An epidermal cell which synthesizes keratin and undergoes a characteristic change as it moves upward from the basal layers of the epidermis to the cornified (horny) layer of the skin. Successive stages of differentiation of the keratinocytes forming the epidermal layers are basal cell, spinous or prickle cell, and the granular cell. [ http://www.ncbi.nlm.nih.gov/pubmed/19727116 http://www.ncbi.nlm.nih.gov/pubmed/15582983 http://en.wikipedia.org/wiki/Keratinocyte http://www.ncbi.nlm.nih.gov/pubmed/15749908 ]