Page 471 - Atlas of Histology with Functional Correlations

intermediate keratin filaments that increase in number as the cells move

superficially. These filaments eventually form the components of keratin in
the superficial cell layer.

STRATUM SPINOSUM—THE SECOND

LAYER

As the keratinocytes divide by mitosis, they move upward in the epidermis

and form the second cell layer of keratinocytes, or stratum spinosum. This
layer consists of four to six rows of cells. Routine histologic preparations
with different chemicals cause these cells to shrink. As a result, the
developed intercellular spaces between cells appear to form numerous

cytoplasmic extensions, or spines, that project from their surfaces. The spines
represent the sites where desmosomes are anchored to bundles of
intermediate keratin filaments, or tonofilaments, and to neighboring cells.
The synthesis of keratin filaments continues in this layer, which are

assembled into bundles of tonofilaments to provide cohesion among cells and
resistance to the abrasion of the epidermis as they terminate at desmosomes.

STRATUM GRANULOSUM—THE THIRD

LAYER

Maturing cells that move above the stratum spinosum accumulate dense

basophilic keratohyalin granules and form the third layer, the stratum
granulosum that consists of three to five layers of flattened cells. These
secretory granules are not surrounded by a membrane and consist of the
protein filaggrin, which induces the aggregation of keratin tonofilaments into

tight bundles. This combination of keratin tonofilaments with protein
filaggrin produces keratin through the process of keratinization. The keratin
formed by this process is the soft keratin of the skin. In addition, the cells of
stratum granulosum contain membrane-bound lamellar granules (bodies)

containing lipid bilayers. These lamellar granules are discharged into the
intercellular spaces between the stratum granulosum and the next layer, the
stratum corneum (or stratum lucidum if present), as a lipid layer to form an
impermeable water barrier that seals and waterproofs the epidermis of the

skin.

STRATUM LUCIDUM—THE FOURTH

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