Skin

•        Covering of the external surface of the body.

•        Continuous with the mucous membranes at the orifices of the body.

•        Regarded as an important organ of the body.

 

•        Functions

–      Protection

–      Sensation

–      Thermoregulation

–      Metabolic function ( Vitamin D)

–      Secretion & excretion

 

•        Epidermis

•        Dermis

•        Hypodermis

•        Epidermal appendages

–      Hair follicle

–      Sweat gland

–      Sebaceous gland

–      Nail

 

Epidermis

•        Stratified epithelium

•        5 Layers

–      Stratum basale

•        Cuboidal cells, sit on a BM, mitotic figures

–      Stratum spinosum

•        Large polyhedral cells (prickle cells), tonofibrils.

–      Stratum granulosum

•        Keratohyaline granules –basophilic granules in the cytoplasm

–      Stratum lucidum

•        Transitional zone, present in thick skin only

–      Stratum corneum

•        Dead & dying cells filled with mature keratin

 

Epidermis – cells

•        Keratinocytes

•        Melanocytes

•        Merkel cells

•        Langerhan’s cells

 

Keratinocytes

•        Cells produced by mitosis in the basal layer undergo maturational changes concerned with the production of keratin.

•        All cells of this lineage are described as keratinocytes.

•        The outer keratinised layer is continuously shed & replaced by progressive movement & maturation of cells from the basal layer.

•        The rate of mitosis = rate of desqamation of keratin (25 -50 days).

 

Melanocytes

•        Responsible for skin pigmentation.

•        Cell processes run between the other cells.

•        The brown colour pigment, melanin is located in membrane-bound organelles called melanosomes.

 

Merkel cells

•        Associated with free nerve endings in thick skin.

•        Function as sensory receptors.

 

Langerhan’s cells

•        Functionally related to macrophages.

•        Important in immune reactions of the epidermis.

 

Dermis

•        Composed of dense connective tissue containing many blood vessels, lymphatic vessels and nerves.

•        Shows variation in thickness:

–      thinner on the anterior than on the posterior surface.

–      thinner in women than in men.

 

•        Two layers

–      Papillary layer

–      Reticular layer

 

Dermis - papillary layer

•        Loose, cell-rich connective tissue,

•        Fills the hollows at the deep surface (dermal papillae) of the epidermis.

•        Capillaries are frequent.

•        Collagen fibres appear finer than in the reticular layer

 

Dermis - reticular layer

•        Denser and contains fewer cells.

•         Thick collagen fibres aggregate into bundles.

 

•        Two types of skins

–      Thick skin – (eg. sole)

–      Thin skin - (eg. abdomen)

Thin skin

•        Individual cell layers are more difficult to discern.

•        Stratum corneum is thiner.

•        Combined thickness of other cell layers are reduced.

•        Epidermal ridges are less prominent.

•        Presence of hair follicles.

 

Skin circulation

•        Functions

–      Nutrition of the skin & appendages

–      Increased blood flow to facilitate heat loss in hot conditions.

–      Decreased blood flow to minimise heat loss in cold conditions.

 

Sweat glands – types

•        Merocrine sweat glands

•        Apocrine sweat glands

 

Merocrine sweat glands

–      Simple coiled tubular type, watery secretion.

–      Thermoregulatory function.

–      Innervated by cholinergic fibers of sympathetic nervous system.

–      Sweating stimulated by excessive heat & fear-provoking stimuli.

 

Apocrine sweat glands

–      Found in the axilla & genital regions.

–      Viscid secretion, discharged into hair follicles.

–      Innervated by adrenergic fibers of the sympathetic nervous system.

 

Sebaceous gland

•        Associated with hair follicles.

•        Secrete sebum – waterproofing & moisturising agent for the skin.

•        In regions of transition of skin to the body tracts (lips, eyelids, glans penis, labia minora, nipples) they are found independent of hair follicles.

 

Hair

•        Highly modified keratinised structures.

•        Produced by hair follicles-invaginations of epidermis into the dermis.

•        Hair growth takes place in – hair bulb, the expanded extremity of the hair follicle.

•        Hair bulb is surrounded by vascular connective tissue – hair papilla.

•        Cells in the hair bulb are mitotically active & form the hair & the cells that surround its root, the root sheath.

 

Arrector pili muscle

•        Each hair follicle has an associated smooth muscle, the arrector pili muscle.

•        Inserts with one end to the papillary layer of the dermis & with the other end to the dermal sheath of the hair follicle.

•        Contraction causes ‘goose-flesh’ effect.