Integumentary System

The integumentary system consists of the skin, hair, nails, and glands, playing a crucial role in protection, sensation, and regulation of body temperature.

  • The term integument is derived from the Latin word integumentum, meaning "a covering."

    • The integumentary system comprises the skin and its accessory structures.

    • The terms "integumentary system" and "skin" are often interchangeable.

    • Accessory structures, also called specialized derivatives, include hair, nails, and skin glands.

    • The skin, being made of various structures participating in related functions, is considered an organ and is the largest organ in the human body by mass.

Epidermis

  • The epidermis is the top layer of skin facing the external environment.

    • It is composed of keratinized stratified squamous epithelium.

    • Keratinocytes produce and store keratin, a protein giving hardness and water resistance to skin, hair, and nails.

    • The term stratified indicates multiple cell layers, while squamous describes flattened cell shape.

    • Epithelial tissue forms a barrier between internal and external environments, regulating substance flow in and out of the body.

    • The epidermis consists of four to five layers, depending on the anatomical location.

Stratum Corneum

  • The outermost layer of the epidermis is the stratum corneum.

    • Composed of dried, keratinized cells that slough off and are replaced by underlying cells.

Stratum Lucidum

  • Found in the palms of the hands and soles of the feet, the stratum lucidum is a clear, thin, translucent layer.

Stratum Granulosum

  • The stratum granulosum lies beneath the stratum lucidum in palms and soles, and directly under the stratum corneum in other skin areas.

Stratum Spinosum

  • The stratum spinosum underlies the stratum granulosum and contains Langerhans cells that break down foreign particles and damaged cells.

Stratum Basale

  • The innermost layer of the epidermis is the stratum basale.

    • Composed of basal cells that divide to produce keratinocytes, pushing these cells toward the surface.

    • Contains melanocytes, responsible for skin and hair pigmentation, and Merkel cells, which act as touch receptors.

Dermis

  • The dermis lies below the epidermis and is composed of connective tissue, providing skin strength and structure.

    • It contains blood and lymph capillaries, delivering oxygen and nutrients to the skin while removing waste.

    • The dermis is divided into two layers:

Papillary Dermis

  • The upper layer, known as the papillary dermis, forms small projections into the stratum basale of the epidermis.

    • Responsible for unique skin patterns like fingerprints and footprints due to its collagen forming loose connective tissue.

Reticular Dermis

  • The lower layer, termed the reticular dermis, is primarily made of collagen, elastin, and reticular fibers, forming dense connective tissue.

    • Contains hair follicles, sebaceous glands, and sweat glands.

Exocrine Glands

  • Sebaceous and sweat glands are classified as exocrine glands, which secrete products externally.

Sebaceous Glands

  • Sebaceous glands consist of cells producing sebum, an oily substance that lubricates hair and adds water resistance to the skin.

    • Most sebaceous glands empty into hair follicles leading to the skin's surface.

    • Present in the dermis throughout the body, excluding palms and soles.

Function and Variants of Sebum

  • Sebum serves specialized functions in various body locations:

    • In the eye, it combines with other secretions to maintain lubrication and cleanliness.

    • In the ear canal, it contributes to cerumen, or earwax.

    • During fetal development, sebum builds up on the skin contributing to vernix caseosa.

    • Sebaceous glands are hormonally responsive, with activity increasing during adolescence, contributing to conditions such as acne and seborrheic dermatitis.

Sweat Glands

  • There are two primary types of sweat glands:

    • Eccrine Sweat Glands:

    • Secretes a thin, watery, salty solution known as sweat.

    • Ducts open at the skin surface across various body locations.

    • Sweat is produced in response to emotional stress or temperature fluctuations.

    • Upon reaching the skin surface, much salt is reabsorbed, resulting in sweat with approximately half the salt concentration of plasma.

    • This reabsorption is notable in cystic fibrosis patients, leading to excessive saltiness in skin.

    • Apocrine Sweat Glands:

    • Secretes a more oily substance that mixes with sebum as it exits to the skin's surface.

    • Largely inactive until puberty, with similar hormonal responsiveness as sebaceous glands.

    • Although initially odorless, apocrine sweat can combine with skin bacteria, producing body odor.

    • Mammary glands are often considered specialized apocrine glands modified to produce milk.

Subcutaneous Tissue

  • The subcutaneous tissue, also referred to as the hypodermis, is situated beneath the dermis.

    • Composed primarily of adipose tissue, it serves functions in fat storage, cushioning, and insulative properties for the body.

Temperature Regulation

  • The human body independently generates and regulates its heat.

    • Heat is produced mainly through metabolism and muscle activity, including exercise and shivering.

    • Heat transfer occurs through radiation, conduction, convection, or evaporation.

    • The skin is crucial for maintaining steady body temperature:

    • Blood Vessels in the dermis can dilate to increase blood flow near the skin surface for heat release or constrict to conserve heat by retaining blood within the body.

    • The production of sweat from eccrine glands facilitates heat loss via evaporation.

Piloerection

  • Piloerection, also known colloquially as goosebumps, occurs through the contraction of the arrector pili muscle, linked to hair follicles.

    • Standing hairs can trap warm air close to the skin, though this effect is less significant in humans with reduced body hair.