Structure and Function of Skin (skin) Flashcards
Integument or Integumentary system
- skin glands, hair and nails and sensory receptors
- largest and heaviest organ of the body - around 15% of adult body weight
- much GP time
- 3 layers of skin:
- epidermis
- dermis
- hypodermis
Functions
- Thermoregulation
- Sensation
- Repair
- Vitamin D production
- Barrier (protection): against dehydration, infection, injury/abrasion and solar radiation
3 layers of the skin
top to bottom:
- epidermis (thin outermost layer)
- dermis (nerves and blood vessels)
- hypodermis (fat and connective tissue)
- muscle
‘derm’=skin
‘epi’=on
‘hypo’=below
- hypodermic can be thickest layer depending on part of the body
Epidermis
- most superficial
- gives skin its coloir
- protection from pathogens and environment
- vitamin D production
- made up of multiple layers of differentiating keratinocytes
- thickness varies in body (thin in abdomen, thick in heel)
4 main layers of Epidermis
top to bottom: - Cornified layer (stratum corneum) (- Stratum Lucidium) - Granular layer (stratum granulosum) - Spiny layer (stratum spinosum) - Basal layer (stratum basale)
- contains keratin which is also main component of horns, hair, nails, claws and hoofs
- other epidermal cell types:
- melanocytes (pigment)
- langerhans cells (defense)
- Merkel cells (sensation)
Basal Layar (B)
- also known as stratum basale
- first single layer from bottom
- contains stem cells which constantly proliferate
- attached to dermis
- dynamic: daughter cells constantly move ‘up’ (distally) through epidermis, differentiating as they go, until they shed from outer surface (takes around 20-50 days)
Spiny layer (S)
- also known as stratum spinosum
- second layer from bottom
- cells (keratinocytes) have many desmosomes (junctions)
- these are visible as ‘spines’ between cells
- strong bonds holding epidermis together
Granular layer (G)
- also known as stratum granulosum
- third layer from the bottom
- 1-4 layers of cells containing prominent granules of ‘keratohyalin’ which are the precursor of protein keratin
- also contain lamellar bodies containing lipids (seen by TEM)
- cells are differentiating to form the outermost layer
Cornified layer (C)
- also known as stratum corneum
- outer protective later of epidermis
- cells are keratinised (cornified) - cytoplasm full of ‘horny’ keratin (from keratohyalin granules), making skin tough and resistant to injury
- cells are flattened and have lost their nuclei
- nonpolar lipids (waterproof) are between cells - form lamellar bodies
- very thick layer
- cells flake off and are constantly replaced
Melanocytes (pigment)
- is viewed through thin sheets of human epidermis
- special (DOPA) stain for melanin shows dendritic form
- synthesises melanosomes (pigment granules) and transfers them to basal keratinocytes through long dendrites
- keratinocytes arrange melanin pigment in a cap distal to the nucleus (sunny side)
- especially in basal layer (stem cells)
- UV protection - black-skinned people have only about 10% as many skin cancers as white people with the same lifestyle
- melanocytes (M) by H&E (white skin) - pale cells, in or protruding from basal layer
- some pale cells in basal layer are Merkel cells (touch-sensors)
- hard to tell difference with H&E alone
Langerhans cells
- function: immune system - seems and deals with invading microbes
- they are antigen-presenting cells (like macrophages)
- they are dendritic cells, forming a network (can be seen with immunoperoxidase staining)
- H&E appearance: small, pale cells in non-basal later of epidermis
- hard to see with H&E only
Vitamin D production
- Vitamin D3 is made in epidermis
- mostly basal cells, also stratum spinosum
- requires UV light
- requires more UV light in dark skin (melanin barrier)
- converted to active form in liver and kidney: 1,25 dihydroxyvitamin D3
- commonly deficient in UK
Structure of Dermis
- made up of layers of connective tissue characterised by interconnected mesh of elastin and collagen fibres
- produced by deal fibroblasts
- fibroblasts are principal cell of dermis
- 2 layers of dermis: thin papillary later and deeper reticular layer
- dense, irregular connective tissue:
- dense = full of collagen fibres
- irregular = fibres run in all directions (not parallel)
Functions of Dermis
- collagen provides tensile strength (strength when pulled), hence protection against abrasion and impact
- also contains elastin, a protein complex that provides elasticity
- dermis also carries blood and nerve supply for epidermis
- rich in blood vessels, sensory receptors and skin glands
Dermal-Epidermal border
- dermal-epidermal border is often wavy to resist shear forces (rubbing sideways)
- dermal papillae - finger like protrusions of dermal connective tissue into epidermal layer
- rete ridges - extensions of epidermis into dermal layer
- more apparent in thick skin of hands = fingerprints, as well as feet
Dermis H&E stain
visible:
- collagen - much of volume of dermis
- fibroblasts - make and maintain collagen
- small blood vessels
Collagen and Elastin fibres in dermis
- Verhoeff van Gieson (VVG) stain shows collagen fibres in red and elastin fibres in black
- elastin fibres are stretchy and give skin its elasticity
- one effect of UV light on skin is damage/loss of elastic fibres resulting in loss of elasticity and wrinkles
Structure of Hypodermis
- below the dermis
- fascia or subcutis (cutis = epidermis and dermis)
- made up of well vascularised, loose, areolar connective tissue
- also made of adipose tissue which functions as a mode of fat storage and provides insulation and cushioning for the integument
- composed of fat containing glands, hair follicles, nerves, blood vessels
- often the thickest layer of skin - thickness varies with age, body site, nutrition etc
Function of Hypodermis
- provides insulation to deeper tissue, cushioning and energy storage
- anchors skin to muscle with connective tissue
- area where you put a hypodermic needle for a subcutaneous injection
Hair
- found on nearly every part of skin except the palms, soles and lips
- every strand of hair is composed of the shaft, root and bulb that sits in a pouch like structure called the hair follicle
- hair follicle is epidermal tissue that dips into dermis and interacts with other structures like apocrine glands, sebaceous glands, the arrector pili muscle and nerve receptors
- bulb contains hair matrix - active site of hair growth and pigmentation
- contains 2 different cell types - follicular keratinocytes and melanocytes
- follicular keratinocytes produce hard keratin = hair
- keratinocytes filled with hard keratin flatten out and are slowly pushed up the follicle
- this results in root and shaft formation
- follicular keratinocytes at bulb of hair replicate only a set number of times, after which the hair follicle stops growing and falls out leading to baldness
- over time these melanocytes stop producing melanin which is why hair turns white as people age
Skin glands
Eccrine sweat gland: - normal sweat glands - watery secretion on to skin surface - cools body by evaporation Apocrine sweat gland: - secrete into hair follicles - found in armpits and anogenital region - oily fluids in humans - function unclear - contains pheromones in some mammals - source of body odour after bacterial action - only present after puberty Sebaceous gland: - secrete oily sebum (lanolin) into hair follicle - conditioner for hair and skin - prevents dryness and flaking - only present after puberty
Nails
Nail folds:
- where skin seals the side edges of the nail
Eponychium:
- proximal skin fold at bottom
- gives rise to cuticle
- semi circular later of dead skin keratinocytes that covers the junction where the nail enters the skin
- prevents entry of pathogens
Nail matrix:
- creates nail plate which is the hard part of nail that is visible as well as the free edge that hangs over skin
- special epidermal tissue that contains nerves, lymphatics and blood vessels that support the nail
- contains modified keratinocytes that replicate and undergo the process of keratinisation
- the youngest keratinocytes are in the nail matrix and older cells keratinise and form the nail plate
Sensory receptors
1) Free nerve endings:
- nociception
- thermal sensation
- light touch
2) Encapsulated endings:
- Pacinian corpuscles (rapidly adapting) → vibration or brief touch
- Merkel’s discs (slowly adapting) → sustained touch/pressure e.g. skin indentation
- Meissner’s corpuscles (rapidly adapting) → moving touch or vibration
- Ruffini endings (slowly adapting) → steady pressure
- Root hair plexus (rapidly adapting) → hair displacement.
- Krause end bulbs → touch/pressure, possibly thermoreceptors.
Main skin functions - Barrier
Dehydration - epidermis - keratin holds water, lipids stop it evaporating
Infection - epidermis - impervious barrier, also immune system
Injury/abrasion - all layers - epidermis: strong, thick, where friction. occurs. dermis: collagen, tough, leathery. hypodermis: cushions
Solar radiation - epidermis - stratum corneum and melanin pigment
Main skin functions - Thermoregulation
Dermis and hypodermis - insulation, thermoreceptors, blood supply regulation, sweating
Main skin functions - Sensation
All layers - nerve supply and various sensory receptors
Main skin functions - Repair
Epidermis - normal proliferation
Dermis - fibroblasts fill gaps with new collagen that epidermis can attach to
Main skin functions - Vit D production
Epidermis - reaction of 7-dehydrocholesterol with UVB radiation to Vitamin D3
Summary
- there are 3 main layers of the skin: epidermis, dermis and hypodermis
- the epidermis is the outer protective layer
- the dermis is the layer beneath the epidermis made up of elastin and collagen fibres
- the hypodermis is the layer beneath the dermis that insulates the skin and anchors it to the muscle
- hair, skin glands and nails are referred to as skin appendages
- both hair and nails are formed from the process of keratinisation
- the skin functions as a barrier (protection from dehydration, injury, infection, solar radiation), thermoregulator, sensory organ and a site for Vit D production
