Discussion

Abstract

One of the most notable peculiarities of the trigeminal sensory system is the presence of two populations of (pseudo)unipolar primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TrG) (Ramón y Cajal 1909; Brodal 1981) but part of them lie in the MTN. Recently Nagtegaal et al. (1996) observed prenatally in the rat an unknown group of neurons located immediately lateral to and partly intermingled with the MTN which they have temporarily named “Gr?”. In shape, Gr? closely resemble the MTN. As we have already stated earlier in the introduction, future studies will be needed to determine the embryonic development of the MTN in an accurate chronology and the principle sites of origin of its neuronal population in different animal species, including humans. However, it is now well established that in the cat all the MTN neurons have a common origin and arise exclusively from neural crest cells (for review see Davies 1988; Davies and Lumsden 1990).