SKIN: Structure and function of skin Flashcards
(37 cards)
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).
What are the functions of the skin?
- acts as a barrier against physical, chemical and biological threats (including immunological aspects) –> these include dehydration (prevents water leaking), infection (immune cells inside), injury/abrasion, solar radiation.
- homeostatic functions in thermoregulation and water balance regulation
- sensory functions –> sensation
- secretory/’nutritional’ function producing Vitamin D
- insulation (really, a property of the hypodermis/subcutis)
- repair
- cosmetic (tattoos, piercings, etc.)
What are the 3 layers of skin and briefly describe them?
Going from top to bottom, the structures found in the different skin layers are:
EPIDERMIS: (has no blood supply, is the thin outermost layer)
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)
- Nerves and blood vessels
- Made of a lot collagen and connective tissue
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. Has fat and connective tissue. This layer is the thickest layer depending on the location in the body.
Derm = skin Epi = on Hypo = below
Briefly describe the epidermis.
EPIDERMIS: (has no blood supply, is the thin outermost layer)
- stratum corneum
- stratum lucidum
- stratum granulosum
- stratum spinosum
- stratum basale
It’s made up of multiple layers of differentiating keratinocytes. The thickness varies depending on the body part (e.g. thin in abdomen and very thick in heel.)
Cells are constantly flaking off at the surface, these are constantly replaced by cells from beneath.
Some functions:
- Most superficial
- Gives skin its colour
- Protection from pathogens and environment
- Vitamin D production
What are the 4 main layers of the epidermis?
From in to out:
- stratum basale
- stratum spinosum
- stratum granulosum
- stratum corneum
Under the palms of the hands and the soles of the feet is a layer called the stratum lucidium. This layer is found between the stratum granulosum and stratum corneum. This layer is translucent under the microscope.
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, differentiating as they go, until they are shed from the outer surface. This takes about 20-50 days.
Describe the stratum spinosum.
The spiny layer. This is right above the basal layer.
These cells (keratinocytes) have many desmosomes (junctions) here, visible as 'spines' between the cells. Strong bonds hold this epidermis layer together.
Describe the stratum granulosum.
The granular layer.
There are 1-4 layers of cells containing prominent granules of ‘keratohyalin’ - the precursor for the protein keratin. The keratohyalin are responsible for the granular appearance.
It also contains lamellar bodies containing lipids (seen by TEM). These cells are differentiating to form the most outermost layer of skin. Some of the cells start to lose their nuclei here.
Describe the stratum corneum.
The cornified later. It is the outer, protective layer of epidermis, many cells thick.
- the cells are also cornified (keratinised) -cytoplasm is full of ‘horny’ keratin (from keratohyalin granules), thus are tough and resistant to injury
- contains squamous (flattened) cells which have lost their nuclei
- 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.
Keratinisation occurs in the stratum corneum.
What is keratin the main component of?
- Hair
- Horns
- Nails
- Claws
- Hoofs
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?
From Sarah Sheikh Flashcards - not present in 2020 lecture
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?
From Sarah Sheikh Flashcards - not present in 2020 lecture
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]
From Sarah Sheikh Flashcards - not present in 2020 lecture
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?
From Sarah Sheikh Flashcards - not present in 2020 lecture
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)
Describe the melanocytes and their function.
A special DOPA stain is used to view melanocytes. They have a dendritic form – resemble neurones.
They synthesise melanosomes (melanin pigment granules) and transfers them to basal keratinocytes through long dendrites. Melanin pigment is mostly found in the basal epidermis.
Once the melanocytes have transferred the pigment granules to the keratinocytes. The keratinocytes then arrange the melanin pigment in a cap like conformation distal to the nucleus (sunny side) – covering the nucleus. The melanosomes are especially transferred to the basal layer (stem cells). This is a protective feature.
As a result, they provide UV protection – black skinned people have only 10% as many skin cancers as white people with the same lifestyle.
How would you be able to identify melanocytes by H&E?
They would be pale cells (cytoplasm), 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 and how can they be seen through staining?
Their function is within the immune system. They seek and deal with invading microbes.
They are antigen-presenting cells (like macrophages).
They’re dendritic cells, forming a network (can be seen through immunoperoxidase staining).
Through H & E staining:
- 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 (inactive precursor) is made in the epidermis
- it’s synthesised 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
Briefly describe the dermis.
- Below the epidermis
- Made of 2 layers: thin papillary layer and deeper reticular layer.
- It is responsible for the tensile strength and elasticity of the skin
- It houses blood vessels, skin glands and sensory receptors.
Describe the structure of the dermis.
The 2 layers of the dermis are the thin papillary layer (contain finger like papillary protrusions) and the deeper reticular layer (containing the network of fibres.).
Its made up of layers of connective tissue characterised by an interconnected mesh of elastin and collagen fibres, produced by dermal fibroblasts.
Fibroblasts are the principal cells of the dermis. They make and maintain collagen.
Describe some of the functions of 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 dermal-epidermal border
The dermal-epidermal border is often wavy, to resist shear forces (rubbing sideways).
- Dermal papillae - finger like protrusions of dermal connective tissue into the epidermal layer.
- Rete ridges - extensions of epidermis into the dermal layer.
- More apparent in the thick skin of the hands and feet. (Fingerprints?)
- see histology pics in slides
