Integumentary System Practice Flashcards

Overview of the Integumentary System

The integumentary system is a vital biological system comprising the skin and its various derivatives, which include hair, nails, and glands. The term integumentary is derived from the Latin roots "in," meaning inward, and "tegere," meaning to cover. Collectively, these components form a protective barrier for the body. Dermatology is the specific medical specialty dedicated to the diagnosis and treatment of disorders affecting the integumentary system.

The skin itself is categorized as the largest organ of the human body. An organ is defined as a structure composed of two or more types of tissues that work together to perform a specialized function. In the case of the skin, it serves as the largest sense organ in the body. Physically, the skin ranges in thickness from approximately $1\,mm$ to $4\,mm$. It covers a total surface area of about $2\,m^2$, which is equivalent to $22\,square\,feet$. In terms of mass, the skin weighs between $4.5\,kg$ and $5\,kg$, or approximately $10\,to\,11\,lbs$.

Classification of Epithelial Membranes

There are three distinct types of epithelial membranes relevant to the study of the human body: serous membranes, mucous membranes, and the cutaneous membrane. Serous membranes line internal body cavities and cover visceral organs. They consist of simple squamous epithelium situated over a layer of loose connective tissue and are divided into parietal portions, which line cavity walls, and visceral portions, which cover the organs themselves. These membranes secrete a watery serous fluid for lubrication; specific examples include the peritoneal, pleural, and pericardial membranes.

Mucous membranes line body cavities that are open to the exterior environment. These consist of a layer of epithelium over connective tissue and contain specialized goblet cells that secrete mucus to keep the surfaces moist and protected. The cutaneous membrane is the skin itself. It covers the entire exterior of the body and serves as the primary organ of the integumentary system.

Cellular Composition of the Epidermis

The epidermis is the most superficial layer of the skin and is composed of stratified keratinized squamous epithelium. The thickness of this layer varies across different regions of the body, reaching its maximum thickness on the palms of the hands and the soles of the feet. This layer is avascular, meaning it contains no blood vessels, and it lacks nerve endings. The deepest layers of the epidermis are bathed in interstitial fluid originating from the dermis, which provides necessary oxygen and nutrients while draining away waste as lymph.

The epidermis contains four primary cell types. Keratinocytes constitute $90\%$ of the epidermal cells. They are the deepest cells and produce keratin, a tough fibrous protein. Melanocytes make up $8\%$ of the epidermal cells and are responsible for producing the dark skin pigment known as melanin. Merkel cells are associated with sensory nerve endings and are essential for sensation. Langerhans cells are macrophage-like dendritic cells that serve an immune function. The epidermis is organized into several layers or strata, extending from the deepest germinative layer to the superficial stratum corneum. As cells move toward the surface, they undergo keratinization, a process where keratin accumulates in the cells, they lose their nuclei, and they eventually die, becoming flat "squames."

Detailed Functions of Epidermal Cells

A keratinocyte is a specialized cell that manufactures and stores the protein keratin. Keratin is an intracellular fibrous protein that provides hardness, strength, and water-resistant properties to the hair, nails, and skin. The primary function of keratinocytes is to form a barrier against environmental damage, including threats from pathogenic bacteria, fungi, parasites, viruses, heat, ultraviolet (UV) radiation, and water loss. Keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by newer cells from deeper layers.

Melanocytes are neural crest-derived cells located in the deepest layer of the epidermis, the stratum basale. These cells produce melanin, the dark pigment primarily responsible for skin color. Once synthesized via a process called melanogenesis, melanin is stored in organelles called melanosomes. These organelles are transported to nearby keratinocytes to induce pigmentation. The purpose of melanogenesis is to protect the hypodermis from UV-B radiation. Melanin is black, allowing it to absorb and block the majority of UV-B light. While light-skinned individuals have lower basal levels of melanogenesis, exposure to UV-B radiation triggers increased production.

Merkel cells are oval-shaped mechanoreceptors essential for light touch sensation. They are found in the deepest layer of the epidermis and are notably abundant in highly sensitive skin areas, such as the fingertips, while being scarce in other regions. Langerhans cells are antigen-presenting immune cells or dendritic cells. They are present in all layers of the epidermis but are most prominent in the stratum spinosum. They also appear in the papillary dermis and the mucosa of the mouth, functioning similarly to macrophages in their morphology and immune response.

The Strata of the Epidermis

The epidermis is organized into five distinct layers, listed here from the deepest to the most superficial. The Stratum Basale, also known as the stratum germinativum, is the deepest layer, consisting of a single layer of cuboidal to columnar basal cells. These are stem cells that serve as precursors to keratinocytes. This layer is constantly undergoing mitosis to produce new cells, which are then pushed superficially. This layer also contains Merkel cells and melanocytes.

The Stratum Spinosum is the next layer, consisting of $8\,to\,10$ layers of keratinocytes that appear spiny due to cell extensions. Langerhans cells are interspersed here to engulf bacteria and damaged cells. Above this is the Stratum Granulosum, consisting of $2\,to\,5$ layers of flattened keratinocytes. Keratinization begins in this layer, and cells begin to die. It contains dark keratohyalin granules and produces glycolipids that waterproof the skin.

The Stratum Lucidum is a smooth, translucent layer consisting of $3\,to\,5$ layers of clear, dead, flattened keratinocytes. This layer is found only in the thick skin of the palms, soles, and digits. Finally, the Stratum Corneum is the most superficial layer, exposed to the outside environment. It contains $15\,to\,30$ layers of dry, dead, keratinized cells (cornification). This layer prevents microbe penetration, dehydration, and provides mechanical protection. The entire epidermal layer is replaced every $4\,to\,6\,weeks$ ($30\,to\,40\,days$).

Anatomy and Physiology of the Dermis

The dermis is a tough and elastic layer formed from connective tissue. Its matrix contains collagen fibers interlaced with elastic fibers. Collagen fibers bind water to provide tensile strength, a capacity that declines with age, leading to wrinkles. Rupture of elastic fibers due to overstretching results in permanent striae, or stretch marks. The primary cells in the dermis are fibroblasts, macrophages, and mast cells. The dermis contains blood vessels, lymph vessels, sensory somatic nerve endings, sweat glands, hair follicles, and sebaceous glands.

The dermis is divided into two layers: the Papillary layer and the Reticular layer. The Papillary layer is the superficial layer made of loose areolar connective tissue. It features finger-like dermal papillae that project into the stratum basale. This layer contains fibroblasts, adipocytes, small blood vessels, phagocytes, lymphatic capillaries, nerve fibers, and Meissner corpuscles (touch receptors). In a fetus, the interface between the stratum basale and the papillary layer forms unique epidermal ridges known as fingerprints, which are permanent and used for forensic analysis.

The Reticular layer lies beneath the papillary layer and accounts for approximately $80\%$ of the dermis. It consists of dense irregular connective tissue that provides flexibility and resistance to multi-directional forces. It is well-vascularized with a rich sensory and sympathetic nerve supply. The name "reticular" refers to the net-like appearance of its tight meshwork of collagen and elastin fibers.

Subcutaneous Layer and Skin Variations

The Subcutaneous layer, also known as the hypodermis, is not technically part of the skin. It is composed of adipose and areolar connective tissue. Its functions include anchoring the skin to underlying structures like muscles, allowing the skin to slide freely, acting as a shock absorber and insulator, and storing fat. This layer thickens with weight gain.

Skin is classified into two types: thick and thin. Thick skin is found only on the palms and soles. It has a thick epidermis ($0.6\,to\,4.5\,mm$), a distinct stratum lucidum, a thick stratum corneum, and lacks hair follicles and sebaceous glands. Thin skin covers most of the body. It has a thinner epidermis ($0.07\,to\,0.15\,mm$), lacks a stratum lucidum and epidermal ridges, and possesses fewer sweat glands and sensory receptors.

Sudoriferous and Sebaceous Glands

Sebaceous (oil) glands consist of secretory epithelial cells that typically secrete an oily substance called sebum into hair follicles. They are present everywhere except the palms and soles and are most numerous on the scalp, face, axillae, and groins. In some areas, such as the eyelids, lips, and nipples, they secrete sebum directly onto the surface. Sebum softens hair, provides waterproofing, and acts as a bactericidal and fungicidal agent. Its production increases at puberty and decreases in old age, which can lead to maceration or skin cracking. Acne is a common clinical condition associated with these glands.

Sudoriferous (sweat) glands are divided into two types. Eccrine (merocrine) sweat glands are simple coiled tubular glands that open directly onto the skin surface. They function in thermoregulation and are found on the forehead, palms, and soles. Eccrine sweat contains water, salts ($Na^{+}$ and $Cl^{-}$), urea, amino acids, glucose, and lactic acid. They function from birth and respond to heat and emotional stress. Apocrine sweat glands are also coiled tubular glands but open into hair follicles. They are found in the axilla, groin, areola, and bearded face regions. Apocrine sweat is viscous and contains fatty acids and proteins. It is odorless until broken down by bacteria and begins functioning at puberty.

Two other specialized glands exist: Ceruminous glands are modified sweat glands in the external ear that secrete cerumen (ear wax), and Mammary glands are specialized for milk secretion. Sweat glands primarily regulate body temperature through heat loss via conduction, convection, evaporation, and radiation.

Anatomy and Growth of Hair

Hair is a protein filament that grows from follicles created by the downgrowth of epidermal cells into the dermis or subcutaneous tissue. At the base of the hair follicle is the papilla or bulb, where cell multiplication occurs. As cells are pushed upward away from their nutrient source, they die and become keratinized. Hair consists of three concentric layers: the medulla (core), the cortex (surrounding the medulla), and the cuticle (overlapping single layers). Parts of the hair include the root (embedded in skin) and the shaft (projecting above the surface).

Hair color is genetically determined by melanin; white hair occurs when melanin is replaced by air bubbles. Genetic and hormonal factors also influence hair types: Vellus (fine baby hair), Intermediate, and Terminal (long, coarse hair). Hair growth averages $2\,mm/week$ and involves active and resting phases. Arrector pili are bundles of smooth muscle attached to the follicle. Stimulated by the sympathetic nervous system during cold or fear, they contract to make hair stand erect ("goose flesh"), trapping air for insulation. Clinical terms include hirsutism (excessive hair) and alopecia (hair loss).

Structure and Components of the Nail

Nails are hard, horny keratin plates that cover the tips of fingers and toes. They are derived from the same cells as the epidermis and hair. The nail structure includes the Root (germinal matrix), which lies below the skin and produces the nail volume; its visible edge is the white crescent called the lunula. The Nail Bed (sterile matrix) contains blood vessels, nerves, and melanocytes. The Nail Plate is the translucent keratin body of the nail, appearing pink due to underlying capillaries.

Protective barriers are formed by the Eponychium (cuticle), which fuses the skin and nail plate, the Paronychium (skin overlapping the sides), and the Hyponychium (area between the free edge and the fingertip). The free edge of the nail appears white. Nails grow faster on the fingers than the toes, and growth rates increase in high environmental temperatures. Their primary function is the protection of the distal digits.

Clinical Definitions and Skin Markings

Skin markings include freckles (flat patches of aggregated melanocytes), moles (elevated patches of melanocytes), friction ridges (patterns on fingertips that create fingerprints), and creases (palmar and flexion creases). Clinical conditions related to skin color include Albinism (inherited inability to produce tyrosinase and melanin), Vitiligo (autoimmune loss of melanocytes), Jaundice (yellowing due to bilirubin build-up), Cyanosis (bluish tint from low oxygen), Pallor (paleness from anemia), Erythema (redness from engorged capillaries), and Bruises (contusions resulting in bluish discoloration).

Comprehensive Functions of the Skin

The skin serves several critical roles: Protection, Regulation of Body Temperature, Sensation, Absorption, Excretion, Blood Reservoir, and Synthesis of Vitamin D. Protection is provided by the waterproof keratinized layer, which defends against microorganisms, chemicals, trauma, UV light, and dehydration. Langerhans cells provide an immune response by phagocytizing antigens. Sensation occurs via receptors for touch, pressure, temperature, and pain, which send impulses to the cerebral cortex. The skin acts as a blood reservoir, holding $8-10\%$ of total blood in a resting adult. Absorption is limited but used for transdermal drug delivery (e.g., nicotine patches), while excretion occurs as the skin removes small amounts of sodium chloride and urea through sweat.