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Physiology 2 > Structure and Function of Skin (skin) > Flashcards

Structure and Function of Skin (skin) Flashcards

1
Q

Integument or Integumentary system

A
  • 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
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2
Q

Functions

A
  • Thermoregulation
  • Sensation
  • Repair
  • Vitamin D production
  • Barrier (protection): against dehydration, infection, injury/abrasion and solar radiation
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3
Q

3 layers of the skin

A

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
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4
Q

Epidermis

A
  • 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)
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5
Q

4 main layers of Epidermis

A 
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)
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6
Q

Basal Layar (B)

A
  • 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)
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7
Q

Spiny layer (S)

A
  • 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
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8
Q

Granular layer (G)

A
  • 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
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9
Q

Cornified layer (C)

A
  • 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
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10
Q

Melanocytes (pigment)

A
  • 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
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11
Q

Langerhans cells

A
  • 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
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12
Q

Vitamin D production

A
  • 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
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13
Q

Structure of Dermis

A
  • 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)
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14
Q

Functions of Dermis

A
  • 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
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15
Q

Dermal-Epidermal border

A
  • 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
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16
Q

Dermis H&E stain

A

visible:

  • collagen - much of volume of dermis
  • fibroblasts - make and maintain collagen
  • small blood vessels
17
Q

Collagen and Elastin fibres in dermis

A
  • 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
18
Q

Structure of Hypodermis

A
  • 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
19
Q

Function of Hypodermis

A
  • 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
20
Q

Hair

A
  • 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
21
Q

Skin glands

A 
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
22
Q

Nails

A

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

23
Q

Sensory receptors

A

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.

24
Q

Main skin functions - Barrier

A

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

25
Q

Main skin functions - Thermoregulation

A

Dermis and hypodermis - insulation, thermoreceptors, blood supply regulation, sweating

26
Q

Main skin functions - Sensation

A

All layers - nerve supply and various sensory receptors

27
Q

Main skin functions - Repair

A

Epidermis - normal proliferation

Dermis - fibroblasts fill gaps with new collagen that epidermis can attach to

28
Q

Main skin functions - Vit D production

A

Epidermis - reaction of 7-dehydrocholesterol with UVB radiation to Vitamin D3

29
Q

Summary

A
  • 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

Physiology 2 (38 decks)

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