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The Integumentary System: Structure, Function, and Clinical Relevance

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The Integumentary System

The Skin and the Hypodermis

The integumentary system consists primarily of the skin, our largest organ, and the hypodermis. Together, they provide protection, regulate body temperature, and perform several other vital functions.

  • Skin

    • Accounts for approximately 7% of body weight.

    • Varies in thickness from 1.5 to 4.4 mm.

    • Divided into two distinct layers: Epidermis (superficial) and Dermis (deep).

  • Hypodermis

    • Lies deep to the dermis.

    • Composed of areolar and adipose connective tissues.

    • Not officially part of the integumentary system but shares some skin properties.

Functions of the Skin and Hypodermis

  1. Protection: Cushions organs and protects from bumps, chemicals, water loss, and ultraviolet (UV) radiation.

  2. Body Temperature Regulation: Capillary network and sweat glands regulate heat loss.

  3. Excretion: Removal of urea, salts, and water through sweat.

  4. Production of Vitamin D: Epidermal cells use UV radiation to synthesize vitamin D.

  5. Sensory Reception: Contains sense organs associated with nerve endings.

Epidermis

Structure and Cell Types

The epidermis is a keratinized stratified squamous epithelium, containing four main cell types:

  • Keratinocytes

    • Most abundant cell type.

    • Arise from the deepest layer of the epidermis.

    • Produce keratin, a tough fibrous protein.

    • Produce antibiotics and enzymes.

    • Are dead at the skin's surface.

  • Melanocytes

    • Located in the basal layer.

    • Manufacture and secrete pigment melanin.

  • Tactile epithelial cells

    • Associated with sensory nerve endings.

  • Dendritic cells

    • Part of the immune system; macrophage-like.

    • Located in the stratum spinosum.

Layers of the Epidermis

The epidermis is organized into distinct layers, each with specialized functions:

  • Stratum basale (germinativum):

    • Deepest layer; attached to underlying dermis.

    • Cells actively divide.

    • Contains tactile epithelial cells and melanocytes.

  • Stratum spinosum (spiny layer):

    • Contains intermediate filaments (tonofilaments).

    • Protein pre-keratin is present.

    • Contains dendritic cells.

  • Stratum granulosum:

    • Consists of a few layers of keratinocytes.

    • Keratinocytes contain keratohyaline granules (help form keratin) and lamellar granules (contain waterproofing glycolipid).

  • Stratum lucidum (clear layer):

    • Occurs only in thick skin (palms, soles).

    • Composed of a few rows of flat, dead keratinocytes.

  • Stratum corneum (horny layer):

    • Thick layer of dead keratinocytes and thickened plasma membranes.

    • Protects skin against abrasion and penetration.

Dermis

Structure and Layers

The dermis is a strong, flexible connective tissue layer richly supplied with blood vessels and nerves. It has two main layers:

  • Papillary Dermis

    • Includes dermal papillae, increasing surface area for exchange of gases, nutrients, and wastes.

    • Dermal papillae lie on dermal ridges (epidermal ridges), which form fingerprints and increase gripping ability.

  • Reticular Dermis

    • Accounts for 80% of dermal thickness.

    • Dense irregular connective tissue with collagen fibers for strength.

    • Contains flexure lines (creases in palms, wrists, soles, fingers, toes).

    • Has two vascular plexuses: dermal and subpapillary.

    • Blood vessels play a role in temperature regulation.

Hypodermis

Structure and Function

The hypodermis, also called superficial fascia, lies deep to the skin and consists of areolar and adipose connective tissue. It anchors skin to underlying structures, insulates the body, and has different distribution in males and females.

Skin Color

Pigments and Determinants

Three pigments contribute to skin color:

  • Melanin: Most important pigment, made from tyrosine.

  • Carotene

  • Hemoglobin: Allows crimson color of blood to show through.

Nails

Structure and Function

Nails are scalelike modifications of the epidermis, made of hard keratin. They protect the distal phalanx and aid in grasping.

  • Parts of the nail:

    • Free edge

    • Nail plate

    • Root

    • Nail folds

    • Eponychium (cuticle)

Appendages of the Skin

Hair and Hair Follicles

Hair is a flexible strand of dead, keratinized cells. It consists of:

  • Root: Embedded in the skin.

  • Shaft: Projects above the skin's surface.

Three concentric layers of keratinized cells:

  • Medulla: Central core.

  • Cortex: Surrounds medulla.

  • Cuticle: Outermost layer.

Hair Follicles

  • Extend from epidermis into dermis.

  • Hair bulb: Deep, expanded end of the follicle.

  • Root plexus: Knot of sensory nerves around hair bulb.

  • Wall of follicle:

    • Peripheral connective tissue sheath

    • Glassy membrane

    • Epithelial root sheath

  • Arrector pili muscle: Bundle of smooth muscle; hair stands erect when it contracts.

Types and Growth of Hair

  • Vellus hairs: Body hairs of women and children.

  • Terminal hairs: Scalp, axillary, and pubic regions.

  • Hair thinning and baldness: Due to aging and male pattern baldness.

Glands of the Skin

Sebaceous Glands

  • Occur over entire body except palms and soles.

  • Secrete sebum (an oily substance) via simple alveolar glands.

  • Most are associated with a hair follicle.

  • Functions: Collects dirt, softens, and lubricates hair and skin.

Sweat Glands

  • Widely distributed over the body.

  • Sweat is a blood filtrate:

    • 99% water with some salts.

    • Contains traces of metabolic wastes (urea, ammonia, uric acid).

  • Two types:

    • Eccrine gland: Most numerous, produce true sweat.

    • Apocrine gland: Confined to axillary, anal, and genital areas; produce a special kind of sweat (may attract mates).

  • Other glands:

    • Ceruminous glands

    • Mammary glands

Burns

Classification by Severity

  • First-degree burn: Only upper epidermis is damaged.

  • Second-degree burn: Upper part of dermis is also damaged; blisters appear.

  • Third-degree burn: Consumes thickness of skin; skin appears white, red, or blackened.

Skin Cancer

Types and Characteristics

  • Basal cell carcinoma: Least malignant and most common.

  • Squamous cell carcinoma: Arises from keratinocytes of stratum spinosum.

  • Melanoma: Cancer of melanocytes; most dangerous type.

The Skin Throughout Life

Developmental Changes

  • Epidermis: Develops from embryonic ectoderm.

  • Dermis and hypodermis: Develop from mesoderm.

  • Melanocytes: Develop from neural crest cells.

  • Fetal skin:

    • Well formed after the fourth month.

    • At 5-6 months, fetus is covered with lanugo (downy hairs).

    • Fetal sebaceous glands produce vernix caseosa.

  • Middle to old age:

    • Skin thins and becomes less elastic.

    • Shows harmful effects of environmental damage.

    • Skin inflammations become more common.

Summary Table: Layers of the Epidermis

Layer

Main Features

Location

Stratum basale

Deepest, mitotically active, contains melanocytes and tactile cells

Base of epidermis

Stratum spinosum

Contains intermediate filaments, dendritic cells

Above stratum basale

Stratum granulosum

Keratohyaline and lamellar granules, waterproofing

Middle layer

Stratum lucidum

Clear, only in thick skin

Palms, soles

Stratum corneum

Thick, dead keratinocytes, protection

Surface layer

Key Equations

  • Vitamin D Synthesis:

Example: Clinical Application

Burns are classified by the depth of skin damage. For example, a second-degree burn damages both the epidermis and part of the dermis, often resulting in blister formation and increased risk of infection.

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.

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