In which layer of the epidermis does keratinization begin?

Keratinization begins in the stratum spinosum, which is the layer of the epidermis situated above the stratum basale. In this layer, keratinocytes go through a process of differentiation and start to produce keratin, a vital structural protein that provides strength and waterproofing to the skin.

As keratinocytes in the stratum spinosum mature, they become more flattened and begin to lose their nuclei as they move toward the outer layers of the skin. The process of keratinization then continues through the stratum granulosum, where cells further accumulate keratin granules and undergo additional changes, ultimately leading to the formation of the stratum corneum, where fully keratinized cells reside.

This foundational step in keratinization occurring in the stratum spinosum is critical for establishing the barrier function of the skin, a key protective aspect against environmental insults and dehydration.