Characterization of the factor in L-cell conditioned medium capable of stimulating colony formation by mouse marrow cells in culture

Medium harvested from cultures of mouse L-cells contains “conditioning factor activity” (CFA) that may be detected by its ability to stimulate colony formation by mouse marrow cells in culture. The active material has been characterized and appears to be a glycoprotein with properties similar to those reported for the colony-stimulating activity in human urine. Characterization of trypsin-digested material indicated that only part of the molecule is essential for biological activity. The CFA has been partially purified from serum-free L-cell conditioned medium, and evidence has been obtained that radioactivity derived from labelled valine or glucosamine may be incorporated into the factor. L-cell conditioned medium appears to be a convenient source of partially-purified, highly active material, suitable for use in studies on its mechanism of action.     

The limiting information capacity of a neuronal link

The maximum rate at which a synaptic link could theoretically transmit information depends on the type of coding used. In a binary modulation system it depends chiefly on the relaxation time, and the limiting capacity equals the maximum attainable impulse rate. In a system using pulse-interval modulation, temporal precision may be a more important limiting factor. It is shown that in a number of typical cases a system of the second type could transmit several times more information per second through a synaptic link than a binary system, and the relation between relative efficiency, relaxation-time, and temporal resolving power is generalized in graphical form. It is concluded, not that interval modulation rather than binary modulation “ought” to be the mode of action of the central nervous system, but that the contrary assumption is unsupported by considerations of efficiency.     

Discrimination of two fibroblast progenitor populations in early explant cultures of hamster gingiva

Summary Numerous metabolic studies have demonstrated heterogeneity of fibroblast populations in culture, yet little is known about the structure of fibroblast populations in adult tissues in vivo. To determine if populations of both cycling and non-cycling cells are present in gingiva, hamsters were labelled with [³H]-thymidine to label cycling cells in vivo, and explanted biopsies were subsequently incubated with bromodeoxyuridine to label cycling cells in vitro. Cycling cells were identified by combined immunohistochemistry and radioautography. Fibroblasts were recognized by the presence of vimentin and the absence of keratin as determined by immuno-fluorescence. The largest proportion of cells were double-labelled with [³H]-thymidine and bromodeoxyuridine (43.8%) indicating the presence of actively cycling populations that maintained their proliferative status upon explantation. Cultures also exhibited a second population of cells labelled only with bromodeoxyuridine (38.7%) that did not cycle in vivo, but retained the capacity for proliferation in vitro. However, limiting dilution analysis of single-cell suspensions revealed only a single class of progenitors capable of forming large colonies in vitro. Approximately 1 in 190 plated cells was capable of colony-formation, indicating that, upon explantation, a subset of the cycling cells in vitro exhibits extensive proliferative capacity. There was also a small population of cells unlabeled with either [³H]-thymidine or bromodeoxyuridine (9.4%) that appeared to be terminally differentiated. Different substrates, including glass and thin films of gelatin and collagen, did not significantly alter the fraction of cells labelled with [³H]-thymidine. These data demonstrate the existence of 2 separate progenitor-cell populations with different capacities for proliferation in vivo and in vitro.     

Obtaining Suspensions of Animal Cells in Metaphase from Cultures Propagated on Glass

Cells of some strains continuously cultivated in vitro have a pronounced tendency to lose their attachment to glass during mitosis. If division of these cells is arrested in meta-phase by treatment with colchicine, many of the cells are set free and float into the medium. Advantage has been taken of this property to collect large numbers of suspended cells in c-metaphase. By applying standard procedures of staining with orcein and manual squashing to these cells, preparations are obtained that are useful for critical chromosome analysis. The procedure consists in treatment of cultures with colchicine (final concentration 0.1 μg/ml) for 18-24 hr; agitation in hypotonic medium (quarter-strength Tyrode's or CMRL-1066) for 5 min; addition of 1:3 acetic-alcohol for 5-10 min; removal and centrifugation of the resulting fluid mixture from the culture; and resuspension of the sedimented cells in a small volume of the fluid. Advantages of the method are: large numbers of metaphase plates can be examined on a single slide, the chromosomes are contracted and well separated, and cytoplasmic boundaries are preserved.