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The Skin: Anatomy & Physiology

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Introduction

Largest organ in the body

Varies in thickness at different parts (< 0.5 mm at eyelids to > 5 mm on middle of upper back)

Anatomy Of The Skin

B e n ea th e p ide rm isT h icke r la yer

C o n n ec tive t is sue

D e rm is

O u te rm os t la yerE p ithe lia l t is sue

E p id e rm is

Skin

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Epidermis

Stratified squamous epithelium

No blood vessels

In palms of hand and soles of feet, epidermis is thicker

Composed of five layers:

[1] Stratum corneum

[2] Stratum lucidum

[3] Stratum granulosum

[4] Stratum spinosum

[5] Stratum basale

Frolich, Human Anatomy, Skin

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[1] Stratum Basale

Single layer of columnar or cuboidal cells

Lower surface of cells attached to dermis

Receives nutrients from blood in the dermal vessels

Mitosis occurs in this layer

Older cells expelled to outer layer

Melanin is produced in this layer

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Stratum Basale

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[2] Stratum Spinosum

Prickle cell layer

Several layers of polyhedral (many-sided) cells

Interlocking spine-like projections help binding of this layer

Active protein synthesis takes place (indicates cell growth and division)

Obtain nutrients through fine elements

Keratinization begins in this layer

Nuclei-containing cells change into flat cells composed of hard durable protein)

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Stratum Spinosum Contain Desmosome

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[3] Stratum Granulosum

Granular + 2 to 4 cells thick

Cells contain keratohyaline in granules

Final stages of keratinization occur

Loss of fluid, nucleus disintegrates

[4] Stratum Lucidum

Transparent layer

Flat + translucent dead cells

Protection against UV

Lucidum appears in palm of hands and soles of feet to protect against sun burn

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[5] Stratum Corneum (Horny Layer)

Thick layer of dead cells

Soft keratin (keep skin elastic)

Cells below contain fatty substrate keep skin waterproof + prevent skin cracking and allowing bacteria inside

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Epidermis

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Dermis

True skin

Highly elastic, tough and flexible tissue

Meshwork of collagenous, reticular and elastic fibres

Collagenous Fibres

Provide support for skin

Reticular Fibres

Thinner, yet still provide support

Elastic Fibres

Skin flexibility

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Cells Of The Dermis

a) Fibroblasts

b) Fat cells

c) Macrophages

Dermis subdivided into two main layers:

{1} Upper papillary layer

{2} Reticular layer

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[1] Upper Papillary Layer

Loose connective tissue

Contain protrusions into epidermis called “PAPILLAE”

Fine capillaries to carry waste away + provide nourishment and oxygen

Nerve endings for heat, pain, cold, pressure and touch (Meissner’s corpuscles)

Double row of papillae better gripping by hands and feet + distinctive fingerprint patterns

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[2] Reticular Layer

Elastic network of tough collagen fibres interwoven with elastic fibres

Collagenous fibres arranged in special pattern

Incisions made parallel to these lines during surgery wound heals faster

Contains sebaceous and sweat glands, arrector pili muscle and hair follicle

Pacinian corpuscles are distributed through the dermis and function as pressure receptors

Stretch marks and pregnancy due to breaks in collagen and elastic fibres

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Dermo-epidermal Junction

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Dermo-epidermal Junction

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Hypodermis

“Subcutaneous” layer

Thicker than dermis

Thicker in females

Ducts of sweat glands and bases of hair follicles

Area for formation and storage of fat due to:

@ Adipose tissue – containing fat cells

@ Areolar tissue – tissue elasticity

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Hypodermis

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Developmental Anatomy of the Skin

Dermis and epidermis are derived from different embryological tissues

Epidermis derived from “ECTODERM”

Dermis derived from “MESENCHYME”

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Glands Of The Skin

Two types: Eccrine and Apocrine

Eccrine Glands

Distributed almost all round the body

Secretory portion in hypodermis

Apocrine Glands

Arm pits and dark regions of nipple

Secretes fatty substances

These react to air YOU STINK

Sweat Glands

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Sebaceous Glands

Located in the dermis

Lubrication and protection

Cluster of cells

Breakdown of inner cells in the cluster sebum formation

Connected to hair follicles

Sebum fight bacteria and fungi

Blackheads due to blocked glands

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Hair

Shaft = protrudes from skin

Root = embedded in skin

Follicle surrounds hair

Arrector pili muscle = bundle of smooth muscle

Arrector pili causes:

a) Hair erection

b) Sebum release from the sebaceous glands

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Anatomy Of The Skin

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Anatomy Of The Skin

Frolich, Human Anatomy, Skin

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Functions Of The Skin

Non-specific immunity

[1] Protection

Anatomical barrier against infection

Melanin = screen out excess UV rays

When melanin is darkened by the tan transferred to outer skin layers (suntan) skin less sensitive to sunrays

Dark skin due to wider distribution of melanin beyond stratum basale into higher levels of epidermis

[2] Thermoregulation

Control of heat production

Shivering

Skeletal m uscle

Motor neurons

Control of heat loss

Skin vasocontriction and vasodilation

Skin blood vessels

Sym pathetic nervous system

Control of heat loss

Sw eating

Sw eat glands

Sym pathetic nervous system

Hypothalam ic therm oregulatory integrating centre

Peripheral Therm oreceptors (Skin)

Skin tem perature

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[3] Sensory Perception

Millions of nerve endings

Receptor for pain, heat and pressure

Therefore, maintain homeostasis

[4] Excretion

Excretion of lactic acid and sodium chloride

Urea (1 g nitrogenous waste eliminated through skin per hour)

[5] Vitamin D Synthesis

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P ro m o tes ab sorp tion o f ca lc iu m a nd ph o sp h ate th ro ug h in tes tine

A c tive v ita m in D

M o d if ica tio n b y live r a nd k id ne y e nzym es

V ita m in D 3 (C h o leca lc ife ro l)

Vitam in D precursor (in skin)

Defining PruritisAn unpleasant localized or generalized

sensation on the skin, mucus membranes or conjunctivae which the patient instinctively attempts to relieve by scratching or rubbing

Diversity of Causes and Presentation

Many Causes, Many Treatments

Trivial to to Life threatening

(mosquito bite) (malignancy)

10-50% of cases with generalized itching have systemic disease

Diseases & ItchingInfectionsInfestations

(scabies)Inflammatory skin

conditions (eczema, contact derm, psoriasis)

Chronic Renal Failure

Cholestatic liver disease

Depression/anxiety

Poorly Understood & ManagedRelies on similar components of the pain

system: receptors, neurotransmitters, spinal pathways and centers in the brain

Stimulating pain can relief itchingTreating pain with some analgesics relieves

itching, others trigger itchingPruritis is a common side-effect of opioid

administration, sometimes worse than the pain

Pruritogenic StimuliPressureLow-intensity electrical or punctate stimuli Histamine: acts directly on free nerve

endings in skin

Pain vs Itch NervesItch transmitted from specialized pain

receptors: a subclass of C-nociceptorsMechano-insensitiveHistamine sensitive

Nerve endings cluster around “itch points” which correspond to areas very sensitive to pruritogenic stimuli

Itch pathwaysFibers originate @ dermal/epidermal jxn Thin unmyelinated axons, lots of branching Ipsilateral dorsal horn of spinal cord Synapse with itch-specific secondary neuronsCross to opposite anterolateral spinothalamic

tract to thalamus Somatosensory cortex of postcentral gyrusSLOW transmission and BROAD receptor field

Itch MediatorsHistamineProstaglandinsLeukotrienesSerotoninAcetylcholine

Substance PProteasesPeptidesEnzymesCytokines

Why do you scratch?Histamine activates both the anterior

cingulate cortex (sensory, emotions) and the supplemental motor area

Lateral Inhibition: “Gate Theory”Noxious stimuli of skin adjacent to pruritic

trigger attenuates initial itch sensationScratching stimulates large fast-conducting

A-fibers adjacent to slow unmyelinated C fibers

A-fibers synapse with inhibitory interneurons and inhibit C-fibers

Pain & ItchPainful stimuli (thermal, mechanical,

chemical) can inhibit itchingInhibition of pain (opioids) may enhance

itching

How to Treat an Itch(Understand the Cause!)Inhibit mediators of itch: histamine,

prostaglandins, substance P, serotonin, cytokinesBlock chemicals that induce pruritis: opioids,

antimicrobialsTreat effects of diseases which induce

itching: eczema, CRF, LF, heme, neuro, endo