The skin is divided into three layers: the epidermis, dermis, and hypodermis or subcutaneous layer (see image on pg. 9).
The epidermis is the top layer of the skin and is composed of four cell types:
The epidermis is further divided into the outermost non-living layer, the stratum corneum, and the living cellular layers of the stratum granulosum, stratum spinosum, and stratum Basale.
The stratum corneum is composed of corneocytes and lipids, which serves as a barrier against microbial pathogens and environmental irritants and keeps the skin hydrated and protected from injury.
Constant renewal is necessary for the epidermis to maintain its integrity and function effectively. In healthy young skin, it takes approximately 1 month for keratinocytes to migrate from the living basal layer of the epidermis to the stratum corneum surface and desquamate, during the process of epidermal renewal.
The image Fig ?? shows the structure of the epidermis with the keratinocyte maturation highlighted. Photoaged skin demonstrates slower keratinocyte maturation and abnormal retention of cells in the epidermis.
This results in a rough and thickened stratum corneum with impaired barrier function, reduced hydration capabilities, and increased pigmentation from retention of melanin-laden keratinocytes in the epidermis.
The image in Fig? illustrates the Epidermis structure showing keratinocyte maturation process from basal keratinocyte to desquamating corneocyte.
The distribution of melanin within the epidermis determines skin colouration.
While the number of melanocytes is similar for both light and dark skin types, melanin is concentrated in small, aggregated melanosomes in light skin and is more disbursed in darker skin.
The dermis lies beneath the epidermis and is divided into the more superficial papillary dermis and deeper reticular dermis. The main cell type in the dermis is the fibroblast, which is more abundant in the papillary dermis and sparse in the reticular dermis.
Below the dermis and above the underlying muscle is the subcutaneous layer, also called the hypodermis, subcutis or superficial fascia. This layer has both fatty and fibrous components.