The outer layer closest to enamel is known as mantle dentin.

This area of mineralization is known as mantle dentin and is a layer usually about 150 μm thick.

Mantle dentin is formed by newly differentiated odontoblasts and forms a layer approximately 150 micrometers wide.

Whereas mantle dentin forms from the preexisting ground substance of the dental papilla, primary dentin forms through a different process.

Unlike primary dentin, mantle dentin lacks phosphoryn, has loosely packed collagen fibrils and is less mineralized.

The different stages of dentin formation result in different types of dentin: mantle dentin, primary dentin, secondary dentin, and tertiary dentin.

Below it lies the circumpulpal dentin, a more mineralized dentin which makes up most of the dentin layer and is secreted after the mantle dentin by the odontoblasts.