SKIN: Structure and Function of Skin Flashcards
What does integument refer to?
Integument refers to the skin and its appendages which are hair, nails and glands
The skin is the largest and heaviest organ of the body (making up about 15% of an adult’s weight).
Structures of the Integument
Skin (3 layers)
- epidermis (outer epithelial layer of the skin)
- dermis (Nerves & blood vessels)
- hypodermis ( fat & connective tissue)
Hairs
Glands
Nails
Sense Organs
What are some functions of the skin?
- acts as a barrier against physical (injury), chemical and biological threats (infections)
- homeostatic functions in temperature and water balance regulation
- sensory functions
- secretory/’nutritional’ function producing Vitamin D
- insulation (really, a property of the hypodermis/subcutis)
- repair
- cosmetic (tattoos, piercings, etc.)
Epidermal Layers (deep to superficial)
EPIDERMIS: (has no blood supply)
- stratum corneum
- stratum lucidum
- stratum granulosum
- stratum spinosum
- stratum basale
DERMIS: (everything below the stratum basale and above the adipose tissue)
- dermal papillae
- papillary layer
- reticular layer
(don’t need to know this in detail, but could help)
Stratum Corneum (cornified)
- outer protective layer of the epidermis
- many cells thick
- squamous keratinocyte cells which have lost their nuclei and are cornified (full of horny keratin from keratohyalin granules), thus tough and resistant to injury
- non-polar lipids from lamellar bodies are now between cells for waterproof purposes
- cells flaking off and constantly being replaced
Stratum Basale (basal layer)
- keratinocyte stem cells attached to the dermis and constantly proliferate
- dynamic daughter cells move “up” distally through the epidermis, differentiating as they go until they are shed from the outer surface (takes 20-50 days)
Stratum Spinosum (spiny layer)
- keratinocyte cells held together by many desmosomes (junctions), which are visible as “spines”
- strong bonds hold the epidermis together
Stratum Granulosum (granular layer)
1-4 cell layers, but cells are flattened, so the layer is thin
Keratinocyte cells contain prominent keratohyalin granules, a precursor of the protein keratin
Cells also contain lamellar bodies (granules) containing lipids and they help with waterproofing the skin
Cells are differentiating to form the outermost layer
Lack of nuclei in these cells as they don’t need to divide
Epidermal cells other than keratinocytes
Melanocytes (pigment) Langerhans cells (defence) Merkel cells (sensation)
What would you see if you stained a section of skin?
The cells in the epidermis are very close together, so they would give a very intense strain, whereas tissue in the dermis is much looser, with cells further apart, so it stains less intensely.
Fat (the rounded bits in the subcutis) doesn’t stain at all.
Melanocytes
synthesize melanosomes (in which melanin pigment is synthesized) and transfer them to basal keratinocytes through long dendrites
How do keratinocytes protect the skin from UV radiation?
they arrange melanin pigment on top of the nucleus (in a cap distal), protecting the nucleus from UV radiation from the sun
Why are melanocyte pale cells?
Why are melanocyte pale cells?
because their melanin pigment tends to be transferred to other basal cells, meaning not much pigment is in the melanocyte itself
What are the two types of melanin?
Two types of melanin account for variations in skin and hair colour. The result depends on how they are mixed.
1) Eumelanin (brown/black)
2) Pheomelanin (red/yellow)
How would you be able to identify melanocytes by H&E?
They would be pale cells, in or protruding from the basal layer.
Some pale cells in the basal layer are Merkel cells: touch-sensors. It’s hard to tell the difference with H&E alone.
What is a frictional blister?
It is a watery type of blister. It occurs when the top layer of skin rubs against the basal cells (bottom layer of the epidermis). This causes the spiny cells to get crushed, so the watery fluid in the content of all the spiny cells that burst leaks out.
Then, because there is a lot of protein in this fluid, there is a lot of osmotic pressure. So, the water gets pulled via osmosis as a transudate.
Why do you go wrinkly if you stay in water for a long time?
The reason is that water can only go as far as the granular layer (which is oily) because the stratum corneum can absorb a lot of water. So, it swells and, as the layer below stays the same, the stratum corneum has to ripple.
Merkel cells
pale cells in the basal epidermis (like melanocytes)
touch-sensors/receptors
Langerhans cells
what are they?
function?
where are they?
dendritic cells which form a network and function as antigen-presenting cells (like macrophages) to protect against infection
small pale cells in the upper layers of the epidermis (non-basal layers)
They’re hard to see with H&E only though.
Describe Vitamin D production in the skin.
- Vitamin D3 is made in the epidermis
- mostly in the basal cells, but also in the stratum spinosum
- it requires UV light
- it requires more UV light in dark skin (due to the melanin barrier)
- it is converted into its active form in the liver and kidney (1,25-dihydroxyvitamin D3)
- it is commonly deficient in the UK, due to the relative lack of sun exposure
The conflict between vitamin D production and protection from UV light
Vitamin D3 needs UV light to produce active vitamin D, however we want to protect ourselves from UV light to prevent DNA damage and cancer
Describe the dermis.
It is dense (full of collagen fibres), irregular (fibres run in all directions, not parallel) connective tissue.
Collagen provides tensile strength (made from fibroblasts), hence it acts as protection against abrasion and impact.
It also contains elastin, a protein complex that provides elasticity.
The dermal-epidermal border is often wavy, to resist shear forces (rubbing sideways).
Circulation of dermis
The dermis also carries blood and nerve supply for the epidermis, as it is rich in blood vessels.
What are the two layers of the dermis
Thin papillary layer
Deeper reticular layer
