The development of the neuromuscular synapse is initiated by an interaction between the motoneuron processes and the skeletal muscle. Previously we showed that basic polypeptide-coated latex beads can mimic the nerve in effecting a postsynaptic differentiation (Peng, H.B. and Cheng, P.-C.: J. Neurosci., 2:1760-1774, 1982). In this study, we examined whether these beads can also induce a presynaptic differentiation along the nerve processes. Explant cultures were prepared from the spinal cords of Xenopus larvae and polyornithine-coated latex beads (0.45-4.5 micron in diameter) were applied. After 1-2 days, the cultures were processed for light and electron microscopy. For light microscopy, the cells were permeabilized and labeled with a monoclonal antibody against a 65-KD antigen on synaptic vesicles. Indirect immunofluorescence revealed that this antigen was concentrated at 60% of the bead-neurite contacts, therefore suggesting the clustering of synaptic versicles at these sites. This phenomenon was not observed at the contacts between neurites and negatively charged (polycarboxylate) beads. However, a concentration of this antigen was also observed at the varicosities along the neurites cultured on polylysine substrate. Thin-section electron microscopy showed the following features: (1) The neurites formed terminal-like varicosities on the bead. (2) Within these varicosities, clusters of 50-60-nm clear vesicles were prominent at the bead-neurite contact. (3) Large (80-100 nm) dense-cored vesicles were also present in the varicosity, but they did not form clusters. (4) Basement membrane did not form at the bead-neurite interface, in contrast to its appearance at the bead-muscle contact from our previous study.(ABSTRACT TRUNCATED AT 250 WORDS)
Counts of differentiating motor cells over the length of the brachial lateral motor column (LMC) indicate that a large decrease in cell number takes place during the larval period. During the same period an increase in nuclear size of the motor cells occurs with a maximum size attained just following forelimb emergence. Comparison between development of the LMC at the brachial and lumbo-sacral levels indicates a slight lag in brachial LMC development. Cell number remains greater in the brachial LMC than in the lumbo-sacral LMC, but nuclear size is consistently less in the brachial column. Probably no significant change in cell number occurs after metamorphosis, though there is an increase in cell size.The normal development of the brachial lateral motor column (LMC) in Rana pipiens has not yet been described. Such a study would serve to complete the picture of normal LMC development. The lumbosacral LMC has been described in terms of its normal development in R. pipiens by Beaudoin ('55). He found that an initial large LMC cell number per section during the early larval period was followed by a large decrease in cell number per section during the mid-larval stages and a much smaller subsequent decrease by the completion of metamorphosis. That the total number of cells follows this pattern is implied by Beaudoin's study and has since been confirmed by Kollros (personal communication) and Pollack (unpublished). The decrease in cell number was accompanied by an increase in the size of the remaining cells. Similar patterns of LMC development have been found in R. temporariu (Race and Terry, '65), Eleutherodactylus ricordii (Hughes, '59), and Xenopus Zaevis (Kollros, '56; Baird, '57; Hughes, '61; Prestige, '67). Species differences are apparently not reflected in the developmental patterns of the LMC, but rather in the number and size of the motor cells (Race and Terry, '65; Hughes, '68).In X . Zaevis, the LMC of the lumbosacral cord appears prior to that of the brachial region (Kollros, '56), and for a large portion of the larval period maintains a lead over the brachial region in its level of differentiation. It would be of interest to ascertain if this situation also occurs in R. pipiens, contrary to the generally observed cephalo-caudal gradient in developmental events. Within the central nervous system of amphibians there are several instances of the more cephalic regions undergoing developmental changes earlier than the more caudal regions. The degenerative processes by which RohonBeard cells disappear in R. pipiens during the larval period proceed in a cephalocaudal direction (Suter, '66). The differentiation and thickening of the layers of the optic lobes in R. pipiens progress from cephalic to caudal poles (Kollros, '53). During the development of the mesencephalic V nucleus in R. pipiens, the cells alter their concentration from the cephalic portion of the optic tectum to the more caudal portion (Kollros and McMurray, '55). The cells of the cephalic half of the mesencephalic V nucleus are larger th...
Neuronal turnover has been demonstrated during the period of differentiation and maturationof motor neurons in the developing lateral motor columns of Rana pipiens larvae. As determined by thymidine-3H labeling, cells continued to migrate into the lateral motor column (LMC)while there was a concurrent reduction in LMC cell numbers. Notably, cells in the lumbosacral LMC ceased to be labeled following thymidine-3H injection at an earlier time than forthe brachial LMC. This is consistent with a caudo-cephalic direction of development amongthe two pairs of motor columns in the spinal cord. Since neuronal turnover occurs followingthe initial formation of the LMC and continues after the maximum number of cells has beenattained, it is proposed that neuronal turnover may be a mechansim that allows for neuralreadjustment and refinement during spinal cord development.
Limb bud mesenchyme enhances and directs the growth of tadpole spinal cord nerve fibers in tissue culture. This effect on elongating neurites may involve alterations in nerve-substratum interactions by the presence of undifferentiated target tissues. The relationship between nerve fibers and their potential innervation sites can explain directed nerve growth to the developing limb.
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