Final answer:
Keratinization refers to the process in the epidermis in which keratinocytes become filled with the protein keratin. This process starts in the stratum basale, continues through the stratum spinosum and stratum granulosum, and ends in the stratum corneum, where dead keratinocytes are replaced by new ones. The process gives hair, nails, and skin their hardness and water-resistant properties.
Step-by-step explanation:
Keratinization Process and Keratinocytes Keratinization is a process that occurs within the epidermis where keratinocytes begin to be filled with keratin, a protein that is essential for the hardness and water-resistant properties of hair, nails, and skin.
As new keratinocytes are produced in the stratum basale, they start to synthesize keratin and release a water-repelling glycolipid in the stratum spinosum. These cells then get pushed into the stratum granulosum, where they fill with keratin and appear granular due to their cytoplasm.
When keratinocytes reach the stratum corneum, they are dead and slough away, allowing cells from deeper layers to replace them. Hair growth is initiated in a follicle when stem cells divide to produce new keratinocytes. In the stratum granulosum, they release lipids to form a lipid barrier and start to die due to the distance from blood vessels and nutrient supply.
All keratinocytes are produced from the stratum basale, and they move outward as new cells form. The stratum basale also contains Merkel cells, which function in touch sensation, and melanocytes, which produce the pigment melanin for skin and hair color and UV radiation protection.