The process by which epidermal cells begin to harden due to the accumulation of the protein keratin is called .

The process by which epidermal cells begin to harden due to the accumulation of the protein keratin is called keratinization.

Here are the steps of this process:

1. Basal Cells: The process begins in the basal layer of the skin, where the youngest cells are found. These cells are constantly dividing and pushing older cells towards the surface of the skin.

2. Keratin Production: As the cells move up through the layers of the skin, they begin to produce keratin, a tough, fibrous protein that provides protection.

3. Cell Death: By the time the cells reach the outermost layer of the skin, they have filled with keratin and died. This process is known as cornification.

4. Desquamation: The dead, keratin-filled cells, now called corneocytes, form the outermost layer of the skin. They eventually shed in a process called desquamation.

5. Replacement: The shed cells are constantly being replaced by new cells from the basal layer, maintaining the thickness of the skin.

This entire process is known as keratinization and it is crucial for maintaining the skin's barrier function.