Laser microsurgery demonstrates that cytoplasmic strands anchoring the nucleus across the vacuole of premitotic plant cells are under tension. Implications for division plane alignment

In epidermal cells of the plant Nautilocalyx lynchii, induced to divide by explantation, the nucleus undergoes a series of movements, on cytoplasmic leading to construction of a division plane across the vacuole. In the stage, the nucleus separates from the cortex, occupying an eccentric the cell, suspended across the vacuole by few thin strands. In the central the nucleus occupies a central position anchored to the cortex by more thicker strands. Finally, the phragmosome forms as a coalescence of strands across the cell, constituting the division plane within which cytokinesis take place. The behaviour and alignment of these strands is important since some are precursors of the division plane. In a previous (Flanders et al (1990) J. Cell Biol. 110, 1111–1122), it was pointed out alignment of cytoplasmic strands showed features common to a variety of under tension. That is, provided they are free to move relative to the strands radiating from the nucleus should tend to seek short rather than routes to the cortex. In this way, strands under tension would move away distant cell corners where two of the cell's edges make a three-way a neighbouring wall. This provides a basis for the avoidance of four-way and the maintenance of three-way junctions which are a characteristic most plant tissues. In addition, tensile elements such as soap bubble contact rigid surfaces perpendicularly. Perpendicular attachment of the to the side wall is embodied in Sachs' rule of cell division and the the premitotic strands in a state of tension would provide a basis for In this study, laser microsurgery has been used to confirm that strands the premitotic nucleus to the cortex are under tension since the severed retract immediately upon severance. However, the response of the nucleus breaking of a cytoplasmic strand by laser depends upon the particular nucleus is most likely to move during the early stage when it is placed and has few cytoplasmic strands. In the middle and phragmosomal nucleus is more resistant. Computer-aided image reconstruction of anti-

Ultrastructure of the embryonic syncytial epithelium in a cestode Hymenolepis diminuta

The embryonic epithelium in Hymenolepis diminuta appears in the early preoncosphere stage. Inside the embryo there is a binucleate cell connected by a cytoplasmic strand with an epithelial layer spreading over the embryonic surface. After the embryo has become covered by tine epithelium the latter delammates into three layers. A basal layer resting on the basal lamina accumulates dense bodies. These bodies are spheroid and membrane-bound in the early embryo. In the late preoncosphere stage rodlike bodies remain in the basal epithelial layer. The basal membrane forms long invaginations into the basal layer. The intermediate epithelial layer is rich in polysomes and it is postulated that secretes extracellular material which cements the intermediate and peripheral layers and the ‘oncospheral membrane’. The continuity in embryonic, larval and adult cestode epithelium is discussed.

Spermiogenesis in Lumbricus herculeus

Small pieces of the sperm sacs of Lumbricus herculeus were fixed for 4 hours in chrome-osmium, embedded in methacrylate, sectioned with a Porter-Blum microtome, and studied with a R.C.A. EMU-2C electron microscope. Each spermatid of a group developing synchronously is attached by a cytoplasmic strand to a common nutrient protoplasmic mass. This mass contains mitochondria and yolk bodies but is anucleate. The proximal centriole, that is, the centriole nearer the nucleus, is at first associated with a small peg which becomes firmly attached to the nuclear membrane. Later these two bodies become separated during the development of the middle-piece which is differentiated in the usual manner from a nebenkern formed by the fusion of 6 or 7 mitochondria. The acrosome develops in relation to the dictyosome (Golgi body), itself composed of 8 or more individual flattened sacs and situated in the cytoplasm opposite the point of attachment of the spermatid to the nutrient mass. Soon after its formation, the acrosome becomes incorporated into a cytoplasmic appendage or acrosome carrier. The carrier moves from its original position, along the lateral border of the elongating nucleus, to the distal margin of the nucleus where the acrosome is deposited. No evidence was found of a centriole located at the point of junction between nucleus and acrosome as suggested by earlier workers.