The keratinocyte is the predominate cell type of the epidermis. These cells originate in the basal epidermal layer. On leaving this layer, keratinocytes assume two essential activities:

• They produce keratin, the major structural protein of the epidermis. Keratin constitutes almost 85% of fully differentiated keratinocytes.

• They participate in the formation of the epidermal water barrier.

The keratinocytes in the basal layer contain numerous free ribosomes, scattered 7- to 9-nm intermediate (keratin) filaments, a small Golgi apparatus, mitochondria, and rough endoplasmic reticulum (rER). The cytoplasm of immature keratinocytes appears basophilic in histologic sections because of the large number of free ribosomes, most of which are engaged in the synthesis of keratin, which will later be assembled into keratin filaments. These filaments are classified as intermediate filaments although they are more commonly called tonofilaments.

As the cells enter and are moved through the stratum spinosum, the synthesis of keratin filaments continues, and the filaments become grouped into bundles sufficiently thick to be visualized in the light microscope. These bundles are called tonofibrils. The cytoplasm becomes eosinophilic because of the staining reaction of the tonofibrils that fill more and more of the cytoplasm.

Keratohyalin granules contain intermediate filament-associated proteins that aid in aggregation of keratin filaments

In the upper part of the stratum spinosum (Fig. 14.4), the free ribosomes within the keratinocytes begin to synthesize keratohyalin granules that become the distinctive feature of the cells in the stratum granulosum. Keratohyalin granules contain the two major intermediate filament-associated proteins, filaggrin and trichohyalin. The appearance of the granules and expression of filaggrin in the keratinocytes is often used as a clinical marker for the initiation of the final stage of apoptosis. As the number of granules increases, the contents of the granules are released into the keratinocyte cytoplasm. Filaggrin and trichohyalin function as promoters in the aggregation of keratin filaments into tonofibrils, thus initiating the conversion of granular cells into cornified cells. This process is called keratinization and occurs in 2 to 6

CHAPTER 14 I Integumentary System 405 keratinized cell envelope

lamellar bodies

_discharge of lamellar body lamellar bodies keratohyalin granules tonofilaments Golgi apparatus lamellar bodies mitochondrion tonofilaments ribosomes basal lamina

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