Skin Notes

SKIN

-Large organ, covers the exterior surface of the body

Functions

-protection-protects the body from damaging external agents such as the

-UV

-infection-barrier to bacteria, fungus- both bacteria and fungus live on skin surface but can’t penetrate tissue unless skin is breached

-excessive wetting

-mechanical insults- frictional forces experienced by soles/ventral aspect of toes

-detection of sensory stimuli- skin is the largest sensory organ and has different receptors-touch, pressure, pain, temperature

-thermoregulation- skin plays an important role in heat conservation. To loose heat, increase blood flow through rich vascular network in skin and secretion of sweat- a watery secretion of eccrine glands onto skin surface and evapouration produces a cooling effect.

-prevention of dehydration

-metabolic functions- synthesis of vitamin D3 occurs in the skin, adipose tissue stores TG

Development

-epidermis formed from the ectodermal gern layer, dermis is formed from the mesoderm

-ectoderm is initially a single layer then divides into a bilaminar- outerperiderm, epidermis proper

-epidermis proper forms all 4 layers of the mature epidermis, periderm is a temporary protective membrane

-after 2 months of fertilisation, the ectodermal epidermis is invaded by cells of the neural crest region- melanocytes

-Hair forms afterwards- epithelial downgrowth into dermis

-sweat glands grow down from the epidermis, sebaceous glands formed as epithelial buds from the hair follicle

-by sixth month, periderm has keratinised and is desqumated.

Structure

-specialised epithelium which has glands (sweat glands, sebaceous glands-lubricate an oily matter(sebum) into the hair follicles to lubricate the skin and hair, mammary glands). The specialised epithelial tissue also is associated with supporting tissue

-skin is arranged in layers

-epidermis (outermost layer)

-dermis

-subcutaneous tissue (hypodermis)

-the relative thickness of these layers is dependent on the area of the body and reates to specific functional specialisations;

Epidermis

-surface layer of skin cells that are in contact with the external environment

-there is a constant need to replenish the epidermis, as outer surface is constantly sloughed off, so the basal layers are constantly proliferating to replace the dead cells

-sweat glands and hair follicles are down growths of this layer

Structure of the epidermis

DRAW LABELLED DIAGRAM IN EXAM

-keratinised stratified squamous epithelium-keratinocytes tough keratinised upper layer formed by keratinocytes which die and produce keratin plates (squames). The outermost layer of keratin is shed and replaced by new keratinocytes from deeper layers

-the epidermis has distinct layers

-the thickness of the skin is classified according to the thickness epidermis. Thick skin has a deeper stratum corneum, stratum granulosum and stratum lucidum. Found in areas which are most exposed to abrasive forces (palm of hands, soles of feet)

a) stratrum basale (basal layer)

-the low cuboidal keratinocytes express small amount of specific keratin isoforms which aggregate to form tonofilaments-merge into desmosmes- single layer of cuboidal/columnar cells attached to each other via desmosomes and attached to the basement membrane via hemidesmosomes, seperates the connective tissue from the epidermis

-Desmosomes: Structural integrity-very important in the skin, allows it to withstand high abrasive forces, keratin isoforms that insert into membrane anchor protein, desmoplankin, transmembrane proteins-CADHERINS-electron dense line

-layer has stem cells-assymetrically divide to reform a stem cell, transit amplifying cell. Large amount of transit amplifying cells-differentiate into keratinocytes-unipotent, stem cells divide at a low rate, prevent the frequency of mutations. Stem cells are responsible for repeated mitotic divisions for constant regeneration of other layers-responsible for kertinocyte production and other epidermal cell types- cells are rich in ribosomes and mitochondria, allows for raid cell turnover and protein synthesis.

-Also found in the basal layer are malanocytes which produce skin pigment melanin (synthesised from amino acid, tyrosine) which colours skin and hair and reduces damage caused by UV radiation. They synthesise melanin in cytoplasmic membrane bound granules, melanosomes, which advance along cytoplasmic processes into the cytoplasm of basal and prickle layer kertinocytes. In keratinocytes when exposed to UV light it deposits melanin. Melanocytes are more active in darker skinned people

Disorders of melonocytes

-Dominant piebald trait- melanocytes are derivatives of neural crest, defects in neural crest migration produces pigmentation defects- mutations in tyrosine kinase leads to the absence of pigmentation in certain areas of the body

-vitiligo- symmetrical areas of depigmentation of skin occurs often on hands, fingers, face- disease destroys meloanocytes in affected skin and skin becomes glaringly white- caused by autoimmune destruction of melanocytes

-moles- benign accumulations of melanocytes in dermis/epidermis

-albinism- is due to a defect in tyrosinase enzyme

b) stratum spinosum (prickle layer)

-layer of polyhedral cells with round central nuclei that sits on the basal layer

-system of intercellular bridges made from cytoplasmic extensions which connect the prickle cells. These cytoplasmic projections terminate as desmosomal junctions on the cell surface.

-large stanining nuclei, prominent nucleoli-active protein synthesis of cytokeratin- intermediate filament- accumulates in cell in form of aggregates known as tonofibrils. Tonofibris converge into desmosomes that form strong contacts between adjacent keratinocytes. Few tonofibrils in the basal layer

c) stratum granulosum (granular layer)

-in this layer, cells are packed with protein keratohyaline which forms dense cytoplasmic granules- basophilic granules that contain sulphated amino acids (cysteine)-promote dehydration of the cell...

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