Chapter 15 Discussion questions and answers

  • Discuss the structural features of the epidermis and describe how its structure relates to its function.

Bullet Point Answers

  • Multiple layers of cells composed of keratinocytes
  • Moored to the basement membrane
  • 5 identifiable layers of keratinocytes (stratum corneum, stratum lucidem, stratum granulosum, stratum spinosum and stratum germinativum).
  • Progressive changes to keratinocytes as they migrate to the outer stratum corneal layer. Keratinocytes are anucleate at this stage.
  • Migration from the basal keratinocyte layer (stratum germinativum) through to final layer (stratum corneum) takes between 22 and 45 days, depending on the skin site.
  • Keratinocytes held together through desmosomal and hemidesmosomal junctions. Intermediate filaments forming the cytoskeletal structure provide rigidity to the keratinocytes.
  • Specialist cells other than keratinocytes exist in the epidermis, including melanocytes (produce melanin), Langerhans cells (immunosurveillance) and neuroendocrine/Merkel cell (specialized keratinocyte with neurotransmitter functions)
  • Hair shafts containing hair run through the epidermis and form part of the pilosebaceous unit.
  • Nails and hair are specialized structures forming the skin adnexa
  • Functionally the epidermis provides:

Protection: Largely due to the properties of the keratinocytes.

Regulation: Thermoregulatory control through adaptations of hair and secretions from the pilosebaceous units.

Sensory: The presence of neurosecretory cells and close associations with mechanoreceptors.

Metabolic: The synthesis of vitamin D derivatives utilising sunlight is an additional role.

  • Describe the range of key cell types that reside within the dermis.

Bullet Point Answers

  • Cells of the dermis include:

Mast cells; Secrete mediators including histamine, tryptase, chymase etc, which function to increase permeability of vascular cells for inflammatory responses.

Fibroblast: Mesenchymally derived and function for the synthesis and breakdown of fibrous and non-fibrous connective tissue within the dermis. The connective tissue includes the dermal collagen, elastin and ground matrix substances of the dermis. Fibroblasts are important in repair processes and wound healing.

Histiocytes: Within the dermis these are usually identified as macrophages. These cells phagocytose cellular and tissue breakdown products. They also act as antigen-presenting cells in cell-mediated immune responses. They are also involved in anti-tumour and wound healing responses. They are also key to the granulomatous responses of the skin.

Dermal dendrocyte: Phagohistiocytic cells. They may also be associated with fibroblast lineage. They are found in greatest numbers in the papillary dermis and also around the pilosebaceous units. They are found in elevated numbers following wound healing, particularly around scar tissue.

  • Discuss the structure of the BMZ. How important is this layer to the integrity of the skin as a functioning unit?

Bullet Point Answers

  • The epidermis rests on a relatively thin but highly complex layer, the basement membrane zone (BMZ). This thin uniformed line of tissue components is not identifiable using conventional H&E staining, requiring the use of stains such as periodic acid Schiff (PAS).
  • As well as acting as a continuous point of attachment for the epidermis to the dermis, the BMZ also acts to resist the effects of frictional forces on the skin.
  • The BMZ is also central to the orchestration of the cytoskeleton of basal keratinocytes, and acts as a semipermeable barrier for the transfer of epidermal developmental signals.
  • The full supramolecular construction and more importantly the associations of the protein complexes in the BMZ are not fully elucidated.
  • The BMZ is broadly subdivided into three structurally different networks:

i) The hemidesmosome and anchoring filament complex
ii) The basement membrane proper
iii) The mooring and supporting anchoring fibrils.

  • The basement membrane proper is composed of two distinctive zones. The first appears as an empty electron-lucent area called the lamina lucida .This area has anchoring filaments containing an assortment of laminin isoforms, entactin/nidogen and fibronectin. This area of the BMZ is thought to be its weakest part, as separation within the epidermis using salt solutions or heat can be relatively easily achieved through this zone. Below the lamina lucida is the lamina densa. Unlike the lamina lucida, the lamina densa is an electron-dense area. This area functions as a barrier/filter that restricts the passage of molecules with a molecular mass >40 kDa. Interestingly, it is penetrated by melanocytes and Langerhans cells as they migrate up and down the epidermal and dermal compartments. This zone is composed mainly of collagen type IV and some laminins. Below the lamina densa we have the third component of the BMZ, the sublamina densa zone.
  • The sublamina densa zone is composed of the anchoring filbrils which are broad (20–60 nm) and elongated (200–800 nm). These fibrils are flexible, fibrillar and come from the lamina densa and are responsible for securing the basal lamina to the dermis. The major component of this area of the BMZ is collagen type VII, although types I, III, V and VI are also present.
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