Basic Anatomy: Parts and Structure of the Integumentary System

The integumentary system consists of the skin and its associated structures: hair, nails, and various glands. The skin itself comprises two main layers—the epidermis and dermis—along with the underlying hypodermis that connects skin to deeper structures.

The epidermis, the outermost layer, consists entirely of epithelial tissue with no blood vessels of its own. This layer varies in thickness from paper-thin on the eyelids to several millimeters thick on the palms and soles. The epidermis contains five distinct sublayers in thick skin areas, though only four are present in thin skin covering most of the body.

The stratum basale, the deepest epidermal layer, contains actively dividing cells that give rise to all other epidermal cells. These basal cells are cuboidal to columnar in shape and attach to the basement membrane separating epidermis from dermis. This layer also contains melanocytes—cells that produce melanin pigment responsible for skin color and UV protection.

The stratum spinosum, or "spiny layer," consists of several layers of cells connected by desmosomes—protein structures that give cells a spiny appearance under microscopic examination. These cells begin producing keratin, the tough protein that will eventually make up the skin's protective surface. Langerhans cells in this layer serve immune functions by detecting and responding to foreign substances.

The stratum granulosum contains flattened cells filled with keratohyalin granules that help bind keratin fibers together. Cells in this layer begin dying as they move farther from their nutrient supply in the dermis. Lamellar granules release lipids that help waterproof the skin and create the barrier that prevents excessive water loss.

The stratum lucidum appears as a clear, thin layer found only in thick skin areas like palms and soles. This layer consists of flattened, dead cells packed with keratin, providing extra protection in areas subject to significant wear and friction.

The stratum corneum, the outermost layer, consists of 20-30 layers of dead, flattened cells filled with keratin and surrounded by lipids. These cells, called corneocytes, are constantly shed from the surface and replaced by cells from deeper layers. This layer provides the primary barrier function of skin, protecting against physical damage, chemical exposure, and water loss.

The dermis, located beneath the epidermis, consists of connective tissue containing collagen and elastin fibers that provide strength, flexibility, and elasticity. The dermis houses blood vessels, nerve endings, hair follicles, and glands, making it the metabolically active portion of skin. This layer is divided into two regions with different characteristics.

The papillary dermis, the upper portion, consists of loose connective tissue forming finger-like projections (dermal papillae) that interlock with corresponding ridges in the epidermis. This arrangement increases the surface area for nutrient and waste exchange while creating the unique patterns we see as fingerprints. The papillary dermis contains many nerve endings responsible for light touch sensation.

The reticular dermis, the deeper and thicker portion, consists of dense irregular connective tissue containing large bundles of collagen fibers. This layer provides most of the skin's strength and houses larger blood vessels, nerve trunks, hair follicles, and glands. Elastic fibers throughout the reticular dermis allow skin to stretch and return to its original shape.

The hypodermis, also called subcutaneous tissue, lies beneath the dermis and consists primarily of adipose (fat) tissue and loose connective tissue. While not technically part of the skin, this layer insulates the body, cushions underlying structures, and provides energy storage. The thickness of the hypodermis varies greatly among individuals and body locations.

Hair follicles extend from the epidermis down into the dermis or hypodermis, producing hair shafts that project above the skin surface. Each follicle consists of multiple layers of cells that divide rapidly to produce the hair shaft. Associated with each follicle is a sebaceous gland that produces oil (sebum) to lubricate the hair and skin.

Nails consist of tightly packed, keratinized cells that form protective coverings over fingertips and toes. The nail body is the visible portion, while the nail root extends beneath the skin. The nail matrix, located beneath the cuticle, contains dividing cells that produce new nail material, causing nails to grow continuously throughout life.

Sweat glands exist in two types with different functions and distributions. Eccrine sweat glands are found throughout the body and produce the watery sweat that helps regulate body temperature. Apocrine sweat glands, located primarily in the armpits and groin, produce a thicker secretion that bacteria can break down to create body odor.