Anatomy, Organization, Histology, Function of Human Skin and Diagnosis

The skin: is the interface (surface) between human & environment

• is one of the largest organs in the body • Its surface area is of 1.8 - 2 m2 & makes up

approximately 16% of our body weight ≈ 4kg• is a window through which the physician can

"see" the entire body

Normal Human Skin

Normal Skin Low Power with Labels

Skin normal histologyEpidermis: thickness vary from 0.1mm in eyelids to 1mm

in palm & soleNo blood vesselsKeratinocytes make 85% of it, other cells are

melanocytes, langerhans cells, merkel cellsTransit time: is the time needed for cell

maturation from basal layer to the surface, normally = 60 days

The skin is often referred to as the "integumentary system" composed of epithelial, mesenchymal, glandular and neurovascular components.

• The integumentary system is synonymous with:

1- The skin and its derivatives: sweat glands, nails, hair, sebaceous glands, arrector pili muscles. 2- Mammary glands and 3- Teeth

Functions of the Skin• barrier to physical agents• protects against mechanical injury• prevents dehydration of body through fluid loss• reduces the penetration of UV Radiation• helps to regulate body temperature• provides a surface for grip• acts as a sensory organ• acts as an outpost for immune surveillance• plays a role in Vitamin D production• has a cosmetic association

Functions of the skin

1. Protection against:• chemicals, particles horny layer• UV radiation melanocytes• Antigens, haptens langerhans cells• Microbes langerhans cells2. Preservation of horny layerbalanced internal Environment3. Prevents loss of horny layerwater, electrolytes, macromolecules

4. Shock absorber, dermis & subcut. fat Strong, elastic, compliant5. Temprature blood vessels Regulation eccrine sweat glands

6. Insulation subcutaneous fat7. Sensation specialized nerve endings

8. Lubrication sebaceous glands9. Protection & prising nails10. Calorie reserve subcut. Fat11. Vitamine D synthesis keratinocytes12. Body odor & apocrine sweat glands pheromones: (chemicals for communication)13. psychological: skin, lip, hair & nails display

Based on the thickness of the epidermis, skin can also be classified as thick or thin:1) Thick skin - covers palms and soles, - has sweat glands, but lacks hair follicles,

arrector pili muscles, and sebaceous glands2) Thin skin - covers most of the rest of the body - contains sweat glands & hair follicles, arrector

pili muscles, and sebaceous glands.

Layers of epidermis1. Basal layer ( stratum basalis): deepest layer, single layer of columnar layer Keratinocytes have hemidesmosomes

attached them to underlying lamina densa In normal skin 30% of basal cells are

preparing for division (growth fraction)

2. Spinous (prickle) cell layer: keratinocytes larger than basal layer 7-9 layers of cellsKeratinocytes firmly attached by cytoplasmic

process (desmosomes) & intercellular glycoprotein cement

Tonofilaments are small fibers running from cytoplasm to desmosomes

Tonofilaments packed into bundles called tonofibrils, more numerous in this layer

3. Granular layer:• 2-3 layers• Cells flatter than spinous layer, • No nucleus• No intracytoplasmic organelles• More tonofibrils• Contain irregular basophilic granules of

keratohyalin.

4. Stratum Luidum: palms & soles5. Stratum corneum: SC:Made of layers of flattened dead cells (corneocytes), has no nuclei or cytoplasmic organelles.Bricks-stuck together by intercellular lipidsCytoplasm is picked with keratin filaments,

embedded in a matrix, enclosed by an envlope derived from keratohyalin granules give corneocytes toughness allow skin to withstand chemical & mechanical insults

DEJ: Dermo-epidermal junction the basement membrane lies at the interface

between epidermis & dermis plasma membrane of basal layer has

hemidesmosomes .

Composed of:Lamina Lucida is electron lucent area lies below basal

cell layer.

Lamina Densa Fine anchoring filaments cross lamina lucida &

connect lamina densa to plasma membrane of basal cell layer.

Anchoring fibrils, dermal microfibril bundles & single small collagen extend from papillary dermis to deep part of lamina densa

Function of DEJ:1.Provide mechanical support2.Encouraging adhesion, growth, differentiation

& migration of overlying basal cells3.Act as semipermeable filter that regulate

transfer of nutrients & cells between dermis & epidermis

Dermis:

Dermal Layers: 1. Papillary layer 2. Reticular layer

Dermal Fibers: 1. Collagen Fibers

2. Elastic (Elastin) Fibers

Dermis:Lies between epidermis & subcut. fatSupport epidermis structurally & nutritionallyThickness greatest in palm & soles & least in

eyelids & penisInterdigitate with epidermis, upward projection

of dermis is dermal papillaeRete ridges: the downward ridges of epidermis is the rete

ridges, which allow adhesion between dermis & epidermis as it increases the surface area between them

Cells & fibers of dermis Collagen fibers: • make 70-80% of dermis dry weight• Compose of thinner fibrils packed in bundles• Made of 3 polypeptide chains• Contain proline, hydroxyproline, glycin• Its tensile strenth prevent tearing when skin is stretchedReticulin fibers: are fine collagen fibers seen in fetal skin & around blood vessels & appendages

in adult skin

Elastic fibers: 2% of dermis dry weight

elastin is made up of polypeptides rich in glycine, desmosine, valine

Have amorphous elastin core surrounded by elastic tissue micrifibrillar components

Ground substance of dermisComposed of glycosaminoglycan (hyaluronic

acid & dermatan sulphate) with smaller amount of heparan sulphate & chondroitin sulphate

Functions:Ground substance binds water, allowing

nutrients, hormones & waste products to pass through dermis

Acts as lubricants between fibersProvides bulk act as shock absorber

MusclesSmooth arrector pili muscle is vestigial in 1- sebum expression 2- goose pimples from cold, 3- nipple erectionRaising of scrotum is by smooth dartos muscleStriated fibers (platysma) & some of muscles

of facial expression found in dermis

Blood vessels

• Regulate body temprature• 2 main horizontal plexus, superficial & deep• Under sympathetic nervous control arteriovenous

anastamoses can shunt blood to venous plexus reducing surface heat loss by convection

Nerves• One million nerve fibers mostly in the face &

extremities• Cell bodies are in dorsal root ganglia• Both mylinated & unmyelinated fibers exist• Itch is follow stimulation of free nerve endings

close to dermo-epidermal junction

Skin appendages (Adnexa): SHANS:

1. Sweat glands2. Hair3. Arrector pilli muscle4. Nails5. Sebaceous glands

Skin appendages (Adnexa): SHANS:• Hair: is human plumage Hair follicle form before nine week of fetal life when hair germ

grows obliquely down into dermis.It includes the following parts: Infandibulum: from surface to entrance of sebaceous g. Asthmus: between entrance of seb. g to attachment of arrector

pili muscle Hair Matrix: is the inferior portion including the hair bulb Free of hair areas: Lip, glans penis, labia minora, palms, soles Hair cycle: anagen, catagen & telogen Typesof hair: 1- Lanugo hair 2- Vellus hair 3- Terminal hair

Nails• Hard keratin of nail plate formed in nail matrix

which lies in an invagination of epidermis on back of terminal phalanx of digits toes

• It provide strength & protection for terminal phalanx & allow handling of fine objects

• Finger nail growth is 0.5-1.2 mm per week• Slower in toe nails• Faster growth in summer

Skin Emberiology:Origins of the Integumentary System: (The Skin)• The skin basically consists of two layers that are

derived from two germ layers: ectoderm and mesoderm.

• The epidermis is from surface ectoderm.• The dermis is derived from mesoderm. • Mesenchyme: is the embryonic meshwork of

connective tissue (CT).

Hair: A hair follicle develops as a proliferation of the epidermal stratum germinativum and extends into the underlying mesenchyme.

Nails: develop from thickened areas of epidermis at the tips of each digit called nail fields.

• Later these nail fields migrate onto the dorsal surface surrounded laterally and proximally by folds of epidermis called nail folds

• Both the sebaceous glands and sweat glands (+ mammary glands) are derived from the epidermis and grow downwards into the dermis.

• The majority of sebaceous glands originate as buds from developing hair follicle root sheath.• Sebaceous glands in the glans penis and labia

minora (independent of hair follicles) develop similarly, but as buds from the epidermis.

Sweat glands: - Eccrine sweat glands • located throughout the body originate as downgrowths from

the epidermis into the underlying mesenchyme. • As a result, these eccrine ducts open onto the surface of the

skin.- Apocrine sweat glands • are located in the axilla, pubic region, perineal region, and

areolae of the nipples. • originate instead as downgrowths from the epidermal

stratum germinativum (which also gives rise to hair follicles). • As a result, these apocrine ducts open into the upper part of

hair follicles above sebaceous gland openings.

The Physiology of Skin• The skin is a metabolically active organ with

vital functions such as protection and homeostasis.

• When discussing the physiology of the skin and its derivatives, the following topics are important:

Keratinocyte Maturation: • The aging of basal cells into the corneocytes

(dead cells) is crucial. • The stratum corneum is important in

preventing all manner of agents from entering the skin, including micro-organisms, water and particulate matter.

• It's the epidermis that also prevents loss of vital body fluids.

• The dividing basal cell on average replicates every 200 to 400 hours, and the resulting cell takes 14 days to differentiate and 14 days to be shed.

Hair Growth • Unlike most mammals, humans are

considered "nude“ رووت ; thus, hair does not play the same vital role of heat conservation.

• However, the hair on the scalp does protect us from UV radiation and minor injury.

• Today, hair is of cosmetic value.Hair growth is cyclical and has three phases. The 3 phases are:

1) Anagen is the growing phase. • Its duration depends on the location of the hair. • For eyebrow hair, this phase only lasts 4 months, but for scalp hair

it lasts from 3-7 years. • At any moment, 80-90% of scalp hairs are in anagen; daily,

approximately 50-100 scalp follicles switch to the catagen phase. 2) Catagen is the resting phase and lasts 3-4 weeks. • There is a stop to hair protein synthesis as the hair follicle retreats

towards the surface. • At any moment, less than 1% of hairs are in the catagen phase.3) Telogen is the shedding phase and • is characterized by hairs with a short club root. • Daily, approximately 50-100 scalp hairs are shed. • At any moment, 10-20% of scalp hairs are in telogen.

Dermatological General & Specific Terms:• Dermatologists have some universal terms

that are used when describing skin conditions:• Lesion:• Eruption (or rash):• 2 Types of Skin Lesions: 1- Primary Skin Lesions 2- Secondary Skin Lesions

1- Primary Lesions: are the first to appear and are due to the disease or abnormal state.

• Must be distinguished from secondary or induced lesions

• Macule• Papule• Nodule• Vesicle• Bulla• Pustule• Cyst• Plaque• Wheal, Erythema, abscess, Angiodema, Tumor,

Petechiae, Purpura, Ecchymosis, Haematoma, Burrow, Comedo, Telangiectasia & Poikiloderma

Macule: small, flat, nonpalpable circumscribed area of change in color or texture of the skin.

• are < 0.5 cm in size, • more than 0.5 called patches1. Macules (individual) may be the result of (A) hyperpigmentation: e.g. brown as in lentigos) (B) depigmentation (e.g. vitiligo), (C) vascular dilation (e.g. erythema) 2. Multiple well-defined macules of various shapes and sizes. In this case, the macules blanch upon pressure

(diascopy) and thus are due to inflammatory vasodilation.

Papule: small solid elevation of skin • < 5 mm in diameter. • Projects above the plane of the surrounding skin • Flat-topped (in lichen planus) or • Dome shaped (in xanthomas) or • Spicular (if related to hair follicles)

Nodule: palpable, solid, round, or ellipsoidal lesion

• Its depth of involvement and/or palpability differentiate it from a papule rather than its diameter

• Larger than papules: > 5 mm diameter (in both depth & width)

• Any layer of the skin • Edematous or solid. • 5 types of nodules: epidermal, epidermal-

dermal, dermal, dermal-subdermal, and subcutaneous

Vesicle (blister): circumscribed, elevated lesion

• is < 5 mm in diameter containing serous (clear) fluid.

• A vesicle/bulla is the technical term for blisters • Vesicle walls can be so thin that the contained

serum, lymph, blood, or extracellular fluid is easily seen.

• Fluid can be accumulated within or below the epidermis

• Pustule: Vesicle contains pus• Bulla: Vesicle more than 0.5 cm

Bulla: A vesicle with a diameter > 5 mm. 1. Subcorneal2. 2. Epidermal3. Subepidermal vesicles

Pustule: superficial, elevated lesion that contains pus (pus in a blister). • may vary in size and shape. • The color may appear white, yellow, or greenish-yellow depending

on the color of the pus. • Pus is composed of leukocytes with or without cellular debris • may also contain bacteria or may be sterile1. A pustule is basically a papule containing pus2. Superficial, subcorneal pustules ( e.g. pustular psoriasis) `

Cyst: an epithelial lined cavity containing liquid or semisolid material (fluid, cells, and cell products)

• A spherical or oval papule or nodule may be a cyst if, when palpated, is resilient (feels like an eyeball).

Plaque: palpable, plateau-like elevation of skin, usually more than 0.5 cm in diameter without substantial growth

• Often formed by a convergence of papules, as in psoriasis. 1. Plaques occupy a relatively large surface area in comparison with its height above the skin

Wheal: elevated, transitory, compressible papule or plaque produced by dermal edema

• The papule or plaque is usually rounded or flat-toped, and evanescent, disappearing within hours.

• The borders of a wheal are sharp, but not stable and can move from involved to adjacent uninvolved areas over hours.

• The epidermis is not affected. • can be pale red or white (especially in the center) if

edema is sufficient to compress superficial vessels.

Erythema: is redness caused by vascular dilatation

Abscess: localized collection of puss more than 1 cm in diameter

Angiodema: is diffuse swelling caused by subcutaneous oedema

Tumor: is an enlargement of the tissue by normal or pathological material or cell greater than 1 cm in diameter

Petechiae: pinhead sized macules of blood

Purpura: larger macule or papule of blood

Ecchymosis: a collection of blood greater than 1 cm in size outside the vascular tree (extravasation of blood), in the skin and within tissue

Haematoma: is swelling from gross bleeding

Burrow: is linear or curvilinear papule as in scabies

Comedo: is a plug of greasy keratin wedged in a dilated pilosebaceous orifice

2- Secondary Lesions• From the natural evolution of primary lesions a. From Eroded area left by bursting vesicle or b. From the patient's manipulation of the primary lesion

(scratching):- • ScaleUlcerCrustErosionExcoriationLichenification

AtrophyScar, fissure, Sinus, Keratosis, Striae & Pigmentation

Scale: is a flake • is accumulation or abnormal shedding arising

from horny layer keratin: (SC) • may be primary or secondary. • usually indicates inflammatory change and

thickening of the epidermis

Erosion, Fissure (fish Your), and Ulcer

Erosion: moist, circumscribed, usually depressed lesion due to loss of all or part of the epidermis,

heals without scar due to: 1. Herpes viruses (vesicles)2. Pemphigus (bullae)3. Toxic epidermal necrosis :TEN

Ulcer: a "hole in the skin" • circumscribed area of skin loss extending

through the epidermis and at papillary dermis• heals with scarring• results from the impairment of vascular and nutrient supply to the skin

Crust: look like scale but composed of dried serum, blood, or pus on the surface of skin.

• May be thin, delicate, and friable or thick and adherent. - Yellow, if from serum; - Green or yellow-green if from pus; or - Brown or dark red if formed from blood. • Characteristic of pyogenic infections • Crusts that occur as honey-coloured, delicate, glistening

particulates are typical of - Impetigo.

Excoriation: • Epidermal linear or punctate • superficial excavations (ulcer or erosion) • caused by scratching, rubbing, or picking.

Lichenification: chronic thickening of the skin along with increased skin markings.

• Results from scratching or rubbing

Atrophy: Thinning of skin caused by diminution of epidermis, dermis or subcut. tissue

• paper-thin, wrinkled skin with easily visible vessels. • Results from loss of epidermis, dermis or both. • Seen in: - aged, - some burns, - long-term use of highly potent topical corticosteroids (Cs).

Scar: is a pattern of healing where normal structures replaced by fibrous CT at a site of injury to the dermis.

1- Hypertrophic Scar 2- Atrophic Scar 3- Sclerotic or hard Scar due to collagen proliferation

1- Hypertrophic or

2- atrophic scar. 3- Sclerotic or hard Scar

Fissure: is a skin slit

Heel fissure

Sinus: is a cavity or channel permits escape of fluid or pus

Keratosis: is a horn like thickening of stratum corneum(SC)

Striae: is a streak like linear atrophic pink purple or white lesions (or called stretch marks).

Pigmentation: more or less darker than surrounding skin

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