At which layer does keratinisation of the epidermis begin?

The process of keratinisation begins in the stratum granulosum, which is the layer of the epidermis where keratinocytes start to flatten and begin to lose their nuclei, leading to the production of keratin. In this layer, cells contain keratohyalin granules, which contribute to the formation of the protein keratin. As the cells progress through the layer, they become filled with keratin, a tough, protective protein that helps form a barrier.

The stratum corneum, while it is composed entirely of dead, flattened keratinized cells, is actually the final layer where keratinisation is observed to culminate, not where it starts. The stratum lucidum is specific to areas of thick skin (like the palms and soles) and is an additional layer above the stratum granulosum, but does not initiate keratinisation. The stratum basale is the deepest layer, where cell division occurs, but it does not participate in the actual keratinisation process. Thus, the stratum granulosum is the correct answer, as it is the layer where the transition into keratinisation is actively taking place.