What is keratinization and where does it occur?

Keratinization is the process by which cells in the outermost layer of the skin, called the epidermis, produce a tough, fibrous protein called keratin. Keratin is a durable protein that provides strength and protection to the skin, nails, and hair.

Keratinization begins in the innermost layer of the epidermis, which is called the basal layer or the stratum basale. Here, cells called keratinocytes divide and push older cells towards the surface of the skin. As these cells move upwards, they undergo a process of transformation and eventually become completely filled with keratin. These fully keratinized cells are known as corneocytes.

The process of keratinization typically takes around 4 to 6 weeks for skin cells to travel from the basal layer to the surface of the skin, where they eventually shed off. During this journey, the cells become flatter, lose their nucleus, and develop a tougher and more rigid structure due to the accumulation of keratin.

Keratinization occurs not only in the skin but also in other structures derived from the epidermis. For example, hair and nails are composed primarily of keratinized cells. In hair follicles, keratinization occurs as hair cells are produced in the hair bulb within the scalp and gradually pushed upward. The cells in the hair shaft keratinize and become harder, forming the hair strand.

In nails, keratinization takes place in the nail matrix at the base of the nail. New cells are continuously added to the nail plate, and as they move forward, they undergo keratinization, contributing to nail growth.

Overall, keratinization is a crucial process for maintaining the integrity, strength, and protective function of the skin, hair, and nails.