Thick skin -- present on fingertips, palms, and soles of feet, it is specialized for protection during friction and contains sensory receptors responsive to pressure, the Pacinian corpuscles.

Examine a section of thick skin (slides 5 and 89) and identify epidermis and dermis (Figs. 18-1 through 18-3).

• In the dermis, note the difference between the papillary and reticular layers.
• Note too how downward epidermal folds or ridges interdigitate with upward projecting dermal papillae.
• In the epidermis, identify characteristic cells, keratinocytes, (as seen in Fig. 18-2) representing the
  • Stratum basale,
  • Stratum spinosum,
  • Stratum granulosum, and
  • Stratum corneum
  • Click image to right for expanded view.
• The strata grade into one another and cells gradually move outward from the basal layer to the cornified layer.

What is the structural significance of the small “spines” on the cells of the spinous layer?

Study the ultrastructural features of the intercellular connections of the keratinocytes (Fig. 18-4).

Describe the cell biology of the keratinization process, accounting for all the histological features seen.

The skin as a route of medication delivery.