G. Arcidiacono

Clusters of nerve cell bodies enclosed within a common connective tissue envelope in the spinal ganglia of the lizard and rat

SummaryA careful search for groups of nerve cell bodies enclosed within a common connective envelope was made in the spinal ganglia of the lizard and rat using a serial-section technique. Nerve cell bodies sharing a common connective envelope were found to be more common in the lizard (9.4%) than in the rat (5.6%). These nerve cell bodies were arranged in pairs, or, less frequently, in groups of three. At times, they appeared to be in immediate contact, with no intervening satellite cells; at other, they remained separated from one another by a satellite cell sheet. The clusters of nerve cell bodies enclosed within a common connective envelope probably result from the arrest of developmental processes in the spinal ganglion. It is possible that, as a result of the cell arrangement here described, certain neurons electrically influence other sensory neurons at the level of the ganglion.

A Quantitative Study of Microtubules in Motor and Sensory Axons

The number, density and distribution of microtubules were compared in the myelinated motor and sensory axons of the spinal roots of lizard (Lacerta muralis). In both motor and sensory axons the average number and density of microtubules were found to be related to the axonal size: the average number of microtubules rose, while the microtubular density decreased with an increase in the cross-sectional area of the axon. More precisely, a linear relationship was observed between the logarithm of the microtubular density and the cross-sectional area of the axon. No significant differences in the microtubular number and density were found between motor and sensory axons of corresponding size. Microtubules were unevenly distributed throughout the cross section of both motor and sensory axons. In particular, a nonaccidental association between microtubules and mitochondria was found in both axon types.

Stability at low temperatures of neuronal microtubules in spinal ganglia and dorsal roots of the lizard (Lacerta muralis).

Thoracic spinal ganglia and dorsal roots central to the latter from three adult lizards (Lacerta muralis) kept at 30°C for 90–95 days and from three other lizards kept at 3°C for the same length of time were examined under the electron microscope. The number, density, and distribution of microtubules were determined in cross sections of unmyelinated and myelinated axons of the dorsal roots. The relationships between the densities of microtubules and the cross-sectional areas of the axons were not found to be significantly different in the hot and cold lizards. Also the perikaryal microtubules, which however, were not quantitatively evaluated, showed no obvious differences under these conditions. The assumption that the microtubules studied in the present research are first disassembled in the cold and then reassembled before fixation has taken effect is probably incorrect because the specimens were fixed at the same temperature at which they had been previously kept, and also because neither C-shaped microtubules nor macrotubules were observed. The most likely explanation of the present results is that the microtubules here studied are cold stable.

Qualitative and Quantitative Observations on the Structure of the Schwann Cells in Myelinated Fibres

Various morphological features of the Schwann cells of myelinated fibres in the lizard thoracic spinal roots were studied, and, when possible, quantified using morphometric methods. About 0.8% of the Schwann cells are binucleate and some display clusters of microvilli along the internodes. The percentages of the cytoplasmic area of the Schwann cell occupied by the following cytoplasmic components were determined: mitochondria, Golgi apparatus, granular endoplasmic reticulum, smooth endoplasmic reticulum, multivesicular bodies, dense bodies, autophagic vacuoles, peroxisome-like bodies, lipofuscin granules and lipid droplets. Linear relationships were found between the sectional areas of the mitochondria and granular endoplasmic reticulum of the Schwann cell and both the length of the profile of the Schwann cell plasma membrane and the size of the related axon. The results obtained are compatible both with the hypothesis that the mitochondria and granular endoplasmic reticulum of the Schwann cell are involved in the production and storage of proteins for the plasma membrane of this cell, and with the hypothesis that these organelles are involved in the production and storage of protein metabolites which are subsequently transferred to the related axons.

Association between microtubules and mitochondria in myelinated axons of Lacerta muralis

SummaryThe spatial relationship between microtubules and mitochondria was studied in myelinated axons of the ventral and dorsal spinal roots of the lizard Lacerta muralis by use of quantitative methods in single and serial sections.Microtubules mainly occurred in groups of 3 to 10. The mean density of microtubules was found to be significantly higher close to mitochondria than in the rest of the axoplasm. In single sections, 59–62% (according to the root region examined) of the microtubule groups were found to be ‘associated’ with mitochondria; this percentage rose to 74–76% in serial sections. The examination in serial sections of progressively longer segments of the same microtubule groups showed that the longer the segments of microtubule groups examined the higher was the percentage of microtubule groups ‘associated’ with mitochondria.The results obtained show that in the axons studied in the present research a non-accidental spatial association exists between microtubule groups and mitochondria. This evidence supports the suggestion that the microtubule groups play a role in the movement of mitochondria along the axon, even though it does not clarify the precise nature of this role.

Microtubules in unmyelinated and myelinated axons: a quantitative analysis

The average density of microtubules was compared in unmyelinated and myelinated axons from the following regions of the peripheral nervous system of lizard (Lacerta muralis): ventral spinal root, the segment of the dorsal root just peripheral to the spinal ganglion and the segment of the dorsal root just central to the spinal ganglion. In both unmyelinated and myelinated axons, the microtubular density was found to decrease with an increase in the cross-sectional area of the axon, while no significant difference in the microtubular density was observed between unmyelinated and myelinated axons of corresponding size. The results obtained in the present research suggest that in the axons of the peripheral nervous system of lizards the microtubular density is related to axonal size rather than to the absence or presence of a myelin sheath.

An electron microscope study of quantitative relationships between axon and Schwann cell sheath in myelinated fibres of peripheral nerves

SummaryThe quantitative relationships between the crossectional area of the Schwann cell sheath (myelin included) and that of its related axon were studied by electron microscopy in the nerve fibres of the spinal roots of lizard (Lacerta muralis). In both ventral and dorsal roots the cross-sectional area of the Schwann cell sheath (myelin included) was found to be directly proportional to that of its related axon (correlation coefficients between 0.88 and 0.92). The ratio between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon tends to diminish as the cross-sectional area of the latter increases. Thus, under normal conditions, in myelinated fibres of the spinal roots of the lizard a quantitative balance exists between the nerve tissue and its associated glial tissue. This result agrees with those previously obtained in the spinal ganglia of the lizard, gecko, cat and rabbit. Some of the mechanisms probably involved in the control of the quantitative balance between nerve tissue and its associated glial tissue in peripheral nerves are presented and discussed.

Internodal microvilli of Schwann cells of myelinated fibres in lizard spinal roots project onto unmyelinated axons

SummaryTufts of microvilli originating from the internodal cytoplasm of Schwann cells associated with myelinated axons in apparently normal lizard spinal roots have been studied under the electron microscope by means of both single and serial sections. More than one tuft of internodal microvilli may arise from a single Schwann cell. Sometimes mitochondria and more frequently an organelle resembling a multivesicular body with a clear matrix can be found in the Schwann cell cytoplasm underlying a tuft of internodal microvilli. The dimensions (length: 0.4–1.0 μm; diameter: 40–70 nm) and structure of internodal microvilli of the Schwann cell are very similar to those of nodal microvilli of the same cell. Each tuft of internodal microvilli projects towards an adjacent unmyelinated axon which at this site is partly devoid of its own Schwann cell sheath. Thus a single Schwann cell may be related to a myelinated axon and an unmyelinated axon at the same time. Patches of a dense axolemmal undercoating (which could be portions of the cytoskeleton) are present in the unmyelinated axon in close spatial correlation with internodal microvilli. The factors which could induce the formation of internodal microvilli as well as the possible role (or roles) of these microvilli are briefly discussed.

Modificazioni della struttura in neuroni dei gangli spinali di lucertole esposte al freddo

We studied structural changes in spinal ganglion neurons that occur in lizards exposed to the cold, at the light and electron microscope levels. In the cold-exposed animals, about a third of the neurons showed marked structural changes. Two types of perikaryal changes were found. (a) In 25% of all neurons, the central region of the perikaryon took up no stain or was weakly basophilic, whereas a narrow peripheral zone was strongly basophilic. Electron microscopic examination of these cells showed that mitochondria, Golgi complexes and other organelles were assembled in the central region of the perikaryon, while most cisternae of granular endoplasmic reticulum and free polysomes were confined to the periphery. These changes seem to take place mainly in dark neurons. (b) In 8.6% of all neurons, Nissl bodies were present throughout the perikaryon, but separated by large, clear spaces. Under the electron microscope, these clear spaces were filled with large numbers of densely packed filaments. Mainly light neurons seem to undergo this type of structural change. The degree of nuclear eccentricity was significantly greater in the neurons of cold-exposed animals than in controls. The nucleolar volume was significantly increased and both the percentages of nuclei with two nucleoli and of nuclei with «vacuolated» nucleoli were significantly greater in neurons displaying structural changes in their perikarya than in the other neurons. All the structural modifications observed closely resemble those seen in the same lizard neurons following axonal section. They could be due to (1) metabolic changes induced by low temperature and fasting, (2) alterations in the flow of nerve impulses from the periphery, or (3) impaired retrograde transport of trophic substances from the periphery to the cell body.RiassuntoLe modificazioni di struttura dei neuroni dei gangli spinali di lucertole esposte al freddo sono state studiate al microscopio ottico e al microscopio elettronico. Dopo esposizione al freddo, circa un terzo dei neuroni presentava modificazioni di struttura. Sono stati individuati due tipi di modificazioni. (a) Nel 25% di tutti i neuroni, la regione centrale del pericarion non si colorava con i coloranti basici, mentre una sottile banda periferica appariva intensamente basofila; la zona centrale era occupata prevalentemente da mitocondri, complessi di Golgi e altri organelli, mentre quasi tutte le cisterne del reticolo endoplasmatico granulare e quasi tutti i polisomi liberi erano confinati alla periferia. Queste modificazioni di struttura si verificano verosimilmente nei neuroni scuri. (b) Nell’8,6% di tutti i neuroni, i corpi di Nissl erano separati da ampi spazi chiari, occupati da accumuli di neurofilamenti. Queste modificazioni di struttura si verificano verosimilmente nei neuroni chiari. Il grado di eccentricità del nucleo era significativamente maggiore negli animali esposti al freddo che in quelli di controllo. Il volume medio del nucleolo, le percentuali dei nuclei con due nucleoli e dei nuclei con nucleoli provvisti di «vacuoli» erano significativamente maggiori nei neuroni con struttura modificata che negli altri neuroni. Le modificazioni di struttura osservate sono molto simili a quelle che si verificano negli stessi neuroni dopo taglio dell’assone e potrebbero dipendere (1) da modificazioni del metabolismo provocate dal freddo e dal digiuno, (2) da alterazioni nel flusso di segnali nervosi provenienti dalla periferia, oppure (3) dalla riduzione o dal blocco del trasporto retrogrado di sostanze trofiche dalla periferia al corpo del neurone.