Page 472 - Atlas of Histology with Functional Correlations

LAYER

In thick skin only, the stratum lucidum is translucent and barely visible; it
lies just superior to the stratum granulosum and inferior to the stratum
corneum. The tightly packed cells lack nuclei or organelles and are dead. The

flattened cells contain densely packed keratin filaments cross-linked with
filaggrin. This layer is not visible in thin skin, although individual cells may
be present.

STRATUM CORNEUM—THE FIFTH LAYER

The stratum corneum is the fifth and most superficial layer of the skin. All

nuclei and organelles have disappeared from the cells of this layer that now
consists of flattened, dead cells filled with soft keratin filaments. The
keratinized, superficial cells from this layer are continually shed, or
desquamated, and are replaced by new cells arising from the deep stratum

basale. During the keratinization process, the hydrolytic enzymes disrupt the
nucleus and all cytoplasmic organelles, which disappear as the cells fill with
keratin.

OTHER SKIN CELLS

In addition to the keratinocytes that form and become the superficial layer of

keratinized epithelium, the epidermis also contains three less abundant cell types.
These are melanocytes, Langerhans cells, and Merkel cells. Unless the skin is
prepared with special stains, these cells are not distinguishable in histologic
slides prepared with only hematoxylin and eosin.

Melanocytes are derived from the neural crest cells. They have long,

irregular cytoplasmic or dendritic extensions that branch into the epidermis and
establish contact with nearby cells. Melanocytes are located between the stratum
basale and the stratum spinosum of the epidermis and in hair matrices.

Melanocytes synthesize the dark brown pigment melanin from the amino acid
tyrosine. The formed melanin granules in the melanocytes then migrate to the
tips of the cytoplasmic or dendritic extensions, from which they are
phagocytized by keratinocytes in the basal cell layers of the epidermis. Melanin
imparts a dark color to the skin, and exposure of the skin to sunlight promotes

synthesis of melanin. The main function of melanin is to protect the skin from

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