The first question outlined in Chapter 1 concerns the evolution of the slow tempo of growth that characterises living humans. Here in Chapter 5, the incremental structures of enamel and dentine which are described in Chapter 4 are used to address this by building sequences from which the rate of tissue formation and the timing of different developmental stages can be estimated.

Underlying principles

Five main conditions are required for building accurate dental development sequences:

1. Unambiguous counts of prism cross striations can be made;

2. prism cross striations and short-period dentine lines represent an approximately 24-hour development rhythm;

3. brown striae of Retzius represent a regular rhythm for which the periodicity can be established;

4. sequences of brown striae of Retzius and long-period dentine lines can be matched between teeth from one individual;

5. childbirth leaves a mark on the enamel and dentine sequences as a neonatal line.

Counting prism cross striations repeatably

First of all, it is necessary to be clear about what is being counted. Seen under the light microscope, cross striations represent the cumulative effect of reflections at all the prism boundaries in the enamel section through which the light passes. Within the depth of field, or thickness of tissue within which structures appear in focus, the striations seen are a combination of several prisms. As the microscope focus is adjusted up and down, the position and prominence of the striations changes because different prism boundaries are included. In a section cut along a buccal-lingual plane to pass through the cusp tip or central mamelon of the crown, prisms in general run parallel to the section surface.