The Integumentary System

Overview

The integumentary system is the body's largest organ system, primarily composed of the skin and its derivatives. It serves as a protective barrier, regulates temperature, and provides sensory information.

• Two main regions:

  • Epidermis: Superficial region, composed of epithelial tissue.

  • Dermis: Deeper connective tissue region.

• Hypodermis: Subcutaneous layer deep to the skin, not part of the skin but shares some functions (mainly adipose tissue for shock absorption and insulation).

Structure of the Skin

Layers of the Skin

• Epidermis: Keratinized stratified squamous epithelium.

• Dermis: Strong, flexible connective tissue with blood vessels, nerves, and glands.

• Hypodermis: Mostly adipose tissue (not technically part of the skin).

Thin vs. Thick Skin

Cells of the Epidermis

• Keratinocytes: Produce the fibrous protein keratin; tightly connected by desmosomes.

• Melanocytes: Synthesize the pigment melanin; protect against UV damage.

• Dendritic (Langerhans) cells: Macrophages that activate the immune system.

• Tactile (Merkel) cells: Sensory touch receptors.

Layers of the Epidermis

Stratum Basale (Basal Layer)

• Deepest epidermal layer; single row of stem cells.

• Cells undergo rapid division (mitosis).

• Contains melanocytes (10–25% of cells in this layer).

Stratum Spinosum (Prickly Layer)

• Several layers thick.

• Abundant melanosomes and dendritic cells.

Stratum Granulosum (Granular Layer)

• Thin; four to six cell layers.

• Cells flatten, organelles disintegrate, and keratinization begins.

• Cells above this layer die due to lack of nutrients.

Stratum Lucidum (Clear Layer)

• Only in thick skin.

• Few rows of flat, dead keratinocytes.

Stratum Corneum

• 20–30 rows of dead, flat, anucleate keratinized membranous sacs.

• Protects deeper cells from environment and water loss.

• Barrier against biological, chemical, and physical assaults.

Dermis

Structure and Function

• Strong, dense irregular connective tissue.

• Contains fibroblasts, macrophages, and sometimes mast cells and white blood cells.

• Richly supplied with blood vessels, nerves, and lymphatics.

• Contains hair follicles, oil, and sweat glands.

Layers of the Dermis

• Papillary Layer: Areolar connective tissue with collagen and elastic fibers; contains dermal papillae (superficial projections).

• Reticular Layer: Dense irregular connective tissue; 80% of dermal thickness; provides strength and resiliency.

Dermal Papillae

• Superficial projections containing capillary loops, Meissner's corpuscles (touch), and free nerve endings (pain, temperature).

• In thick skin, form dermal ridges that create fingerprints.

Skin Color

Pigments Contributing to Skin Color

• Melanin: Only pigment made in skin; ranges from yellow to brown to black.

• Carotene: Yellow to orange pigment, most obvious in palms and soles.

• Hemoglobin: Pinkish hue due to oxygenated blood in dermal capillaries.

Melanin

• Produced in melanocytes and transferred to keratinocytes.

• Protects DNA from UV radiation.

• Freckles and pigmented moles are local accumulations of melanin.

• Sun exposure stimulates melanin production.

Skin Color in Diagnosis

• Cyanosis: Blue skin color due to low oxygenation.

• Erythema: Redness due to fever, hypertension, inflammation, or allergy.

• Pallor: Pale skin due to anemia, low blood pressure, fear, or anger.

• Jaundice: Yellow skin due to liver disorder.

• Bruises: Black-and-blue marks due to clotted blood beneath skin.

Appendages of the Skin

Hair and Hair Follicles

• Dead keratinized cells of hard keratin.

• Functions: sense insects, guard scalp, protect from heat loss, shield skin from sunlight.

• Hair pigments: melanins (yellow, rust, brown, black); trichosiderin in red hair; decreased melanin and increased air bubbles in gray/white hair.

Nails

• Scale-like modifications of the epidermis.

• Protective cover for distal, dorsal surface of fingers and toes.

• Contain hard keratin; nail matrix responsible for growth.

Sweat Glands

• Also called sudoriferous glands; two main types:

• Eccrine (merocrine) sweat glands: Most numerous; abundant on palms, soles, and forehead; ducts connect to pores; function in thermoregulation.

• Apocrine sweat glands: Confined to axillary and anogenital areas; ducts empty into hair follicles; begin functioning at puberty; may act as sexual scent glands.

Sebaceous (Oil) Glands

• Widely distributed except in thick skin of palms and soles.

• Most develop from hair follicles and secrete into hair follicles.

• Secrete sebum (oily holocrine secretion) that softens hair and skin.

Skin Cancer

General Information

• Most skin tumors are benign (do not metastasize).

• Risk factors: overexposure to UV radiation, frequent irritation of skin.

Basal Cell Carcinoma

• Most common, least malignant skin cancer.

• Stratum basale cells proliferate and invade dermis and hypodermis.

• Cured by surgical excision in 99% of cases.

Additional info: The notes above include inferred details for completeness, such as the functions of the hypodermis, the role of dermal papillae in fingerprints, and the clinical significance of skin color changes.