Introduction to the skin Flashcards
What is the largest organ in the human body?
- Skin: up to 16% of total body weight.
What does the cutis consist of?
- Epidermis and dermis.
What does the subcutis consist of?
- hypodermis (this is the buffer layer between skin and underlying material).
What are the functions of the skin?
- barrier (abrasion, fluid, immune).
- thermoregulation.
- VitD3 production.
- storage (fat and fluid).
- sensory information.
What is hirsute skin?
- Hairy skin.
What is glabrous skin?
- Hairless skin.
What is acral skin?
- Skin affecting body protrusions e.g. Finger tips, knuckles, elbows, knees.
- this skin will be hairless.
Where is hairless skin found?
- Acral skin: e.g. Fingertips, elbows etc.
- Epithelial transition zones of the lips and anus.
- Parts of outer genitals.
Name some appendages to skin.
- Nail, hair and glands (all epidermal in origin).
What are the three layers of skin?
- Epidermis (closest to surface), dermis and hypodermis (buffer layer between skin and underlying material).
Where do the nail, hair and glands originate?
- Although derived from the epidermis the base of all skin appendages is located in the lower dermis and subcutis.
What is the collective name for the skin + appendages?
- integument.
In wound healing can the appendages be restored?
- No.
What secretes next to the hair shaft?
- holocrine sebaceous glands.
- apocrine sweat glands.
What happens when the growing hair germ invades the underlying stroma?
- It forms a hair follicle with associated glands.
- it induces the formation of the arrector pili muscle.
What does the follicular unit (or pilo-sebaceous) consist of?
- Hair follicle + arrector pii muscle + sebaceous gland.
Name 2 procedures for hair restoration?
- Follicular unit extraction and transplantation.
What does a hair transplant consist of?
- Transplanted are small bundles consisting of 1-4 hair follicles and the sebaceous glands, arrector pii muscles and connecting tissue that accompany and support them.
What is an infundibulum?
- Cup or funnel in which a hair follicle grows, it’s continuous with surface, site where the ducts of the hair-associated glands end, initial site of inflammation.
What is acne?
- Result of blocked infundibulum (usually due to an initial plug of keratin and sebum).
Where are stem cells for hair located?
- In the bulge (of mature follicle).
What are the characteristics of a sweat gland?
- An eccrine germ invades the underlying stroma and forms an eccrine sweat gland .
- predominant type of sweat gland in primates.
- only gland not associated with hair, I.e. They have their own separate opening.
- have several million eccrine glands over the body surface.
- estimated aggregate mass of about 100 grams.
Name unusual apocrine glands.
- the glands of Moll at the eye lashes: active from brith.
- the ceruminous glands in the external ear canal, already active before birth.
- thought to have an antimicrobial function.
Name some hair germs that may develop into glands without associated hair.
- sebaceous meibomain glands forming the tarsal plate of the eyelids.
- apocrine mammary glands of the breast.
What are the five different cell types of the epidermis?
- keratinocytes (barrier) - 90%.
- melanocytes (uv protection) - pigment cells 5%.
- merkel cells (transduction of fine touch) mechanoreceptor cells 1%.
- T cells, NK cells (immune defence) patrolling lymphocytes.
- langerhans cells (immune defence) resident dendritic cells together about 4%.
What are the characteristics of skin cancer?
- Most common form of cancer, can derive from any epidermal cell type, each has a different prognosis and treatment.
Name different types of skin cancer.
- squamous cell carcinoma (20%).
- basalioma (75%) = slow but destructive.
- malignant melanoma (5%) = 75% of deaths.
- langerhans cell histiocytosis = rare, variable.
- cutaneous T cell lymphoma (type of non Hodgkin lymphoma) = relatively rare, variable.
- merkel cell carcinoma = very rare, highly aggressive.
What is a keratinocyte?
- Predominant cell type in the epidermis, the outmost layer of the skin, constituting 90% of cells found there.
- Primary function is the formation of a barrier against environmental damage by pathogenic bacteria, fungi, viruses etc.
- once pathogens invade the upper layers of epidermis keratinocytes react by inducing proinflammatory mediators e.g. Chemokines which attract leukocytes to the site of pathogen invasion.
When thinking about keratinocyte cross linking, explain epithelial sheet formation?
- Adherens junctions = actin- actin.
- Desmosomes = keratin-keratin.
When thinking about keratinocyte cross linking, explain anchorage and signalling?
Focal adhesions = actin - fibronectin
Hemi-desmosomes = keratin - laminin
Name the keratinocyte layers from outmost to innermost.
- Cornified layer, granular layer, spinous layer, basal layer.
What is proliferation?
- Constant cell renewal.
What is desquamation?
- Shedding of surface cells.
What are corneocytes?
- “Bricks”.
- Outer cell envelope: lipids (template for extracellular lipid bilayers).
- Inner cell envelope: insoluble proteins, structural stability of the individual cells.
What are extruded lipids?
- “Mortar” arranged in multi lamella membrane sheets.
What are corneodesosomes?
- “Rivets” hold the cellular sheet together - overall integrity.
Where do the keratinocytes form a waterproof barrier?
- In the granular layer this is one of the most important of all epidermal functions.
The cornified layer needs to be hydrated in order to retain water, how does it stay hydrated?
- Natural moisturising factor (NMF) is the collective term for filaggrin-derived cornification-specific compounds.
- They absorb water allowing the cornified layers to stay hydrated. As NMFs are water soluble, they easily leach from the cells upon water contact (I.e. Too much contact with water can actually make skin drier).
What is transdermal patch?
- A medicated adhesive patch placed onto the skin to deliver a specific dose of medication through the skin and into the blood stream.
Name some examples of of commonly used transdermal patches.
- nicotine patch.
- hormone delivery.
- antidepressants and ritalin.
- vitamin b12/ cyanocobalamin.
- colonising.
When does transdermal absorption become a problem?
- There are high transdermal absorption rates for
- most organochlorine insecticides.
- solvents.
- fumigants.
Skin is most common route of poisoning from pesticides.
What is sunlight?
- Electromagnetic radiation of varying wavelengths.
Name the ionising rays.
- Ultraviolet.
Name the non-ionising rays.
- Visible light.
- Near infrared.
- Far infrared.
What is the radiation intensity of uv light?
7%.
What is the radiation intensity of visible light?
44%
What are the different kinds of uv light?
- UVC.
- UVB.
- UVA.
What are the effects of the different kinds of UV light?
UVC - directly damaging to DNA but not penetrating, only effects the epidermis.
UVB - directly damaging to DNA, causes sunburn (shorter wavelength than A and penetrates epidermis and dermis).
UVA - indirectly damaging to DNA via free radicals (penetrates all layers of skin).
What are the characteristics of melanocytes?
- Highly branched cells in the basal layer of the epidermis. 1 melanocyte supplies upto 35 keratinocytes.
- UV-induced DNA damage in keratinocytes triggers release of alpha-MSH.
- Alpha-MSH induces production of the pigment melanin in melanocytes.
- Melanin containing organelles (melanosomes) are passed on to keratinocytes.
- Melanin absorbs uv light and transforms the energy into heat.
What are the three basic types of melanin?
- Eumelanin (2 types of this brown and black).
- Pheomelanin (reddish).
- Neuromelanin.
What does blonde hair have a lack of?
- Eumelanin.
DOPA -> DOPAquinone is catalysed by what and where does this take place?
- Catalysed by tyrosinase and in the skin.
Phenylalanine -> tyrosine is catalysed by what and where does this take place?
- Catalysed by phenylalanine hydroxylase in the liver.
Discuss the pathway leading to melanin production.
- Phenylalanine -> tyrosine -> DOPA -> DOPAquinone.
Give some examples of hyperpigmentation.
- liver spots (lentigenes).
- freckles.
- moles.
- melasma.
Give some examples of hypopigmentation.
- Albinism.
- Vitiligo.
What are liver spots?
- Sun induced and age related - not fading.
What are freckles?
- Sun induced - fading.
What are melamas?
- Hormonal overstimulation of melanocytes.
What are moles?
- Proliferation of melanocytes.
What is albinism?
- General lack of tyrosinase.
What is vitiligo?
- Local loss of melanocytes.
What factors determine skin colour?
- All humans have similar number of melanocytes so not this.
- rate of melanin production (melanogenesis).
- chemical structure of melanin.
- degree of melanization of melanosomes.
- melanosome numbers, dispersion and turnover.
- pigments other than melanin (haemoglobin, carotene).
How would you go about whitening skin?
- Inhibition of tyrosinase.
How would you go about making your skin darker?
- Maillard reaction (browning of aa).
- deposits of canthaxanthin.
- hormonal induction of melanogenesis.
What was driving the evolution of different skin colour?
- In colder places people had body hair so their skin was lighter.
- In sunnier places people had no/less body hair so they had dark skin.
Vitamin d production is proportional to …
- Light penetration (both Uva and uvb rays are required for this).
Folic acid destruction is proportional to …
- Light penetration (uva needed).
Vitamin d can only be synthesised by a photochemical process so …
- we either eat foods that contain already synthesised vit d.
- or we have to be exposed to sunlight to photosynthesize vit d in our skin.
- a lack of production has to be fully compensated by dietary intake.
Which type of skin is the most efficient in producing vitamin d?
- Dark skin needs 6x more uv light or 8x more time than light skin to produce the same amount of vitamin d.
Name the intra-epidermal immune cells and what is there function?
- Intra-epidermal lymphocytes and langerhans cells - constantly monitor for damage or disease and regulate homeostasis and tissue repair.
What are intra-epidermal lymphocytes?
- Adult human skin contains nearly twice number of T cells than the blood. Most (80%) memory T cells.
- Significant fraction (5-20%) regulatory T cells which limit the function of effector T cells.
What are langerhans cells?
- Epidermis-specific dendritic cells located in the spinous layer.
What are the characteristics of the dermo-epidermal junction?
- dermal papillae greatly increases the contact area between dermis and epidermis (folds increase surface area).
- they contain a plexus of capillaries and lymphatics.
- can contain encapsulated mechanoreceptors (meissners corpuscles).
What are the dermal layers?
- Papillary layer: loose connective tissue.
- Reticular layer: dense connective tissue.
What are the characteristics of the papillary layer (one of two dermal layers)?
- Thin fibre bundles, arranged randomly.
- Highly capillarised and hydrated (80% water).
What are the characteristics of reticular layer (one of two dermal layers)?
- Thick fibre bundles, arranged orthogonally.
- Abundant elastic and collagen fibres (70% collagen and 1% elastin).
What is the relationship between skin turgor and hydration?
- slow recoil = mild dehydration (10%).
Hydration of which layer of skin is reflected in skin turgor?
- Dermal papillary layer.
Describe the blood supply to the skin.
- The cutaneous blood supply is organised as 3 horizontal plexuses.
- The epidermis is avascular and supplied by diffusion from the dermis. - The 3 plexus are: superficial vascular plexus (between the papillary layer of dermis and the epidermis I.e. Closest to surface).
- Deep vascular plexus between the reticular layer and papillary layer of the dermis).
- Subcutaneous vessel ( between the muscle fascia and subcutis I.e. Closest to inside of body).
Does the skin have a low or high metabolic rate?
- The skin has a very low metabolic rate I.e. Most blood flowing to the skin has a thermoregulatory rather than a metabolic function.
What is the direction of blood flow to the skin?
- Deep artery -> muscle -> subcutaneous vessels -> deep vascular plexus -> superficial vascular plexus -> epidermis.
What are the two independent modes of thermoregulation?
- cutaneous blood flow.
- sweating.
Explain thermoregulation (cutaneous blood flow)?
- Vasodilation: we lose heat by radiation/conduction if skin temperature > surrounding.
- Vasoconstriction: we retain heat by reducing skin perfusion.
Thermoregulation (sweating)
More nerve activity = sweating. We lose heat by evaporation if humidity is low.
Not sweating is not an option (we can’t retain heat by not sweating)
Of the mechanoreceptors which types have encapsulated endings?
- Meissners corpuscle and pacinian corpuscle.
Of the mechanoreceptors which types have endings with expanded tips?
- Ruffini ending and merkels disc.
Mechanoreceptors sense mechanical pressure or distortion of surrounding. There are four main types in glabrous skin, what are they?
- Meissners corpuscle (dermal papillae/ changes in texture).
- pacinian corpuscle (deep dermis/subcutis - surface roughness).
- ruffini ending (reticular dermis - tension).
- merkels disc (basal epidermis - sustained touch and pressure).
What are the usual problems with wounds?
- Infection and scarring.
What are the main kinds of wounds?
- Superficial wounds - can regenerate.
- Partial/full thickness wounds - can repair but always result in a scar, the appendages and elastin are not replaced (no melanocytes either).
- hypodermis and dermis - much worse = scar.
Why do scars always remain white?
- They don’t replace melanocytes.
How is wound repair different in foetus different before and after 28 weeks?
- Before 28 weeks: repairs and don’t scar.
- After 28 weeks: scars present.
Wound Healing can be of first or second intention. What is healing by first intention?
- small wounds/narrow margins, regeneration»_space; fibrosis, healing is fast with minimal scarring e.g. Sharp cuts.
Wound Healing can be of first or second intention. What is healing by second intention?
- Larger wounds/ wide margins, fibrosis»_space; regeneration, healing is slower more scarring inevitable infection e.g. Large burns and ulcers.
Wound closure - why do they stitch/glue it?
- They aim to initiate healing by 1st intention via well-approximated wound edges.
- Remodelling and maturation of the scar will continue for a year or more.
What can be the problems with wound healing?
- Too much = excessive healing, hypertrophic and keloid scars.
- too little = delayed healing, “dirty” wounds (infection, debris) wounds > 4cm: radiation: secondary ulcers.
When the growing hair germ invades the underlying stroma, what happens?
- forms a hair follicle with associated glands.
- induces the formation of the arrector pili muscle.
