Observations on the ultrastructure of the enamel organ: III. Internal and external enamel epithelia

Abstract

The internal and external enamel epithelia of the cat were studied by means of electron microscopy before the formation of dentine and enamel. In stage I, at the boundary between internal and external enamel epithelia and the surrounding mesenchyme, there is a continuous electron-dense lamella of about 100–250 A in thickness (basement membrane according to electron microscopists). This structure between the external enamel epithelium and the dental sac persists into the next stage, but it cannot be seen at the boundary of the dental papilla after the odontoblastic differentiation. At this stage the enamel organ and the dental papilla are separated by an interval of 150–4000 A, bounded by the opposed surfaces of the internal enamel epithelial cells and the odontoblasts. The internal enamel epithelial cells undergo a series of gradual changes as a result of which they assume a highly organized morphological polarity, and become secretory cells (ameloblasts). The process is a true cytological differentiation. The metabolic materials reach the enamel organ mainly from the capillaries of the dental sac. They penetrate mainly through the external epithelium, at which level there is evidence of a submicroscopic pinocytosis, and reach the internal enamel epithelium after passing through the stellate reticulum and stratum intermedium. This flow of metabolic materials is clearly polarized from the external toward the internal enamel epithelium. Before enamel formation the internal enamel epithelium undergoes structural changes which facilitate the exchange of materials between the cells and the intercellular spaces.