • Hyponychium: Area under free edge that accumulates dirt.
  • Abnormal nail properties can indicate underlying diseases.

Sweat Glands

  • Called sudoriferous glands; found all over the body except nipples and external genitalia. There are about 3 million sweat glands per person.
  • Two types:
    1. Eccrine (Merocrine) Sweat Glands:
    • Most numerous and important for thermoregulation.
    • Ducts connect to pores; secretion is primarily water and salts, along with other substances.
    1. Apocrine Sweat Glands:
    • Found in axillary and anogenital areas; larger than eccrine glands.
    • Their secretion contains fatty substances and proteins, leading to body odor after bacterial breakdown.

Functions of Skin

  • Primary functions of the skin include:
    • Protection: Guards against microorganisms, abrasions, temperature extremes, and chemicals.
    • Body Temperature Regulation: Maintains a stable internal temperature.
    • Cutaneous Sensations: Allows for perception of stimuli.
    • Metabolic Functions: Synthesizes vitamin D.
    • Blood Reservoir: Holds about 5% of body's blood.
    • Excretion of Wastes: Eliminates waste products through sweat.

Clinical Homeostatic Imbalance

  • Alterations in skin color can indicate various diseases:
    • Cyanosis: Blue skin color due to low oxygen in hemoglobin.
    • Pallor: Pale skin indicative of anemia or low blood pressure.
    • Erythema: Redness from fever, hypertension, or inflammation.
    • Jaundice: Yellow cast associated with liver disorders.
    • Bruises: Indicators of clotted blood beneath skin, changing color during healing.