Integumentary System: Structure, Function, and Clinical Relevance

A) Integumentary System: Overview

The integumentary system is the body's outer covering, providing protection and sensory functions. It consists of several components that work together to maintain homeostasis and defend against environmental hazards.

• Skin

• Hair, nails, glands

• Muscles

• Sense receptors (nervous)

• Hypodermis

B) Skin - Layers

The skin is composed of two main layers, each with distinct structures and functions. Below the skin lies the hypodermis, which is not technically part of the skin but is closely associated with it.

1. Epidermis

• Definition: The superficial (outermost) layer of the skin.

• Composed of stratified squamous epithelium.

• Avascular (lacks blood vessels).

• Separated into 4–5 strata (sub-layers) based on cell types and location.

a) Stratum Basale

• Single row of cells at the base of the epidermis.

• Contains two main cell types:

  • Keratinoctyes (90%):

    • Actively undergo mitosis (cell division).

    • Produce keratin (a tough, protective protein).

    • New cells are pushed upwards as more are produced.

  • Melanocytes (10%):

    • Produce melanin, a pigment that protects against UV light.

    • Melanin granules are transferred to keratinocytes.

    • Skin color is determined by the type and amount of melanin produced.

    • All humans have the same number of melanocytes, but produce different amounts/types of melanin.

    • Blood (hemoglobin) in dermal capillaries can give a pinkish tint to fair-skinned people.

b) Stratum Spinosum

• Several layers of keratinocytes with low levels of mitosis.

c) Stratum Granulosum

• Cells begin to die; keratinization increases.

d) Stratum Lucidum

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

• Flat, dead cells (no nuclei or organelles).

e) Stratum Corneum

• Many layers of flat, dead keratinized cells.

• Glycolipids between cells create a waterproof barrier.

• Cells are shed and replaced from below.

2. Dermis

• Definition: The deep layer of the skin, located beneath the epidermis.

• Contains blood vessels, nerves, glands, hair follicles, and arrector pili muscles.

• Composed of two sub-layers of connective tissue:

  • Papillary Layer (superficial):

    • Made of areolar connective tissue.

    • Contains dermal papillae that project into the epidermis, forming fingerprints and improving grip.

    • Contains capillaries and sensory receptors.

  • Reticular Layer (deep):

    • Dense irregular connective tissue.

    • Provides strength and elasticity to the skin.

C) Thin vs. Thick Skin

Skin is classified as either thin or thick based on the number of epidermal layers and the presence of certain structures.

• Thin Skin:

  • Covers most of the body.

  • Contains hair follicles, sebaceous glands, and arrector pili muscles.

  • Lacks the stratum lucidum.

• Thick Skin:

  • Found on the palms of the hands and soles of the feet.

  • Contains all five epidermal layers, including the stratum lucidum.

  • Lacks hair follicles, sebaceous glands, and arrector pili muscles.

D) Epidermal Derivatives

Structures derived from the epidermis include hair, nails, and various glands. These play important roles in protection, sensation, and homeostasis.

1. Hair

• Composed of dead, keratinized cells.

• Parts of hair:

  • Root: Part embedded within the skin.

  • Shaft: Visible part above the skin surface.

  • Hair follicle: Surrounds the root.

• Associated structures:

  • Outer CT sheath: Holds follicle in place (from dermis).

  • Hair papilla: Contains blood supply for growing hair (from dermis).

  • Root hair plexus: Free nerve ending for touch.

  • Sebaceous (oil) gland: Opens into follicle.

  • Arrector pili muscle: Smooth muscle causing "goosebumps".

2. Nails

• Very heavily keratinized epidermal cells.

• Consist of:

  • Root: Embedded in skin.

  • Body: Visible portion.

  • Free edge: Extends past the finger or toe.

3. Skin Exocrine Glands

• Sebaceous glands:

  • Connected mainly to hair follicles.

  • Secrete sebum (oil) to soften and lubricate hair and skin.

  • Antibacterial properties.

• Sudoriferous (sweat) glands:

  • Secrete portions in dermis of thick and thin skin.

  • Types:

    • Eccrine: Most numerous, open onto skin surface, function in temperature regulation.

    • Apocrine: Open into hair follicles, found in axilla and groin, produce odor.

  • Sweat contains water, electrolytes, urea.

• Ceruminous glands:

  • Modified sweat glands in the ear canal.

  • Produce wax (cerumen) to prevent insects and debris from entering.

• Mammary glands:

  • Modified sweat glands that produce milk.

E) Cutaneous Sense Receptors

Specialized cells or structures in the skin that detect and transmit sensory information to the nervous system.

• Touch Receptors: Free nerve endings in the epidermis and dermis, sensitive to light touch.

• Pressure Receptors:

  • Encapsulated nerve endings (e.g., Lamellar/Pacinian corpuscles) found deep in the dermis or hypodermis.

• Thermoreceptors: Free nerve endings for temperature detection.

• Nociceptors: Free nerve endings for pain. Third-degree burns can destroy these, resulting in loss of pain sensation in affected areas.

F) Hypodermis

The hypodermis, also known as the subcutaneous layer or superficial fascia, is not technically part of the skin but is closely associated with it.

• Composed mainly of adipose (fat) tissue and loose connective tissue.

• Functions as insulation and energy storage.

• Anchors the skin to underlying structures such as muscles and bones.

• Contains about 1/2 of the body's adipose tissue.

G) Related Medical Conditions

Certain disorders are associated with the skin and its components, often due to genetic or autoimmune causes.

• Albinism: Genetic condition characterized by lack of melanin production by melanocytes, resulting in very light skin, hair, and eyes.

• Psoriasis: Autoimmune disorder causing accelerated mitosis of keratinocytes in the stratum basale, leading to thick, scaly plaques on the skin surface.

• Additional info: Other common skin conditions include eczema, dermatitis, and skin cancers (e.g., basal cell carcinoma, squamous cell carcinoma, melanoma).