DEVELOPMENT OF TRIGEMINAL MESENCEPHALIC AND MOTOR NUCLEI IN RELATION TO MASSETER MUSCLE INNERVATION IN MICE

This study evaluated temporospatial changes in the central organizatio n of trigeminal mesencephalic (mesV) and motor (motV) nuclei during th eir development. Very little is known regarding the timing of formatio n of these trigeminal nuclei and the role that target tissue interacti ons may have on their spatial organization. Cells located in motV inne rvate muscles of mastication while the mesV nucleus contains populatio ns of primary afferent cells that innervate muscle spindles in jaw clo sing muscles and periodontal mechanoreceptors around the roots of teet h. To label mesV afferents and motV efferents during their development , lipophilic dyes, -dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) or 4-(4-dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA), were inserted into various jaw muscles, the dorsomedial mesencephalic region or tooth buds of maxillary or mandibular teeth of embryonic or postnatal mouse pups. Parasagittal sections were evaluat ed under epifluorescence to determine the temporospatial organization of trigeminal nuclei and the timing of outgrowth of their processes to target tissues. Early in development, motV motoneurons were organized in columns or clusters closely associated with groups of motoneuron a xon fascicles that innervated a specific muscle. Double labelling of m asseter and temporalis muscles showed that the columns containing moto neurons associated with these muscles were interdigitated early in dev elopment but later condensed into respective motoneuron pools. In cont rast, no spatial organization of mesV afferent cell bodies was observe d throughout the developmental sequence examined. The results of this study also demonstrated that motV processes enter into jaw muscle at l east 1 day prior to proprioceptive afferents. MesV afferent appearance at the tooth was further delayed by 10 days suggesting different sign aling mechanisms for these two targets. (C) 1998 Elsevier Science B.V. All rights reserved.