Skin Flashcards
What is the difference between dermis and epidermis?
Epidermis is the thin outside layer of skin, many cells packed closely together, mainly keratinocytes (produce keratin proteins and lipids).
Basal cells sit on the basement membrane and daughter cells move upwards from it towards the surface.
Keratinocytes also produce granules, the granules become a layer on the surface of the keratinocytes and do programmed cell death to become corneocytes on the external skin.
Within the epidermis there is also a few cells that aren’t keratinocytes; there are 2 types of special Dendritic cells - Langerhans cells (present antigens) and Melanocytes (produce melanin and pass it to keratinocytes).
Basement membrane separates the dermis from epidermis.
The eccrine sweat is formed from keratinocytes in the epidermis but a large part of it is found in the dermis.
The sebacceous gland is also made from cells in the epidermis but found within the dermis.
Hair follicle is an epidermal down growth down into the dermis.
There is a vascular supply to the dermis but not the epidermis so nutrients have to diffuse.
Rich neural supply to dermis but some fine nerves in the dermis too.
What is the dermis
Dermis is the layer beneath the epidermis, much larger area but much fewer cells.
Mainly accellular - collagen fibres, elastin fibres and ground substance. The fibroblast synchronise and remodel the extracellular materials.
There are also mast cells (like tissue basophils) which contain lots of granules.
The eccrine sweat gland is formed from keratinocytes in the epidermis but a large part of it is found in the dermis.
The sebacceous gland is also made from cells in the epidermis but found within the dermis.
Hair follicle is an epidermal down growth down into the dermis.
There is a vascular supply to the dermis but not the epidermis.
Rich neural supply to dermis but some fine nerves in the dermis too.
What are skin appendages?
Epidermal structures made from modified keratinocytes but found geographically in the dermis:
Eccrine sweat gland,
Sebaceous gland,
Hair follicle.
What are some of the constituents of skin?
Keratins: there are over 50 keratin genes — mutations of keratins are associated with a range of different disorders. Keratinisation is the process of epidermal (keratinocytes) differentiation that leads to the dead anucleate cell compartment called the stratum corneum (or horny cell layer).
The cell compartments (or layers) within the epidermis are: basal layer, spindle cell layer, granular layer, and stratum corneum.
Filaggrin: is a key protein, so called because is a keratin filament aggregating protein. Mutations of filaggrin are associated with atopic dermatitis (‘eczema’) and some other skin diseases.
Lipids: skin is relatively water impermeable; extracellular lipids produced by keratinocytes are key to this ability.
Cell types of the epidermis: The other main resident cells in the epidermis are keratinocytes, melanocytes and Langerhans’ cells; both of the latter are dendritic (star like shape) but they are not related to each other. Melanocytes are neural crest derived cells, and Langerhans’ cells are bone marrow derived.
The picture of the Langerhans’ cell is en face (rather than the conventional cross section when the plane of the image is parallel to the skin surface).
Melanin: whilst melanin is produced in melanocytes, the melanin is passed to the surrounding keratinocytes.
Mast cells are similar to basophils.
Appendageal structures: the collective name for the hair follicle, the sebaceous glands and the eccrine glands. The dominant cell type of these structures is the keratinocyte, even though they are principally located in the dermis.
What is a sunburn?
The chromophore is responsible for absorbing the UV light (technically it is the part of a molecule responsible for its colour) which leads to erythema, DNA is a chromophore, as it melanin and haemaglobin.
Erythema (sunburn) is around 1000 more potent at 300nm (UV) than 400nm (visible light), erythema exponentially decreasing as wavelength increases.
DNA damage has a very similar pattern.
UVB is shorter wavelength, than UVA, and causes more DNA damage.
The development of pyrimidine dimers in DNA is characteristic of UV radiation induced damage, but there are many other types of DNA damaged that it can induce as well.
We quickly resolve this DNA damage normally, but there is a small amount unresolved which can accumulate if repeatedly exposed.
Freckles/freckling is a focal overproduction of melanin, and a marker of sun damage from UVR.
Xeroderma pigmentosum (XP) is an autosomal recessive condition. There are a variety of types of XP which vary in their phenotypes, reflecting mutations of different genes. The clinical severity depends on the type, but also the amount of UVR the individual is subject to.
What is melanin?
Constitutive pigmentation is the baseline of pigmentation in skin, facultative pigmentation is due to UV exposure (i.e. a tan) which increases the amount of melanin in the skin.
Melanin is produced by melanocytes in little organs called melanosomes and pass it into the keratinocytes.
Within the cell, the melanocytes cluster together in the side of the keratinocyte which UV light is coming from. The nucleus is thus protected from the harmful UV since the melanocytes absorb it before it gets to it.
‘sunhat of melanosomes’
There are two main broad classes of melanin - eumelanin (brown/black) and pheomelanin (red/yellow).
It mainly acts on the visible spectrum, not the UV, evolutionary for camouflage or signalling. Sometimes it becomes adaptive for protection against UV also.
What are the classes of melanin?
There are two main broad classes of melanin - eumelanin (brown/black) and pheomelanin (red/yellow).
The ratio of eumelanin to pheomelanin is important for hair colour. Those with red hair have a high pheomelanin:eumelanin ratio. Those with black hair have a low pheomelanin:eumelanin ratio. If you have very little of either melanin, you have blonde hair.
How does skin cancer develop?
Exposure to UV happens continuously, which damages DNA;
DNA is usually quickly and efficiently repaired but occasionally mutations occur;
The mutates cells can clone, expand and acquire more mutations;
This can lead to skin cancer.
Melanin pigmentation protects - paler the skin, more likely to get mutations and cancer.
Rates of skin cancer affected by the accumulative damage.
Immunology also matters - more likely to develop skin cancer.
