The appearance of the skin is affected by both the outer layer of the skin, the epidermis, and the middle layer of the skin, the dermis. The deepest layer is composed of a subcutaneous fat layer. The thickness and contour of the fat layer can also affect the appearance of the skin. The fat layer and muscle layer below it are too deep to be influenced by topical skin care agents so they will not be discussed in this book. Instead, the epidermis and dermis will be explored. (For a more in-depth review of basic skin science, see Cosmetic Dermatology: Principles and Practice, 2nd edition, Chapters 1–8.) The epidermis and dermis contain many types of cells. Only the ones known to play an important role in skin care will be discussed here (Figure 5-1).
Cross-section of skin as seen from a microscope.
Keratinocytes constitute the outer layer of skin called the epidermis. The epidermis is very important in skin appearance because it provides protection, keeping skin hydrated while preventing the penetration of caustic agents, and significantly contributes to skin radiance, evenness of color, and smoothness.
The epidermis is made up of layers of keratinocyte cells that resemble a brick wall. The youngest cells are found at the base and are called the basal layer. The stem cells also reside in the basal layer. The next layer contains spiny attachments that tightly hold the keratinocyte cells together to give skin strength. This layer is called the spinous layer. The next layer of cells is very productive, synthesizing fats, proteins, and sugars as well as other cellular components in “factories” inside the cells. When viewed through a microscope these factories (cellular organelles) appear as multicolored granules. For this reason, this important layer is called the granular layer. After the granular layer, the cells begin to lose activity, the granules disappear, and the cells flatten out. This layer of flattened inactive cells is the most outer layer and is termed the stratum corneum (SC).
The outermost cells are known as squamous cells from the Latin squama meaning “scale.” They are “dead” or apoptotic and flake off into the environment. At some point in the SC the attachments that hold these squamous cells together loosen, allowing the cells to fall from the surface in an action known as desquamation, from the Latin desquamare meaning “to scrape the scales off” (Figure 5-2).
Normal epidermis showing the cuboidal basal cells, the spinous layer, granular layer, and stratum corneum. The top layer of the SC is exhibiting the process of desquamation.
Aged skin is thinner and more fragile than young skin because ...