Morphometric analysis of the nucleolus during the life cycle of human odontoblasts.

Developing first premolars were used as a model system to obtain information on the nucleolar structure of human odontoblasts at several stages of their life cycle. Four stages were defined by their location within the tooth: a) preodontoblasts were located at the growing tip of the root; b) secretory odontoblasts in the apical region; c) transitional odontoblasts in the middle region; and d) aged odontoblasts in the coronal region. Preodontoblasts have a small nucleolus (0.55 micron 2) with few strands of dense fibrillar material radiating from the fibrillar center. Secretory odontoblasts are characterized by a large (1.24 micron 2), irregular, and reticulated nucleolus. The fibrillogranular material, the largest component in all nucleoli, reaches maximal size at this stage (0.88 micron 2). Fibrillar centers occupy about the same area (0.1 micron 2) throughout the odontoblast's life cycle. As the formation of primary dentin is completed, the nucleolus of transitional odontoblasts is reduced in size (0.54 micron 2). Finally, the aged odontoblasts have a small, compact nucleolus (0.39 micron 2), with segregated components. Morphologic analysis and quantification of size and component areas of nucleoli obtained with an image analyzer indicated that secretory odontoblasts had the most active, and aged odontoblasts the least active, nucleolus.