Structure and function of the skin Flashcards
What system is the skin part of?
• Integument or Integumentary system
Skin, Hair and Nails
How much of the body does the integumentary system comprise of?
Largest and heaviest organ of the body - ~15% of adult body weight
What other features of the human body does the skin encompass?
Hair, skin glands, nails and sensory receptors.
What are the functions of the skin?
Barrier (protection) against: • Dehydration • Infection • Injury/abrasion • Solar radiation
- Thermoregulation
- Sensation
- Repair
- Vitamin D production
What are the 3 layers of the skin?
- Epidermis = outer epithelial layer - thin outermst layer
- Dermis = Middle connective tissue/collagen layer - nerves and blood vessels
- Hypodermis = Inner fatty layer and connective tissue
What are the roles of the epidermis?
• Gives skin its colour • Protection from Pathogens Environment • Vitamin D production
What cell type is prominent in the epidermis?
Keratinocytes - flat pancake-shaped cells that are named for the keratin -producing capabilities.
What are the 5 main layers of the epidermis?
Cornified layer [C] (Stratum corneum)
• The outer protective layer of the epidermis.
• Cell are keratinised (cornified) – cytoplasm full of “horny” keratin (from keratohyalin granules), thus tough and resistant to injury.
• Cells are flattened and have lost their nuclei…
• Nonpolar lipids (waterproof) are between the cells – from lamellar bodies.
• STRATUM LUCIDIUM (Clear layer) [only in thick skin e.g. palms and soles of feet]
Granular layer [G] (Stratum granulosum)
• 1-4 layers of cells containing prominent granules of “keratohyalin” – precursor of the protein keratin. Also contain lamellar bodies containing lipids (seen by TEM). Cells are differentiating to form the outermost layer.
• Stratum Granulosum - Keratinocytes in this layer begin the process of keratinization, which is the process where the keratinocytes flatten out and die, and in the process they create the epidermal skin barrier. To do this, keratinocytes in the stratum granulosum layer produce large amounts of keratin precursor proteins and glycolipid which remain within granules called keratohyalin granules and lamellar granules, respectively. Keratohyalin granules eventually start to aggregate and cross-link forming enormous bundles of keratin within the keratinocyte. Lamellar granules, on the other hand, get secreted and stick to the outer cell surface. It forms a sort of cement between the cells, making them more resistant to external forces and water loss.
Spiny layer [S] (Stratum spinosum)
• Cells (keratinocytes) have many desmosomes, (junctions) here visible as “spines” between the cells. Strong bonds holding the epidermis together.
Basal layer [B] (Stratum basale)
• First single layer, containing stem cells, and attached to dermis.
• Stem cells constantly proliferate.
• Dynamic - Daughter cells constantly move “up” (distally) through the epidermis, differentiating as they go, until they are shed from the outer surface. This takes ~20-50 days.
Give some examples of other epidermal cell types
- Melanocytes (Pigment)
- Langerhans cells (defense)
- Merkel cells (sensation)
What forms do melanocytes take and what do they synthesize?
Where are they mainly found?
How do keratinocytes arrange melanin?
What is the function of melanocytes?
Synthesizes melanosomes (pigment granules) and transfers them to basal keratinocytes through long dendrites.
Basal layer
Keratinocytes arrange melanin pigment in a cap distal to the nucleus (sunny side)
UV protection
What other cell types are found in the basal layer?
Merkel cells - pressure sensors
What is the function of Langerhans cells and describe their structure
How do they appear when stained with H&E?
Function: Immune system. Seeks and deals with invading microbes.
Antigen-presenting cells (like macrophages).
They are dendritic cells, forming a network – seen here with immunoperoxidase staining.
H&E appearance: Small, pale cells in non-basal layers of epidermis. Hard to see with H&E only.
Where is vitamin D produced?
What does it require in order to be produced?
- Vitamin D3, made in the epidermis.
- Mostly basal cells, also stratum spinosum.
- Requires UV light.
- Requires more UV light in dark skin (melanin barrier).
- Converted to active form in liver and kidney: 1,25 dihydroxyvitamin D3.
- Vitamin D3 (cholecalciferol) is synthesized in the skin by the reaction of 7-dehydrocholesterol with UVB radiation, present in sunlight with an UV index of three or more.
What are the 2 roles of the dermis?
What does it contain?
How many layers has the dermis got?
- Tensile strength & elasticity
- Blood vessels, skin glands & sensory receptors
2
Describe the structure of the dermis
What is the most prominent cell type?
What gives the dermis high tensile strength and elasticity?
What runs in the dermis?
Made up of layers of connective tissue characterized by interconnected mesh of elastin and collagen fibres, produced by dermal fibroblasts.
Fibroblasts are the principal cell of the dermis.
Functions: Collagen provides tensile strength, (strength when pulled), hence protection against abrasion and impact.
Also contains elastin, a protein complex that provides elasticity.
Dermis also carries blood and nerve supply for epidermis. Rich in blood vessels, sensory receptors and skin glands.
