Measurements of noradrenaline fluorescence in axons of chick embryo sympathetic ganglia grown in three day culture

Summary Chicken embryo sympathetic ganglion fragments, grown in culture for three days, were incubated for short periods in appropriate experimental and control media to provide information on axonal uptake of exogenous noradrenaline in the presence and absence of the uptake-blocking drugs, protriptyline and phenoxybenzamine. The cultures were dried, gassed in formaldehyde vapour and examined by fluorescence microscopy. Spot readings were taken on axon-bundles using a microspectrophotometer. Readings were corrected for the amount of tissue present in each field, using graticule and interferometric methods. Brightness was uniform along individual axon-bundles, but varied from bundle to bundle within the same culture. Despite this range in brightness the readings fell into a normal distribution about the mean. Dose-dependent increases in brightness were obtained following incubation with noradrenaline. A clear indication of the effect of the blocking drugs was observed but the wide range in brightness prevented reliable quantitative data on these effects from being obtained. It was concluded that the wide range of brightness within each culture reflected a lack of equivalence in the intrinsic noradrenaline-uptake properties of the axon-bundles developing in such cultures.This work was supported by a grant from the Medical Research Council. We wish to thank Mrs. G. O'Shea for valuable technical assistance, and Messrs. P.F. Hire and K. Twohigg for help with the illustrations. Phenoxybenzamine was a gift from Smith, Kline and French Ltd.     

Cholinoreception of chick embryo myocardial explants]

The effects of acetylcholine, subecholine (selective nicotinomimetic) and methylfurmetide (selective muscarinomimetic) on the spontaneous contractions of atrial and ventricular explants from the heart of 408 day embryos as well as of mixed explants, have been studied by means of a special photoelectric device. In most of the experiments, acetylcholine reduced the rate of contractions, the sensitivity of preparations being increased with the age of embryos. Preliminary atropine treatment reversed or significantly decreased acetylcholine effect. Subecholine usually increased the contraction rhythm, whereas methylfurmetide decreased the spontaneous activity.     

Developmental decrease in neurite extension in cultured chick embryo retina and spinal cord neurons

Retina and spinal cord neurons from chick embryos attach to culture substrates and extend neurites. There is a statistically significant age-related decrease in the percentage and average length of neurites formed in 24-hr cultures of chick retina and spinal cord neurons between 6 and 16 days of embryonic age. The developmental decrease of neurite extension may be important for synaptogenesis in the developing nervous system.     

Tissue interactions affecting the migration and differentiation of neural crest cells in the chick embryo.

A series of microsurgical operations was performed in chick embryos to study the factors that control the polarity, position and differentiation of the sympathetic and dorsal root ganglion cells developing from the neural crest. The neural tube, with or without the notochord, was rotated by 180 degrees dorsoventrally to cause the neural crest cells to emerge ventrally. In some embryos, the notochord was ablated, and in others a second notochord was implanted. Sympathetic differentiation was assessed by catecholamine fluorescence after aldehyde fixation. Neural crest cells emerging from an inverted neural tube migrate in a ventral-to-dorsal direction through the sclerotome, where they become segmented by being restricted to the rostral half of each sclerotome. Both motor axons and neural crest cells avoid the notochord and the extracellular matrix that surrounds it, but motor axons appear also to be attracted to the notochord until they reach its immediate vicinity. The dorsal root ganglia always form adjacent to the neural tube and their dorsoventral orientation follows the direction of migration of the neural crest cells. Differentiation of catecholaminergic cells only occurs near the aorta/mesonephros and in addition requires the proximity of either the ventral neural tube (floor plate/ventral root region) or the notochord. Prior migration of presumptive catecholaminergic cells through the sclerotome, however, is neither required nor sufficient for their adrenergic differentiation.     

Protein turnover in senescent cultured chick embryo fibroblasts

The over-all rates of protein synthesis, degradation and net accumulation were estimated in rapidly growing young and slowly doubling old cultures of chick fibroblasts. We find that not only the rate of protein synthesis is reduced in senescent cultures, but the average rate of protein degradation is also slowed down considerably. This decrease in the rate of protein breakdown in aging cells stands in contrast with the previously observed acceleration of this process by other conditions (such as serum deprivation or overcrowding) that lead to the cessation of cellular growth. Though the retarded protein degradation may contribute to the acculation of abnormal proteins in senescent cells we find that the breakdown of grossly abnormal puromycin peptides proceeds equally rapidly in young and old cultures. The protein content of senescent cells increases by 1.8-fold as compared to young cells, while the average cell volume is increased even more (almost 5-fold). By contrast, consideration of the over-all balance of protein metabolism in these cells indicates that the average concentration of metabolically turning-over proteins is somewhat higher in senescent than in young fibroblasts.     

Oxygen enhances fusion of cultured chick embryo myoblasts

Fusion of mononucleate myoblasts to form multinucleated myotubes increases when skeletal muscle cells are grown in progressively higher oxygen concentrations (5%, 20%, and 40% oxygen). At four days of growth fusion of myoblasts (as expressed by the percent of all muscle nuclei that are located in myotubes) is 57 ± 2% in 5% oxygen, 68 ± 1% in 20% oxygen, and 78 ± 2% in 40% oxygen (P<0.001). However, at a concentration of 40%, oxygen depresses the rate of cell division and thereby affects the number of myoblasts available for fusion. Thus, oxygen concentration significantly modifies growth of skeletal muscle in vitro. Its net effect on myotube formation results from the interaction of its separate effects to enhance cell fusion and to depress cell proliferation.