The integumentary system contains the skin, as well as associated hair, nails, glands, and nerves, and it is organized into three distinct layers. Listed from superficial to deep, the layers are the epidermis, dermis, and hypodermis. However, the “skin” is only composed of the epidermis and dermis. The integumentary system is key to many homeostatic functions such as heat and moisture regulation, and it is needed for production of vitamin D and protection from pathogens.
The epidermis is the most superficial layer of the skin. The prefix epi- means above; hence, epidermis means “above the dermis”. ****It is thin and avascular, meaning it does not contain blood vessels. Thus, it relies on diffusion of oxygen and nutrients from the neighboring dermis.
The epidermis is composed of stratified squamous cells, which are flattened, disk-shaped cells arranged into multiple layers. There are four to five layers of cells in the epidermis, which are mostly keratinocytes. As the keratinocytes divide and differentiate, they migrate from the deeper to more superficial layers.
Along the way they expel their nuclei and produce additional keratin. By the time they reach the most superficial layer, they are fully differentiated keratinocytes. Eventually, these keratinocytes are shed during epidermal turnover.
Note: There are three types of epithelial cells based on their shape. Squamous are flattened; cuboidal are as wide as they are tall; and columnar are taller than they are wide.