The skin
- The skin consists of three major layers
- Epidermis – outer superficial layer
- Dermis – middle layer
- Hypodermis – deepest layer
- Outer portion of the skin is exposed to the external environment and
functions in protection
The epidermis
- Composed of keratinized stratified squamous epithelium
- Made up of four distinct cell types and five layers
- Cells of the epidermis
- Keratinocytes – produce the fibrous protein keratin
- Melanocytes – produce the brown pigment melanin
- Langerhans’ cells – epidermal macrophages that help activate the
immune system
- Merkel cells – function as touch receptors in association with
sensory nerve endings
Layers of the Epidermis
- Stratum Basale (Basal Layer)
- Deepest epidermal layer firmly attached to the dermis
- Consists of a single row of the youngest keratinocytes
- Cells undergo rapid division, hence its alternate name, stratum
germinativum
- Stratum Spinosum (Prickly Layer)
- Cells contain a system of intermediate filaments attached to
desmosomes
- Melanin granules and Langerhans’ cells are abundant in this layer
- Stratum Granulosum (Granular Layer)
- Thin layer in which drastic changes in keratinocyte appearance
occurs
- Keratohyaline and lamellated granules accumulate in the cells of
this layer
- Stratum Lucidium (Clear Layer)
- Thin, transparent band superficial to the stratum granulosum
- Consists of a few rows of flat, dead keratinocytes
- Present only in thick skin
- Stratum Corneum (Horny Layer)
- Outermost layer of keratinized cells
- Accounts for three quarters of the epidermal thickness
- Functions include reducing water loss and protection from abrasion
and penetration
- Renders the body relatively insensitive to biological, chemical,
and physical assaults
The dermis
- Second major skin region containing strong, flexible connective
tissue
- Cell types include fibroblasts, macrophages
- Also mast cells and white blood cells
- Composed of two layers called the papillary and reticular dermis
- Papillary dermis
- Areolar connective tissue with collagen and elastic fibres
- Its superficial surface contains peg-like projections called
dermal papillae
- Dermal papillae contain capillary loops, Meissner’s corpuscles and
free nerve endings
- Reticular dermis
- Accounts for approximately 80% of the thickness of the skin
- Collagen fibres in this layer add strength and resiliency to the
skin
- Elastin fibres provide stretch-recoil properties
The hypodermis
- Subcutaneous layer deep to the skin
- Composed of adipose and areolar connective tissue
Sweat glands
- Different types prevent overheating of the body; secret cerumen and
milk
- Eccrine sweat glands – found in palms, soles of the feet, and
forehead
- Apocrine sweat glands – found in axillary and anogenital areas
- Ceruminous glands – modified apocrine glands in external ear canal
and secrete cerumen
- Mammary glands – specialized sweat glands that secret milk
Functions of skin
- Protection – chemical, physical, and mechanical barrier
- Temperature regulation
- Achieved by dilation (cooling) and constriction (warming) of
dermal vessels
- Sweat glands increase secretions to cool the body
- Cutaneous sensation – receptors sense touch and pain
- Metabolic functions – synthesis of vitamin D in dermal blood vessels
- Blood reservoir – skin blood vessels store up to 5% of the body’s
blood volume
- Excretion – limited amounts of nitrogenous wastes are eliminated
from the body in sweat
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