Mechanical stimulation promotes osteogenic differentiation of human bone marrow stromal cells on 3-D partially demineralized bone scaffolds in vitro.

Bone is a dynamic tissue that is able to sense and adapt to mechanical stimuli by modulating its mass, geometry, and structure. Bone marrow stromal cells (BMSCs) are known to play an integral part in bone formation by providing an osteoprogenitor cell source capable of differentiating into mature osteoblasts in response to mechanical stresses. Characteristics of the in vivo bone environment including the three dimensional (3-D) lacunocanalicular structure and extracellular matrix composition have previously been shown to play major roles in influencing mechanotransduction processes within bone cells. To more accurately model this phenomenon in vitro, we cultured human BMSCs on 3-D, partially demineralized bone scaffolds in the presence of four-point bending loads within a novel bioreactor. The effect of mechanical loading and dexamethasone concentration on BMSC osteogenic differentiation and mineralized matrix production was studied for 8 and 16 days of culture. Mechanical stimulation after 16 days with 10 nM dexamethasone promoted osteogenic differentiation of BMSCs by significantly elevating alkaline phosphatase activity as well as alkaline phosphatase and osteopontin transcript levels over static controls. Mineralized matrix production also increased under these culture conditions. Dexamethasone concentration had a dramatic effect on the ability of mechanical stimulation to modulate these phenotypic and genotypic responses. These results provide increased insight into the role of mechanical stimulation on osteogenic differentiation of human BMSCs in vitro and may lead to improved strategies in bone tissue engineering.

Correlation of Stomatal Conductance with Photosynthetic Capacity of Cotton Only in a CO(2)-Enriched Atmosphere: Mediation by Abscisic Acid?

Some evidence indicates that photosynthetic rate (A) and stomatal conductance (g) of leaves are correlated across diverse environments. The correlation between A and g has led to the postulation of a "messenger" from the mesophyll that directs stomatal behavior. Because A is a function of intercellular CO(2) concentration (c(i)), which is in turn a function of g, such a correlation may be partially mediated by c(i) if g is to some degree an independent variable. Among individual sunlit leaves in a cotton (Gossypium hirsutum L.) canopy in the field, A was significantly correlated with g (r(2) = 0.41, n = 63). The relative photosynthetic capacity of each leaf was calculated as a measure of mesophyll properties independent of c(i). This approach revealed that, in the absence of c(i) effects, mesophyll photosynthetic capacity was unrelated to g (r(2) = 0.06). When plants were grown in an atmosphere enriched to about 650 microliters per liter of CO(2), however, photosynthetic capacity remained strongly correlated with g even though the procedure discounted any effect of variable c(i). This "residual" correlation implies the existence of a messenger in CO(2)-enriched plants. Enriched CO(2) also greatly increased stomatal response to abscisic acid (ABA) injected into intact leaves. The data provide no evidence for a messenger to coordinate g with A at ambient levels of CO(2). In a CO(2)-enriched atmosphere, though, ABA may function as such a messenger because the sensitivity of the system to ABA is enhanced.

Photosynthesis of cotton plants exposed to elevated levels of carbon dioxide in the field.

The cotton (Gossypium hirsutum L.) plant responds to a doubling of atmospheric CO2 with almost doubled yield. Gas exchange of leaves was monitored to discover the photosynthetic basis of this large response. Plants were grown in the field in open-top chambers with ambient (nominally 350 µl/l) or enriched (nominally either 500 or 650 µl/l) concentrations of atmospheric CO2. During most of the season, in fully-irrigated plants the relationship between assimilation (A) and intercellular CO2 concentration (ci) was almost linear over an extremely wide range of ci. CO2 enrichment did not alter this relationship or diminish photosynthetic capacity (despite accumulation of starch to very high levels) until very late in the season, when temperature was somewhat lower than at midseason. Stomatal conductance at midseason was very high and insensitive to CO2, leading to estimates of ci above 85% of atmospheric CO2 concentration in both ambient and enriched chambers. Water stress caused A to show a saturation response with respect to ci, and it increased stomatal closure in response to CO2 enrichment. In fully-irrigated plants CO2 enrichment to 650 µl/l increased A more than 70%, but in water-stressed plants enrichment increased A only about 52%. The non-saturating response of A to ci, the failure of CO2 enrichment to decrease photosynthetic capacity for most of the season, and the ability of the leaves to maintain very high ci, form in part the basis for the very large response to CO2 enrichment.