Pulpal Repair and Secondary Dentin Formation

The dentine-pulp complex is not a passive target of caries: it defends itself by laying down new dentine. Beyond the secondary dentine that forms physiologically throughout life, the pulp responds to the advancing carious lesion with tertiary dentine—reactionary dentine secreted by surviving odontoblasts and reparative dentine formed by newly differentiated cells—which can wall off and slow the disease.

Definition

Pulpal repair through dentinogenesis is the deposition of new dentine by the dentine-pulp complex in response to injury: reactionary dentine secreted by surviving primary odontoblasts under a mild stimulus, and reparative dentine formed by newly differentiated odontoblast-like cells where the original odontoblasts have died, both forms collectively termed tertiary dentine.

Scope

This topic covers the defensive dentinogenic responses of the dentine-pulp complex: physiological secondary dentine, and the two forms of tertiary dentine (reactionary and reparative) produced in response to caries and other irritants. It outlines the cellular and signalling basis of these responses and their role in repair. It is a reference account of biology, not a guide to vital pulp therapy or treatment.

Core questions

• How does the dentine-pulp complex respond defensively to an advancing carious lesion?
• What distinguishes physiological secondary dentine from tertiary dentine?
• What is the difference between reactionary and reparative dentinogenesis?
• What cellular and molecular signals trigger tertiary dentine formation?
• How does dentine deposition help wall off or slow the carious process?

Key concepts

• Dentine-pulp complex as a vital, responsive unit
• Secondary (physiological) dentine
• Tertiary dentine
• Reactionary dentinogenesis (surviving odontoblasts)
• Reparative dentinogenesis (newly differentiated cells)
• Growth factors sequestered in dentine matrix (e.g. TGF-β)
• Dentine deposition as a defensive barrier

Mechanisms

Throughout life the odontoblasts that line the pulp slowly add secondary dentine, gradually reducing the size of the pulp chamber. When a carious lesion or other irritant challenges the tooth, the complex mounts a defensive response by forming tertiary dentine. Under a relatively mild stimulus the original odontoblasts survive and up-regulate their secretory activity, producing reactionary dentine in continuity with the existing tubular dentine (Smith 1995). Under a stronger insult that kills the primary odontoblasts, progenitor cells in the pulp differentiate into new odontoblast-like cells that secrete a more irregular, atubular reparative dentine to seal the exposed surface (Smith 2002). These responses are driven in part by bioactive molecules, including growth factors such as TGF-β that are sequestered within the dentine matrix and released as the matrix is demineralized by the carious process, signalling the underlying cells to repair. By depositing new dentine at the pulpal aspect of the lesion, the complex increases the distance and barrier between the bacteria and the pulp, helping to wall off and slow disease progression (Mjör 1995; Pitts 2017).

Clinical relevance

The dentine-pulp complex's capacity to form tertiary dentine is the biological basis for regarding the pulp as a tissue that can defend and repair itself rather than merely succumb to caries, and it underlies the rationale for preserving pulp vitality. This entry describes the repair biology as reference material; it does not provide indications, materials, or protocols for vital pulp therapy or other treatment.

History

Recognition that the pulp responds to injury by forming new dentine led to a distinction between physiological secondary dentine and stimulus-induced tertiary dentine, and then to separating reactionary dentinogenesis by surviving odontoblasts from reparative dentinogenesis by newly differentiated cells (Smith 1995). Subsequent work identified the role of growth factors held within the dentine matrix in signalling these repair responses (Smith 2002).

Debates

The distinction between reactionary and reparative dentinogenesis

Distinguishing reactionary dentine, made by surviving primary odontoblasts, from reparative dentine, made by newly differentiated cells, is conceptually clear, but in a real lesion the responses can overlap and the stimulus intensity that separates them is graded rather than sharply defined.

Key figures

• Anthony J. Smith
• Hervé Lesot
• Jean-Victor Ruch
• Ivar A. Mjör

Seminal works

• smith-1995
• smith-2002

Frequently asked questions

Can a tooth repair itself against decay?

The dentine-pulp complex can mount a defensive repair by laying down new tertiary dentine beneath the lesion, which thickens the barrier between bacteria and the pulp and can slow the disease, though it cannot remove an established lesion.

What is the difference between reactionary and reparative dentine?

Reactionary dentine is made by the original odontoblasts that survive a mild stimulus, whereas reparative dentine is made by newly differentiated odontoblast-like cells when the original odontoblasts have died under a stronger insult.