Final answer:
Under the microscope, the stratum spinosum can be identified by its eight to 10 layers of keratinocytes with desmosomes, and the presence of Langerhans cells. The spiny appearance is a result of staining and keratinocytes start accumulating keratin in this layer.
Step-by-step explanation:
When identifying the stratum spinosum under the microscope, you can look for several characteristics. The stratum spinosum, known for its spiny appearance under microscopic examination, is actually an artifact of the staining process and in unstained samples, this distinctive spiny appearance would not be seen. This layer is typically composed of eight to 10 layers of keratinocytes, which are produced from cell division in the lower stratum basale. These keratinocytes begin to accumulate keratin, giving them a tougher and flatter structure over time.
Other key features include desmosomes, which are structures that protrude and interlock to strengthen the bond between cells, and the presence of Langerhans cells. Langerhans cells are a type of dendritic cell that functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer. Thus, under the microscope, you will not only see the polygonal shapes of keratinocytes and their desmosomes, but may also identify Langerhans cells interspersed among them.