What is Hertwig's epithelial root sheath (HERS) and its function?

• A. A neural tissue that signals tooth eruption.
• B. A layer of odontoblasts responsible for dentin formation.
• C. A bilayer of epithelial cells at the cemento-enamel junction that guides root dentin formation and shapes root formation; it later disintegrates to permit cementogenesis.
• D. A mineralized layer covering root surface.

Answer: (C)

Hertwig's epithelial root sheath is a bilayer of epithelial cells that forms at the cemento-enamel junction and guides how the root develops. It signals the underlying dental papilla to form odontoblasts that lay down root dentin and helps shape the root itself. Once the root dentin is formed, the sheath breaks down, allowing the dental follicle cells to contact the root surface and differentiate into cementoblasts to form cementum. Remnants of HERS can remain in the periodontal ligament as the rests of Malassez. This is why neural tissue would relate to eruption, odontoblasts are the dentin-forming cells themselves, and a mineralized layer covering the root is cementum—not the guiding sheath that shapes the root.