Anatomy and Physiology of the Skin

Anatomy and Physiology of the Skin

• The skin is the second largest organ in the body
• The skin weighs about eight pounds, excluding fat, in the average adult
• It covers a surface area exceeding 20,000 square centimeters
• The skin receives about one third of the body's blood flow
• The skin protects against physical and chemical attacks
• The skin acts as a thermostat to maintain body temperature and as an insulator
• The skin shields from microorganisms and ultraviolet (UV) rays
• The skin plays a role in regulating blood pressure
• The skin is an access point for local and systemic drug administration
• Normal skin thickness ranges from 3 to 5 mm
• The thickest skin is located on the palms and soles
• The thinnest skin is on the eyelids and parts of the genitals
• Anatomically, the skin consists of many histological layers
• Generally, the skin is described in terms of three tissue layers: epidermis, dermis (corium), and hypodermis (subcutaneous fat layer)
• The epidermis is the outermost layer; it is compact, nonvascular, and consists of stratified squamous epithelial cells
• The dermis is the next layer; it is vascular and composed of connective tissue
• The hypodermis is the innermost layer

Stratum Corneum

• The stratum corneum (horny layer) is the outermost layer
• The stratum corneum consists of compacted, dead, keratinized cells
• The stratum corneum is a limiting barrier, restricting the movement of chemical substances
• The stratum conjunctum is a separate barrier that exists within the stratum corneum
• The barrier to penetration is made of a keratin phospholipid complex in the stratum corneum's dead cells
• The stratum corneum is about 15 microns thick when dry
• The stratum corneum increases to about 48 microns when fully hydrated
• When hydrated, the stratum corneum contains about 75% water, 20% protein, and 5% lipid
• Removing the upper layers of the skin via sandpapering causes water loss and increased skin permeability
• Beneath the stratum corneum are the stratum lucidum and stratum granulosum layers
• The stratum lucidum and stratum granulosum layers are parts of the epidermis

Epidermis Distinct Layers

• The stratum germinativum is the innermost layer, closely associated with the dermis
• The stratum germinativum consists of columnar/cuboidal epithelial cells
• The prickle cell layer (stratum spinosum) consists of layers of polygonal epithelial cells
• The prickle cell layer is thicker in the palms than in hairy skin
• Mitotic processes in two layers contribute to epidermal regeneration and repair
• Prickle cells contain keratinocytes that are produced by cellular division in the epidermis
• Keratinocytes migrate to the skin's surface, changing from living cells to dead, thick-walled, flat, nonnucleated cells that contain keratin
• The granular layer (stratum granulosum) lies above the prickle cell layer
• The granular layer consists of several layers of flattened, polygonal cells
• The granular layer contains granules of keratohyaline, which change to keratin in the stratum corneum
• The stratum lucidum is present only in areas of thick skin and is between the granular layer and the stratum corneum
• The stratum lucidum is a narrow band of flattened, closely packed cells thought to contain eleidin
• The stratum corneum is made of flat, scaly, dead tissue (keratinized)
• The outermost cells of the stratum corneum are flat or squamous plates that are constantly shed

Stratum Germinativum and Dermis

• The stratum germinativum (stratum basale) is the innermost layer of the epidermis, closely associated with the dermis
• The stratum germinativum undergoes constant mitosis to replace cells exfoliated from the skin's surface
• The stratum germinativum contains melanocytes, which produce melanin pigments responsible for giving skin its color
• Melanin accumulates in melanosomes, which are transported to dendrites and then transferred to keratinocytes
• Dendritic processes exist between adjacent keratinocytes throughout the basale layer of the epidermis
• The dermis (corium) is the next distinctive histological layer
• The dermis thickness ranges from 0.6 mm on the eyelids to 3 mm on the back, palms, and soles
• The dermis constitutes the main mass of the skin
• Numerous blood vessels, lymphatics, epidermal appendages, hair follicles, sebaceous glands, and sweat glands are contained and supported within the dermis
• Electrolytes, blood, and water may be stored in the dermis, especially in edematous patients
• The dermis supports the epidermis and separates it from the lower fatty layer
• The dermis consists mainly of collagen and elastin embedded in a mucopolysaccharide substance
• Fibroblasts and mast cells are found throughout the dermis
• The dermis contains a network of nerves that are the neurovascular supply to the appendages in the dermis
• The main sublayers of the dermis are the papillary and reticular layers
• The papillary layer is adjacent to the epidermis and is the dermis's superficial layer
• The papillary layer projects into the stratum basale of the epidermis to form finger-like dermal papillae
• Dermal papillae results in the formation of ridges on the fingers that are recognized as fingerprints
• Dermal papillae are rich in blood vessels
• Dermal papillae act as conduits to bring blood nutrients near the vascular epidermis and regulate temperature
• The reticular layer lies below the papillary layer and contains coarser tissue that connects the dermis with the hypodermis
• The reticular layer is well vascularized and has a rich sensory and sympathetic nerve supply
• The reticular layer appears reticulated (net-like) due to a tight meshwork of fibers
• Elastin fibers in the reticular layer provide elasticity to the skin, enabling movement
• Denser than the papillary dermis, it strengthens the skin, providing structure and elasticity and supports hair follicles, sweat glands, and sebaceous glands

Hypodermis and Sensory Receptors

• The subcutaneous tissue or fat layer is beneath the epidermis and dermis
• The hypodermis is made of relatively loose connective tissue of varying thickness
• The hypodermis provides pliability for the skin
• In most areas, the hypodermis includes layers of fat (panniculus adiposus) involved in thermal control, food reserve, and cushioning
• Fat distribution in the hypodermis changes as bodies mature and age
• Specialized sensory receptors embedded in the skin and mucous membranes respond to hot and cold, light touch, low-frequency vibration, and flutter
• Specialized sensory receptors can detect skin stretch, joint activity, and warmth
• Pacinian corpuscles are encapsulated, rapidly adapting receptors that detect transient pressure and high-frequency vibration and are present in the deep epidermis
• Hair receptors are rapidly adapting nerve endings wrapped around the base of hair follicles to detect hair movement and deflection
• Langerhans cells are in all layers of the epidermis
• Langerhans cells are most prominent in the stratum spinosum participating in immune responses to microbes that invade the skin
• Langerhans cells help other immune system cells recognize an antigen to be destroyed

Hair Structure

• A single hair consists of a central medulla of soft keratin surrounded by the cortex of hard keratin
• Outer surfaces are covered with very thin flat overlapping scales that interlock to form the cuticle
• The medulla always contains air spaces in coarse hair
• No medulla is present in fair or blond hair
• Hair color depends on the amount of pigment (melanin) present in the cortex
• Melanin is responsible for brown and black hair and other hair colors caused by varying proportions of melanin pigment
• Melanin is formed in the epithelial cells of the matrix
• These cells carry melanin as they are pushed up the follicle to become keratinized
• Keratinization of the epithelial cells that form hard keratin (cortex and cuticle) is like the formation of fingernails and epidermal tissue's horny layers
• Keratin includes a large group of compounds obtained from keratinous structure
• Keratin includes at least the keratin of the inner and outer cuticle, the cortex, and medulla
• The keratin varies in composition and so they are separately involved in the chemical reactions and physical behavior of hair.

Skin Appendages

• Hair follicles are specialized structures contained within the skin
• Hair is produced by tube-like follicles that are continuous with the epidermis
• Sebaceous glands are associated with hair follicles
• Sebaceous glands are found all over the body
• Sebaceous glands secrete an oily substance known as sebum
• A hair shaft is generated by a hair germ at the base of a hair follicle
• The follicle is an inward tubular folding of the epidermis into the dermis
• The hair within the follicle is a fiber of keratinized epithelial cells which grow as the result of multiplication of cells in the hair germ bud
• Acne is caused by the formation of plugs of sebum within the ducts of these glands
• The arrector pili muscle is a tiny muscle that attaches to the base of a hair follicle at one end and to dermal tissue on the other end
• The arrector pili muscle is contracted by cold and emotional stress, like fear
• Arrector pili muscles contract to generate heat when the body is cold
• Arrector pili muscle contraction causes the hair to "stand up straight" on the skin, causing goosebumps, which is accompanied by shivering to raise body temperature (traps heat)
• The arrector pili muscle effect is controlled by adrenergic sympathetic nerves

Glands

• Sebaceous glands are appendages of the hair follicles
• The ducts of sebaceous glands are lined with epithelial cells, continuous with those of the basal layers of the epidermis
• Sebaceous glands produce sebum
• Sebaceous glands are holocrine because the gland cells are destroyed in sebum production
• Not all sebaceous glands are associated with hair follicles
• Sebaceous glands are found in genital areas, around the nipples of the breast, and on the edges of the lips
• Sweat glands develop from epithelial cells that proceed downward from the epidermis and are independent of hair follicles
• Sweat glands are long, hollow tubes that reach into dermal and hypodermal areas
• The secretory parts of sweat glands are in the hypodermis
• The excretory ducts proceed upward through the epidermis as wavy or curved channels
• Sweat is an eccrine secretion, and no part of the gland cell is destroyed
• Sweat glands are found over much of the body
• Eccrine sweat glands (small coil glands) are the true sweat glands that occur over almost all of the body surface
• Eccrine sweat glands originate in the dermis's deeper layers or in the sub dermis and open via a thin duct directly on the skin
• Eccrine sweat glands are cholinergically innervated, although the nerve fibers are sympathetic
• The hypothalamus's heat regulation centers control overall secretion of the sweat glands
• Emotional stress and heat can produce sweating
• Sweat is a saline solution with some electrolytes, but lacks carbohydrates, fats, or proteins
• True sweat glands produce much more sweat (several liters per day) than apocrine glands
• Apocrine glands (large coil glands) are associated with sexual development, being post-pubertal in occurrence
• Apocrine glands occur in relatively small numbers and are found in such areas as axillae, around the nipples, on the abdomen, and in perianal and genital areas
• Apocrine glands produce a milky secretion that appears in very small amounts on the skin and has a characteristic odor
• Apocrine glands are larger and deeper in the skin than true (eccrine) sweat glands
• Apocrine glands are attached to a hair follicle by a duct leading down into the coiled, secretory tubules of the gland
• The tubules are covered by myoepithelium
• Myoepithelium allows contraction to adrenergic stimulation
• Stimulation, as in stress, releases the secretion
• The secretion is odorless, until the bacteria of the skin utilize the fats present
• This causes the characteristic pungent odor of apocrine "sweat"
• Sweat is odorless in the absence of apocrine secretions, such as in prepuberty

Perspiration, Body Odor, and Nails

• Odour in the human skin is produced from secretions of sebaceous and sweat glands
• Sebaceous glands are found with every hair
• Sebum is secreted by sebaceous glands and is made mainly of cholesterol and its esters, palmitic and stearic acid and their esters, and various other substances
• Sebum is generally oily and may solidify on the skin surface
• Pure sebum is not a critical factor in skin odour
• The known constituents of sebum are odourless
• Perspiration assists in regulating body temperature by dispelling heat through evaporation of moisture from the surface of the skin
• Perspiration eliminates lactic acid, which is formed during muscular exercise
• Perspiration protects the skin from dryness
• There are about 2,380,000 sweat glands distributed over the body surface
• Eccrine and apocrine are the two types of sweat glands
• The axilla is virtually an apocrine organ and the profuse flow of sweat is called hyperhidrosis and is the result of intense activity of the eccrine
• Eccrine glands in each axillary vault, number around 25,000
• Eccrine glands can secrete large quantities of sweat
• In hyperhidrotic individuals, each armpit may produce upwards of 12 grammes per hour
• Eccrine and apocrine sweat are sterile and adourless at the time of discharge
• Odour is produced later through bacterial action on primarily the apocrine sweat, which is rich in organic material
• Bacteria is an ideal substrate for bacterial growth
• Eccrine sweat is a dilute aqueous solution and is far more abundant
• Eccrine sweat is much less important as a source of axillary odour
• Axillary hair also promotes the development of odour
• Axillary hair acts as a collecting site for apocrine sweat and increases the surface area available for bacterial proliferation
• Decomposition of secretions from sudoriferous and sebaceous glands by the skin microflora, and decomposition of proteins on the surface of the skin give rise to numerous odorous substances
• These substances produce the natural odour of human skin
• Actual odour of the human being is the sum of the natural and acquired odour
• Body odour is an entirely individual property of a human being like fingerprints
• Odour intensity differs from person to person, depending upon personal circumstances, environment, social and psychological conditions
• Nails are modifications of the keratinized layer of the epidermis
• The nail bed on which the nail plate lies derives from the basal epidermal layers
• The body of the plate, at its periphery, is surrounded by the nail root
• The root is derived from the nail groove, which is a process of the basal epidermal unit
• The white area at the base is called the lunula
• The part of the nail groove that enfolds the plate at its margin is the eponychium, which is the thickened layer of skin at the base of the fingernails and toenails
• It can also be called the medial or proximal nail fold
• The eponychium comprises live skin cells whilst the cuticle is dead skin cells
• The cuticle is a layer of clear skin located along the bottom edge of your finger or toe
• The cuticle functions to protect new nails from bacteria when they grow out from the nail root
• The area around the cuticle is delicate
• The hyponychium is a thick layer of stratum corneum immediately beneath the plate of the distal tip of the nail
• It is a part of the epidermis under the free edge of the nail plate
• The hyponychium protects the nail bed from infection

Skin Function and Hydration

• Skin is a barrier between the environment and the body The skin protects against harmful external agents (e.g., pathogenic organisms and chemicals)
• Skin contributes to sensory experiences and involved in temperature control, development of pigment, and synthesis of some vitamins
• Skin is important in hydroregulation because it controls loss of moisture from the body and penetration of moisture into the body
• The skin, except for the stratum corneum, is living tissue
• Stratum corneum requires nutrition, including oxygen
• Cells use nutrients and excrete water and carbon dioxide
• Most oxygen comes from the blood; a small amount comes from the air
• Carbon dioxide exits tissues mainly via the blood, but some is "exhaled" directly to the atmosphere
• Water content of the skin is important to its normal health
• Topical, nonmedicated preparations are available for the physical control of moisture in the skin
• The amount of water impacts percutaneous absorption
• The degree of skin hydration could promote percutaneous absorption too far
• The degree of skin hydration could restrict the degree of penetration
• Lack of skin hydration can cause poor therapeutic activity of a drug
• Corneous layer elasticity is affected if dries out
• Returning water to the skin reverses induced brittleness
• Hydration of the stratum corneum occurs by transfer of water from lower layers or by accumulation of water (perspiration) caused by occlusive coverings at the surface (bandages, vehicles)
• Accumulation of moisture by occlusive coverings cause making penetration of this area by molecules easier

Sebum and pH

• The surface of the skin is covered with sebum
• Sebum contains free fatty acids (mainly palmitic and oleic), triglycerides, waxes, cholesterol, squalene, and other hydrocarbons and traces of fat-soluble vitamins
• Sebum is produced by the sebaceous glands
• Sebum lubricates the skin surface for suppleness
• Sebum acts as a surface barrier to prevent moisture loss from the deeper layers
• Sebum chemically prevents penetration
• Sebum has some antiseptic and antifungal properties
• Sebum is an emulsion of sweat and waste from cutaneous cells
• Skin pH is normally between 4.8 and 6
• Acidity from amphoteric amino acids, lactic acid, and fatty acids in the secretions from the sebaceous glands
• Buffering capacity is partly from secretions in the horny layer
• The ability to neutralize acids and alkalis varies
• Greater buffering is where the horny layer is thickest
• Secretions at the skin are weakly acidic with a pH of 4.5 to 5.5
• The pH varies slightly
• It is higher where sweat evaporates slowly The acidic nature causes the "acid mantle"
• The acid mantle is a protective mechanism, because microbes tend to grow better at pH 6 to 7.5 and infected areas have higher pH
• Fatty acids in sweat inhibit microbial and fungal growth (propionic, caproic, and caprylic)
• The acid mantle is not only in the pH, but the compounds for acidity
• Sebum has also fungicidal and bacteriacidal properties
• Buffer capacity of the secretions of the skin surface is a protection
• The skin can readjust to its normal pH effectively
• Skin that are slow to adjust are prone to skin diseases

Skin Temperature and Flora

• Skin temperature is the temperature of the outermost surface
• Trunk temperature is between 33.5 and 36.9 °C (92.3 and 98.4 °F)
• Skin temperature is lower over protruding parts, like the nose
• Skin temperature is higher over muscles and active organs
• A variety of microorganisms lives on the surface of healthy skin
• Species make a normal ecologic balance
• The flora acts as a defense mechanism control the presence of potential pathogenic organisms
• Infections relate to breakdown of protective mechanism
• The skin has an abundance of colonies of pathogenic organisms
• The stratum corneum is about 10 percent water
• The stratum corneum ensures elasticity
• An increase in moisture causes imbalance in microbial growth
• Infection is made of scrubbing especially strong detergents
• The infection is caused by exposure to water, sweating, occlusion increases temperature, excessive bathing and injury
• The presence of a infection depend on the extent of breakdown, defensive mechanisms, the pathogenic organisms present, and the supportive nutrient environment