The present study was conducted to examine the plasticity of 5-hydroxytryptamine (5-HT)-immunoreactive terminals in the rat phrenic nucleus following an ipsilateral C2 spinal cord hemisection and 30-day survival period. A retrograde horseradish peroxidase (HRP) labeling technique was used to identify the phrenic motoneurons at the electron microscopic (EM) level. After employing a pre-embedding immunocytochemical technique, the ultrastructural characteristics of 5-HT-immunoreactive terminals were qualitatively and then quantitatively analyzed with a computerized morphometric system before and after injury in separate groups of rats. The results indicated that the majority of the 5-HT-labeled terminals formed axodendritic contacts, but some 5-HT-labeled terminals made axosomatic contacts. 5-HT terminals were associated with either asymmetrical or symmetrical synapses, and some displayed postsynaptic dense bodies. Approximately 2% of the 5-HT terminals had dense-core vesicles. Although the total number of labeled and unlabeled terminals in the phrenic nucleus was reduced after hemisection, the number of 5-HT terminals in the hemisected group was greater than that of the control group. Moreover, the total number and length of asymmetrical and symmetrical synaptic active zones per 5-HT terminal were significantly greater after injury. Finally, the total number of 5-HT terminals with multiple synapses was significantly greater in the hemisected group as compared to controls. It is possible that 5-HT synaptic plasticity may be part of the morphological substrate for the unmasking of the latent crossed phrenic pathway which mediates recovery of the ipsilateral hemidiaphragm paralyzed by C2 spinal cord hemisection.

The present study was designed to: 1) morpholog ically characterize cat glutamate and GABAergic synaptic term inals in lamina IX in the intact spinal cord at the electron microscopic leve l using postembedding immunochemical techniques and 2), begin an analysis of how the synaptic architecture of glutamate and GABAergic termi nals changes after an ipsilateral spinal cord hemisection. The present study shows that glutamate immunoreactive terminals are characte rized by a wide synaptic cleft, asymmetric synaptic membrane densities and spherical synaptic vesicles. Most of the glutamatergic terminals are presynaptic to small or medium size dendrites. In contrast, GABAergic terminals display typical pleomorphic ·synaptic vesicles , a narrow synaptic cleft and a symmetrical membrane density. Qualitative ana lys is indicated that 13-17 months after hemisection , the length of the s ynaptic active zones in both glutamatergic a nd GABAergic terminals ipsilateral to hemisection is longer tha n those observed in the terminals cont ra lateral to hemisection or in normal control cats. Furthermore, the perimeters of both dendrites and either glutamate or GABA immunoreactive terminals are longer on the hemisected side compared with those observed in the nonhemisected s ide of the spina l co rd . These results are important for complete understand ing of the mechanisms which unde rlie locomotor recovery in mammals following spinal cord injury. (J Spinal Cord Med 1997;20:311 -318)

Fish chromatophores serve as excellent study models for cytoskeleton-dependent organelle translocations because the distribution of pigmentary organelles can be observed against a time frame by microscopy. In this study the distribution of microfilaments along with microtubules in cultured melanophores of the killifish (Fundulus heteroclitus Linneaus) are examined using whole-cell transmission electron microscopy (WCTEM), fluorescence, and laser scanning confocal microscopy. Dispersing, dispersed, aggregating and aggregated states of pigment are induced by adding either caffeine (for dispersion) or epinephrine (for aggregation) to the cells in a standard culture medium. The cells that exhibited a random melanosome distribution in the standard culture media without these two reagents, served as the control. The results indicate that: (i) a structure considered to be the actin-filament organizing center (AFOC) is in close proximity to the microtubule-organizing center (MTOC); (ii) the radial layout of microfilaments remains similar over four physiological states of pigmentary response with the exception of epinephrine-aggregated pigment, in which the aggregate blocks the viewing of the AFOC and central microfilament rays, yet radial microfilaments, whether central and/or peripheral, are apparent in all physiological states of distribution; and (iii) microfilaments serve, together with microtubules, as scaffolding for melanosomes which migrate in bi-directional rows on cross-bridges, thus shedding light on the mechanisms for orderly melanosome translocations in a structural continuum.

The present study was conducted to examine the plasticity of 5-hydroxytryptamine (5-HT)-immunoreactive terminals in the rat phrenic nucleus following an ipsilateral C2 spinal cord hemisection and 30-day survival period. A retrograde horseradish peroxidase (HRP) labeling technique was used to identify the phrenic motoneurons at the electron microscopic (EM) level. After employing a pre-embedding immunocytochemical technique, the ultrastructural characteristics of 5-HT-immunoreactive terminals were qualitatively and then quantitatively analyzed with a computerized morphometric system before and after injury in separate groups of rats. The results indicated that the majority of the 5-HT-labeled terminals formed axodendritic contacts, but some 5-HT-labeled terminals made axosomatic contacts. 5-HT terminals were associated with either asymmetrical or symmetrical synapses, and some displayed postsynaptic dense bodies. Approximately 2% of the 5-HT terminals had dense-core vesicles. Although the total number of labeled and unlabeled terminals in the phrenic nucleus was reduced after hemisection, the number of 5-HT terminals in the hemisected group was greater than that of the control group. Moreover, the total number and length of asymmetrical and symmetrical synaptic active zones per 5-HT terminal were significantly greater after injury. Finally, the total number of 5-HT terminals with multiple synapses was significantly greater in the hemisected group as compared to controls. It is possible that 5-HT synaptic plasticity may be part of the morphological substrate for the unmasking of the latent crossed phrenic pathway which mediates recovery of the ipsilateral hemidiaphragm paralyzed by C2 spinal cord hemisection.