Skin: Structure and Function

Question 1

What is integument?

Answer 1

A collective word to describe the Skin, Hair and Nails

Question 2

List some functions of the skin.

Answer 2

Barrier - Protects against dehydration, infection, injury/abrasion, solar radiation

Thermoregulation - helps in regulation of body temperature

Sensation - skin also functions as a sensory organ

Repair - skin is capable of repair

Vitamin D production - skin is a site of vitamin D production

Question 3

List the three layers of the skin.

Answer 3

Epidermis - outermost epithelial layer

Dermis - middle layer that is made up of lots of connective tissue, particularly collagen

Hypodermis - inner fatty layer.

Question 4

What are some functions of the epidermis?

Answer 4

• Most superficial layer of the skin (outermost)
• Gives skin its colour
• Protection from pathogens and environment
• Vitamin D production
• Made up of multiple layers of upwards differentiating keratinocytes

Question 5

What are the 4 main layers of the epidermis?

Answer 5

Basal Layer (Stratum basale) - Contains stem cells, and attached to dermis. Stem cells constantly proliferate. Dynamic since there is constant motion of the keratinocytes up through the epidermis.

Spiny Layer (Stratum spinosum) - Keratinocytes have many desmosomes, (junctions) between the cells. These spiny structures have strong bonds holding the epidermis together.

Granular layer (Stratum granulosum) - 1-4 layers of cells containing granules of keratohyalin (precursor of keratin). Also contain lamellar bodies containing lipids. Cells start to lose their nuclei because they are differentiating into the cornified layer.

Cornified layer (Stratum corneum) - outer protective layer of the epidermis. Cell are keratinised so the skin is tough and resistant to injury. Flattened cells and have lost their nuclei. Nonpolar lipids (waterproof) between the cells - formed from lamellar bodies. Layer constantly shed and replaced.

Question 6

List some epidermal cell types

Answer 6

• Keratinocytes (most prominent epidermis cell)
• Melanocytes (responsible for skin pigment)
• Langerhans cells (responsible for immune function/defense)
• Merkel cells (sensation e.g. touch)

Question 7

Describe melanocytes.

Answer 7

• Melanocytes synthesise melanosomes (granules which pigment skin) and transfers them to basal keratinocytes through long dendrites.
• Melanocytes then arrange melanin pigment in a cap-like conformation distal to the nucleus (sunny side). This protects the nucleus from radiation damage
  from UV rays.
• Present especially in basal layer
• Hard to tell the difference between melanocytes and Merkel cells using H&E alone because they both have pale cytoplasm so can use DOPA stain for melanin.

Question 8

Describe Langerhans cells.

Answer 8

Immune system defence - seeks and deals with invading microbes

Antigen-presenting cells - behave like macrophages

They are dendritic cells, forming a network

Hard to see with H&E so seen with immunoperoxidase staining.

Question 9

Describe Vitamin D production.

Answer 9

• Vitamin D3 (inactive precursor), made in the epidermis.
• Synthesised mostly in basal cells, and also somewhat in the stratum spinosum.
• Requires UV light
• Requires more UV light in dark skin to make the same amount of Vitamin D in individuals with lighter skin (due to the melanin barrier).
• Converted to active form in liver and kidney: 1,25 dihydroxyvitamin D3

Question 10

Describe the dermis.

Answer 10

• The skin layer that lies below the epidermis
• It is made up of two layers
• It is responsible for the tensile strength and elasticity of the skin
• The dermis houses blood vessels, skin glands and sensory receptors
• Fibroblasts are the principal cell of the dermis
• Made up of layers of connective tissue characterized by interconnected mesh of elastin and collagen fibres produced by dermal fibroblasts.
• Dense (full of collagen fibres), irregular (fibres run in all directions) connective tissue

Question 11

What are the 2 layers of the dermis?

Answer 11

Thin papillary layer - consists of finger-like protrusions (which go into the epidermal region) called papilla.

Deeper reticular layer - interconnected mesh of fibres.

Question 12

What are some functions of the dermis?

Answer 12

• Collagen provides tensile strength so protects against abrasion and impact e.g. hits to the skin.
• 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 13

Describe the dermal-epidermal border.

Answer 13

• 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
• The wavy appearance more apparent in thick skin of hands and feet = fingerprints

Question 14

Explain how you would stain to see the fibrous dermis network.

Answer 14

Verhoeff - van Gieson (VVG) stain

• To visualise elastin fibres the first stain with the Verhoeff stains elastin in black
• Then counter stained with the van Gieson which stains collagen in red.

UV light causes damage/loss of elastic fibres, resulting in loss of some of its elasticity and wrinkles.

Question 15

Describe the hypodermis.

Answer 15

• Skin layer below the dermis
• Helps insulate deeper tissue - due to fatty cells
• Anchors skin to muscle with connective tissue
• Hypodermis also referred to as Fascia (fatty) or subcutis (Cutis = epidermis + dermis)
• Made up of well-vascularized, loose, areolar connective tissue and adipose tissue for fat storage, insulation and cushioning for the integument.
• Composed of fat, glands, hair follicles, nerves, blood vessels.
• Often the thickest layer of skin. Thickness varies with age, body site, nutrition etc.

Question 16

What are functions of the hypodermis?

Answer 16

• Provides insulation
• Cushioning
• Energy storage (can absorb new energy intake from the fat deposits)
• The hypodermis is where you put a hypodermic needle, for a subcutaneous injection.

Question 17

Describe hair.

Answer 17

• Composed of the shaft, root, and bulb
• Bulb sits in the hair follicle.
• 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
• Bulb contains the hair matrix - active site of hair growth and pigmentation.

Question 18

Describe the hair matrix.

Answer 18

Contains two different cell types - follicular keratinocytes and melanocytes.

Follicular keratinocytes produce hard keratin - the hair. Keratinocytes at the bottom produce keratin, they flatten out and begin to stack and get pushed up resulting in formation of the root and shaft of the hair. Eventually, the hair follicle stops growing and falls out, which leads to baldness.

The melanocytes give the hair its colour. Over time these melanocytes stop producing melanin, which is why hair turns white as people age.

Question 19

Describe the skin glands.

Answer 19

Eccrine sweat gland - normal sweat glands that secrete watery secretions to the skin surface. This functions to cool our body by evaporation which takes the heat away from the body surface.

Apocrine sweat gland - secrete into the hair follicles. Found in armpits and genital regions. The secretions are oily. Known to contain pheromones in some animals. Apocrine sweat responsible for body odour after bacterial action. Only active after puberty.

Sebaceous gland - secrete oily sebum into hair follicle. Acts as natural conditioner for hair and skin - prevents dryness and flaking. Only active after puberty

Question 20

Describe nails

Answer 20

• Grow in a proximal to distal direction.

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

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

• Nail originates from the nail matrix. Keratinisation of the modified keratinocytes.

Question 21

Describe the nail matrix.

Answer 21

• Creates the nail plate
• Contains special epidermal tissue that contains nerves, lymphatics, and blood vessels
• Contains modified keratinocytes that replicate and undergo the process of keratinisation.
• Youngest keratinocytes are in the nail matrix, and older cells keratinize to form the nail plate.

Question 22

What are the two categories of sensory nerve endings?

Answer 22

1. Free nerve endings - these are responsible for nociception, thermal sensation, light touch on skin
2. Encapsulated endings
   - Pacinian corpuscles (rapidly adapting) - respond to vibration or brief touch
   - Merkel’s discs (slowly adapting) - respond to sustained touch/pressure e.g. skin indentation
   - Meissner’s corpuscles (rapidly adapting) - respond to moving touch or vibration
   - Ruffini endings (slowly adapting) - sense steady pressure
   - Root hair plexus (rapidly adapting) - responds to hair displacement.
   - Krause end bulbs - responds to touch/pressure, possibly thermoreceptors.