Morphogenesis and ultrastructure of Geotrichum candidum septa.

The ultrastructure and mode of formation of septa of Geotrichum candidum were investigated by light and electron microscopy. The invaginations of the lateral membrane and wall appear to initiate at multiple points around the circumference of the cell; the immature septum subsequently assumes a cart-wheel shape, with branched spokes radiating from the center of the septum. Each face of the septum is covered with a membrane possessing hitherto undescribed structural differentiation; the membrane substructures are comprised of two central subunits encircled by 12 identical subunits. The diameter of the entire 12 plus 2 structure is 24 to 25 nm, and the diameter of each individual subunit is approximately 4 nm. The maturation of the septum appears to occur by further deposition of material along the branched skeletal regions. Numerous small openings (micropores), formed as a result of incomplete deposition, ultimately give rise to plasmodesmata. During arthrospore formation, the plasmodesmal canals and associated micropores are occluded by electron-dense materials, rendering each segment of the hyphae completely independent of the rest of the hyphae.