The formation of axonal projections of the mesencephalic trigeminal neurones in chick embryos.

Horseradish peroxidase-wheat germ agglutinin conjugate (HRP) was injected into masticatory and eye muscles of 6- to 15-day-old chick embryos and posthatched chicks to establish the timetable of axonal outgrowth and distribution of central and peripheral terminations of mesencephalic trigeminal neurones (MTN). HRP-labelled MTN first appeared on the 10th day of incubation and by the 14th day most of MTN became labelled, indicating that axonic outgrowth to peripheral targets occurred between the 10th and 14th days of incubation. Peripheral targets included the pterygoideus lateralis and medialis, protractor quadratus, pseudotemporalis superficialis and profundus, and adductor mandibulae of jaw-closing muscles. HRP-filled central axonic processes of MTN were identified first on the 13th day of incubation and they terminated exclusively in the motor nucleus of the trigeminal nerve. Between the 10th and 13th days of incubation, at the peak of naturally occurring cell death in the MTN pool, a consistently lower percentage of neurones could be labelled with HRP than in older embryos and in posthatched chicks. This finding suggests that many MTN die before establishing contact with peripheral targets.