Quick Facts
The enamel striae are lines showing the successive layers deposited in a tissue. In the enamel, they are brown striations visible under transmitted light and colorless in reflected light. They may be observed under the microscope in longitudinal sections as oblique lines running inward from the surface and toward the root and in cross-sections as rings similar to those in a tree trunk. Dry dentin often shows a series of somewhat parallel lines caused by imperfectly calcified dentin arranged in layers (Dorland, 2011).
Related parts of the anatomy
Structure and/or Key Feature(s)
Incremental growth lines known as enamel striae of Retzius are evident in tooth enamel. Histologically, they appear as a series of dark bands, with each line or band representing a period of between 6 and 12 days of enamel growth. This value is referred to as an individual’s periodicity, which remains constant in all teeth of one individual but varies considerably between individuals (Standring, 2016).
These bands travel in an oblique direction from the dentinoenamel junction near the surface of the crown. The striae subsequently end as a series of shallow furrows or curved grooves called perikymata. The proximate cause of the enamel striae of Retzius is changes in ameloblast activity; however, their ultimate cause is unknown (Reid and Ferrell, 2006).
While long-term striae of Retzius are more commonly reported in the literature, short-term daily growth lines are also evident in tooth enamel. These lines appear in intervals of between 2–3 µm at the dentinoenamel junction and 4–6 µm in outer enamel. When viewed microscopically, a small stria is evident across each prism, which likely reflects the diurnal swelling and shrinking of the prism diameter during growth. These short-term striae are called cross-striations (Birch and Dean, 2014).
Function
While the etiology of the enamel striae of Retzius remains unclear, the striae occur with a constant periodicity in the teeth of an individual such that it can facilitate the determination of crown formation times. Knowledge of crown formation times is useful in forensic anthropology and for determining the age of juvenile remains. These incremental enamel lines are also useful for determining enamel growth trajectories and ages at death for humans (Birch and Dean, 2014; Reid and Ferrell, 2006).
References
Birch, W. and Dean, M. C. (2014) 'A method of calculating human deciduous crown formation times and of estimating the chronological ages of stressful events occurring during deciduous enamel formation', J Forensic Leg Med, 22, pp. 127-44.
Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.
Reid, D. J. and Ferrell, R. J. (2006) 'The relationship between number of striae of Retzius and their periodicity in imbricational enamel formation', J Hum Evol, 50(2), pp. 195-202.
Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41 edn.: Elsevier Limited.
