Exploration of opportunity costs and consideration exploration of opportunity costs and consideration for future generations: Two lasting contributions to systems thinking

In his numerous writings C. West Churchman has shown how the systems approach can be used to secure improvements in the human condition. Specifically one must think holistically. Two concepts—exploration of opportunity costs and consideration for future generations—underline whole systems thinking. The author argues based on his own experience that these tools of the human intellect are among the most lasting contributions Churchman has made to systems thinking.     

Transferrin receptor on endothelium of brain capillaries

The blood/brain barrier prevents the passive diffusion of proteins and metabolites from cerebral blood vessels into tissue spaces around neuronal and glial cells. To provide nutrients for these cells, transport mechanisms must exist and indeed have been demonstrated for metabolites. We now show that monoclonal antibodies against rat and human transferrin receptors label blood capillaries in the brain but not in other tissues. In the rat this labelling occurs after injection of antibody into the blood, thus the receptors seem to be accessible at the endothelial surface. It is possible that transferrin receptors are expressed on these cells to allow transport of transferrin (and thus iron) into brain tissues.     

Mechanisms of glial-guided neuronal migration in vitro and in vivo

Summary Our laboratory has developed an in vitro model system in which glial-guided neuronal migration can be observed in real time. Cerebellar granule neurons migrate on astroglial fibers by apposing their cell soma against the glial arm, forming a specialized migration junction, and extending a motile leading process in the direction of migration. In vitro assays indicate that the neuronal antigen astrotactin functions as a neuron-glia ligand, and is likely to play a role in the movement of neurons along glial fibers. In heterotypic recombinations of neurons and glia from mouse cerebellum and rat hippocampus, neurons migrate on heterotypic glial processes with a cytology, speed and mode of movement identical to that of neuronal migration on homotypic glial fibers, suggesting that glial fibers provide a permissive pathway for neuronal migration in developing brain. In vivo analyses of developing cerebellum demonstrate a close coordination of afferent axon ingrowth relative to target cell migration. These studies indicate that climbing fibers contact immature Purkinje neurons during the migration and settling of Purkinje cells, implicating a role for afferents in the termination of migration.     

Sarcomere shortening and tension development during 'isometric' tetanus of muscle

Laser diffraction and tension measurements from frog muscle during isometric tetanus reveal that the sarcomeres contract more rapidly than tension develops.     

Influence of grassland type, nest type, and shrub encroachment on predation of artificial nests in Chihuahuan Desert grasslands

Nest predation on artificial nests was examined in relation to nest type, grassland type, and shrub encroachment in Chihuahuan Desert grasslands in southern New Mexico. Open-cup ground, open-cup shrub, and spherical shrub nests ( n = 210), mimicking Eastern Meadowlarks ( Sturnella magna ), Black-throated Sparrows ( Amphispiza bilineata ), and Cactus Wrens ( Campylorhynchus brunneicapillus ), were placed in 10 grasslands of tobosa ( Pleuraphis mutica ) and black grama ( Bouteloua eripoda ) with low and heavy levels of mesquite encroachment. Nest predation varied among nest types, grassland types, and shrub encroachment, with highest levels of predation occurring on open-cup shrub nests in tobosa grasslands with heavy shrub encroachment. We detected a significant interaction between nest type and shrub encroachment, but not between grassland type and nest type or grassland type and shrub encroachment. Combined predation rates from the 3 nest types were positively associated with shrub density. The encroachment of shrubs into desert grasslands may act as a corridor for a diversity of species historically not associated with desert grasslands to occupy or move through a patch, increasing vulnerability to nest predation.     

Muscle crossbridge action in excitation and relaxation

Summary Pulse-propagation measurements on a muscle stimulated into tetanus show that the stiffness develops earlier and starts to relax later than the tension. It is concluded that the myosin heads which move towards the actin filaments during excitation become mechanically attached to them.We gratefully acknowledge financial help from the Australian Research Grants Committee.     

Effect of vitamin A on tumor development in burned, unburned, and glucocorticoid-treated mice inoculated with an oncogenic virus.

High doses of vitamin A decreased the severity of tumor development in mice inoculated with a murine sarcoma virus; the same doses of vitamin A had no effect on the increased tumorigenesis seen in animals severely stressed with thermal injury or the increased tumorigenesis induced by exogenous glucocorticoid administration.