Time lapse studies on the motility of fibroblasts in tissue culture

Regulation of motility of myogenic cells in filling limb muscle anlagen by Pitx2

Cells of the ventrolateral dermomyotome delaminate and migrate into the limb buds where they give rise to all muscles of the limbs. The migratory cells proliferate and form myoblasts, which withdraw from the cell cycle to become terminally differentiated myocytes. The myogenic lineage colonizes pre-patterned regions to form muscle anlagen as muscle fibers are assembled. The regulatory mechanisms that control the later steps of this myogenic program are not well understood. The homeodomain transcription factor Pitx2 is expressed specifically in the muscle lineage from the migration of precursors to adult muscle. Ablation of Pitx2 results in distortion, rather than loss, of limb muscle anlagen, suggesting that its function becomes critical during the colonization of, and/or fiber assembly in, the anlagen. Microarrays were used to identify changes in gene expression in flow-sorted migratory muscle precursors, labeled by Lbx1(EGFP/+), which resulted from the loss of Pitx2. Very few genes showed changes in expression. Many small-fold, yet significant, changes were observed in genes encoding cytoskeletal and adhesion proteins which play a role in cell motility. Myogenic cells from genetically-tagged mice were cultured and subjected to live cell-tracking analysis using time-lapse imaging. Myogenic cells lacking Pitx2 were smaller, more symmetrical, and had more actin bundling. They also migrated about half of the total distance and velocity. Decreased motility may prevent myogenic cells from filling pre-patterned regions of the limb bud in a timely manner. Altered shape may prevent proper assembly of higher-order fibers within anlagen. Pitx2 therefore appears to regulate muscle anlagen development by appropriately balancing expression of cytoskeletal and adhesion molecules.

Motility of mouse fibroblasts in tissue culture

The growth and motion of mouse L-cells in vitro have been studied by means of time-lapse photography. In particular, the mitotic period and the motility, defined in terms of [R2], the mean square displacement of an ensemble of cells, have been measured as a function of temperature. The motility is a function of the phase of the cell cycle. For approximately the first one-eighth of the mitotic period the motility is well described as a random walk with persistence, the duration of the persistence being determined by the time of extension of the filopodic spindle. The temperature dependence of the diffusion constant follows the Arrhenius factor. The mitotic period, which varies exponentially as (1/T), exhibits a large variance, and the time difference in replication of daughter pairs follows approximately a Poisson distribution with a mean difference of 138 min at T = 37 degrees C. There is no evidence of mirror symmetry in the motion of daughter pairs for fibroblast cells plated in vitro in Corning tissue culture flasks.

Mutual potentiation by magnesium and calcium of growth in animal cells

The effects on DNA synthesis of various combinations of Mg2+ and Ca2+ in cultures of chick embryo cells have been studied. When [Mg2+] larger than or equal to 0.24 mM, reduction of Ca2+ from the standard concentration of 1.72 mM to 0.01 mM had no effect on the incorporation of [3H]thymidine ([3H]dThd) into DNA over a 16-hr period. When Mg2+ was reduced to 0.04 mM, [3H]dThd incorporation into DNA decreased directly with [Ca2+] below 1.72 mM and increased slightly up to [Ca2+] = 5.02 mM, where cell damage began to occur. The change in [Ca2+] necessary to maintain a half-maximal rate of [3H]dThd incorporation was found to depend inversely on the fourth power of the change in [Mg2+]. Chelation of Ca2+ with approximately equimolar ethylene glycol-bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) in the presence of [Mg2+] larger than or equal to 0.24 mM reduced [3H]dThd incorporation about 10-fold, and large excesses of EGTA did not further reduce it. The amount of EGTA required to produce a detectable inhibition of [3H]dThd incorporation was independent of [Mg2+] larger than or equal to 0.24 mM, as was the level of residual incorporation in excess EGTA. When [Mg2+] was reduced to 0.04 mM, however, [3H]dThd incorporation declined even when [EGTA] less than [Ca2+], and vanished when EGTA was in large excess. The results are discussed within the framework of a model for the regulation of cell metabolism and growth in which the availability of free Mg2+ is the central coordinating factor. The metabolic effects of Mg2+ depend on its distribution between elements such as ATP and binding sites on membranes. We propose that the major metabolic effects of varying [Ca2+] are produced indirectly through its competition with Mg2+ for membrane sites, thereby making more or less Mg2+ available for rate-limiting transphosphorylation reactions.