The fine structure of secretory ameloblasts in rat incisors.

Abstract

AbstractThe fine structure of the ameloblasts which secrete the inner enamel matrix in rat incisors was described using light and electron microscopy. The tissue was fixed by perfusion with 2.5% glutaraldehyde and gently decalcified in isotonic EDTA.The ameloblasts are tall cells forming a simple columnar epithelium. The base is adjacent to the stratum intermedium and the apex (Tomes' process) extends into newly‐formed enamel. The infranuclear zone is divided by the basal cell web into a small basal bulge adjacent to the stratum intermedium, and a larger compartment containing most of the cell's mitochondria. The supranuclear zone contains the rough endoplasmic reticulum and the Golgi apparatus. The rough endoplasmic reticulum predominates in the proximal and distal regions of this zone where it occupies most of the cell width. In the intermediate region, the rough endoplasmic reticulum surrounds a central tubule‐shaped Golgi apparatus, the tubule wall being made up of flattened saccules. The Golgi region of ameloblasts is associated with coated vesicles, two types of granules (light and dark) which may be lysosomes, and a characteristic dense content granule shown to be the enamel precursor (0.16 μ diameter).The supranuclear zone is separated from Tomes' process by the apical cell web. Tomes' process is devoid of endoplasmic reticulum and Golgi material, but contains numerous dense content granules as well as microtubules and coated vesicles. The amorphous dense content granules are the precursors of the highly orientated fibrous enamel matrix. The proximity of the process to the fibrous enamel suggests that it is involved in orientation of these fibrils. Since bundles of fibrils constitute rods, the process would seem also to be involved in enamel rod orientation.