Structure, differentiation, and multiplication of Golgi apparatus in fungal hyphae

Golgi apparatus in subapical regions of hyphae consist of paranuclear dictyosomes with 4-5 cisternae each. Transverse and tangential sections provide ultrastructural evidence for a three-dimensional architectural model of the Golgi apparatus and a stepwise mechanism for dictyosome multiplication. The dictyosomes are polarized, with progressive morphological and developmental differentiation of cisternae from the cis to the trans pole. Small membrane blebs and transition vesicles provide developmental continuity between the nuclear envelope and the adjacent dictyosome cisterna at the cis face. Cisternae are formed as fenestrated plates with extended tubular peripheries. The morphology of each cisterna depends on its position in the stack, consistent with a developmental gradient of progressive maturation and turnover of cisternae. Mature cisternae at the trans face are dissociated to produce spheroid and tubular vesicles. Evidence in support of a schematic sequence for increasing the numbers of dictyosomes comes from images of distinctive and unusual forms of Golgi apparatus in hyphal regions where nuclei and dictyosomes multiply, as follows: (a) The area of the nuclear envelope exhibiting ''forming-face'' activity next to a dictyosome expands, which in turn increases the size of cisternae subsequently assembled at the cis face of the dictyosome. (b) As subsequent large cisternae are formed and mature as they pass through the dictyosome, an entire dictyosome about twice normal size is built up. The number of cisternae per stack remains the same because of continuing turnover and loss of cisternae at the trans face. (c) This enlarged dictyosome becomes separated into two by a small region of the nuclear envelope next to the cis face that acquires polyribosomes and no longer generates transition vesicles. (d) As a consequence, assembly of new dictyosomes is physically separated into two adjacent regions. (e) As the enlarged cisternae are lost to vesiculation at the trans pole, they are replaced by two separate stacks of cisternae with typical ''normal'' diameters. (f) The net result is two adjacent dictyosomes where one existed previously. Dictyosome multiplication is thus accomplished as part of the normal developmental turnover of cisternae, without interrupting the functioning of the Golgi apparatus as it continues to produce new secretory vesicles from mature cisternae at the trans face. Coordination of Golgi apparatus multiplication with nuclear division ensures that each daughter nucleus receives a complement of paranuclear dictyosomes.