Violet-blue light is toxic to mammalian cells, and this toxicity has been linked with cellular production of H 2 O 2 . In this report, we show that violet-blue light, as well as UVA,

The mechanism by which epithelial cells rearrange is a process that is central to epithelial morphogenesis, yet remains poorly understood. We have investigated epithelial cell rearrangement in the dorsal hypodermis of the Caenorhabditis elegans embryo, in which two rows of epithelial cells rearrange in a morphogenetic process known as dorsal intercalation. The intercalating cells extend basal protrusions which squeeze between their opposing neighbors beneath their adherens junctions. As the intercalating cells move forward, these protruding tips become broader in the anterior-posterior and dorsoventral dimensions, effectively "plowing through" the adherens junctions and forcing an opening for the remainder of the intercalating cell to insert between the contralateral cells. These cell movements are dependent upon intact cytoarchitecture, since the pharmacological disruption of microtubules or actin filaments blocks cell rearrangement. The cells appear to intercalate independently of immediately adjacent neighboring hypodermal cells because dorsal intercalation is not blocked by the ablation of the progenitors for either half of the lateral hypodermal cells or the posterior half of the dorsal hypodermis. This is the first case in which the protrusive mechanism underlying epithelial cell rearrangement has been characterized, and we propose a model describing how epithelial cells rearrange within the confines of an epithelial monolayer, and discuss the mechanisms that may be guiding these directed cell movements.

We have used transmission electron microscopy to examine plasmodesmata of the charophycean green alga Chara zeylanica, and of the putatively early divergent bryophytes Monoclea gottschei (liverwort), Notothylas orbicularis (hornwort), and Sphagnum fimbriatum (moss), in an attempt to learn when seed plant plasmodesmata may have originated. The three bryophytes examined have desmotubules. In addition, Monoclea was found to have branched plasmodesmata, and plasmodesmata of Sphagnum displayed densely staining regions around the neck region, as well as ring-like wall specializations. In Chara, longitudinal sections revealed endoplasmic reticulum (ER) that sometimes appeared to be associated with plasmodesmata, but this was rare, despite abundant ER at the cell periphery. Across all three fixation methods, cross-sectional views showed an internal central structure, which in some cases appeared to be connected to the plasma membrane via spoke-like structures. Plasmodesmata were present even in the incompletely formed reticulum of forming cell plates, from which we conclude that primary plasmodesmata are formed at cytokinesis in Chara zeylanica. Based on these results it appears that plasmodesmata of Chara may be less specialized than those of seed plants, and that complex plasmodesmata probably evolved in the ancestor of land plants before extant lineages of bryophytes diverged.

Extracellular matrix (ECM) polymers secreted by the diatoms Achnanthes longipes Ag. and Cymbella cistula (Ehr.) Kirchn. completely encase the cell and are responsible for adhesion and other interactions with the external environment. To preserve details of the highly hydrophilic ECM in the native state and to preserve, with a high degree of fidelity, the intracellular structures involved in synthesis of extracellular polymers, we applied a suite of cryotechniques. The methods included high-resolution visualization of surfaces using cryo-field emission SEM (cryo-FESEM) and preservation for TEM observation of thin sections by high-pressure freezing (HPF) and freeze substitution (FS). The extracellular structures of diatoms plunge-frozen in liquid ethane, etched at low temperature, and observed on a cryostage in the FESEM showed overall dimensions and shapes closely comparable to those observed with light microscopy. Cryo-FESEM demonstrated the pervasive nature of the extracellular polymers and their importance in cell-substratum and cell-cell associations and revealed details of cell attachment processes not visible using other SEM techniques or light microscopy. The layer of ECM coating the frustule and entirely encapsulating cells of A. longipes and C. cistula was shown to have a significant role in initial cell adhesion and subsequent interaction with the environment. Trails of raphe-associated ECM, generated during cell motility, were shown at high resolution and consist of anastomoses of coiled and linear strands. Cryo-FESEM revealed a sheet-like mucilage covering stalks. HPF/FS of A. longipes resulted in excellent preservation of intra-and extracellular structures comparable to previous reports for animals and higher plants and revealed several organelles not described previously. Three distinct vesicle types were identified, including a class closely associated with Golgi bodies and postulated to participate in formation of the extracellular adhesive structures. HPF/FS showed a number of continuous diatotepic layers positioned between the plasma membrane and the silicon frustule and revealed that extracellular adhesive extrusion through frustule pores during stalk production was closely related to the diatotepum. The stalks of A. longipes consist of highly organized, multilayered, fine fibrillar materials with an electron-opaque layer organized as a sheath at the stalk periphery.