Human skin biology Flashcards
Human Skin Structure
- Epidermis
- Dermis
- Hypodermis
Human Skin Structure - Epidermis
- Top layer of skin
- Provides a barrier and continued renewal
- No structural strength
- Mainly consists of layers of keratinocytes
- Thin skin has 4 layers
- Thick skin – fingertips, palms, soles
- Thick skin has 5 layers, 5thlayer is Stratum lucidum
- No vasculature – all nutrient supply and waste removal through the dermis
Epidermis - Stratification
Crucial for barrier function and continued renewal of the epidermis
Stratification – Stratum Basale
- The bottom layer of the epidermis
- Keratinocyte stem cells are the reservoirs of cells for a lifetime of renewal
- Transit amplifying keratinocytes proliferate a lot to provide cells for all the top layers
Stratification – Stratum Spinosum
- 8-10 layers of cells
- Keratinocytes start to flatten out
- Keratin intermediate filaments and desmosomes hold cells together
Stratification – Stratum Granulosum
- Flattened keratinocytes undergoing apoptosis
- Lamellar granules -fuse to plasma membrane and release lipid rich secretions to help form the barrier.
- Keratohyalin- dark granules, help form keratin intermediate filaments into keratin.
Stratification – Stratum Lucidum
- Only present in thick skin
* Fingertips, palms, soles
Stratification – Stratum Corneum
- 25-30 layers of flattened, dead keratinocytes
- Keratinocytes overlap like scales of snake
- Barrier formed to keep moisture in and the outside world out
Stratification - Process
- Proliferating keratinocytes on the bottom of the epidermis push cells up and away from the dermis.
- Undergo programmed cell death
- Complete epidermal turnover approximately once a month
Basement Membrane
- Interface between the dermis and epidermis
- Collagen IV, Perlecan, Nidogen, Laminin 332
- Important for epidermal attachment to the dermis
- Mutation in basement membrane proteins can result in Epidermolysis Bullosa
Rete Ridges
- Dermal papillae/Rete ridges
* Contour provides resistance to shear forces
Pigmentation - Melanocytes
- reside at the basement membrane, on the epidermal side
- Make melanosomes, which contain the pigment melanin - Melanosomes are transferred to keratinocytes- Contacts on average 36 keratinocytes
Pigmentation - Melanin
- pigment that gives skin its colour
- Pheomelanin is yellow-red, eumelanin is brown-black
- Melanin protects from UV
- Nuclear cap protects keratinocyte DNA
Epidermis – Langerhan’sCells
• Langerhan’scells surveil the epidermis for foreign organisms
Human Skin Structure - Dermis
- Dense matrix made up of collagen and elastin fibres
- Strong and supple
- Thickness varies, again thickest on soles and palms
- Unlike epidermis, dermis is very stable, turnover is minimal
Dermis - Fibroblasts
- Fibroblasts produce collagen and elastin
- Collagens - strength
- Elastin - elasticity
Dermis - Papillary and Reticular
- Transverse section of human skin
- Papillary dermis – high cell density, loose connective tissue
- Reticular dermis – low cell density, dense connective tissue
Dermis – Vasculature
• Vasculature –- supply nutrients and remove waste for both dermis and epidermis
Wound Types
- Superficial
- Partial-thickness
- Full-thickness
Superficial Wounds
Damage to the epidermis only
Superficial Wound Healing
Healing occurs by migration of keratinocyte from the wound edges and dermal appendages (sweat glands, hair follicles, sebaceous glands)
Superficial Wound Healing
Once all keratinocytes are in contact on all sides, then stratification can occur to reform the epidermis
Partial Thickness Wounds
All of the epidermis and some of the dermis is destroyed
Partial Thickness Wound Healing
• Four phases of healing –
- Inflammatory phase - immune cells come in and clean up the wound.
- Migratory phase-keratinocytes migrate from the wound edge and appendages, fibroblasts migrate into the clot make collagen fibres
- Proliferative phase -keratinocytes proliferate
- Maturation phase - epidermal stratification then scab falls off.
Full Thickness Wounds
- All of the epidermis and dermis is destroyed
* Hypodermis can be destroyed too, exposing bone and muscle
Full Thickness Wounds
- Wound repair is very difficult because all of the reservoirs of epidermal stem cells have been destroyed.
- Keratinocytes have to migrate from the wound edges
- Heals as scar tissue
- Intervention is required to improve patient outcomes
Full Thickness Wound Treatment
- Split thickness skin graft is the gold standard
- Split thickness skin graft - all of the epidermis, part of the dermis
- Donor site – undamaged skin
- Split thickness skin graft covers the wound
- Donor site will heal in 10-14 days
