Ultrastructure of frozen, fractured and etched pycnidiospores of Coniothyrium minitans

Pycnidiospores of Coniothryium minitans Campb. were freeze-etched and examined in the electron microscope. The cell wall is composed of an outer granular layer, with large protuberances giving the outer surface a verrucose appearance, a fibrillar middle layer and possibly a thin amorphous inner layer. The plasmalemma is characterized by the presence of invaginations of various lengths and also by the presence of randomly distributed particles on its outer and inner surface. Within the cytoplasm are spherical vacuoles, whose inner and outer surfaces also bear particles, a large storage vesicle with lamellae, mitochondria with cristae, and a nucleus with nuclear pores. By comparison, examination of ultrathin sections of chemically fixed pycnidiospores embedded in resin yielded little information on the fine structure of the cells. The freeze-etching technique with its unique fracturing process gives much clearer views of the cell walls and cell organelles and furthermore reveals their surface features.