Morphology of developing rat genioglossal motoneurons studied in vitro: Changes in length, branching pattern, and spatial distribution of dendrites

Abstract

The aim of this study is to describe the postnatal change in dendritic morphology of those motoneurons in the hypoglossal nucleus that innervate the genioglossus muscle. Forty genioglossal (GG) motoneurons from four age groups (1-2, 5-6, 13-15, and 19-30 postnatal days) were labeled by intracellular injection of neurobiotin in an in vitro slice preparation of the rat brainstem and were reconstructed in three-dimensional space. The number of primary dendrites per GG motoneuron was approximately 6 and remained unchanged with age. The development of these motoneurons from birth to 13-15 days was characterized by a simplification of the dendritic tree involving a decrease in the number of terminal endings and dendritic branches. Motoneurons lost their 6th-8th order branches, in parallel with an elongation of their terminal dendritic branches maintaining the same combined dendritic length. The elongation of terminal branches was attributed to both longitudinal growth and the apparent lengthening caused by resorption of distal branches. The elimination of dendritic branches tended to increase the symmetry of the tree, as revealed by topological analysis. Later, between 13-15 days and 19-30 days, there was a reelaboration of the dendritic arborization returning to a configuration similar to that found in the newborn. The length of terminal branches was shorter at 19-30 days, while the length of preterminal branches did not change, suggesting that the proliferation of branches at 19-30 days takes place in the intermediate parts of terminal branches. The three-dimensional distribution of dendrites was analyzed by dividing space into six equal volumes (hexants). This analysis revealed that GG motoneurons have major components of their dendritic tree oriented in the lateral, medial, and dorsal hexants. Further two-dimensional polar analysis (consisting of eight sectors) revealed a reconfiguration of the tree from birth up to 5-6 days involving resorption of dendrites in the dorsal, dorsomedial, and medial sectors and growth in the lateral sector. Later in development (between 13-15 days and 19-30 days), there was growth in all sectors, but of a greater magnitude in the dorsomedial, medial, and dorsolateral sectors.

Developmental changes in spinal motoneuron dendrites in neonatal mice

“…For example, in cat phrenic MNs Cameron et al (1991a) reported a considerable decrease of the number of bifurcations per tree that paralleled dendritic elongation between the second and fourth week of age, such that the combined surface area during this period remained nearly constant. A similar phenomenon was found in rat hypoglossal (genioglossus) MNs between birth and the end of the second week of age (Nuñ ez-Abades et al, 1994). Again, the observed reduction in the number of dendritic terminals and maximum branching order per tree occurred while branch lengths increased, resulting in a constant total combined length throughout the whole 2-week period.…”

GAD67-GFP+ Neurons in the Nucleus of Roller: A Possible Source of Inhibitory Input to Hypoglossal Motoneurons. I. Morphology and Firing Properties

“…However, if at least some of these boutons are synaptic connections between GFPϩ cells and HMs, then this implies that these synapses are located on HM dendrites that extend beyond the boundaries of the XII nucleus. Dendrites that extend well beyond the border of the XII nucleus and into the lateral reticular formation have been described in the rodent brain stem (Altschuler et al 1994;Nunez-Abades et al 1994; TarrasWahlberg and Rekling 2009) and we confirm this observation. These distal dendrites are thus a possible location for synaptic contacts between the axon terminals of GFPϩ cells in the NR and HMs.…”

“…2A). The morphology of the filled HMs in this study was similar to what has been described in previous studies for this type of cell (Nunez-Abades et al 1994). These cells have extensive dendritic trees that branch repeatedly within the nucleus and often extend well beyond the borders of the XII nucleus into the reticular areas adjacent to the nucleus ( Fig.…”

Synaptic Targeting of Retrogradely Transported Trophic Factors in Motoneurons: Comparison of Glial Cell Line-Derived Neurotrophic Factor, Brain-Derived Neurotrophic Factor, and Cardiotrophin-1 with Tetanus Toxin

“…The labeling densities of organelles within hypoglossal motoneuron cell bodies and proximal dendrites were quantified by applying the circle probability method with a half distance of 90 nm (Salpeter et al, 1978) to printed photomicrographs as detailed below. Hypoglossal motoneurons were distin-guished from other cell types (e.g., glia and interneurons) in electron micrographs using established criteria (Takasu and Hashimoto, 1988;Peters et al, 1991;Nunez-Abades et al, 1994). Hypoglossal motoneuron proximal dendrites were distinguished from distal dendrites by the presence of ribosomes in the former and their absence in the latter.…”