58. Brown, Jr. R. M. and D. K. Romanovicz. 1976. Biogenesis and structure of Golgi-derived cellulosic scales in Pleurochrysis . I. Role of the endomembrane system in scale assembly and exocytosis. Applied Polymer Symp. 28:537-585. John Wiley & Sons, Inc., New York.

58. Synopsis

The Golgi apparatus of a marine haptophycean alga, Pleurochrysis scherfellii, has been studied with respect to its role in the assembly and transport of cell wall scales. In the vegetative cell the scale consists of three morphologically recognizable subunits: a quadriradial network of microfibrils rich in galactose and acidic sulphated polysaccharides, a spiral network of cellulosic microfibrils associated with a peptide rich in hydroxyproline, and amorphous coating substances rich in acidic polysaccharides. Scale structure, size, and composition is dependent upon specific phases of the life history. Unlike vegetative scales, those from the zoospore phase lack the amorphous coating substances and are about half the dimensions. The ratio of radial-to-spiral subcomponents remains unchanged. In the nutritionally induced coccolith phase, the scale rim is modified to consist of two concentric microfibrils. The ratio of radial-to-spiral subcomponents is dramatically altered. Scale subcomponents are synthesized and assembled in association with the membranes of the endoplasmic reticulum and Golgi apparatus. Incipient Golgi membranes originate from budding regions of the endoplasmic reticulum. Precursor pools for radial subcomponents are differentiated from the spiral subcomponents by dense reaction products of silver methenamine when preceded by periodate oxidation. These pools which are about to be incorporated into the forming face of the Golgi membranes first appear within specific topographic regions of the endoplasmic reticulum synthesized in a folded configuration. Cytochemical enzyme localization reveals aryl sulfatase and alkaline phosphatase activity in the region of synthesis of the radial microfibrils. The radial microfibrils undergo an elaborate unfolding as observed by the presence of distinct "Z-stages." Following unfolding of the radial microfibrils, the spiral cellulosic microfibrils are presumably crystallized within a central feeding tubule on the distal face of the cisterna. Alkaline phosphatase activity is pronounced in the zone of centripetal amorphous polysaccharide addition. In vegetative cells, this sequence immediately follows the synthesis and deposition of cellulosic microfibrils. Scale transport to the surface and ultimate exocytosis involve a subsurface cisterna which has the capacity to guide and direct Golgi cisternal membranes to the plasma membrane and to transfer "spent" Golgi membranes to an autophagic vacuolar system. Acid phosphatase activity was prominent in the subsurface cisterna as well as the distal region of the Golgi apparatus. Colchicine prevents scale exocytosis and promotes autophagic consumption of accumulating scales and their cisternal membranes. Control of scale exocytosis seems to be under specific control of plasma membrane "acceptor" regions and Golgi cisternal coated vesicle "donor" regions. Protoplast rotation and exocytosis is discussed in terms of the "receptor-donor" hypothesis.

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Last modified 27 October 2005.
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