Structure And Function Of Skin

Question 1

What are the 3 major functions of the skin?

Answer 1

• protection (physiological and immunological barrier): mechanical impacts, protects and detects pressure, detects extremes in temperature, barrier to microorganisms and radiation/chemicals
• regulation (physiological): body temperature via sweat, hair, and changes in peripheral circulation, fluid balance via sweat and insensible loss, synthesis of vitamin D
• sensation: network of nerve cells that detect and relay changes in the environment

Question 2

List the layers and contents of the skin

Answer 2

• epidermis (stratum corneum, stratum granulosum, stratum spinosum, stratum basale)
• basement membrane
• dermis (thick fibrous layer with glands and follicles)
• sub-cutaneous fat (anchoring element between fascia and skeleton containing blood vessels)

Question 3

Describe the embryologic development of the skin

Answer 3

• derived from the ectoderm
• week 5: skin of the embryo is covered by simple cuboidal epithelium
• week 7: single squamous layer (periderm) and a basal layer
• month 4: an intermediate layer containing several cell layers is interposed between the basal cells and periderm (+ formation of anatomical structures eg. Melanocytes/nerve endings)
• hair: 3rd month as epidermal proliferation into dermis
• cells of epithelial root sheath proliferate to form sebaceous gland bud, sweat glands develop as down-growth of epithelial cords into dermis

Question 4

Describe the immune function of the skin

Answer 4

• Langerhan cells are members of dendritic cells residing in the basal layers of the skin
• specialise in antigen presenting, acquire antigens in peripheral tissues, transport them to regional lymph nodes, present to naive T cells and initiate adaptive immune response
• activated T cells initiate cytokine release cascade (help immune system damage antigens)
• involved in antimicrobial immunity, skin immunosurveillance, induction hypersensitivity and pathogenesis of chronic inflammatory diseases of the skin

Question 5

Describe the mechanism of allergy of the skin

Answer 5

• skin irritation by non-allergenic and allergenic compounds induces Langerhan cell migration and maturation
• Langerhan cells migrate from epidermis to draining lymph nodes
• initial sensitisation takes 10-14 days from initial exposure to allergen
• once an individual has been exposed to a skin allergy contact dermatitis can occur

Question 6

Describe the effects of UV on the skin

Answer 6

• direct cellular damage and alterations in immunological function
• direct effects = photoaging, DNA damage and carcinogenesis
• P53 tumour suppressor genes mutated by DNA damage (implicated in melanoma and non-melanoma skin cancers)
• keratinocytes and melanocytes work together to protect cells from UV DNA damage
• chronic exposure leads to loss of skin elasticity, fragility, abnormal pigmentation and haemorrhage of blood vessels
• wrinkles and premature ageing

Question 7

Describe the action of vitamin D

Answer 7

• during exposure to UVB photons in sunlight are absorbed by 7-dehydrocholesterol in the skin and converted to previtamin D(3)
• undergoes transformation in the plasma membrane to active vitamin D(3)
• deficiency is associated with increased risk of common cancers, autoimmune disease, infections and CV disease

Question 8

Describe how the skin sensation works

Answer 8

• cutaneous receptors can be encapsulated (Meissner, Pacini, and Ruffini corpuscles) or free nerve endings (Merkel cells)
• Merkel cells (base of epidermis): response to sustained gentle/localised pressure, assesses shape/edge
• Meissner (immediately below epidermis and on fingertips): light touch
• Ruffini (dermis): deep pressure and stretching
• Pacini (mechanoreceptors in deep dermis): deep touch, rapid deformation of skin surface and around joints for position/proprioception