Study of the developing chick retina with the electron microscope revealed that dyad ribbon synapses begin to form in the inner plexiform layer before synaptic ribbons begin to appear in photoreceptor terminals of the outer plexiform layer. This centrifugal (inner to outer) sequence of synaptogenesis in the predominantly cone retina of the chick differs from the centripetal sequence that has been reported for the predominantly rod retinas of the mouse and rat. This difference does not favor the hypothesis, suggested by others, that the photoreceptor may influence the maturation of inner retinal elements. The different patterns of synaptogenesis are discussed briefly with reference to anatomical differences between the retinas of different species.
The possibility that the arrival of retinal ganglion cell axons in their efferent target field could influence the subsequent progress of cytomorphogenesis in the ganglion cells was tested by destroying all or portions of the primordial optic tectum at four or five days of incubation. The lesions were made essentially prior to the outgrowth of axons from the eye. Complete bilateral or unilateral destruction of the primordial tectal field did not detectably alter the morphogenesis of the inner retina or of the ganglion cells until 11 days of incubation. After this time, extensive loss of ganglion cells occurred within the retinas contralateral to the projection fields that were destroyed. The cytology of this degeneration is described as seen with the light and electron microscope. After smaller lesions involving the anterior or posterior half-tectum on one side, however, variable results occurred both in tectal development and in the retinal locus of ganglion cell degeneration observed in flat mount preparations. Many partial or punctate lesions, in fact, resulted in no detectable loss of cells from the ganglion cell layer. These results are discussed in regard to specification and possible regulatory capabilities of the tectum. Cell death in normal retinas, not previously reported in the chick, is also considered in relation to the experimental results.
The early nuclear and nucleolar responses at 0.5, 1, 2 and 4 days after axotomy were observed in neurons just before and after the completion of nuclear maturation. Axotomy of hamster facial motor neurons at a postnatal age of 15 days did not produce any changes within the nucleus that were significantly different from those of control cells. In addition, no significant changes were evident in the adults at 0.5 and 1 day after axotomy. At postoperative days 2 and 4, however, the adult neurons showed enlargement of the nucleus and nucleolus. Nucleolonemal strands became more rounded and distinct, and the large cluster of granules located centrally in the nucleolus disaggregated. The irregularly distributed clumps of nucleolus-associated chromatin dispersed to form a thin shell about the nucleolar periphery. In adults at postoperative day 4, the nucleoplasmic granules became more homogeneous and less distinctly outlined than normal. The peak of both nucleolar and nuclear responses coincided at 2 days after injury in the adult, i.e. 2 days before the previously documented chromatolytic peak at 4 days after injury. These studies on the ultrastructural level support our previous hypothesis that the 15-day neurons are synthesizing at peak capacity related to their rapid growth phase and cannot be stimulated further by axotomy. The adult neurons, however, do undergo a metabolic reorganization for regenerative synthesis, and the nucleolar and nuclear changes observed are indicative of transcriptive alterations involving the underlying genome.
In this study, the effects of axotomy on the ultrastructure of the nucleolus and associated organelles were examined in fetal, newborn, and early postnatal facial motoneurons of the hamster. Golden hamsters used for this study were the 14-day fetus, newborn (0 days; less than 6 hr) and 2, 4, 7, and 9 days postnatal ages, with 3 animals per group. For prenatal surgeries, pregnant hamsters were anesthetized and the facial nerves severed in the fetuses via electrocautery through the uterine wall and amniotic membrane. For postnatal surgeries, the animals were anesthetized and the right facial nerve exposed and severed at its exit from the stylomastoid foramen. At the appropriate postoperative times, the animals were reanesthetized and perfused-fixed. The facial nuclear groups were dissected and processed for routine electron microscopy. Microbody and coiled body frequencies were determined from the number of neurons containing these structures per number of neurons sampled per animal in each experimental or control group and subjected to statistical analysis. Nucleolar reactive changes that occurred during this developmental sequence fell into two major categories. The first category displayed by most injured cells consisted of an initial compacting of fibrillar material and reduction in vacuolar space. The second category appeared to represent a progression from this first stage of nucleolar reactivity into degenerative changes involving a striking segregation of nucleolar components into five distinct regions. The incidence of microbodies increased as a result of axotomy, whereas the presence of coiled bodies decreased at the later postoperative stages in the older animals. With increasing age and nucleolar maturation, the nucleolar reactive pattern became less pronounced and severe, and neuronal survival predominated. It appears, therefore, that the two categories of nucleolar changes following axotomy during early development correlate with changes observed in nucleoli under conditions of rRNA downregulation. It is hypothesized from these results that a key step in the ability of neurons to survive axotomy and successfully regenerate at these early developmental stages occurs at some point in ribosomal RNA transcription and/or processing. Complementary information at the molecular level concerning changes in nucleolar synthetic activity and ribosome production will be necessary to test this hypothesis.
One or two healthy structures frequently have been observed attached to nucleoli in facial motor neurons of the golden hamster. These round-to-oval structures, called "coiled bodies", were seen at 15, 19, and 24 days postnatal and in the adult, both in normal neurons and in chromatolytic neurons which had been axotomized 4 days previously. With one exception, the coiled bodies were seen to be attached via fibrillar material to the nucleolar periphery. Although the numbers of coiled bodies may be altered during neuronal maturation and as a result of axon section, the bodies revealed no structural alterations that could be attributed to developmental age or to experimental trauma.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.