Photobiology of the Skin - Biophotonics - page 294

Stratum Corneum

Skin cell Types
- Keratinocytes
- Melanocytes

Stratum granulosum

- Langerhans cells

Stratum Spinosum

Stratum Basale

- Fibroblasts

- Vascular cells
- Mast cells

- Adnexal cells

Fig. 16.1. Skin is a multilayered organ. Epidermis comprises the most superficial
layer. The cells of the epidermis divide in the basal layer, then differentiate, moving
upward to the skin surface to slough. The dermis underlies the epidermis, providing
structural support for blood vessels and adnexae, as well as the epidermis

then differentiate, flattening and progressing toward the surface of the skin as
the process of differentiation proceeds, until they are no longer viable. Dur-
ing this differentiation process, keratinocytes synthesize and secrete an array
of lipids intercellularly in organelles termed lamellar bodies. As differentia-
tion is completed, these organelles are secreted into the extracellular space
where their lipid contents spread and disperse among the cells of the stratum
corneum, creating a “brick and mortar” motif that is extraordinarily imper-
meable [2,11]. The barrier is resistant to moisture, invasion of microorganisms,
and even oxygen. The constant sloughing of fully differentiated epidermal cells
also is an adaptive strategy that prevents microorganism invasion and pro-
vides a constant new supply of skin as we wash and scrub the skin surface in
the course of work and play. Keratinocytes are also capable of innate immune
responses, secreting inflammatory cytokines and chemotactic mediators that
help to repel invading organisms [7], and are responsible for the synthesis of
the vitamin D precursor, Vitamin D 3 [9, 12].
A second important cell type located in the epidermis is the melanocyte,
which represents 4% of the cells in the epidermis [13]. It is responsible for

our skin color because of its capacity to synthesize melanin granules and se-
crete them to be taken up by keratinocytes [14,15]. The melanin produced by
melanocytes is the chief photoprotective substance within the skin, as it is ca-
pable of absorbing light throughout the visible and ultraviolet light spectrum.
One melanocyte provides melanin for about 36 keratinocytes (Fig. 16.2).
In addition to these two key cell types, the epidermis also contains ∼ 4%
Langerhans cells, which are the resident immune cell of the epidermis. When
microorganisms do successfully penetrate the epidermal stratum corneum

∼