10. skin Flashcards
what are appendages? what origin are they derived from? where are their bases located?
nails, hairs and glands
epidermal
their bases are located in the lower dermis and subcutis
what is an integument
skin and appendages
what are the two basic types of skin and what does it depend on?
hirsute skin= hairy, dominant
glabrous skin= hairless
only found at
- the epithelial transition zones of the lips and anus
- parts of the outer genitals
- all acral skin
(palmoplantar areas and body protrusions
i.e. toe tips, finger tips, knuckles, elbows, knees)
how can the two different types of skin act in different ways? whats an example of this
acral erythema (redness, swelling)
a common adverse reaction to certain chemotherapy drugs
others: different
sets of mechanoreceptors
innervation of sweat glands
temperature regulation
types of tumours
skin variation
general pattern is the same but layers can vary depending on the body
eg. thinner dermis in flexor side, thick on back
thickness of subcutis varies upon fat storage
function of epidermis
- barrier (permeability & abrasion)
- VitDproduction
- UV protection
- immune defense
- sensory information
epidermal cell types
keratinocytes (90%) : barrier and vit D production
melanocytes: pigment cells, UV protection
* can cause malignancy- 75% death (5% cases)
merkel cells: mechanoreceptor
langerhans cells: dendritic
skin-homing lymphocytes
malignancy in epidermal cell types
basalioma most common
squamous cell carcinoma
melanoma
cutaneous t cell lymphoma, merkel cell carcinoma and langerhan cell histiocytosis are rare
skin cancer: what is the staging based on?
skin layers involved
keratinocyte epithelium
stratified keratinised squamous
granular layer contains dead cells
keratinocyte maturation
- proliferation: constant cell renewal
- differentiation: exit from cell cycle
- mass production: of epidermis-specific compounds (lipids in lamellar bodies, keratins and structural proteins in keratohyalingranules)
- functional re-organisation: of epidermis specific compounds
- cornification= programmed cell death
- desquamation= shedding of surface cells
what is the concept of the barrier of keratinocytes
brick wall
corneocytes= bricks: strengthened cornifiedenvelope made out of structural proteinsand tonofibrils
Corneodesmosomes= Rivets hold the cellular sheet together
Extruded lipids = Mortar
arranged in multi-lamellar membrane sheets prevent trans-epidermal water loss
how does the stratum corneum (outermost layer of the skin) act as a permeability barrier?
highly lipophillic structure
the intercellular space is filled with multi-lamellar sheets of extruded lipids
sealed with natural moisturizing factor (NMF)
- collective term for filaggrin-derived compounds
- bind about 20% of water
properties of stratum corneum
water repellent
can be crossed by low Mr lipophillic substances
prevents trans epidermal loss of water and solutes
what must the cornified layer be in order to repel water?
hydrated!
- NMFs absorb water and swell, sealing the intercellular space
- this allows the cornified layer to stay hydrated
- as NMFs are water soluble, they easily leach from the cells upon water contact
- i.e. too immersion in water actually makes the skin drier
what substances can be absorbed trans dermally
- most organochlorineinsecticides
- solvents
- fumigants
(skin absorption common route for pesticide poisoning)
skin patches (eg. nicotine)
keratinocyte adhesion
what is desquamation
is strictly controlled process where proteases are degrading the corneodesmosomes
The integrity of the epidermis depends on the cohesion between keratinocytes, with desmosomesas the main adhesion structures
issues arising relating to adhesion
not strong enough:
defective adhesion leads to fluid accumulation between cells and the formation of blisters
too strong:
hereditary altered structure of keratins or structural proteins delay the rate of desquamation
UV damage to skin
UVA indirect DNA damage via formation of free radicals
photodegradationof certain vitamins (e.g. folate) in dermal capillaries?
UVB direct DNA damage (disrupted DNA replication)
and sunburn
endocrine function of skin
- Vitamin D is naturally present in very few foods
- It is also produced endogenously in the epidermis upon UVB radiation
- lack of epidermal production has to be fully compensated by dietary intake (or vice versa)
UVBrays are required for photoconversion of 7-DHC into VitD3 by keratinocytes in the Malpighian layer
protection against UV rays
- UV-induced DNA damage in keratinocytes leads to p53-mediated up-regulation of proopiomelanocortin (POMC)
- posttranslational cleavage of POMC produce α-MSH and ACTH which bind to the melanocortin1-receptor (MC1R) on melanocytes and induce melaninproduction
- melanin-containing organelles = melanosomesare passed on to keratinocytes
- melanin absorbs UV light and transforms the energy into heat
happens when skin is already damaged
what is natural skin colour determined by
- the rate of melanin production (melanogenesis)
- the chemical structure of melanin
- the degree of melanizationof melanosomes
- melanosomenumbers, dispersion and turnover
- pigmentsother than melanin (haemoglobin, carotene)
what type of trait is skin colour?
polygenic
determined by the additive effects of an estimated 125 different genes
- perhaps 7 gene products have the most impact, with MC1Rbeing the most important
- MC1R regulates both the quality and quantity of melanin production
the two different waves of melanocyte migration from neural crest
ventral wave- schwann cell precursor cells in dermis-> epidermal melanocytes
dorsal wave: melanocyte stem cells->hair bulb melanocytes
why can hair and skin have different colours?
why can children from biracial parents show different skin colour?
different melanocyte populations
alleles of skin colour randomly distribute
2 main types of melanin
eumelanin: pale, medium, dark tan and blonde- black hair
pheomelanin: no tan, red hair (no protection)
pigmentation problems
function of dermis
supplies the epidermis
- fluid storage
- thermoregulation
- wound healing
- strengthand elasticity
- immune defence
- sensory structures
- appendages
function of hypodermis/ subcutis
- fat storage
- insulation
- padding
- sensory structures
- appendages
how is the cutaneous blood supply organised?
3 horizontal plexus
The skin has a low metabolic rate, i.e. most blood flowing to the skin has a thermoregulatory rather than a metabolic function
blood supply of the skin is directly linked to the blood supply of the underlying skeletal muscles
what is the epidermis in terms of blood supply?
avascular
supplied by diffusion of dermis
- Dermal papillae greatly increase the contact area between dermis and epidermis
- they contain a plexus of capillaries and lymphatics
- can contain encapsulated mechanoreceptors (Meissner’s corpuscles)
- cause ridges in the overlaying epidermis = fingerprints
discuss the papillary layer in the dermis
loose connective tissue
supplies epidermis
stores fluid
immune defence
papillary layer
- thin fibre bundles; arranged randomly
- highly capillarisedand hydrated
discuss reticular layer of dermis
dense connective tissue
strength and elasticity
- thick fibre bundles; arranged orthogonally
- abundant elastic and collagen fibres
(70% collagen and 1% elastin)
what do mechanoreceptors do?
sense mechanical pressure or distortion of surrounding
types of mechanoreceptors in glabrous skin
“pilo-sebaceous unit” or “follicular unit”
The actual hair follicle
+ arrectorpili muscle
+ sebaceous gland
acne- result of blocked infibidulum
sebaceous glands
what is sebum
- mixture of mostly nonpolar lipids
- coat the fur as a hydrophobic protection against overwetting and for heat insulation
- Increased sebum excretion is a major factor involved in the pathophysiology of acne
where are apocrine glands found and what are they
what is apocrine sweat
sweat glands
certain regions (eg. axilla)
become active during puberty
cloudy, viscous fluid- proteins, lipids, steroids
presence of bacteria causes odour
evolved to coat and stick to hair
eccrine sweat glands
dominant type of sweat gland (secretes directly to surface)
only gland not associated with hair
own separate excretory duct: acrosyringium
clear hypotonic fluid
evolution of sweat glands
primates show a transition from apocrine to eccrine gland-dominant skin
- original eccrine glands remain in the palmoplantar and cranial regions
- original apocrine glands remain in the axilla and perineal region
- new eccrine glands take over the limbs and trunk
thermoregulation
Our evolutionary old and new glands are differently regulated
cholinergic (PNS): newly evolved eccrine glands (trunk and limbs)
a-adrenergic (SNS): conserved glands
eccrine= palmoplantar & cranial
apocrine = axilla, perineum
(explains sweating patterns when under stress)
why can glabrous skin get colder than hirsute
glabrous skin has extra atriovenous shunts so more blood reaches surface
open during hypothermia
