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Physiology 2 > SKIN: Structure and function of skin > Flashcards

SKIN: Structure and function of skin Flashcards

1
Q

What does integument refer to?

A

Integument refers to the skin, hair, and nails.

The skin is the largest and heaviest organ of the body (making up about 15% of an adult’s weight).

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2
Q

What are the functions of the skin?

A
  • acts as a barrier against physical, chemical and biological threats (including immunological aspects) –> these include dehydration (prevents water leaking), infection (immune cells inside), injury/abrasion, solar radiation.
  • homeostatic functions in thermoregulation and water balance regulation
  • sensory functions –> sensation
  • secretory/’nutritional’ function producing Vitamin D
  • insulation (really, a property of the hypodermis/subcutis)
  • repair
  • cosmetic (tattoos, piercings, etc.)
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3
Q

What are the 3 layers of skin and briefly describe them?

A

Going from top to bottom, the structures found in the different skin layers are:
EPIDERMIS: (has no blood supply, is the thin outermost layer)

DERMIS: (everything below the stratum basale and above the adipose tissue)
- dermal papillae
- papillary layer
- reticular layer
(don’t need to know this in detail, but could help)
- Nerves and blood vessels
- Made of a lot collagen and connective tissue

Beneath the dermis is the HYPODERMIS (or subcutis), which is looser connective tissue containing (characteristically) adipose tissue. Some definitions include this as the third layer of skin. Has fat and connective tissue. This layer is the thickest layer depending on the location in the body.

Derm = skin
Epi = on
Hypo = below
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4
Q

Briefly describe the epidermis.

A

EPIDERMIS: (has no blood supply, is the thin outermost layer)

  • stratum corneum
  • stratum lucidum
  • stratum granulosum
  • stratum spinosum
  • stratum basale

It’s made up of multiple layers of differentiating keratinocytes. The thickness varies depending on the body part (e.g. thin in abdomen and very thick in heel.)

Cells are constantly flaking off at the surface, these are constantly replaced by cells from beneath.

Some functions:

  • Most superficial
  • Gives skin its colour
  • Protection from pathogens and environment
  • Vitamin D production
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5
Q

What are the 4 main layers of the epidermis?

A

From in to out:

  • stratum basale
  • stratum spinosum
  • stratum granulosum
  • stratum corneum

Under the palms of the hands and the soles of the feet is a layer called the stratum lucidium. This layer is found between the stratum granulosum and stratum corneum. This layer is translucent under the microscope.

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6
Q

Describe the stratum basale.

A

It is the first, single layer containing stem cells, attached to the dermis. The stem cells constantly proliferate.

It is a dynamic layer - the daughter cells gradually move ‘up’ (distally) through the epidermis, differentiating as they go, until they are shed from the outer surface. This takes about 20-50 days.

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7
Q

Describe the stratum spinosum.

A

The spiny layer. This is right above the basal layer.

These cells (keratinocytes) have many desmosomes (junctions) here, visible as 'spines' between the cells.
Strong bonds hold this epidermis layer together.
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8
Q

Describe the stratum granulosum.

A

The granular layer.

There are 1-4 layers of cells containing prominent granules of ‘keratohyalin’ - the precursor for the protein keratin. The keratohyalin are responsible for the granular appearance.
It also contains lamellar bodies containing lipids (seen by TEM). These cells are differentiating to form the most outermost layer of skin. Some of the cells start to lose their nuclei here.

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9
Q

Describe the stratum corneum.

A

The cornified later. It is the outer, protective layer of epidermis, many cells thick.

  • the cells are also cornified (keratinised) -cytoplasm is full of ‘horny’ keratin (from keratohyalin granules), thus are tough and resistant to injury
  • contains squamous (flattened) cells which have lost their nuclei
  • non-polar lipids (waterproof) are between the cells - from lamellar bodies

Most of the skin is oily and soft; this is because the cells contain soft keratin. However, other parts, such as nails and hair, contain hard keratin.

Keratinisation occurs in the stratum corneum.

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10
Q

What is keratin the main component of?

A
  • Hair
  • Horns
  • Nails
  • Claws
  • Hoofs
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11
Q

What would you see if you stained a section of skin?

A

The cells in the epidermis are very close together, so they would give a very intense stain, whereas tissue in the dermis is much looser, with cells further apart, so it stains less intensely.
Fat (the rounded bits in the subcutis) doesn’t stain at all.

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12
Q

What is a frictional blister?

From Sarah Sheikh Flashcards - not present in 2020 lecture

A

It is a watery type of blister. It occurs when the top layer of skin rubs against the basal cells (bottom layer of the epidermis). This causes the spiny cells to get crushed, so the watery fluid in the content of all the spiny cells that burst leaks out.
Then, because there is a lot of protein in this fluid, there is a lot of osmotic pressure. So, the water gets pulled via osmosis as a transudate.

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13
Q

Why do you go wrinkly if you stay in water for a long time?

From Sarah Sheikh Flashcards - not present in 2020 lecture

A

The reason is that water can only go as far as the granular layer (which is oily) because the stratum corneum can absorb a lot of water. So, it swells and, as the layer below stays the same, the stratum corneum has to ripple.

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14
Q

There are some differences between thick skin and thin skin.
What is an extra ‘layer’ that is found in thick skin?

[unlikely to be examined on]

From Sarah Sheikh Flashcards - not present in 2020 lecture

A

Thick skin has an extra layer that is halfway between the granular cells and stratum corneum.

The layer is called the stratum lucideum (clear layer).
It contains immature keratin.

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15
Q

List some other epidermal cell types, besides keratinocytes.

A
  • melanocytes (pigment)
  • langerhans cells (defence)
  • merkel cells (sensation)
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16
Q

What are the two types of melanin?

From Sarah Sheikh Flashcards - not present in 2020 lecture

A

Two types of melanin account for variations in skin and hair colour. The result depends on how they are mixed.

1) Eumelanin (brown/black)
2) Pheomelanin (red/yellow)

17
Q

Describe the melanocytes and their function.

A

A special DOPA stain is used to view melanocytes. They have a dendritic form – resemble neurones.

They synthesise melanosomes (melanin pigment granules) and transfers them to basal keratinocytes through long dendrites. Melanin pigment is mostly found in the basal epidermis.

Once the melanocytes have transferred the pigment granules to the keratinocytes. The keratinocytes then arrange the melanin pigment in a cap like conformation distal to the nucleus (sunny side) – covering the nucleus. The melanosomes are especially transferred to the basal layer (stem cells). This is a protective feature.

As a result, they provide UV protection – black skinned people have only 10% as many skin cancers as white people with the same lifestyle.

18
Q

How would you be able to identify melanocytes by H&E?

A

They would be pale cells (cytoplasm), in or protruding from the basal layer.

Some pale cells in the basal layer are Merkel cells: touch-sensors. It’s hard to tell the difference with H&E alone.

19
Q

What are the functions of Langerhans cells and how can they be seen through staining?

A

Their function is within the immune system. They seek and deal with invading microbes.

They are antigen-presenting cells (like macrophages).
They’re dendritic cells, forming a network (can be seen through immunoperoxidase staining).

Through H & E staining:

  • They are small, pale cells in the non-basal (upper) layers of the epidermis.
  • They’re hard to see with H&E only though.
20
Q

Describe Vitamin D production in the skin.

A
  • Vitamin D3 (inactive precursor) is made in the epidermis
  • it’s synthesised mostly in the basal cells, but also in the stratum spinosum
  • it requires UV light
  • it requires more UV light in dark skin (due to the melanin barrier)
  • it is converted into its active form in the liver and kidney (1,25-dihydroxyvitamin D3)
  • it is commonly deficient in the UK, due to the relative lack of sun exposure
21
Q

Briefly describe the dermis.

A
  • Below the epidermis
  • Made of 2 layers: thin papillary layer and deeper reticular layer.
  • It is responsible for the tensile strength and elasticity of the skin
  • It houses blood vessels, skin glands and sensory receptors.
22
Q

Describe the structure of the dermis.

A

The 2 layers of the dermis are the thin papillary layer (contain finger like papillary protrusions) and the deeper reticular layer (containing the network of fibres.).

Its made up of layers of connective tissue characterised by an interconnected mesh of elastin and collagen fibres, produced by dermal fibroblasts.

Fibroblasts are the principal cells of the dermis. They make and maintain collagen.

23
Q

Describe some of the functions of the dermis.

A

It is dense (full of collagen fibres), irregular (fibres run in all directions, not parallel) connective tissue.

  • Collagen provides tensile strength (strength when pulled), hence it acts as protection against abrasion and impact.
  • It also contains elastin, a protein complex that provides elasticity.
  • The dermis also carries blood and nerve supply for the epidermis, as it is rich in blood vessels.
  • The dermal-epidermal border is often wavy, to resist shear forces (rubbing sideways).
24
Q

Describe the dermal-epidermal border

A

The dermal-epidermal border is often wavy, to resist shear forces (rubbing sideways).

  • Dermal papillae - finger like protrusions of dermal connective tissue into the epidermal layer.
  • Rete ridges - extensions of epidermis into the dermal layer.
  • More apparent in the thick skin of the hands and feet. (Fingerprints?)
  • see histology pics in slides
25
Q

Describe the elastic fibres in the dermis and the effect of UV light.

A

Elastin Verhoeff van Gieson (VVG) stain shows the collagen fibres in red, and the elastic fibres in black.
These are stretchy and give the skin its elasticity.

One effect of UV light on the skin is damage to elastic fibres, resulting in the loss of elasticity (ie. wrinkles).

26
Q

Briefly describe the hypodermis.

A
  • Below the dermis
  • Helps insulate deeper tissue
  • Anchors skin to the muscle with connective tissue
27
Q

Describe the structure and function of the hypodermis.

A

The hypodermis is also known as Fascia/subcutis. (cutis = epidermis + dermis, sub = below)

They are made up of well vascularised, loose, areolar (circular) connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.

It is composed of fat, containing glands, hair follicles, nerves, and blood vessels. It is often the thickest layer of skin. The thickness varies with age, body site, nutrition, etc.

Its function is that it provides insulation, cushioning and energy storage.

The hypodermis is the location where you put the hypodermic needle, for a subcutaneous injection.

28
Q

What are the appendages?

A

Appendages are things that are attached to something larger.

29
Q

Describe hair as an appendage.

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 thehair follicle.

Thehair follicleis epidermal tissue that dips down into thedermis, and interacts with other structures likeapocrine glands,sebaceous glands, the arrectorpilimuscle, and nerve receptors.

30
Q

How does hair get its strength and colour, and how does balding and the whiting of hair occur?

A

The Bulb contains the hair matrix – active site of hair growth and pigmentation.

It contains 2 different cell types – follicular keratinocytes and melanocytes. Follicular keratinocytes produce hard keratin = hair. The melanocytes give the hair its colour.

Keratinocytes filled with hard keratin flatten out and are slowly pushed up the follicle - resulting in root and shaft formation.

Follicularkeratinocytesat the bulb of the hairreplicateonly a set number of times, after which thehair folliclestops growing and falls out, which leads tobaldness.

Overtimethesemelanocytes stop producingmelanin, which is why hair turns white as people age.

31
Q

Describe the skin glands as an appendage.

A

Eccrine sweat glands:
- Normal sweat glands. Watery secretion on to the skin surface, cools body by evaporation (takes heat away from body surface).

Apocrine sweat glands:

  • Secrete into the hair follicles.
  • Found in armpits and anogenital regions.
  • They secrete oily fluids in humans, their function is unclear (contains phermones in some mammals)
  • Source of body odour after bacterial action.
  • Only active after puberty.

Sebaceous glands:

  • Secrete oily sebum (‘lanolin’) into the hair follicle. Conditioner for hair and skin, prevents dryness and flaking.
  • Only present after puberty.
  • Can lead to causing acne.
  • The 3 different ways for a gland/cell to secrete something?
  • MEROCRINE: release via exocytosis (these are accrine glands)
  • APOCRINE: pinch bits off, give bits away
  • HOLOCRINE: blow up
  • from Sarah Sheikh flashcards - not in 2020 lecs.
32
Q

Describe the nails as an appendage.

A

Nails grow in a proximal to distal direction.

Nail folds → where the skin seals of the edges of the nail.

Eponychium (located before the nail plate)→ Proximal skin fold that gives rise to the cuticle, a semi-circular layer of dead skinkeratinocytesthat covers the junction where the nail enters the skin, preventing the entry of pathogens.

Nail matrix → creates thenail plate (through modified keratinocytes undergoing keratinisation), which is the hard part of nail we can see as well as the free edge that hangs over the skin.

The nail matrix also has a special epidermal tissue that contains nerves, lymphatics, and blood vessels that support the nail.

The nail matrix also contains modifiedkeratinocytesthatreplicateand undergo the process ofkeratinisation. The youngest keratinocytesare in the nail matrix, and older cells keratinize, and form thenail plate.

The nail grows about 3 mm/month, so it takes several months to replace.

33
Q

Describe the vasculature of the skin.

From Sarah Sheikh flashcards - not in 2020 lectures.

A

Note that the epidermis is avascular and fed by capillary networks in the dermal papillae. The arteriovenous anastomoses are important in redistributing blood flow.

Blood supply arises from the subcutaneous tissue; this is where vessels pop up and you have a deep plexus that talks to a superficial plexus, This gives two levels of control.

Having two plexuses allows for different degrees of ‘shutting off blood to the skin’. This is controlled by sympathetic vasomotor neurons innervating the AVA and other blood vessels.

34
Q

Describe the sensory receptors in the skin.

A

The nerve endings originate and run along the dermis. Merkel disc end and protrude in the epidermis.

  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.

Each of these sensory structures responds to slightly different modalities (pressure, vibration, temperature, etc.). They also differ in how they adapt (rapidly versus slowly). Finally, they differ in their abundance in the skin from different parts of the body.

Nociception – detection of noxious stimuli (painful, harmful). Rapidly-adapting nerve fibers send information related to changing stimuli.

35
Q

What is the importance of Vitamin D3 production?

From Sarah Sheikh flashcards - not in 2020 lectures.

A
  • vitamin D3 regulates calcium and phosphate metabolism
  • most (so, 90%) of out Vitamin D is produced by the skin, but dietary sources include oily fish
  • low Vitamin D can lead to rickets in children (=osteomalacia in adults)
  • there is also a lot of talk about how low Vitamin D contributes to other diseases, but there is not much hard data
36
Q

As a summary, with the different skin functions, explain how each of them is achieved.

A

1) BARRIER AGAINST:
- DEHYDRATION: epidermis - keratin holds water, lipids stop it from evaporating

  • INFECTION: epidermis - impervious barrier, also houses cells of the immune system (Langerhans cells)
  • INJURY/ABRASION: all layers - the epidermis is strong, rapidly heals, and is thick where frictions occurs; the dermis has collagen, which is tough and leathery; the hypodermis fat cushions
  • SOLAR RADIATION: epidermis - stratum corneum and melanin pigment (protects nuclei of keratinocytes)
    2) THERMOREGULATION: dermis and hypodermis. hypodermis acts as insulation; thermoreceptors, blood supply, regulation, sweating
    3) SENSATION: all layers - nerve supply and various receptors
    4) REPAIR: epidermis - normal proliferation; dermis - fibroblasts fill the gap with new collagen that the epidermis can attach to.

5) VITAMIN D3 PRODUCTION: epidermis
- reaction of 7-dehydrocholestrol with UVB radiation –> Vitamin D3 (inactive form present in skin)

37
Q

Glossary

A

Integument – The skin and its appendages, which are hair, nails and glands.

Epidermis – The outer layer of the skin: stratified, squamous, keratinized epithelium.

Dermis – The second skin layer: fibrous, dense connective tissue

Hypodermis or subcutis – the third layer, of fatty connective tissue

Cutis (adj. cutaneous) – Epidermis + dermis

Keratin – A protein made inside keratinocytes; the main component of hair, hooves, horns, nails and the outer epidermis, giving strength and (in large masses) hardness.

Keratinocytes – epithelial cells, main cells composing the epidermis

Melanocytes – cells that make melanin, the pigment of skin, hair and eyes

Collagen – a strong, fibrous, extracellular protein, main component of the dermis (and of leather)

Fibroblasts – main cell type of the dermis (and other soft connective tissue), generating the extracellular matrix including collagen

Sebaceous gland – gland producing oily conditioning fluid (sebum) for skin and hair

Eccrine sweat gland – produces normal sweat

Apocrine sweat gland – produces different sweat in armpits etc, after puberty

Physiology 2 (30 decks)

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