NBME Histology Practice Test

The main idea being tested is how the outermost epidermal layer prevents water loss. The barrier is formed by keratinized, flattened cells in the stratum corneum that are surrounded by a lipid-rich envelope. Keratinization makes these corneocytes tough, compact, and effectively dead, so they stack tightly like bricks. The lipids that surround and fill the spaces between these cells act as a hydrophobic mortar, greatly reducing water diffusion through the skin. This combination—the keratinized cells plus their surrounding lipid envelopes—provides the most effective barrier to transepidermal water loss. Desmosomes mainly hold cells together and contribute to structure, not the primary barrier. The basement membrane lies below the epidermis and is not the key player in preventing water loss.