Final answer:
Terminally differentiated keratinocytes constitute the major part of the epidermis, producing keratin to form a protective layer. They originate from the base, move upwards, become keratin-rich, die, and slough off, being replaced by new cells. The epidermis also contains melanocytes, Langerhans cells, and Merkel cells, each with specific functions.
Step-by-step explanation:
Terminally differentiated keratinocytes are the primary cell type in the epidermis, making up to 90 percent of this outer skin layer. These cells produce keratin, a strong fibrous protein that provides the skin with its protective and water-resistant qualities. As new keratinocytes are formed at the base of the epidermis, they begin to produce keratin, fill with this protein, and gradually die. The dead keratinocytes reach the outer surface to create the stratum corneum, a protective layer that also aids in preventing water loss from the body. These cells act as a barrier and, upon reaching the terminal stage of differentiation, slough off to be replaced by newer cells from below.
The outer layer of skin also includes other cell types such as melanocytes, which produce the pigment melanin, Langerhans cells that help fight pathogens, and Merkel cells involved in the sense of touch. Although the epidermis is primarily made of the non-living protective keratinocytes, these other living cells are essential in the stratum basale, with each playing its own role in skin health and function.