No abstract

Freeze-etched preparations of sporangiospores of Rhizopus were examined for ultrastructural alterations during spore maturation and to determine when invaginations of the cell membrane appear in relation to these changes. First, membrane irregularly delimitated the sporangium. The membrane appeared thick. Then, the walls of the sporangiospore became apparent. Simple ridges appeared on the thin walls. Near wall surfaces, particle-like structures were characteristically observable only in this stage of maturation. The cell membrane was devoid of invaginations, particulated less densely than the cell membrane of mature spores and concaved along the ridges. In crossfractures, immature spores revealed a nucleus, lipid droplets and mitochondria, indistinct both in cristae and furrows of the membrane. Spores then resembled mature spores in shape but differed in the following points : (i) freeze-fracture was not observed along the spore surface; (ii) the ridges of the spore wall were etched deep, indicating a low content of structural material; and (iii) the cell membrane was devoid yet of invaginations. Other new findings on dormant spores were described, and the function of the cell membrane invaginations was discussed. 259 260 K. Takeo, T. Nonaka and T. Ikeda

Key Words: Internal structures, Rat incisor odontoblasts, Scanning electron microscopy (SEM) using the aldehyde prefixation-osmium tetroxide postfixation -DMSO freeze fracture -Osmium teroxide maceration (AODO) method Summary: The internal configuration of rat incisor odontoblasts was studied mainly by scanning electron microscopy (SEM) using the AODO method (low concentration aldehyde prefixation, osmium tetroxide postfixation, dimethyl sulfoxide (DMSO) freeze-fracture, osmium tetroxide maceration). The present SEM findings were compared with the results obtained by conventional transmission electron microscopy (TEM) of epon-embedded specimens. The following results were obtained: 1) Functioning odontoblasts were characterized by a concentric, laminar rough endoplasmic reticulum (rER) with many long mitochondria interposed. 2) A network of tubular smooth endoplasmic reticulum (sER) was observed in the odontoblast process and distal portion of both functioning and resting odontoblasts.3) The tubulo-vesicular elements which have been found to present a modified Golgi-GERL organelle with secretory and absorptive functions were demonstrated in both the functioning and resting odontoblasts. Structurally they consist of the sER network and strings of granules and vesicles. 4) Various types of cytoplasmic bodies, e.g., lysosomes, cytosomes and multivesicular bodies, related to the sER were also noted in both functioning and resting odontoblasts. 5) Microapocrine secretion of membranous vesicles of varous sizes into the predentin and along the lateral branchings of odontobalst processes in the circumpulpal dentin was observed during the matrix apposition stage of the odontoblasts. The present morphological study revealed the threedimensional configuration of the intra-and extra-cellular structures related to dentinogenesis by odontoblasts.There have been many transmission electron microscopy (TEM) studies on the morphological changes and the fine structure related to secretion and absorption by odontoblasts (OBs) at different developmental stages