Ref ID : 2041
Harald Netzel; [The Origin of the Hexagonal Shell Structure in the Testate Amoeba Arcella vulgaris var. multinucleata (Rhizopoda, Testacea)]. Arch.Protistenk.Bd. 117:321-357, 1975
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The theca of Arcella vulgaris var. multinucleata is built out of a single type of prefabricated building parts, the thecagenous granules, by a cytoplasmic bud or anlage, which protrudes from the old shell. The construction process preceeds cell division and takes up to 10 minutes to complete. For about 6 minutes at the end of this process the testanlage is surrounded by a cupola-like pseudopodium. The thecagenous granules are membrane-bounded, bipolar units, the electron density of which is somewhat less pronounced at one pole, referred to as the Golgi-pole. Near the opposite, blunt end of the granule there is a lumen. This end has been designated the lumen pole. Two circular series of narrow, curved, blunt-ending channels project from the lumen into the matrix at different levels. The granules arise as spheroidal or droplet-like secretory products in close proximity and polar relationship to dictyosomes. Prior to thecagenesis from 1 to 3% of the estimated maximum cell volume consists of these granules. They are stored in the perpheral layer of the cytoplasm inside the old shell. During thecagenesis they move to the surface of the test-anlage, where they are orientated so that the lumen pole points towards the interior of the cell. The granules are thus arranged into a single, discontinuous layer. At this stage they possess the form of rodlets 1-1.35 µm in length and 480-600 nm in diameter. The basically hexagonal pattern of the Arcella-shell then becomes established. It results from the closest physically possible packing of the rod-like thecagenous granules, which are circular in cross section and similar in size. Faults in the arrangement of the granules later appear as disturbances of the alveolar pattern. When the arrangement of the granules is complete they are rapidly and almost synchroneously extruded. The membrane of the granules fuses with the plasmalemma and opens to the exterior near the Golgi-pole. By enlargement of the lumen and spreading of the matrix the granules swell, become vase-shaped and are finally transformed into alveoli. During the transformation process the granules remain embedded in the bud cytoplasm. The peripheral layer, which at first surrounds each granule completely, is confined by their expansion to villus-like cell projections in the angles between the alveoli. The alveoli flatten off hexagonally. Contiguous faces of adjacent alveoli fuse to give rise to common (long) walls. At the (usually 6) vertices around each alveolus fusion of the alveolar matrix is prohibited by the cytoplasmic villi. Here the (short) alveolar walls remain separate and constitute the walls of pore channels surrounding the villi. Thus a continuous layer of laterally connected, hollow chambers with orthogonal side-plan and dodecagonal ground-plan emerges. Deformations of the alveoli and distortions of pattern frequently occur in the ventral side of the theca. These appear to result from the slight invagination of the anlage, which produces the funnel-shape of this side. The invagination occurs after the granule-alveolus-transformation has taken place. Vestiges of the original polarity of the granules are visible in some structural features of the alveoli: the outer walls are thicker that the remaining walls and they have short projections, which narrow the external orifices of the pore channels. During morphogenesis the cell enlarges to about 150% of its premorphogenetic volume (= volume of the theca, 272,000 µm3), probably by dilution of the ground cytoplasm owing to reduced activity of the contractile vacuoles. The volume of the thecal wall (around 20,000 µm3) is about 7-8% of the maximum cell volume. The increase in volume of the granules by transformation into alveoli is between 250 and 800% of the initial granular volume. During thecagenesis the surface of the plasmamembrane temporarily increases to about 300 to 400% of the previous maximum cell surface, which is of the order of 25,000 µm2. Around 1/3 to 2/5 of this increase could result from the incorporation of thecagenous granule membrane into the plasmalemma.