Keratinocytes

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General histology of the skin.

The keratinocytes are the primary cell of the epidermis. They originate in the deepest layer of the epidermis- stratum basale and continue to the outer most layer - stratum corneum.

In the basal layer, the keratinocytes contain a number of cells including : free ribosomes, small golgi apparatus, mitochondria, intermediate filaments and the rough endoplasmic reticulum. 

In histologic sections, the cytoplasm of immature keratinocytes appears basophilic due to the abundance of free ribosomes, most of which are involved in synthesizing keratins that will later form keratin filaments. These keratin filaments are known as intermediate filaments but are commonly referred to as tonofilaments.

As keratinocytes progress through the stratum spinosum, the synthesis of keratin filaments continues, and they start grouping together into bundles called tonofibrils. These bundles are thick enough to be visible under a light microscope. As a result, the cytoplasm becomes eosinophilic due to the staining reaction of the tonofibrils that occupy an increasing portion of the cytoplasm.

As the keratinocytes start entering the stratum granulosum, they synthesise keratohyalin granules. These granules are a characteristic feature of cells in the stratum granulosum. They contain two essential intermediate filament-associated proteins: filaggrin, and trichohyalin. 

Keratinocytes are also known for an important process called keratinization. During keratinization, the number of keratohyalin granules increases in the keratinocytes and their contents are released into the cytoplasm. Filaggrin and trichohyalin, play a crucial role in this process. They act as promoters, facilitating the aggregation of keratin filaments into structures called tonofibrils. This aggregation initiates the transformation of granular cells into cornified cells. 

Keratinization is a time-consuming process, taking about 2 to 6 hours for cells to move from the stratum granulosum to the outermost layer of the skin, known as the stratum corneum. The keratin fibril formed during this process is called soft keratin, which is different from the hard keratin found in hair and nails. This transformation results in the formation of a protective and durable outer layer for the skin. 

The keratohyaline granules also produce lamellar bodies. These lamellar bodies contribute to the formation of the intercellular epidermal water barrier. This the e barrier is crucial for preserving body homeostasis and maintaining a vital role in mammalian "dry" epithelia,