Changes in the fine structure of the conidia-bearing apparatus of Aspergillus niger during the course of its development and degeneration was studied by the technique of ultrathin sectioning followed by electron microscopy.
The conidiophore growing from a foot cell is characterized by its fine structure which resembles that of growing hyphae, having an infolded cell membrane connected to other intracytoplasmic membranes and tubules as well as to mitochondria and nuclei. At this stage mitochondria are rather irregular in shape, and nuclei are delimited by double membranes with pores. With the progress of ripening of conidiophore its fine structure becomes more and more granulous; mitochondria appears to be globular, nucleus becomes difficult to discern, and the vesicles are then formed at the tip.
At the initial stage of vesicle formation the granulous structure is gradually reorganized into a structure characteristic of growing hyphae, and sterigmata begin to appear from the buds formed on the cell wall of the vesicle. Mitochondria, nuclei and other reticular structures then extrude into the developing sterigmata, which are further septated by the median cell plates developing centripetally from the cell walls of sterigmata. Thus, the formation of primary and secondary sterigmata with organized cytoplasmic structure similar to that of growing hyphae, are completed.
The formation of conidia then follows at the apical portion of each secondary sterigma initiated by the inward thickening of the wall. During this process, nucleus, mitochondria and reticular structures in the sterigma appear to move into the apical cell. When the neck stem (thick-wall portion) has been closed completely, a round conidium with a thick exosporium is produced.
Vacuolization takes place first in the conidiophore with the formation of a vesicle, and the latter is then vacuolated with the formation of sterigmata, which in turn undergo a similar vacuolization with the formation of conidia.