Cornification

Definition of Cornification: A unique, terminal differentiation and programmed cell death of epidermal keratinocytes leading to the formation of the cornified layer which acts as the outer skin barrier, as well as contributing to hair and nail formation.
Process Summary:
- Gene Expression: Specific genes are expressed in waves that contribute to structural and regulatory aspects of cornification.
- Cytoskeletal Changes: Keratin intermediate filaments form a scaffold that fills the cell and provides mechanical strength after cell organelles are removed.
- Cornified Envelope Formation: Proteins in the cell periphery are cross-linked by transglutaminases, forming the cornified envelope.
- Extracellular Lipid Modification: Lipids fill the intercellular spaces, creating a water-repellent barrier; allows for corneocyte shedding through desquamation.
- Variations in Cornification: Different cornified structures (e.g., stratum corneum, nails, hair) exhibit unique cornification processes.

Cornification vs. Classical Cell Death: Unlike apoptosis (programmed cell death aimed at removing damaged cells), cornification leads to mechanically stable structures from keratinocytes that become biologically inactive cell corpses.
Role of Proteases and Other Enzymes: Evidence suggests that proteases, nucleases, and transglutaminases are involved in executing the event of keratinocyte death, yet the full mechanistic understanding is still developing.
Anti-Apoptotic Mechanisms: Keratinocytes activate pathways that prevent premature cell death during the differentiation process which is vital for epidermal homeostasis.

The skin protects against physical, chemical, and microbial threats, acting as a barrier against infections. Antimicrobial proteins are produced by keratinocytes under stress conditions.
Continuous renewal: The outer skin layers are shed and replaced by keratinocytes derived from stem cells in the basal layer, maintaining a functional barrier.

Keratinocyte Differentiation Process:
- Origin: Epidermal stem cells located in the basal layer and hair follicles.
- Asymmetric Division: Proliferating keratinocytes generate daughter cells that stop dividing and initiate differentiation into suprabasal layers.
- Transcription Factors: Controlled by transcription factors like p63, leading to changes in keratin expression (from K5/K14 to K1/K10).

1. Suprabasal Layer Changes:
- Cells stop dividing, express keratins associated with differentiation.
- Caspase-14 plays a role in the transition to the granular layer, aiding keratin aggregation.
2. Formation of Cornified Envelope:
- Involucrin and loricrin expressed; cross-linked by transglutaminases, reinforcing the outer structure.
3. Extrusion of Lipids:
- Lamellar bodies release lipids to fill intercellular spaces, crucial for forming the skin barrier.

Characterized by degradation of organelles (e.g., nucleus, mitochondria). The exact mechanisms and enzymes are still not fully understood, although certain proteases and lysosomal activities are implicated in this process.

Breakdown of cellular structures indicates a process of cornification that leads to cell death, driven by:
- Energy Supply Shut Down: Through removal of mitochondria.
- Nuclear Disassembly: DNA is degraded by specific enzymes during keratinocyte differentiation.
- Proteolytic Activation: Involvement of proteases in finalizing the cornification process and potentially contributing to cell death.

Understanding the mechanisms of cornification can lead to potential therapeutic strategies for skin conditions like atopic dermatitis and psoriasis.
Identifying specific gene expressions and enzyme functions involved during differentiation and cornification will help refine treatment approaches.