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The page below is a sample from the LabCE course The Histology of Dermatological Specimens - Part 1. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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The epidermis is the outermost layer of the skin and protects the body from the environment. The epidermis is comprised of the following cells:
  • Keratinocytes: Most common types of cells in the epidermis
  • Melanocytes: Contain melanin pigment; where melanoma develops
  • Langerhans cells: Involved in the immune system in the skin
  • Merkel cells: Involved in sensory communication
The epidermal layer itself is made up of five sub-layers that work together to continually rebuild the surface of the skin. The five sub-layers include:
  • Keratin
  • Lucid
  • Granular
  • Spinous
  • Basal
The keratinocytes originate in the basal layer and differentiate as they are pushed up to the surface layer and shed in the keratin layer.
The upper-most layer is called the stratum corneum, which is composed of dead keratinocytes (most common type of cell in the epidermis). The cells in this layer are identified by their flat shape and are called squamous cells.
The deepest layer is called the stratum basale (referred to as basal), where the keratinocytes originate and where basal cell carcinoma (BCC) originates as well.
In the layer between the corneum and basal layers, the keratinocytes differentiate as they are pushed up toward the surface of the skin and go through several morphological changes, gradually losing their nucleus and organelles. As the keratinocytes are pushed upward, they begin to flatten (squamous) and become flattened polyhedrons. It is in this layer that cellular changes occur leading to squamous cell carcinoma (SCC).
BCC and SCC are basal or squamous cells that grow out of control, usually due to DNA damage from the sun's ultra-violet (UV) light. These cancers typically stay in place (do not metastasize) and can be surgically removed. Malignant melanoma (MM) has a very high rate of metastasis (or spreading), therefore, early detection improves the cure rate.
The images show the different layers of cells in the epidermis and how the cells differentiate as they move toward the surface of the skin from the basal layer.

Layers of skin
Cellular differentiation in epidermis