SKIN: Structure and Function of Skin Flashcards
What are some functions of the skin?
- acts as a barrier against physical, chemical and biological threats (including immunological aspects)
- 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.)
What are the two basic layers of skin, and what structures come under them?
The two basic layers of skin are the epidermis and the dermis.
Going from top to bottom, the structures found in the different skin layers are: 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)
Beneath the dermis is the HYPODERMIS (or subcutis), which is looser connective tissue containing (characteristically) adipose tissue. Some definitions include this as the third layer of skin.
What does integument refer to?
Integument refers to the skin, hair, and nails.
The skin is the largest and heaviest organ of the body (making up about 15% of an adult’s weight).
Describe the stratum corneum.
It is the outer, protective layer of epidermis, many cells thick.
- contains squamous cells which have lost their nuclei
- the cells are also cornified - full of ‘horny’ keratin (from keratohyalin granules), thus are tough and resistant to injury
- non-polar lipids (waterproof) are between the cells - from lamellar bodies
Most of the skin is oily and soft; this is because the cells contain soft keratin. However, other parts, such as nails and hair, contain hard keratin.
Describe the stratum granulosum.
There are 1-4 layers of cells containing prominent granules of ‘keratohyalin’ - the precursor for the protein keratin.
It also contains lamellar bodies containing lipids. These cells are differentiating to form the most outermost layer of skin.
Describe the stratum spinosum.
These cells have many desmosomes (junctions) here, visible as ‘spines’ between the cells.
Strong bonds hold this epidermis layer together.
Describe the stratum basale.
It is the first, single layer containing stem cells, attached to the dermis. The stem cells constantly proliferate.
It is a dynamic layer - the daughter cells gradually move ‘up’ (distally) through the epidermis until they are shed from the outer surface, taking about 20-50 days.
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 stain, 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.
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.
There are some differences between thick skin and thin skin.
What is an extra ‘layer’ that is found in thick skin?
[unlikely to be examined on]
Thick skin has an extra layer that is halfway between the granular cells and stratum corneum.
The layer is called the stratum lucideum (clear layer).
It contains immature keratin.
List some other epidermal cell types, besides keratinocytes.
- melanocytes (pigment)
- langerhans cells (defence)
- merkel cells (sensation)
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)
What is the function of a melanocyte (in the epidermis)?
It synthesises melanosomes (pigment granules) and transfers them to basal keratinocytes through long dendrites.
In which layer is melanin pigment mostly found?
Melanin pigment is mostly found in the basal epidermis.
Keratinocytes arrange melanin pigment in a cap distal to the nucleus (sunny side), especially in the basal layer (stem cells).
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 are the functions of Langerhans cells?
Their function is within the immune system.
They are antigen-presenting cells (like macrophages).
They’re dendritic cells, forming a network.
How would you identify Langerhans cells with H&E?
They are small, pale cells in the non-basal (upper) layers of the epidermis.
They’re hard to see with H&E only though.
Describe Vitamin D production in the skin.
- Vitamin D3 is made in the epidermis
- it’s done 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
Describe the dermis.
It is dense (full of collagen fibres), irregular (fibres run in all directions, not parallel) connective tissue.
Collagen provides tensile strength (strength when pulled), hence it acts as protection against abrasion and impact.
It also contains elastin, a protein complex that provides elasticity.
The dermis also carries blood and nerve supply for the epidermis, as it is rich in blood vessels.
The dermal-epidermal border is often wavy, to resist shear forces (rubbing sideways).
Describe the elastic fibres in the dermis.
Elastin van Gieson stain shows the collagen fibres in red, and the elastic fibres in black.
These are stretchy and give the skin its elasticity.
One effect of UV light on the skin is damage to elastic fibres, resulting in the loss of elasticity (ie. wrinkles).
Describe the hypodermis.
It is composed of fat, containing glands, hair follicles, nerves, and blood vessels. It is often the thickest layer of skin. The thickness varies with age, body site, nutrition, etc.
It is where you put a hypodermic syringe, for a subcutaneous injection.
Its function is that it provides insulation, cushioning and energy storage.
Appendages are things that are attached to something larger.
What are the three main appendages of the skin?
- hair follicles: each has an erector pili muscle which controls the angle of the hair
- sweat glands: of which there are two types (eccrine, all over the body, and apocrine, smelly in the armpit and groin)
- sebaceous glands: these are holocrine glands which release sebum (the oily/waxy substance that waterproofs the skin and hair, to prevent dryness and flaking) [when these sebaceous glands become overactive, we get acne]
What are the three different ways for a gland/cell to secrete something?
MEROCRINE: release via exocytosis (these are accrine glands)
APOCRINE: pinch bits off, give bits away
HOLOCRINE: blow up
