Intrasomatic changes in the maturing hypoglossal nucleus after axon injury

Motoneuron survival is not affected by the proximo-distal level of axotomy but by the possibility of regenerating axons to gain access to the distal nerve stump

The aim of this study was to examine whether axotomy-induced motoneuron death in adult mammals differ: (1) with the distance between the site of injury and the nerve cell body, and (2) if contact between the transected nerve stumps is established after the injury, compared with cases where contact is prevented. The hypoglossal nerve of adult rats was transected either proximally in the neck (proximal injury) or close to the tongue (distal injury). The nerve stumps were then either deflected from each other in order to prevent axon regeneration into the distal nerve stump, or sutured. Three months later, the extent of nerve cell loss was examined bilaterally in cresyl violet-stained sections of the hypoglossal nucleus. In addition, we examined hypoglossal neuron survival twelve months after a proximal nerve transection with prevented regeneration. Our results show that there was no significant difference in neuronal survival after a proximal nerve transection compared with a distal one, neither if contact between the nerve stumps was established nor if it was prevented. However, contact between the transected nerve stumps increased the likelihood of neuronal survival significantly after both proximally and distally located injury compared to nerve injury with prevented regeneration. There was no significant decrease in nerve cell survival after twelve months with prevented reinnervation compared with survival after three months. These observations indicate that the extent of axotomy-induced motoneuron death in adult mammals does correlate with the proximo-distal level of peripheral injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in nuclear envelope invaginations in axotomized fetal and early postnatal hamster facial motoneurons

In this study, changes in the amount of nuclear envelope invaginations (NEI) were morphometrically assessed after axotomy during late fetal and early postnatal developmental stages in hamster facial motoneurons. These changes were expressed as boundary density or BA (length of nuclear envelope per unit area of nucleus). Axotomy-induced changes in nuclear area and perimeter were also quantitatively determined. At 17 h after axotomy in the fetal operative series, no changes in any of the parameters were seen. At 1 day postoperative (dpo) in newborn, 2 and 4 postnatal day animals, the boundary densities of the total and invaginated portion of the nuclear envelope increased significantly. No corresponding qualitative changes were observed. At 2 dpo in 4 and 7 postnatal day animals, there were significant increases in the boundary densities of both invaginated and total nuclear envelope and a decrease in nuclear area. These changes were not seen at 2 dpo in the 9-day operative series. At 4 dpo in 7 and 9 postnatal day animals, scalloping of the normally smooth nuclear profile, as well as a flattening and elongation in nuclear shape, occurred. These qualitative changes in the 7 and 9 day operated groups were also accompanied by significant changes in all the measured parameters. The boundary density of the invaginated, non-invaginated and total nuclear envelope increased; whereas, nuclear area and perimeter decreased. These results argue against the generally held hypothesis that an increase in nuclear envelope invaginations is indicative of an allied increase in cellular metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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.

Organelle changes in cat thoracic alpha- and gamma-motoneurons following axotomy

Horseradish peroxidase was used as a retrograde marker to identify the cell bodies of cat thoracic alpha- and gamma-motoneurons 8 days following axotomy. A quantitative ultrastructural comparison revealed changes in Nissl bodies, the Golgi apparatus, mitochondria and lysosomes for axotomised alpha-motoneurons, while axotomised gamma-motoneurons only showed changes in Nissl bodies and nuclear pores.

Ultrastructure of axotomized alpha and gamma motoneurons in the cat thoracic spinal cord

Using horseradish peroxidase as a retrograde marker, the ultrastructural response of alpha and gamma motoneuronal cell bodies in the cat thoracic spinal cord has been compared 1-8 days following intercostal nerve transection and ligation. By light microscopy, reduction of Nissl body size, together with nuclear and nucleolar alterations were seen in alpha motoneurons 4-8 days following axotomy, but not at any stage in axotomized gamma motoneurons. In the electron microscope, disorganization of Nissl body ultrastructure was seen in both alpha and gamma motoneurons 2 days following axotomy. Only in alpha motoneurons, however, did these disorganized Nissl bodies subsequently fragment into smaller pieces. Both alpha and gamma motoneurons lost synapses following axotomy, but the proportional loss from gamma motoneurons was two-fold greater than that from alpha motoneurons. Loss of synaptic terminals with flattened synaptic vesicles was two-fold higher than that of synaptic terminals with round synaptic vesicles from axotomized gamma motoneurons, whereas axotomized alpha motoneurons lost both types of synaptic terminal equally.

A morphometric analysis of the somata and organelles of regenerating hypoglossal motoneurons from the rat

A detailed morphometric evaluation of the somata and organelles of regenerating hypoglossal motoneurons from the rat was conducted. The volume of the hypoglossal nucleus and various parameters used to appraise neuronal size were estimated from 50 microns sections. The subcellular composition of randomly selected neurons was quantified from 1 micron and ultrathin sections. The volume of neuronal nuclei, nucleoli, mitochondria and lysosomes as well as the surface area of intracellular membranes were determined. Seven to 30 days following axotomy the volume of the hypoglossal nucleus was significantly diminished, undoubtedly reflecting dendritic retraction (P less than 0.05). Concomitantly, all estimates of neuronal size indicated significant neuronal enlargement (P less than 0.05). Ultrastructural alterations were most prominent 7 days following nerve transection: nucleolar volume was significantly increased, rough endoplasmic reticulum surface area was reduced, and non-Golgi smooth membrane surface area increased (P less than 0.05). In general, other organelles resisted the influence of axotomy and all ultrastructural parameters returned to control levels 21 to 30 days following the nerve transection. Functional recovery was detected in all animals 21 and 30 days following axotomy. The measured responses of axotomized hypoglossal motoneurons are similar to those reported for retinal ganglion cells of the goldfish (Whitnall & Grafstein, 1982, 1983), suggesting common metabolic events among these distinct neuronal populations following axonal transection.

Differential effects of axotomy on immature and mature hamster facial neurons: a time course study of initial nucleolar and nuclear changes

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.

Changes in nuclear envelope invaginations in axotomized immature and mature hamster facial motoneurons

In this study, changes in the amount of nuclear envelope invaginations (NEI) were quantitatively assessed after axotomy during the late nuclear maturation stages (15, 20 and 25 days postnatal age) and in the adult (100-day-old) hamster facial motoneurons. These changes were expressed as boundary density or BA (length of nuclear envelope per unit area of nucleus). Absolute nuclear areas and perimeters were also estimated after axotomy at these ages. At 1/2 and 1 day after axotomy, no differences in the above parameters were noted at any of the operative ages. At 4 days postoperative, the peak of chromatolysis for all these ages, axotomy resulted in significant decreases in BA and nuclear perimeter in the immature neurons and no changes in BA and nuclear perimeter in the adult neurons. In addition, 4 days after axotomy at 20 days postnatal and later ages, pronounced increases in nuclear area occurred. These quantitative results are interpreted as evidence that the accelerated loss of NEI after axotomy during the final stages of nuclear maturation in these neurons is related to the formation of rough endoplasmic reticulum (RER) or Nissl substance. The hypothesis that an 'excess' of RER is accumulated during the late maturation stages and may account for the lack of NEI in the adult axotomized facial motoneuron is presented.

Glycogen, its transient occurrence in neurons of the rat CNS during normal postnatal development

Progressive changes in the postnatal incidence, distribution and duration of glycogen in neurons of the pons, medulla and spinal cord were studied by light and electron microscopy using cytochemical and quantitative methods. Albino rats of 11 ages ranging from newborn to adult were used for this investigation. Methacrylate sections, stained with periodic acid-Schiff-dimedone (PAS) were surveyed to identify nerve cell groups containing the polysaccharide, glycogen. The PAS reaction was positive in neuronal cell groups of the hypoglossal nucleus, the mesencephalic nucleus of V, nucleus ambiguus, the abducens nucleus, the facial motor nucleus and anterior horn cells of the spinal cord. The intensity and duration of the PAS reaction appeared greatest in the hypoglossal nucleus. Neurons of the mesencephalic nucleus of V demonstrated a reaction of moderate intensity and duration. The remaining nerve cell groups exhibited a weak, diffuse reaction of brief duration. Postnatal differences in the incidence and patterns of disposition of glycogen were quantified using ultrathin sections of the hypoglossal nucleus, the site richest in glycogen. The presence of glycogen was verified by the periodic acid-thiosemicarbizide-silver proteinate (PA-TSC-SP) ultracytochemical stain. The incidence of glycogen in neuronal perikarya of hypoglossal nuclei was related to age. All neurons contained some glycogen during the first postnatal week. By 24 days postnatal (dpn), the majority of hypoglossal neurons lacked glycogen and all neurons of adult rats were glycogen-free.(ABSTRACT TRUNCATED AT 250 WORDS)