Skin 3, Epidermis

Skin

The skin is the largest organ of the body and completely covers the body.
Continuous with mucus membranes
8% of body mass in adults (liver is less than 3%)
Average adult 2m2 5Kg
There are two main layers of the skin: Epidermis and Dermis
Underneath the skin is a layer of subcutaneous fat, the hypodermis.
Basement membrane zone
Skin is normally under tension


The Epidermis
A keratinised stratified squamous epithelium.
Consists of 4 or 5 layers each containing different cells at different stages.
Cells are produced in the germinating layer and move up through the other layers to the surface, 4-8 weeks.
Keratinocytes secrete a variety of lipids, including dehydrocholesterol
Cells on the surface contain keratin.
Keratin is a waxy then hard, water repellent, protein like substance which helps to protect the skin.
Varies in thickness, thickest on soles of feet and palms of hand and thinnest on lips and eyelids.
No blood vessels, only light tough sensory nerve endings.
Deep layers are bathed in tissue fluid to provide nutrients and oxygen which is drained as lymph.
10% of keratinocytes are stem cells


Cell types
Keratinocytes
Melanocytes with dendrites
Dendritic (Langerhans) cells
Merkel cells and discs


Epidermal strata


Stratum basale (base)
One layer thick
Cuboidal / columnar
Germinative with stem cells
Mitosis rate increased by superficial injury, epidermal growth factor
Tonofilaments will be later converted in to keratin
Desmosomes connect cell to cell


Stratum spinosum (spiny, thorn like)
Closely packed keratinocytes, 8-10 layers thick
Tonofilaments will be later converted in to keratin
Desmosomes connect cell to cell
Dendritic cells


Stratum granulosum (little grains)
Flattened cells undergoing apaptosis, first dead layer
3-5 layers thick
Tonofilaments converted in to keratin by granules of keratohyalin
Lamellar granules secret lipids, waterproofing, eg. burns, swimming
Dehydrocholesterol

Stratum lucidum (in thick skin)
Fingertips, palms and soles
3-5 layers thick, flattened dead cells

Stratum corneum (horny)
25 -- 30 layers
Dead cells, mostly keratin
Intercellular lipid preserved
Pressure leads to callus of this layer
Shed cells form dust