In animals, such as vertebrates, the epidermis is made up of four or five layers (each called stratum). These layers protect the underneath layers of the skin against physical damage, infection, and water loss. In humans, the epidermis consists of the following layers: (1) stratum corneum, (2) stratum lucidum, (3) stratum granulosum, (4) stratum spinosum, and (5) stratum basale or germinativum).
The stratum spinosum is a Latin term, which literally means spinous layer. This layer is found in in between the stratum basale and the stratum granulosum. The keratinocytes in the stratum spinosum are referred to as prickle cells. These keratinocytes are polyhedral in shape and with large pale-staining nuclei. They are actively synthesizing fibrillary proteins that are essential for the formation of desmosomes. The desmosomes anchor the cells to each other and strongly connect the cells together. During tissue fixation for histologic microscopy, the cells tend to shrink but remain tightly bound to each other through the desmosomes and thereby the sites of connection between cells appear like spines or prickles (hence, the name). Langerhans cells are also found in the stratum spinosum. Keratinization begins in this layer.
- spinous layer (of the epidermis)
- prickle cell layer