# Odontoblasts in Equine Hypsodont Teeth—How They Cope with Permanent Occlusal Wear

## TL;DR

This paper explores how equine teeth, which face constant wear, maintain dentin production through continuous odontoblast renewal.

## Contribution

The study reveals that equine odontoblasts are continuously replaced, challenging the belief that they are postmitotic and lifelong.

## Key findings

- Equine odontoblasts express CD90, indicating continuous renewal rather than being postmitotic.
- Hypsodont equine teeth have higher CD90-positive cells compared to brachydont and hypselodont teeth.
- Nestin, a marker of mature odontoblasts, is largely absent in equine hypsodont teeth.

## Abstract

In short-crowned teeth, such as human teeth, the tooth surface is sealed by enamel. The underlying dentin is produced by odontoblasts. Odontoblasts are known as postmitotic cells that produce relatively small amounts of dentin continuously throughout the tooth’s lifespan. However, equine high-crowned teeth are exposed to massive dental wear due to their abrasive diet. This is compensated for by the constant eruption of the teeth. In equines, the dentin is exposed to the tooth surface, so the pulp cavity underneath the dentin is permanently at risk of being opened. Thus, one might ask how odontoblasts compensate for this loss, given that they produce only small amounts of dentin in short-crowned teeth. We discovered that equine odontoblasts express CD90, a marker typically found in immature cells. Therefore, we assume that odontoblasts, which are only replaced after major damage in short-crowned teeth, must undergo continuous renewal in high-crowned equine teeth.

Horses’ hypsodont (high-crowned) teeth face permanent dental wear. This is compensated for by a continuous eruption, which requires a high adaptability of odontoblasts; otherwise, the dental pulp would be exposed. Here, we report on how equine odontoblasts respond to the challenge of maintaining a high production rate of dentin. We analyzed CD90, a marker of odontoblastic differentiation, and nestin, a marker of mature odontoblasts, in equine pulpal tissue via immunofluorescence. For comparison, we examined the hypselodont (ever-growing) incisors and brachydont (short-crowned) molars of rats. Immunofluorescence and Western blot analysis of pulpal tissue revealed a higher content of CD90-positive cells in hypsodont equine teeth than in brachydont and hypselodont rat teeth. The odontoblastic layer of hypsodont teeth was positive for CD90 (marker for differentiating odontoblasts), which was not the case for brachydont and hypselodont rat teeth. Most samples of hypsodont teeth were negative for nestin, whereas in hypselodont and brachydont teeth, odontoblasts were positive for nestin (marker for mature odontoblasts). Our findings suggest that there is a constant replacement of odontoblasts in the equine dentition, enabling a continuous high production rate of dentin. These results contradict the idea of lifelong vital, postmitotic and productive odontoblasts.