9.3:
Layers of the Epidermis
The epidermis, the topmost layer of the skin, is made up of four to five distinct layers of cells.
The stratum basale is the deepest layer attached to the underlying dermis. It is primarily made up of keratinocytes and a small number of melanocytes, tactile epithelial cells, and stem cells.
As the stem cells divide, the newly formed keratinocytes are pushed up into the stratum spinosum, which has multiple layers of keratinocytes attached together by desmosomes.
Superior to spinosum, the stratum granulosum has 3 to 5 layers of keratinocytes, which undergo keratinization, making the cells thinner and flatter.
Moreover, the keratohyalin granules promote cell dehydration and increase cross-linking of keratin fibers. Eventually, cells lose organelles and die.
The Stratum lucidum is a thin translucent layer of keratin-rich dead keratinocytes found only in the thick skin of the palm and the sole.
The stratum corneum is the outermost layer of the epidermis, largely consisting of anucleated dead cells called corneocytes. This layer protects the skin from abrasions and is water-resistant.
9.3:
Layers of the Epidermis
The epidermis, the outermost layer of the skin, is composed of several distinct layers. From deep to superficial, the layers of the epidermis are as follows:
Stratum Basale
Stratum basale, also known as the stratum germinativum, is the deepest layer of the epidermis. It is composed of a single layer of actively dividing cells called basal cells or basal keratinocytes. These cells constantly undergo cell division to replenish the upper layers of the epidermis. Additionally, melanocytes, which produce the pigment melanin, are found in this layer.
Stratum Spinosum
As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome. The desmosomes interlock with each other and strengthen the bond between the cells. It is interesting to note that the "spiny" nature of this layer is an artifact of the staining process. Unstained epidermis samples do not exhibit this characteristic appearance.
Stratum Granulosum
The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum.
Stratum Lucidum
The stratum lucidum is a smooth, seemingly translucent epidermis layer located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits.
Stratum Corneum
The stratum corneum is the epidermis's most superficial layer and is exposed to the outside environment. Its name is derived from the increased keratinization (also called cornification) of the keratinocytes present in this layer. There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues and provides mechanical protection against abrasion for the more delicate, underlying layers.
The layers of the epidermis work together to provide protection, regulate water loss, and maintain the integrity of the skin. The constant renewal of cells from the basal layer to the outermost layer ensures the skin's ability to regenerate and maintain its barrier function.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 5.1: Layers of the Skin