Scanning electron microscopy of the organelles of rat incisor odontoblasts--in particular the tubulo-vesicular elements.

The fine structure of rat odontoblasts (OBs) showing the developmental sequence of cellular maturation along the long axis of the incisor was studied. The cytomorphological changes and relationships of the intracellular components, particularly the membranous component, were investigated mainly by scanning electron microscopy (SEM) using the AODO (A: aldehyde prefixation; O: osmium tetroxide postfixation; D: dimethyl sulfoxide (DMSO) freeze-fracture; O: osmium tetroxide maceration) method and transmission electron microscopy (TEM) using the conventional methods. The following results were obtained: 1) The elongated young OBs were highly polarized and characterized by well-developed concentric cisternal granular (rough) endoplasmic reticulum (rER) layers and Golgi complex. SEM showed the rER was continuous to the agranular (smooth) ER (sER). 2) Transformation of the concentric rER to a tubular network was closely related to the maturation and secretory activity of the OBs. 3) Many branched and elongated tubular mitochondria were observed in the supranuclear region of young OBs. On the other hand, many round-to-ovoid mitochondria were observed particularly in the distal clear area of old OBs. 4) SEM also revealed tubulo-vesicular elements, which are believed to be structures relating to secretory and absorptive functions, composed of secretory granules and small tubules, having connections with the cell membrane in both young and old OBs. Furthermore, the small tubules were continuous with the larger sER tubules in the distal clear area and cell process of the OBs. 5) Using TEM, long lysosomes, various cytosomes and multivesicular bodies (MVBs) were usually observed in the peripheral region and in the distal cytoplasm and major processes of the OBs. The present SEM and TEM studies indicated the MVBs were closely associated with sER tubules of the tubulo-vesicular elements. 6) Both TEM and SEM showed cytosegresomes containing laminated ER and other organelles particularly in the distal clear area of old OBs. 7) SEM also showed some thin lateral branchings of the OBs running along the fine matricial fibrils, as well as matrix-vesicle-like structures deeply invaginated at the basal portion of the cell processes and distributed in the predentine. The present morphological study demonstrated a three-dimensional configuration of the structures relating to secretory and absorptive functions of young and old OBs.