W8 - Skin: Structure and Function of Skin

Question 1

What is the integument or integumentary system?

Answer 1

-> Skin, Hair and Nails
* Largest and heaviest organ of the body -
~15% of adult body weight
* Much GP time

Question 2

What is the skin made of?

Answer 2

 3 Layers of the skin
- Epidermis
- Dermis
- Hypodermis
 Hair, skin glands, nails and sensory receptors.
 Main functions of the skin

Question 3

What are the functions of skin?

Answer 3

• Barrier (protection) against:
• Dehydration - waterproof
• Infection - residential immune cells in skin
• Injury/abrasion
• Solar radiation
• Thermoregulation
• Sensation
• Repair
• Vitamin D production

Question 4

What are the 3 layers of skin?

Answer 4

Hypodermis can be the thickest layer depending on the part of the body.
Derm’ = Skin
‘Epi’ = On
‘Hypo’ = Below

-Epidermis - Thin outermost later
-Dermis - Nerves and blood vessels
-Hypodermis - Fat & connective tissue - a lot thicker than the other two layers.
-Muscle

Question 5

What is the epidermis?

Answer 5

Most superficial
Gives skin its colour
Protection from
- Pathogens
- Environment
Vitamin D production

Made up of multiple layers of differentiating keratinocytes.

Eg. Abdomen skin = thin
Heel skin = thick

Question 6

What are the 4 main layers of the epidermis?

Answer 6

CORNIFIED LAYER - Stratum Corneum

STRATUM LUCIDIUM - this is only present in some places. It can be translucent when viewed under the microscope. Only seen on soles of feet and on the palms.

GRANULAR LAYER - Stratum Granulosum

SPINY LAYER - Stratum Spinosum

BASAL LAYER - Stratum Basale - innermost layer made up of constantly dividing, proliferating, mitotic stem cells. These cells then get pushed up to differentiate into different layers of keratinocytes.

Question 7

What is in the Basal layer?

Answer 7

Basal layer [B] (Stratum basale)
* First single layer, containing stem cells, and attached to dermis.
* Stem cells constantly proliferate.
* Dynamic - Daughter cells constantly move “up” (distally)
through the epidermis, differentiating as they go, until they are shed from the outer surface. This takes ~20-50 days.

Question 8

What is in the Spiny layer?

Answer 8

Spiny layer [S] (Stratum spinosum)
* Cells (keratinocytes) have many desmosomes, (junctions) here
visible as “spines” between the cells. Strong bonds holding the epidermis together.

Under the microscope, the white spiny structures are called Desmosomes. They hold the cells of the layer together.

Question 9

What is the Granular layer?

Answer 9

Granular layer [G] (Stratum granulosum)
* 1-4 layers of cells containing prominent granules of “keratohyalin”
– precursor of the protein keratin. Also contain lamellar bodies
containing lipids (seen by TEM). Cells are differentiating to form the outermost layer.
The cells in this layer starts to lose their nuclei because they are differentiating into the granulated area that is completely flattened and doesn’t have any nuclei.

Question 10

What is the Cornfield layer?

Answer 10

Cornified layer [C] (Stratum corneum)
* The outer protective layer of the epidermis.
* Cell are keratinised (cornified) – cytoplasm full of “horny” keratin
(from keratohyalin granules), thus tough and resistant to injury.
* Cells are flattened and have lost their nuclei…
* Nonpolar lipids (waterproof) are between the cells – from lamellar bodies.

Keratinisation occurs in the Stratum Corneum. This is the thick layer that needs constant shedding and replenishing.
Keratin: Also main component of horns, hair, nails, claws and hoofs.

Question 12

What are other epidermal cell types?

Answer 12

• Melanocytes (Pigment)
• Langerhans cells (defense)
• Merkel cells (sensation)

Question 13

What are melanocytes (pigment)

Answer 13

Melanocytes as viewed through thin sheets of human epidermis. Special (DOPA) stain for melanin shows their dendritic form. To do this, they separated the epidermis layer leaving behind a very thin sheet of epidermis.

Synthesizes melanosomes (pigment granules)
and transfers them to basal keratinocytes through long dendrites. These get transferred to the basal laters of keratinocytes.

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. The melanocytes exist within the epidermis layer.

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.

In images, the ones with the melanocytes being transferred up to the upper layers of the keratinocytes are wrong. It should only be the basal layer.

Question 14

What are Langerhans cells?

Answer 14

Function: Immune system. Seeks and deals with invading microbes.
Antigen-presenting cells (like macrophages).
They are dendritic cells, forming a network – seen here with
immunoperoxidase staining.

H&E appearance: Small, pale cells
in non-basal layers of epidermis.
Hard to see with H&E only.

Question 15

How is Vitamin D produced?

Answer 15

• Vitamin D3, made in the epidermis.
• Mostly basal cells, also stratum spinosum.
• Requires UV light.
• Requires more UV light in dark skin to make the same amount of Vitamin D. (melanin barrier).
• Converted to active form in liver and kidney: 1,25 dihydroxyvitamin D3.
• Commonly deficient in UK - indoors a lot and not much sun.

Question 16

What is the Dermis?

Answer 16

Below the epidermis
Two layers
Tensile strength & elasticity
Blood vessels, skin glands & sensory receptors

Made up of layers of connective tissue
characterized by interconnected mesh
of elastin and collagen fibres, produced
by dermal fibroblasts.
Fibroblasts are the principal cell of the
dermis.

Two layers of the dermis:
Thin papillary layer and deeper reticular layer

Question 17

What is the function of the dermis?

Answer 17

• Dense, irregular connective tissue.
  Dense = full of collagen fibres;
  Irregular = fibres run in all directions (not parallel)
• Functions: 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.

Question 18

What is the Dermal - Epidermal border?

Answer 18

• 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. (in between the fingers)
• Rete ridges – Extensions of epidermis
  into the dermal layer (tip of the fingers)
• More apparent in thick skin of hands =
  fingerprints, as well as feet.

Under a microscope, the collagen (containing much of volume of dermis) shows as pink. The dark spots are the nuclei of the fibroblasts (makes and maintains the collagen). The tiny structures inside are the blood vessels.

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

Question 19

What are the effects of UV light on skin?

Answer 19

One effect of ultraviolet light on skin is
damage/loss of elastic fibres, resulting in
loss of elasticity - wrinkles.

Question 20

What is the hypodermis?

Answer 20

Below the dermis
Helps insulate deeper tissue.
Anchors skin to the muscle with connective tissue.
* Fascia, or subcutis (Cutis = epidermis + dermis)
* Made up of well-vascularized, loose, areolar
connective tissue and 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: provides insulation, cushioning and energy storage.
• Where you put a hypodermic needle, for a subcutaneous injection.

Question 21

What is hair?

Answer 21

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.
The hair follicle is epidermal tissue that dips down into the dermis, and interacts with other structures like apocrine glands, sebaceous glands, the arrector pili muscle, and nerve receptors.

Question 22

What is hair made of?

Answer 22

Bulb contains the hair matrix – active site of hair growth and pigmentation.
It contains two 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 - resulting in root and shaft formation.
Follicular keratinocytes at the bulb of the hair replicate only a set number of times, after which the hair follicle stops growing and falls out, which leads to baldness.
Over time these melanocytes stop producing melanin, which is why hair turns white as people age.

Question 23

What are skin glands?

Answer 23

Normal sweat glands. Watery
secretion on to skin surface,
cools body by evaporation.

Secrete oily sebum (“lanolin”) into
hair follicle. Conditioner for hair and
skin, prevents dryness and flaking.
Only present after puberty.

Secrete into hair follicles. Found
in armpits and anogenital region.
Oily fluid in humans, function
unclear (contains pheromones in
some mammals). Source of body
odour after bacterial action. Only
present after puberty.

Question 24

What are nails made of?

Answer 24

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

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

Nail matrix → creates the nail plate, which is the hard part of nail we can see as well as the free edge that hangs over the 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 keratinize, and form the nail plate.

Question 25

What are sensory receptors?

Answer 25

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.