SKIN: Structure and Function of Skin

Question 1

What are some functions of the skin?

Answer 1

• acts as a barrier against physical, chemical and biological threats (including immunological aspects)
• homeostatic functions in temperature and water balance regulation
• sensory functions
• secretory/’nutritional’ function producing Vitamin D
• insulation (really, a property of the hypodermis/subcutis)
• repair
• cosmetic (tattoos, piercings, etc.)

Question 2

What are the two basic layers of skin, and what structures come under them?

Answer 2

The two basic layers of skin are the epidermis and the dermis.

Going from top to bottom, the structures found in the different skin layers are:
EPIDERMIS: (has no blood supply)
- stratum corneum
- stratum lucidum
- stratum granulosum
- stratum spinosum
- stratum basale

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)

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.

Question 3

What does integument refer to?

Answer 3

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).

Question 4

Describe the stratum corneum.

Answer 4

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

• contains squamous cells which have lost their nuclei
• the cells are also cornified - full of ‘horny’ keratin (from keratohyalin granules), thus are tough and resistant to injury
• 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.

Question 5

Describe the stratum granulosum.

Answer 5

There are 1-4 layers of cells containing prominent granules of ‘keratohyalin’ - the precursor for the protein keratin.
It also contains lamellar bodies containing lipids. These cells are differentiating to form the most outermost layer of skin.

Question 6

Describe the stratum spinosum.

Answer 6

These cells have many desmosomes (junctions) here, visible as ‘spines’ between the cells.
Strong bonds hold this epidermis layer together.

Question 7

Describe the stratum basale.

Answer 7

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 until they are shed from the outer surface, taking about 20-50 days.

Question 8

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

Answer 8

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.

Question 9

What is a frictional blister?

Answer 9

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.

Question 10

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

Answer 10

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.

Question 11

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]

Answer 11

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.

Question 12

List some other epidermal cell types, besides keratinocytes.

Answer 12

• melanocytes (pigment)
• langerhans cells (defence)
• merkel cells (sensation)

Question 13

What are the two types of melanin?

Answer 13

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)

Question 14

What is the function of a melanocyte (in the epidermis)?

Answer 14

It synthesises melanosomes (pigment granules) and transfers them to basal keratinocytes through long dendrites.

Question 15

In which layer is melanin pigment mostly found?

Answer 15

Melanin pigment is mostly found in the basal epidermis.

Keratinocytes arrange melanin pigment in a cap distal to the nucleus (sunny side), especially in the basal layer (stem cells).

Question 16

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

Answer 16

They would be pale cells, 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.

Question 17

What are the functions of Langerhans cells?

Answer 17

Their function is within the immune system.

They are antigen-presenting cells (like macrophages).
They’re dendritic cells, forming a network.

Question 18

How would you identify Langerhans cells with H&E?

Answer 18

They are small, pale cells in the non-basal (upper) layers of the epidermis.

They’re hard to see with H&E only though.

Question 19

Describe Vitamin D production in the skin.

Answer 19

• Vitamin D3 is made in the epidermis
• it’s done 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

Question 20

Describe the dermis.

Answer 20

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).

Question 21

Describe the elastic fibres in the dermis.

Answer 21

Elastin van Gieson 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).

Question 22

Describe the hypodermis.

Answer 22

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.

It is where you put a hypodermic syringe, for a subcutaneous injection.

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

Question 23

Appendages are things that are attached to something larger.
What are the three main appendages of the skin?

Answer 23

• hair follicles: each has an erector pili muscle which controls the angle of the hair
• sweat glands: of which there are two types (eccrine, all over the body, and apocrine, smelly in the armpit and groin)
• sebaceous glands: these are holocrine glands which release sebum (the oily/waxy substance that waterproofs the skin and hair, to prevent dryness and flaking) [when these sebaceous glands become overactive, we get acne]

Question 24

What are the three different ways for a gland/cell to secrete something?

Answer 24

MEROCRINE: release via exocytosis (these are accrine glands)

APOCRINE: pinch bits off, give bits away

HOLOCRINE: blow up

Question 25

Describe a nail as another appendage.

Answer 25

A nail is basically a broad, thick hair, and is similarly composed of hard keratin (all bricks, no mortar, so not oily).

The nail matrix produces keratinised cells much like a hair follicle. Its wedge-shape forces the nail along as it grows. The nail grows about 3 mm/month, so it takes several months to replace.

You can see the distal part of the nail matrix as the lunula under the nail.

Question 26

Describe the vasculature of the skin.

Answer 26

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.

Question 27

List the different sensory innervation in the skin.

Answer 27

1) free nerve endings (multimodal: pain, heat, mechanosensitive)
2) hair follicle receptor (abundant mechanoreceptors)
3) Ruffini endings/corpuscles (slowly-adapting mechanoreceptors)
4) Meissner corpuscles (sense stretching of incorporated collagen fibres, sense light touch, present in hairless skin, such as palms and lips)
5) Pacinian corpuscles (rapidly-adapting mechanoreceptors, sense pressure, lots in fingertips)
6) Merkel cell disks (slowly adapting mechanoreceptors)

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.

Question 28

What is the importance of Vitamin D3 production?

Answer 28

• 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

Question 29

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

Answer 29

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

• INFECTION: epidermis - impervious barrier, also houses cells of the immune system
• 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
  2) THERMOREGULATION: hypodermis acts as insulation; thermoreceptors, blood supply, regulation, sweating
  3) SENSATION: 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

Question 30

Describe the dermal-epidermal border.

Answer 30

The dermal-epidermal border is often wavy, to resist shear forces (rubbing sideways). We can see this especially on the hands (fingerprints) and the feet.

The epidermis can have rete ridges.
The dermis can have finger-like dermal papillae.