Keratin is formed in the skin through a process called keratinization, primarily occurring within specialized skin cells called keratinocytes.
Keratinocytes, which make up over 90% of the cells in the epidermis (the outer layer of the skin), are responsible for the production of keratin. These cells go through a differentiation process as they move from the basal layer (the deepest layer) of the epidermis towards the surface.
Here's a breakdown of the keratin formation process:
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Keratinocyte Production and Migration: New keratinocytes are constantly produced in the basal layer of the epidermis. As new cells are formed, older keratinocytes are pushed towards the skin's surface.
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Keratin Filament Synthesis: While migrating upwards, keratinocytes begin to synthesize keratin proteins. These proteins are building blocks called intermediate filaments. Specific types of keratin are produced depending on the location within the skin (e.g., hard keratins in nails and hair, soft keratins in skin).
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Filament Assembly: Keratin filaments assemble into thicker, more organized structures within the keratinocyte cytoplasm.
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Keratohyalin Granule Formation: In the granular layer of the epidermis, keratinocytes produce keratohyalin granules. These granules contain proteins like profilaggrin, which is later processed into filaggrin.
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Filaggrin Release and Aggregation: Filaggrin, derived from profilaggrin, binds to the keratin filaments, causing them to aggregate and become more tightly packed. This aggregation contributes to the flattening of the keratinocytes.
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Cell Death and Cornification: As the keratinocytes reach the stratum corneum (the outermost layer of the skin), they undergo programmed cell death (apoptosis). The nucleus and other organelles disintegrate, leaving behind a flattened, dead cell filled with keratin. This process is called cornification. These dead, keratin-filled cells form a protective barrier on the skin's surface.
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Desquamation: The outermost layer of cornified cells is constantly shed (desquamation) and replaced by new cells from below, completing the cycle of keratin formation and skin renewal.
In summary, keratin formation in the skin is a dynamic process involving the production, assembly, and aggregation of keratin filaments within keratinocytes, culminating in the formation of a protective barrier of dead, keratin-filled cells on the skin's surface.
