phase 1_week 1_the skin ppp

7/30/2019 Phase 1_week 1_the Skin Ppp

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The Skin and WoundHealing


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Structure of the Skin

The skin is organized into 2/3 layers

Superficially, the epidermis is the topmost layer of the skin

and itself consists f 4/5 layers

Stratum basale

Stratum spinosum

Stratum granulosum

Stratum lucidum*

Stratum corneum

The dermis, which is divided into 2 layers

And the hypodermis (subcutis)


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The epidermis: stratum basale

The deepest of 5 layers

Continuous layer of epithelium 2-3 layers thick (simple cuboidal / columnar)

Made up of germiantive (basal) cells, considered stem cells of the epidermis, and

some keratinocytes

Divide by mitosis to form keratinocytes of S. spinosum

Other cells include MERKEL CELLS (touch receptors), LANGERHANS CELLS(immune cells); MELANOCYTES (pigment-producing cells)

K-5 and K-14 filaments bind cells of this layer to HEMIDESMOSOMES (basal lamina)

and DESMOSOMES (adjacent cells)

Attached to basal lamina through HEMIDESMOSOMES

Forms epidermal ridges which protrude into dermis and interlock with dermal

projections known as dermal papillae larger surface area / more strength for

attachment / dermal papillae contain capillary loops which perfuse basal layer of

epidermis


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The epidermis: stratum spinosum

Numerous keratinocytes arranged in 8-10 layers

Keratinocytes pushed superficially by continually dividing basal cells

Adjacent ells are bound by DESMOSOMES

Cells have spine-like projections only in histological preparations as cells shrinkand pull apart leaving only points of attachment at desmosomes

Keratinocytes produced coarser K-1 and K-10 intermediate filaments(tonofilaments)

Langerhans cells and melanocyte projections from basal layer are seen

Dome divisions occur in this layer as some cells retain their ability to divide

Production ofINVOLUCRIN occurs (forms cell envelope that protectscorneocytes)


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The epidermis: stratum granulosum

3-5 layers of keratinocytes undergoing APOPTOSIS

Keratinocytes begin to synthesize large amounts of keratin and KERATOHYALIN

Keratohyalin forms dense cytoplasmic granules within the cell that promote cellulardehydration and aggregation and also crosslinking or keratin intermediate filaments toform keratin

Cells become thinner and flatter

Cells STOP DIVIDING in this layer

ORGANELLES BEGIN TO DISINTEGRATE AND DIE as they move further away from dermal

vessels and diffusion is no longer effective

Concomitantly, keratinocytes secrete LAMELLAR GRANULES within the extracellular spacebetween cells of the upper layers, promoting formation of a HYDROPHOBIC LIPIDENVELOPE WHICH ACTS AS A WATER REPELLANT AND SEALANT


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The epidermis: stratum lucidum

Only visible in palmoplantar / thick areas of skin

lucida = clear

4-6 layers of flattemned. Clear and dead keratinocytes

containing large amounts of keratin

Thickened plasma membranes

Surrounded by exocytosed oily substance consisting of

proteins and lipids secreted by lamellar bodies


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The epidermis: stratum corneum

Consists of up to 30 layers of flattened dead keratinized corneocytes

No longer contain a nucleus or organelles

Cells in each layer overlap and are interconnected by

(corneo)desmosomes

Filaggrin connections

Cells are continually shed in the process of desquamation

Replaced by proliferating cells migrating from basal layer renewaltakes 15-30 days

Cells form a protective barrier to underlying structures


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THE DERMIS


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The dermis: papillary layer

Consists of (loose) AREOLAR CONNECTIVE TISSUE: thin collagen and fine elastic fibres

1/5th of total dermal thickness

Surface area greatly increased by DERMAL PAPILLAE projections of papillary dermis intobasal lamina of overlying epidermis

Dermal papilla interlock with EPIDERMAL RIDGESof epidermis

All dermal papillae contain capillary loopswhich nourish accessory structures of the skin

May also contain tactile receptors known as MEISSNERs CORPUSCLESnerve endingssensitive to touch

Others may contain free nerve endings

Capillary loops extend into dermal papillae from the papillary plexus derived from deepervessels in the hypodermal layer


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The dermis: reticular layer

Deep to papillary dermis, constituting 4/5ths of dermal thickness

Consists of a deep interwoven network ofDENSE IRREGULR

CONNECTIVE TISSUE

Contains bundles of thick collagen fibres, scattered fibroblasts and

roaming macrophages

Some adipose cells may also be present in the deepest layers, along

with coarse elastic fibres

Collagen fibers have a more regular arrangement that those in the

papillary region


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FUNCTIONS OF THE SKIN


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phase 1_week 1_the skin ppp

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