Subunit structures in hydroxyapatite crystal development in enamel: implications for amelogenesis imperfecta

Previous freeze-etching studies of developing enamel revealed collinear arrays of spherical structures (approximately 50 nM dia) of similar width to the crystals of mature tissue. Concomitant with matrix degradation/processing, spherical structures became less distinct until, coincident with massive matrix loss, only crystal outlines were seen. More recently, using Atomic force microscopy technology, early crystals exhibited topology reminiscent of these collinear spherical structures. After matrix loss these were replaced by similarly sized bands of positive charge density on the crystal surfaces. The data suggest enamel crystals may form from mineral-matrix spherical subunits. Matrix processing may generate mineral nuclei and lead to their fusion and transformation into long apatite crystals. Support for this view derives from the appearance of short crystal segments in amelogenesis imperfecta (hypoplastic AI) or abnormally large crystals alongside 50 nM diameter spherical mineral subunits (hypomaturation AI). Mutation of matrix or processing enzymes leading to defective processing may have impaired mineral initiation, fusion, and subsequent growth.