Early development of the mesencephalic trigeminal nucleus

The cells of the mesencephalic trigeminal nucleus (MTN) are the propriocept ive sensory neurons that innervate the jaw muscles. Interestingly, their ev olution is generally thought to have been concomitant with that of the jaws . They are also the first born neurons of the mesencephalon, and their axon s pioneer some of the major tracts within the brain. The cells of the MTN a re also paradoxical in being the only group of intramedullary primary senso ry neurons in amniotes. However, we know little about the early development of these important neurons, and we have analysed this here. To study the e arliest stages of MTN development, we have used a battery of neural crest m arkers to try and pinpoint the progenitors of the MTN. We find that, contra ry to current perceptions, the progenitors of the MTN are not highlighted b y these markers, suggesting that they are not neural crest derived. However , the cells of the MTN are marked by means of their expression of Brn-3a. T his gene labels cells that arise either side of the dorsal midline, extendi ng rostrally from the isthmus across the roof of the mesencephalon. We have further demonstrated that the MTN develops under the influence of the Fgf- 8 secreted by the isthmus. Ectopic Fgf-8 application promotes MTN developme nt, whereas inhibiting Fgf-8 function in vivo drastically affects MTN devel opment. (C) 2001 Wiley-Liss, Inc.