The stratum spinosum (or spinous layer/prickle cell layer) is a layer of the epidermis found between the stratum granulosum and stratum basale. Their spiny (Latin, spinosum) appearance is due to shrinking of the microfilaments between desmosomes that occurs when stained with H&E. Keratinization begins in the stratum spinosum. This layer is composed of polyhedral keratinocytes. They have large pale-staining nuclei as they are active in synthesizing fibrilar proteins, known as cytokeratin, which build up within the cells aggregating together forming tonofibrils. The tonofibrils go on to form the desmosomes, which allow for strong connections to form between adjacent keratinocytes.
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