The Skin: Structure and Function

Question 1

What are the three layers of the skin?

Answer 1

Epidermis = outer epithelial layer
Dermis = Middle connective tissue/collagen layer
Hypodermis = Inner fatty layer
Hypodermis can be the thickest layer depending on the part of the body

Question 2

What are the functions of the skin?

Answer 2

Barrier against:
	- Dehydration
	- Infection
	- Injury/abrasion
	- Solar radiation
Thermoregulation
Sensation
Repair
Vitamin D production

Question 3

What is the epidermis?

Answer 3

Most superficial
Gives skin its colour
Protection from:
	- Pathogens
	- Environment
Vitamin D production
Made up of multiple layers of differentiating keratinocytes.
Keratinocytes- flat pancake-shaped cells that are named for the keratin producing capabilities

Question 4

What are the four main layers of the epidermis?

Answer 4

All starts with the basal layer (the stratum basal) the innermost layer of the skin
Then the layer above it is the spiny layer (stratum spinosum)
Above that is the granular layer (stratum granulosum)- due to its granular appearance
The outermost layer is the cornified layer (stratum corneum)
Sometimes you have a stratum lucidium (clear layer)- only in thick skin e.g. palms and soles of feet

Question 5

What happens in the basal layer (stratum basalis)?

Answer 5

Basal layer (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 6

What happens in the spiny layer (stratum spinosum)?

Answer 6

• Cells (keratinocytes) have many desmosomes (junctions) here visible as spines between the cells
  Strong bonds holding the epidermis together

Question 7

What happens in the granular layer (stratum granulosum)?

Answer 7

• 1-4 layers of cells containing prominent granules of keratohyalin
  
  • Also contain lamellar bodies containing lipids (seen by TEM)

Question 8

What happens in the cornified layer (stratum corneum)?

Answer 8

• The outer protective layer of the epidermis
  
  • Cells are keratinized (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

Question 9

Describe the process of keratinization?

Answer 9

• Occurs in the stratum corneum
• Keratinocytes in this layer begin the process of keratinization, which is the process where the keratinocytes flatten out and die, and in the process they create the epidermal skin barrier.
• To do this, keratinocytes in the stratum granulosum layer produce large amounts of keratin precursor proteins and glycolipid which remain within granules called keratohyalin granules and lamellar granules, respectively.
• Keratohyalin granules eventually start to aggregate and cross-link forming enormous bundles of keratin within the keratinocyte.
• Lamellar granules, on the other hand, get secreted and stick to the outer cell surface.
• It forms a sort of cement between the cells, making them more resistant to external forces and water loss.
• Over time, the intracellular organelles disintegrate so the cells flatten out and die.

Question 10

What are melanocytes role?

Answer 10

Synthesise melanosomes (pigment granules) and transfers them to basal layer keratinocytes through long dendrites
So melanin is synthesised in melanocytes and then transferred to keratinocytes
Melanocytes as viewed through thin sheets of human epidermis
Special (DOPA) stain for melanin shows their dendritic form

Question 11

What do keratinocytes do to melanin?

Answer 11

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

Question 12

What do Langerhans cells do?

Answer 12

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 (on the power point)

Question 13

How is vitamin D produced?

Answer 13

Vitamin D3, the inactive precursor, made in the epidermis
Mostly basal cells, also stratum spinosum
Requires UV light
Require more UV light in dark skin ( melanin barrier)
Converted to active form in liver and kidney; 1,25 dihydroxyvitamin D3
Commonly deficient in UK, as we don’t have that much sun in our climate and we spend a lot of time indoors

Question 14

What is the dermis like?

Answer 14

Below the epidermis
Made up of two layers
Responsible for tensile strength and elasticity
Blood vessels, skin glands and sensory receptors
Made up of layers of connective tissue characterised by interconnected mesh of elastin and collagen fibres, produced by dermal fibroblasts
Fibroblasts are the principal cell of the dermis.

Question 15

What are the two layers of the dermis?

Answer 15

Thin papillary layer- consists of papillae

Deeper reticular layer- called the reticular layer due to that network of fibres

Question 16

Why is the dermis dense and irregular and what function does it serve?

Answer 16

Dense, irregular connective tissue:
- Dense because it’s full of collagen
- Irregular because fibres run in all direction (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 17

What is the dermal-epidermal border like?

Answer 17

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 thick skin of hands; fingerprints as well as feet

Question 18

What is the hypodermis?

Answer 18

Below the dermis
Helps insulate deeper tissue
Anchors skin to the muscle with connective tissue
Also known as a fascia or subcutis
(Cutis= epidermis + dermis)
Made up of well-vascularised, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument

Question 19

What is the structure of the hypodermis like?

Answer 19

Composed of fat, containing glands, hair follicles, nerves, blood vessels
Often the thickest layer of skin
Thickness varies with age, body site , nutrition etc.
Well vascularised with capillaries so the body can take up fat for nutrition/energy when needed
Function- provides insulation, cushioning and energy storage
Where you put a hypodermic needle, for a subcutaneous injection

Question 20

What is the structure of the hair?

Answer 20

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 21

How is a hair strand formed?

Answer 21

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

Question 22

What are the three different types of glands in the skin? What purpose do they serve?

Answer 22

Eccrine sweat gland:
- Normal sweat glands
- Watery secretion on to skin surface, cools body evaporation
Apocrine sweat gland:
- Secrete into hair follicle
- 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
Sebaceous gland:
- Secrete oily sebum (lanolin) into hair follicle
- Conditioner for hair and skin, prevents dryness and flaking
- Only present after puberty

Question 23

Describe the structure of the nails.

Answer 23

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 contain 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 keratinse and form the nail plate

Question 24

What are the different types of sensory receptors in different layers of the skin?

Answer 24

1. Free nerve endings- responsible for nociception, thermal sensation, light touch
   
   2. Encapsulated ending:
      - Pacinian corpus(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, possible thermoreceptors
      Nociception- detection of noxious stimuli (painful, harmful
      Rapidly- adapting nerve fibres send information related to changing stimuli