Pavement cells

Abstract: This review focuses on the structure and function of the branchial chloride cell in freshwater fishes. The mitochondria-rich chloride cell is believed to be the principal site of trans-epithelial Ca2+ and Cl- influxes. Though currently debated, there is accruing evidence that the pavement cell is the site of Na+ uptake via channels linked electrically to an apical membrane vacuolar H(+)-ATPase (proton pump). Chloride cells perform an integral role in acid-base regulation. During conditions of alkalosis, the surface area of exposed chloride cells is increased, which serves to enhance base equivalent excretion as the rate of Cl-/HCO3- exchange is increased. Conversely, during acidosis, the chloride cell surface area is diminished by an expansion of the adjacent pavement cells. This response reduces the number of functional Cl-/HCO3- exchangers. Under certain conditions that challenge ion regulation, chloride cells proliferate on the lamellae. This response, while optimizing the Ca2+ and Cl- transport capacity of the gill, causes a thickening of the blood-to-water diffusion barrier and thus impedes respiratory gas transfer.

Abstract: Stomata are specialized cellular structures in the epidermis of aerial plant organs that control gas exchange (H(2)O release and CO(2) uptake) between leaves and the atmosphere by modulating the aperture of a pore flanked by two guard cells. Stomata are nonrandomly distributed, and their density is controlled by endogenous and environmental factors. To gain insight into the molecular mechanisms regulating stomatal distribution, Arabidopsis thaliana mutants with altered stomatal characteristics were isolated and examined. The sdd1-1 mutant exhibits a two- to fourfold increase of stomatal density and formation of clustered stomata (i.e., stomata that are not separated by intervening pavement cells), whereas the internal leaf architecture is not altered. The SDD1 gene was identified by map-based cloning. It encodes a subtilisin-like serine protease related to prokaryotic and eukaryotic proteins. We propose that SDD1 acts as a processing protease involved in the mediation of a signal that controls the development of cell lineages that lead to guard cell formation.

Abstract: In fish of all groups examined including Teleostei, Chondrostei, Holostei, Chondrichytes, and Dipnoi, the primary epithelium that surrounds the primary lamellae has a close relationship with the venous compartment. Except in Dipnoi that displays a specialized epithelial drainage, the venous compartment consists of a central venous sinus that is connected with the systemic vasculature by noninnervated muscular arteriovenous anastomoses and drains into the branchial veins. Primary epithelium contains the chloride cells, which vary in morphology and number according to the milieu where the fish lives. The presence of an accessory cell beside the chloride cell is characteristic of seawater or seawater-adapted fish. The secondary epithelium that covers the free part of the secondary lamellae has an exclusive relationship with the arterioarterial vasculature, i.e., the pillar capillary compartment. This compartment is actively controlled by innervated sphincters located in the primary lamellae and in lower species by pre- and postlamellar noninnervated sphincters. Contraction of pillar cells may also contribute to this control. The secondary epithelium consists of an outermost layer of pavement cells that exhibited structural characteristics suggestive of cell coat secretion and an innermost layer of less differentiated cells. In contrast to the primary epithelium, the secondary epithelium does not exhibit any obvious differences between freshwater and seawater fish or undergo any obvious change during transfer of fish from fresh- to seawater. However, in conditions which exaggerate the absorptive functions of freshwater chloride cells, the secondary epithelium become modified by an intensive differentiation of freshwater chloride cells from its innermost layer of cells. These observations suggest possible specialized functional relationships between seawater chloride cells and the central venous sinus, and freshwater chloride cells and the arterioarterial compartment.

Abstract: The surfaces of plants are covered in epithelial cells that come in many different shapes, suggesting that individual cells must have some control over their own shape. An unusually shaped epithelial cell is the pavement cell, which looks like a jigsaw puzzle piece and is found in the leaves of many flowering plants. Relatively little was known about the exact contribution of mechanical properties of the wall to this shape. Furthermore, although it was known that parts of pavement cells are rich in microtubules—tubes of protein that act as a scaffold inside the cell— the possibility that shape impacts the behavior of microtubules was not fully addressed. Now, using a combination of computer modelling and experiments, Sampathkumar et al. reveal that the shape of the pavement cells relies in part on the response of the microtubules to stress. In an individual cell, microtubules align along the direction of the largest stress, with a protein severing those microtubules that are not aligned in this direction. As the stress inside a cell is determined in part by the cell’s shape, this sets up a feedback loop: the stress resulting from the cell shape aligns the microtubules that reinforce the cell wall, thus maintaining the shape of the cell. An external stress applied to the epithelium can override this internal stress. Because all of the plant cells are under turgor pressure from the inside, pressure from the outside, like squeezing a balloon, changes the stress pattern, causing the realignment of the microtubules so as to resist the new stress. This shows that the microtubules respond to local stresses within a cell, and are continually responsive to stress changes.