The Integumentary System: Structure and Function

The Integumentary System

Overview and Primary Functions

The integumentary system consists of the skin, hair, nails, and glands (sweat and sebaceous). Its primary function is protection of the body from external threats and regulation of internal conditions.

• Thermoregulation: Regulates body temperature via sweat and blood flow.

• Sensation: Detects environmental changes through nerve endings in the skin.

• Vitamin D Synthesis: Produces vitamin D when exposed to UV light.

• Excretion: Removes waste products through sweat.

• Water Resistance: Prevents dehydration by limiting water loss.

Skin Structure

Superficial Epidermis and Deep Dermis

The skin is divided into two main layers: the epidermis (outer layer) and the dermis (deeper layer). Each has distinct structures and functions.

• Epidermis: The superficial, outer layer made mostly of epithelial cells. It provides a protective barrier and is avascular (lacks blood vessels).

• Dermis: The deeper, inner layer made mostly of connective tissue. It contains blood vessels, nerves, hair follicles, and glands, and provides structural support and nourishment to the epidermis.

Major Cell Types in the Epidermis

There are four major cell types in the epidermis, each with specialized functions:

• Keratinoctyes: Produce keratin, a tough, fibrous protein that provides strength and waterproofing for the skin. They form most of the epidermis.

• Melanocytes: Produce the pigment melanin, which gives skin its color and protects underlying cells from ultraviolet (UV) radiation damage.

• Langerhans Cells: Act as immune cells by detecting pathogens and initiating immune responses to protect the skin from infection.

• Merkel Cells: Function as sensory receptors involved in the sensation of touch, especially in areas of high tactile sensitivity.

Layers of the Epidermis

The epidermis is organized into layers (strata). There are five layers in thick skin and four in thin skin:

1. Stratum corneum: Outermost, thickest layer of dead, flattened keratinized cells.

2. Stratum lucidum: Thin, clear layer found only in thick skin (e.g., palms, soles).

3. Stratum granulosum: Where cells start to die and accumulate keratohyalin granules.

4. Stratum spinosum: "Spiny" appearance due to desmosomes; active keratinocyte production.

5. Stratum basale (germinativum): Deepest layer, where new keratinocytes are formed from stem cells.

Note: The stratum lucidum is present only in thick skin (palms, soles).

Significance of the Stratum Basale

• Contains stem cells that continuously divide to produce new keratinocytes.

• These new cells move upward through the layers to replace older cells shed from the skin surface.

Thick vs. Thin Skin

• Thick skin: Found on soles of feet and palms of hands; contains all five epidermal layers.

• Thin skin: Covers most of the body; lacks the stratum lucidum.

Subcutaneous Layer (Hypodermis)

The skin rests on the subcutaneous layer (hypodermis), which is not technically part of the skin but connects it to underlying tissues.

• Adipose tissue: Stores fat, cushions the body, and insulates against heat loss.

• Areolar connective tissue: Provides elasticity and support.

Skin Color

Determinants of Skin Color

• Hemoglobin: Red pigment found in red blood cells; gives skin a pinkish hue.

• Carotene: Yellow-orange pigment found in orange/yellow vegetables and fruits (e.g., carrots, sweet potatoes, pumpkins); accumulates in the stratum corneum and fatty tissues.

• Melanin: Brown to black pigment produced by melanocytes; protects DNA in skin cells from UV radiation.

Melanin is stored in melanosomes and transferred to keratinocytes.

The Dermis

Structure and Layers

• Papillary layer: Composed of areolar connective tissue; forms dermal papillae that increase surface area for exchange of gases, nutrients, and waste products with the epidermis.

• Reticular layer: Composed of dense irregular connective tissue; provides strength and elasticity.

Functions of Hair

• Protection: Scalp hair protects from UV radiation and physical injury. Eyelashes and eyebrows protect the eyes from dust and sweat.

• Sensation: Hair follicles are surrounded by nerve endings that help detect light touch or movement.

• Thermoregulation: Hair helps retain heat by trapping a layer of air close to the skin.

Glands of the Skin

Sebaceous (Oil) Glands

• Produce sebum, an oily substance that lubricates and softens the skin and hair.

• Has antibacterial properties that protect the skin from infections.

• Are exocrine glands (use ducts to release contents).

Sweat (Sudoriferous) Glands

• Also exocrine glands; two major types:

  • Apocrine glands: Found mainly in the axillary (armpit) and areolar (nipple) regions, and in the groin and anal regions. Secrete a thicker, milky sweat that can produce body odor.

  • Eccrine glands: Widely distributed; produce watery sweat (primarily water and salts) for thermoregulation (cooling the body by evaporation).

Nails

Structure and Function

• Composed of keratinized dead cells (keratin).

• Located at the ends of fingers and toes.

• Functions:

  • Protect the distal tips of fingers and toes from injury.

  • Enhance fine touch and manipulation by providing support and counterpressure to the fingertips.

  • Assist in scratching and picking up small objects.