Alterations of Rhizopus Sporangiospores, Especially the Cellular Envelope during Maturation, as Observed by Freeze-Etching

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