Integumentary System: Skin, Hair, and Glands

• The integumentary system includes the skin, hair, glands and nails
• The skin is composed of the epidermis and the dermis

Functions of the Integumentary System

• Protection against UV light, microorganisms, and water loss
• Sensation is facilitated via sensory receptors for heat, cold, touch, pressure, and pain
• Temperature regulation occurs through modulation of blood flow in the skin and sweat gland activity
• Vitamin D production from a molecule in the skin exposed to UV light
• Excretion of small amounts of wast products

The Skin

• The epidermis is the superficial layer of stratified squamous epithelial tissue
• The dermis is the deep layer of connective tissue providing structure and strength
• The subcutaneous tissue, not part of the skin, is loose connective tissue connecting skin to underlying structures

Epidermis

• The epidermis is avascular
• It's nourished by diffusion from capillaries in the papillary layer of the dermis
• It's composed of epithelial cells in layers or strata
• It's separated from the dermis by a basement membrane

Epidermal Cells

• Keratinocytes, the most common cell, produce keratin for strength
• Melanocytes contribute to skin color and melanin is transferred to the keratinocytes. The number of melanocytes is the same for everyone
• Langerhans cells are part of the immune system
• Merkel cells detect light touch and superficial pressure
• Keratinization occurs as cells move outward, filling with keratin and resisting abrasion while forming a permeability layer

Epidermal Strata

• The stratum basale (germinativum) is the deepest layer with cuboidal or columnar cells
• This layer has high mitotic activity; cells undergo mitosis about every 19 days and become keratinized
• Desmosomes hold the keratinocytes together for structural strength
• It takes 40-56 days for cells to move from the stratum basale to the surface and be sloughed off
• The stratum spinosum has 8-10 layers of many-sided cells that flatten as they are pushed upward
• This layer contains new desmosomes, lipid-filled lamellar bodies, and additional keratin fibers
• The stratum granulosum is 2-5 layers of flattened, diamond-shaped cells containing granules of keratohyalin that accumulate in the cytoplasm
• Lamellar bodies release lipids into the extracellular space
• In superficial layers, nuclei/organelles degenerate but keratin fibers/keratohyalin granules do not
• The stratum lucidum is a thin, clear zone of dead keratinocytes, only found in palms and soles
• The stratum corneum is most superficial, having 25+ layers of dead squamous cells joined by desmosomes called cornified cells
• Cell remnants have soft protein envelop of keratin, a mixture of keratin fibers and keratohyalin

Thick and Thin Skin

• Thick skin has all 5 epithelial strata, and is found in areas subject to pressure or friction like palms, fingertips, and soles
• Fingerprints and footprints are formed with papillae of underlying dermis in parallel rows
• Thin skin is composed of 4 strata (no stratum lucidum), is more flexible and covers the rest of the body where hair grows
• Calluses increase the number of layers in the corneum and a corn forms over bony prominences

Skin Color

• Determined by pigments in the skin, blood circulating in skin and the thickness of the stratum corneum
• Melanin provides protection against UV light
• It's a group of colored chemicals from amino acid tyrosine in colors ranging from brown to black, and yellowish to reddish
• Melanocytes processes extend between keratinocytes to deposit melanosomes
• Albinism is the deficiency or absence of pigment due to genetics, hormones, or light exposure
• Carotene is a yellow pigment from vegetables
• It accumulates in the stratum corneum, adipose cells of the dermis, and subcutaneous tissue
• Racial variations are due to the types and amount of melanin produced, as well as the size/number/distribution of melanosomes
• Hormones like estrogen and MSH can increase melanin production during pregnancy
• Blood circulating through the skin imparts a reddish hue, increasing during blushing, anger, and inflammation
• Cyanosis is a blue color caused by decreased blood oxygen content
• Erythema is a red color caused by increased blood flow
• Thicker areas of the stratum corneum can be yellowish and pigments/substances in the dermis or subcutaneous tissue can impart a bluish color

Skin Cancer

• Skin cancer is the most common type of cancer
• It most often results from UV radiation damage to epidermal cells’ DNA
• Melanin affects the likelihood of developing skin cancer. Fair-skinned people are at greater risk
• The three major types of skin cancer are: basal cell carcinoma, squamous cell carcinoma, and melanoma

Types of Skn Cancer

• Basal cell carcinoma is the most common, affects cells of the stratum basale, has varied appearances, and is cured by removal or destruction
• Squamous cell carcinoma affects cells of the stratum spinosum and has varied appearances; may bleed, and is mostly cured by removal or destruction
• Melanoma is the most deadly
• ABCDE rule for signs of melanoma includes:
  • A - asymmetry
  • B - border irregularity
  • C - color not uniform
  • D - diameter greater than 6mm
  • E - evolving or changing

Dermis

• The dermis provides structural strength and is connective tissue with collagen, elastic, and reticular fibers, fibroblasts, macrophages, and adipocytes
• It contains nerves, blood vessels, hair follicles, smooth muslces, glands, and lymphatic vessels
• The sensory functions include: pain, itch, tickle, temperature, tough, pressure, two-point discrimination
• There are two dermis layers:
  • The Papillary layer is the superficial areolar with lots of elastic fibers, dermal papillae, and capillary beds
  • Friction ridges form fingerprints as whorls of ridges
  • This layer Contains touch receptors (Meissner, Pacinian, and Ruffini), and free nerve endings sensing pain
  • The Reticular layer is deep and has dense irregular C.T. of collagen and elastic fibers, adipose tissue, hair follicles, nerves, oil glands, ducts of sweat glands, and heat sensors

Cleavage Lines

• Cleavage(tension)Lines are elastic and collagen fibers oriented in some directions more than others
• Of importance during surgery, incisions parallel to the lines result in less gapping, faster healing, and less scar tissue
• If skin is overstretched, stretch marks occur

Subcutaneous Tissue

• The subcutaneous tissue is deep to the skin (hypodermis)
• It has loose connective tissue with collagen and elastic fibers
• Cell types are fibroblasts, adipose cells, macrophages, and contains approximately one-half of body's adipose tissue
• Its functions as an energy source, insulation, and padding

Injections

• Three types of injections exist:
  • Intradermal - into the dermis, skin taut, shallow needle angle
  • Subcutaneous - into subcutaneous layer; pinch skin using a short needle
  • Intramuscular - needle long and perpendicular to skin

Accessory Skin Structures: Hair

• Hair is found everywhere on the human body except palms, soles, lips, nipples, parts of external genitalia, and distal segments of fingers and toes
• Hair structure and coloration changes with age
• There are three types of hair:
  • Lanugo (delicate, unpigmented hair fo the fetus)
  • Terminal hair (long, corse, pigmented, hair of the scamp, eyelids, eyebrows, and with puberty, axilla, pubis, and face)
  • Vellus hair (fine, short hair covering the rest of the body)

Hair Structure

• The shaft protrudes above skin surface
• The root is located below surface, and the expansion that forms the base of root is the hair bulb
• Hair is comprised of 3 concentric layers, which include:
  • Medulla (central axis)
  • Cortex (bulk of the hair)
  • Cuticle (forms surface)
• The internal matrix is the source of hair; and the dermis projects into bulb as har papilla, serving as a blood supply

Hair Follicle

• The dermal root sheath is part of dermis surrounding the epithelial root sheath
• Epithelial root sheath has internal/external parts
• The internal part contains stratum basale that remains after an injury and supplies a source of new epidermis
• When hairs are pulled out, the internal part comes out and is visible as a white bulb

Hair Growth

• Growth and resting stages are cyclic
• During the Growth stage, cells are added and then hair elongates
• During the resting stage, the follice shortens and holds into place causing new hair to begin
• The amount of time spent in each stage depends on hair type and location
• Regular hair loss means hair is being replaced
• Permanent hair loss, called pattern baldness, is the most common cause
• Alopecia areata is spot baldness likely due to an autoimmune response

Hair Color & Muscles

• Color results from varying amounts and types of melanin which can be black-brown and red, and is controlled by several genes
• Arrector pili muscles are of smooth muscle type, extends from the dermal root sheath of the follicle to the papillary layer of dermis
• Muscle contraction causes hair to "stand on end" causing the skin to push up by movement of hair follicle to produce "goose bumps"

Accessory Skin Structures: Glands

• Sebaceous glands produce oily secretions called sebum
• They prevent drying and inhibit some bacteria, with most emptying into the hair follicle
• These are holocrine glands, meaning they secrete via death of secretory cells
• Exceptions to sebaceous glands are lips, meibomian glands of eyelids, and genitalia
• Sweat (Sudoriferous) glands have traditionally been known as two types: apocrine and merocrine. Although apocrine may secrere in a merocrine or holocrine fashion

Eccrine/merocrine glands

• They are the most common in the body
• They're simple called tubular glands that open directly onto the skin's surface
• Coiled parts are in the dermis, with ducts exiting through the epidermis
• Their isotonic fluid (sweat) contains mostly water with some wastes, which aids in body temperature regulation

Apocrine Glands

• Active at puberty, glands open into hair follicles
• The secretions are generally odorless; however, when acted upon by bacteria, may become adiferous
• Found in armpits, genitalia and around the anus.
• The most common glands do not help regulate temperature

Other Glands

• Ceruminous glands are modified eccrine glands in the external auditory canal
• Earwax (cerumen) is composed of sebum/ceruminous secretion and combines with hairs to prevent dirt/insects from entry
• Mammary glands are modified apocrine glands that produce milk

Accessory Skin Structures: Nails

• Nails are thin plated made up of multiple layers of dead, hard keratinzed stratum corneum cells
• Nail body: the stratum corneum's visible portion
• Eponychium/cuticle is corneum superficial to nail body and the hyponychium is the corneum beneath the free edge
• Matrix and nail bed: cells that give rise to the nail
• Nail root: covered by skin, extends towards the nail matrix

Nail Growth

• Is a continuous process unlike the growth cycles of hair
• The nail matrix and nail bed are composed of epithelial tissue containing stratum basale that gives rise to the cells from the nail
• Fingernails grow from 0.5-1.2 mm/day, faster than toenails
• The lunula is the small part of the nail matrix seen through the nail body, forms at the base of the nail and is often crescent shaped

Physiology of the Integumentary System

• Protection against abrasion, bacteria through desquamation, microorganisms/foreign substances
• Glandular secretions are also bacteriostatic/skin contains cells of the immune system
• Melanin protects against UV radiation
• Hair on head insulates/protects against light/abrasion
• Eyebrows keeps sweat out of the eyes and eyelashes help protect eyes of foreign objects/ hair in nose and ears helps stop dust/bugs from entering the body
• Nails protect the ends of digits aiding in self defense
• Acts as a barrier to diffusion of water

Physiology of the Integumentary System

• Sensation can include, pressure, temperature, pain, heat, cold, touch, movement of hairs
• Temperature regulation is mediated by sweating and radiation which causes evaporation cooling
• Arterioles in dermis change diameter as temperature changes, controlling blood flows

Physiology of the Integumentary System

• Vitamin D production begins at the skin which aids in Ca2+ absorption via Vitamin D otherwise known as calcitriol(hormone)
• The uptake of Ca2+ and PO from the intestines is stimulated, and promotes Ca2+and PO4 release from bone and reduces Ca2+ loss from the kidneys which increases blood Ca2+levels
• 7-dehydrocholesterol when exposed to UV radiation, turns into cholecalciferol which is released in the blood to be modified in liver/kidneys to form calcitriol also known as active vitamin D
• People in cold climates/or those who cover their body can be deficient of vitamin D, but calcitriol is absorbable through the intestinal wall(sources: dairy, liver, egg yolks, supplements
• Excretion removes waste products from the body via the skin
• Sweat typically consists of salt, water, urea, ammonia and uric acid
• Compared to the Kidneys, the contribution to excretion is small

Burns

• Burns are characterized by partial thickness and full thickness
• Partial thickness consists of first degree and second degree
• First degree burns affect the epidermis
• Second degree burns affect the epidermis an part of the dermis
• Burns that are full thickness are characterized by third degree and fourth degree
• The third and fourth burns, affect deeper structures, are characterized by skin grafts that can include slit skin, artificial skin and/or pig/ cadaver skin
• Burns cause a hyper metabolic state and cause loss of muscle mass and edema.
• Burns cause the body thermostat to reset itself to high levels
• It increases the amount of blood clotting factors released,
• It activates the release the hormones epinephrine and norepinephrine.
• Shock or blood loss can damage to the kidneys may also occur.

Effects of Aging on the Integumentary System

• Skin becomes more easily damaged because the epidermis thins and amount of collagen decreases, causing infections
• Wrinkling occurs from the loss of elastic fibers that make the skin dry and limits the blood supply which reduces the ability to regulate body temperature.
• Melanocytes decrease which leads to age spots that are caused by the increase of melanin production.
• Sunlight causes the skin to age more rapidly.