Staphylococci: Colonizers and Pathogens of Human Skin

Rosanna Coates; Josephine Moran; Malcolm J Horsburgh


Future Microbiol. 2014;9(1):75-91. 

In This Article

Structure of the Human Epidermis

To understand the challenges faced by bacteria on human skin, it is important to first consider the features of this niche and its barrier function.

The human epidermis comprises many pilo-sebaceous units, containing a hair follicle, sebaceous gland and interfollicular epidermis. These units enable the skin to perform its roles in thermoregulation and protection. Each unit contains a pool of stem cells that maintain cell number throughout skin homeostasis and in the event of injury. The interfollicular epidermis has a multilayered structure consisting of keratinocytes, which move closer to terminal differentiation as they migrate through the layers (Figure 1).[16] Suprabasal cell division occurs during formation of the epidermis; these progeny cells differentiate into suprabasal spinous keratinocytes, then granular cells and finally corneocytes to complete development of the stratified structure.[17] Corneocytes are attached to one another through corneodesmosomes that, when proteolyzed, are shed from the upper layer of the epidermis, a process known as desquamation.[18] This outer skin layer is replaced by differentiation of the underlying granular cells into corneocytes. Migration of keratinocytes is accompanied by a series of morphological changes. Their round appearance changes to flattened and polygonal, and the plasma membrane becomes permeable. Following enucleation, development ends with the maturation of a cornified envelope.[19] Lipids such as ceramide are synthesized and secreted from lamellar bodies[20] and form covalent attachments to the proteins of the cornified envelope.[21]

Figure 1.

Human epidermis.
The structural organization of the epidermis is derived from basal keratinocytes that migrate through the layers of the epidermis and undergo differentiation. This migration is accompanied by morphological changes resulting in the flattened, enucleated corneocytes of the stratum corneum. Corneocytes produce lamellar bodies, which are storage vesicles that secrete antimicrobial peptides, organic acids, antimicrobial fatty acids, cholesterol, sphingosine and ceramides, which in turn comprise the lipid matrix. Proteases are also secreted from the lamellar bodies resulting in desquamation of the upper layer of the epidermis.

The skin extracellular matrix (ECM) has an important role in tissue organization since different profiles of ECM molecules and adhesins create different environments that may influence the recruitment of different cell types. The ECM also plays an important role in both wound healing and aging.[22] The stratum corneum ECM is comprised mostly of skin lipids, with ceramides, free fatty acids and cholesterol, which in a ratio of 1:1:1, are organized in a stacked bilayer.[23]