Fine structure in ascosporogenesis of freeze-substituted Arthroderma simii

We applied the freeze-substituion method to the detailed observation of the ascosporogenesis in Arthroderma simii. The ascospore formation of A. simii underwent the same processes as did that of other dermatophytes, namely an enveloping membrane system (EMS) composed of two invaginated unit membranes that delimit the daughter nuclei in the cytoplasm of the ascus. The inner leaflet of the EMS was found to have changed into an ascospore plasma membrane, whereas the outer leaflet and intercisternal space became the ascospore cell wall. In the freeze-substituted asci, we could not observe the lomasomal structure thought to be the origin of the EMS in dermatophytes.