Anatomy and Physiology of SkinDr Ashwini Kalantri

Skin

Largest organ of the body

Completely covers the body

Continuous with membranes lining body orifices

Average thickness: 1-2 mm. (0.5mm eyelids, 6mm palms and soles)

pH: 4-5.6

Renewal of skin takes place in 28-50 days by shedding of the outer layer.

Structure of Skin

Layers 1. Epidermis2. Dermis

Epidermis

Most superficial layer of the skin composed of stratified epithelium

No blood vessels

Nutrition provided by the capillaries of dermis

Approx. thickness 0.4-1.5mm, thickest on palms and soles.

Layers of Epidermis

Stratum Corneum

Stratum LucidumStratum Granulosum

Stratum Mucosum

Stratum Germinativm

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Cells in the Epidermis

Keratinocytes: Major building block of the epidermis

Melanocytes: Large cells interspaced among keratinocytes, produce melanin.

Langerhans Cells: Antigen presenting cells

Merkel cells: Represent special nerve endings within epidermis

Hair, Sebaceous glands and ducts of sweat glands.

Colour of the Skin

Pigmentation of skin: Melanocytes contain the pigment melanin which darkens the skin on exposure to sunlight

Hemoglobin content in blood: The level of oxygenation of blood and amount of blood circulating in the dermis play an important role in skin colouration.

Dermis

Connective tissue layer made up of dense and stout collagen fibers, fibroblasts and histocytes. Collagen fibers have elastic property and are capable of storing water.

Layers:1. Superficial Papillary Layer

2. Reticular Layer

Superficial Papillary Layer

Projects in to the epidermis

Contains blood vessels, lymphatics and nerve fibers.

Has some pigment containing cells called chromatophore.

Dermal papillae are finger like projections arising from this layer

Reticular Layer

Made up of reticular and elastic fibers

These fibers are found around hair bulbs, sweat glands and sebaceous glands.

Also contains mast cells, nerve endings, lymphatics, epidermal appendages and fibroblasts.

The hair follicles with hairs, sweat glands, sebaceous glands and nails.

Glands of the Skin1. Sebaceous Glands2. Sweat Glands

Sebaceous Glands

Structure: Ovoid or spherical in shape, developed from hair follicles and covered by connective tissue capsule

Secretion: Secrete a oily substance called Sebum.

Composition: Contains free fatty acids, sterols, paraffin, waxes, squalene and triglycerides.

Functions: FFA has antibacterial and antifungal properties. Lipids keep skin smooth and oily, protecting from unnecessary desquamation and injury.

Sweat Glands - Eccrine

Distributed throughout the body with exterior opening through sweat pore with watery and clear discharge.

Temperature regulation

Secretion increases with increase in temperature and emotions under nervous control

Nerve supply by sympathetic cholinergic fibers.

Sweat Glands - Apocrine

Distributed only in a limited area – Axilla, pubis, areola and umbilicus.

Opens in to the hair follicle having thick and milky secretion.

Starts functioning with puberty and has no role in temperature regulation

Secretions increase under emotional conditions under hormonal control.

Supply by sympathetic adrenergic fibers.

Functions of Skin

Protective

Protection from bacteria and toxic substances by secreting lysozyme.

Protection from mechanical blow

Protection from UV rays with the help of melanin pigment.

Sensory

Skin is considered the largest sensory organ

Many nerve endings forming a specialized cutaneous receptors.

Storage

Stores fat, water, chloride and sugar.

Can also store blood with vasodilation of the cutaneous blood vessels

Synthetic

Vitamin D3 is synthesized in the skin with the action of UV rays on cholesterol.

Temperature Regulation

Excess heat is lost from the body by radiation, conduction, convection and evaporation.

Active role in heat loss by secreting sweat.

The lipid content of sebum prevents heat loss in cold temperature.

Water and Electrolyte balance

Excretion: Waste materials like urea, salts and fatty substances are also excreted.

Absorption: skin can absorb fat soluble substances and some ointments.

Secretion: Sweat and sebum are secreted by the sweat and sebaceous glands. Helps in temperature and water balance. Sebum helps in keeping the skin smooth and provides protection.