Skin

18.5.06

Skin disease is a major and disabling problem for many people all over the world.

Skin
• Largest organ of the body.
• Many different functions:
-Thermoregulations.
-Protection.
-Metabolic functions.
-Sensation.

2 main regions
• Epidermis.
• Dermis.
• Each provides distinct role in overall function.
• Dermis attached to underlying hypodermis (also called subcutaneous connective tissue).
• Superficial fascia of gross anatomy.
• Stores adipose tissue.

Epidermis
• Most superficial layer of skin.
• Provides 1st barrier of protection from invasion of foreign substances into body.
• Principal cell: keratinocyte.
• Subdivided into 5 strata.
• In stratum corneum, keratinocytes migrate to surface and are sloughed off - desquamation.

Stratum germinativum
• Single layer of cuboidal cells - relatively large nuclei - basophilic cytoplasm.
• Only layer where cell division normally occurs.
• Provides germinal cells necessary for regeneration of layers of epidermis.
• Basal cells disivide asymmetrically - 1 daughter cell remain attached to basement membrane.

Stratum spinosum
• Cell in Stratum germinativum accumulate - many desmosomes on outer surface.
• Provide characteristic "prickles" of stratum spinosum ("intercellular bridges").
• Often called prickle-cell layer.

Stratum granulosum
• Keratinisation - accumulation of keratin by progressive maturation of keratinocytes.
• Cekks accumulate dense basopohilic keratohyalin granules.
• Contains lipids.
• Cytoplasm significantly more prominent.
• Nucleus elongated.

Stratum lucidum
• Normally only well-seen in thick epidermis.
• Represents transition from Stratum granulosum to stratum corneum.

Stratum corneum
• As cell accumulates keratohyalin granules - rupture of lysosomal membranes - release lysosomal enzymes - causes cell death.

Dermis
• 2 areas:
-Reticular dermis.
-Papillary dermis.
• Papillary dermis projects as round pegs into overlying epidermis.

Functions of dermis
• Thermoregulation.
• Support of vascular network to supply avascular epidermis with nutrients.
• Consists mostly of fibroblasts.
• Secrete:
-Collagen.
-Elastin.
-Ground substance.
• Gives support and elasticity to skin.
• Immune cells provide defence against foreign particles etc. passing through epidermis.

Papillary dermis
• Contains free sensory nerve endings.
• Meissner's corpuscles in highly sensitive areas.
• Composed of Type III collagen.
• Increase/decrease in blood flow, heat can either be conserved/dissipated.

Reticular dermis
• Consists of dense, irregular tissue.
• Gives skin overall strength and elasticity.
• Includes glands and hair follicles.
• Composed of Type I collagen.

Inflammation and wound healing
Vascular response
• Initial vasoconstriction as direct response to trauma.
• Exposed subendothelial tissue activates coagulation and complement cascades.
• Platelet adhesion and aggregation causes clot formation.
• Degranulation of platelets releases growth factors and chemotactin factors.
• Inflammatory response due to histamine and 5HT release.

Cellular response
• Migration fo neutrophils, macrophages and lyphocytes.
• Macrophages produce growth factors, leading to migration of fibroblast and epithelial cells.

Wound healing
• Epithelial barrier important - prevents infection - maintains fluid balance.
• Achived by both migration and proliferation of epithelia cells.
• Maximum collagen production occurs at 20 days.
• Maximum wound strength at 3-6 months.
• Initial collagen production disorganised.
• Remodelling lines it up with stresses in skin.
• Reduced vascularity and strength.

Scars
• Become red and thickened during healing.
• Takes several months for flattening of wound.
• Excess collagen formation may occur.