Biomedical subjects
Publications and source records attributed to A Robertson.
Cells disaggregatedfrom 1-day chick embryos show three morphologies,two reminiscent of mammalian erythrocytes and one epithelial. The presence of intermediate forms suggests that the three morphologies are interconvertible.
The effects of cholinergic blockade of neurons by atropine or serotonergic blockade by methysergide was investigated in rats responding for brain-stimulation reward. Bipolar stimulating electrodes were placed either in the subfornical organ (SFO) or the lateral hypothalamus (LH). Atropine sulphate and methysergide significantly suppressed self-stimulation of the SFO but not of the LH, suggesting that cholinergic and serotonergic neurons are involved in brain-stimulation reward associated with this site.
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Acrodermatitis enteropathica is a rare disease of childhood, characterized by periorificial and perioral dermatitis, acrodermatitis, diarrhea, and alopecia. Two patients were successfully treated with diiodohydroxyquin (Diodoquin). Lipid concentrations disclosed abnormal distribution of fats that resembled those of less mature infants fed diets low in linoleic acid even though the patients were actually on diets with high linoleic acid levels. Fatty acid distribution showed increased levels of linoleic acid that correlated with clinical improvement in the patient who was more ill. Skin fatty acids showed decreased levels of linoleic acid. Electron micrographs and histochemical staining disclosed increased lipids droplets in epidermal cells. Linoleic acid and zinc may have a role in the origin of the disease, and measurement of both skin and serum linoleic acid is suggested in the evaluation of its progress.
We report periodic movements during erection of Dictyostelium discoideum (Dd) sorocarps. Our observations lead to the working hypothesis that Dd sorocarp erection occurs by two superimposed processes: one periodic, with a modal period of 6 1/2 min, and one continuous. We tentatively identify the periodic process with cell movement into the apex of the Dd stalk, and the continuous process with cell vacuolation, together with stalk sheath extension.
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The fraction, X2, of a population of D. discoideum cells competent to relay an aggregative signal has been measured as a function of time, t, spent in interphase. X2(t) is less than 0.I up to 5 h, increases slowly to 0.4 by 8 h 20 min, and saturates at I by 10 h. The hypothesis that cellular interactions might enhance X2(t) was tested by mixing populations of cells set into interphase at different times. No interactions were found. External stimulation did not affect X2(t).
The results of experiments on small populations of Dictyostelium discoideum, directed towards the measurement of the development in time of the competence of the cells to signal autonomously, are reported. This competence is quantified by X3, the intrinsic probability that a given cell may turn autonomous. The data show an early exponential growth in time of X3, followed by saturation. The saturation value depends on the population size suggesting that the differentiation is a co-operative phenomenon. The differentiation of autonomous cells starts roughly 7 h after the removal of food and saturates within 21 h.
Centre formation in Polysphondylium violaceum is delayed for 2 h on buffered agar containing 10(-3) M c-AMP, and for up to 22 h on unbuffered agar with the same c-AMP concentration. With ambient c-AMP concentrations as low as 10(-6) M, P. pallidum forms numerous, small, atypical aggregates which do not fruit. This effect is independent of whether the agar is buffered. P. violaceum amoebae are weakly attracted to the tip of a microelectrole containing 10(-3) or 10(-4) M c-AMP, but the electrode cannot compete when natural centres form nearby. P. pallidum amoebae are not attracted. Aggregates of P. violaceum and of P. pallidum are strongly attracted to a microelectrode releasing c-AMP. The observation of Shaffer that Polysphondylium grex switch over from secreting an acrasin that attracts homologous amoebae to one that attracts the larger Dictyostelium species suggests that the second acrasin might by c-AMP. The above results strengthen this conjecture. As c-AMP inhibits centre formation, the secretion of c-AMP by older aggregates may explain an inhibition of centre formation in the 'overlay' experiments of Shaffer.
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The design, calibration, and operation of a source of controlled amounts of cyclic AMP (c-AMP) are described. Typically, 1.5 s pulses containing 10(10)-10(-12) molecules of c-AMP can be delivered to a region about 10 mum in diameter on an agar plate. The resulting concentration profiles are given as functions of distance and time. The diffusion coefficient of c-AMP in agar was measured to be 0.97 times 10(-5) cm2-s-1 at 21 degrees C.
The microelectrode system described in the accompanying paper was used to investigate properties of fields of Dictyostelium discoideum amoebae in late interphase. Cells in the fields were competent to respond chemotactically to, and to relay, a c-AMP signal, but not to produce an aggregative signal autonomously. The experimental results are generally consistent with c-AMP being the sole compound required for chemotaxis and signal relaying. A periodic signal from the microelectrode can initiate and control aggregation and can complete with spontaneously arising aggregates. The electrode was used to measure the refractory period for relaying which decreases from 9 min or more to between 2 and 3 min with increasing developmental age, and to measure thresholds for chemotaxis and signal relaying. The results are discussed in relation to models for the control of aggregation in D. discoideum.
In the context of frequency-dependent selection, the effect of conditioning the medium, by the previous culture of larvae of a given strain, on the survival of subsequent larvae of the same or different strains was measured. Six strains were sued, some inbred and some lines selected for low or high sternopleural bristles. The survival of larvae was not significantly less when the food had been conditioned by the same strain rather than by another strain, nor did conditioning by one strain have any special effect on the survival of another. The survival of lines on medium conditioned by the same strain was not less than on fresh medium, and in some cases was significantly greater.