May 15, 2012 Dr. Larry Jaeger is a well known and respected board certified dermatologist and dermatologic surgeon who is the medical director of Advanced Dermatology Associates of New York, PC. Dr Larry Jaeger specializes in all aspect of medical, cosmetic and surgical dermatology. Epidermis Anatomy – Keratinocyte Maturation There are four major layers of keratinocytes (the structural cells) in the epidermis and one layer that is present only in certain parts of the body. The bottom layer, the stratum basale, has cells that are shaped like columns. In this layer the cells divide and push already formed cells into higher layers. As cells move into the higher layers, they flatten and eventually die. We will take a closer look at the characteristics of each of these layers. Stratum Basale The stratum basale is the bottom layer of keratinocytes in the epidermis and is responsible for constantly renewing epidermal cells. This layer contains just one row of undifferentiated columnar stem cells that divide very frequently. Half of the cells differentiate and move to the next layer to begin the maturation process. The other half stay in the basal layer and divide over and over again to replenish the basal layer. Stratum Spinosum Cells that move into the spinosum layer (also called prickle cell layer) change from being columnar to polygonal. In this layer the cells start to synthesize keratin. Stratum Granulosum The cells in the stratum granulosum, or granular layer, have lost their nuclei and are characterized by dark clumps of cytoplasmic material. There is a lot of activity in this layer as keratin proteins and water-proofing lipids are being produced and organized. Stratum Corneum The cells in the stratum corneum layer are known as corneocytes. The cells have flattened out and are composed mainly of keratin protein which provides strength to the layer but also allows the absorption of water.