Ultrastructural changes in the nucleolus of facial motor neurons following axotomy during an early critical period in development

Abstract

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.

Presumptive Renshaw cells contain decreased calbindin during recovery from sciatic nerve lesions.

“…Peripheral nerve lesions are known to induce several morphological (16), biochemical (20)(21)(22)(23), and physiological (17)(18)(19) changes in a-motoneurons. The alterations in a-motoneuron physiological properties seen after sciatic nerve lesions are known to be reversible upon target reinnervation (33)(34)(35) and have been ascribed primarily to the loss of functional contact with the muscular targets, since they can be reproduced by the intramuscular administration of botulinum toxin (36) and are not prevented by chronic electric stimulation (37).…”

“…After peripheral nerve lesions, motoneurons undergo complex morphological (16), physiological (17)(18)(19), and biochemical (20)(21)(22)(23)) changes. We have tested whether the presumptive Renshaw neurons might respond to peripheral nerve injuries by modulating their calbindin-expression as a result of the reduced motoneuronal activity and the consequent reduced need for Ca2+-buffering power.…”

Lateral magnocellular nucleus of the anterior neostriatum (LMAN) in the zebra finch: Neuronal connectivity and the emergence of sex differences in cell morphology

“…It is most likely that sex differences in nucleolar size are also present in other sexually dimorphic system, but this special feature has not been investigated, so far. During development, changes in nucleolar size have been observed in facial motor neurons (Clark et al, 1991). The nucleolus of these neurons matures progressively from a small, densely compact structure to a large one having a distinctly reticulated organization early in development.…”

Ribosomal RNA transcriptional activation and processing in hamster facial motoneurons: Effects of axotomy with or without exposure to testosterone

“…Evidence obtained from numerous systems has demonstrated that the nucleolus, site of polymerase I-directed ribosomal RNA transcription and processing, is one of the earliest cellular organelles to be affected by axon disconnection from the periphery (Watson, 1968;Lieberman, 1971;Jones and LaVelle, 1986). Ultrastructural analysis of axotomized FMN nucleoli in developing and adult hamsters suggests that a key step in the ability of these neurons to survive injury and successfully regenerate occurs at some point in rRNA transcription, processing, or both (Kinderman and LaVelle, 1976;Jones and LaVelle, 1986;Clark et al, 1991). Steroid hormones have also been shown to regulate ribosomal gene expression and nucleolar ultrastructure (Cohen and Pfaff, 1981;Whelly, 1985;Jones et al, 1990).…”

“…It is unknown, however, which aspects of ribosome biogenesis are altered by axotomy and what effects steroid hormone treatment coincident with injury might have on the nucleolar reaction. The morphologic evidence presented in the hamster FMN system suggests that axotomy induces changes in rRNA transcription, rather than stability (Jones and LaVelle, 1986;Clark et al, 1991). In this study, we used molecular and stereologic approaches to determine the effects of both axotomy and steroid treatment on rRNA transcription and processing in axotomized hamster FMN.…”