MORPHOLOGY OF DEVELOPING RAT GENIOGLOSSAL MOTONEURONS STUDIED IN-VITRO - CHANGES IN LENGTH, BRANCHING PATTERN, AND SPATIAL-DISTRIBUTION OF DENDRITES

The aim of this study is to describe the postnatal change in dendritic morphology of those motoneurons in the hypoglossal nucleus that inner vate the genioglossus muscle. Forty genioglossal (GG) motoneurons from four age groups (1-2, 5-6, 13-15, and 19-30 postnatal days) were labe led by intracellular injection of neurobiotin in an in vitro slice pre paration of the rat brainstem and were reconstructed in three-dimensio nal space. The number of primary dendrites per GG motoneuron was simil ar to 6 and remained unchanged with age. The development of these moto neurons 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 b ranches, in parallel with an elongation of their terminal dendritic br anches maintaining the same combined dendritic length. The elongation of terminal branches was attributed to both longitudinal growth and th e apparent lengthening caused by resorption of distal branches. The el imination 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 arborizatio n returning to a configuration similar to that found in the newborn. T he length of terminal branches was shorter at 19-30 days, while the le ngth of preterminal branches did not change, suggesting that the proli feration of branches at 19-30 days takes place in the intermediate par ts of terminal branches. The three-dimensional distribution of dendrit es was analyzed by dividing space into six equal volumes (hexants). Th is analysis revealed that GG motoneurons have major components of thei r dendritic tree oriented in the lateral, medial, and dorsal hexants. Further two-dimensional polar analysis (consisting of eight sectors) r evealed a reconfiguration of the tree from birth up to 5-6 days involv ing resorption of dendrites in the dorsal, dorsomedial, and medial sec tors and growth in the lateral sector. Later in development (between 1 3-15 days and 19-30 days), there was growth in all sectors, but of a g reater magnitude in the dorsomedial, medial, and dorsolateral sectors,