Determination of the embryonic origin of the mesencephalic nucleus of the trigeminal nerve in birds

The precise site of origin and the cells of origin of the mesencephalic nucleus of the trigeminal nerve in birds have been studied by the method of growing embryonic fore-, mid- and hind brain grafts inside a millipore filter chamber filled with amniotic fluid, upon a host chorioallantoic membrane, and by the method of transplantation of cranial neural crest of the mesencephalon between quail embryo-donors and duck embryo-hosts. The growth and differentiation of grafts grown in the chambers resemble very closesly those of normal embryos at corresponding stages of development. Data obtained from a total of 42 grafts representative of the three species of birds used in this study, show that the cells of the mesencephalic nucleus are present only in grafts of embryonic midbrain and totally absent in forebrain and hindbrain grafts. The presence of cells of the mesencephalic nucleus in midbrain grafts suggests that these neurons arise chiefly from the developing mesencephalon. Histological examination of such grafts has shown that these neurons are observed in the leptomeningeal tissue overlying the midbrain region and extend into the various laminae of the optic tecta. A progressive increase in the size of the cells from an immature state confined to the mesodermal tissue to fully mature neurons in the stratum griseum periventriculare of the Optic tectum is observed. This is also indicative of a downward migration of these cells. In a few embryonic midbrain grafts mature cells clearly identifiable as belonging to the mesencephalic nucleus are observed in the leptomeningeal tissue and outside of the external limiting membrane of the neural tube. These appear to be neurons which had failed to migrate. The resemblance of these latter neurons to those in the stratum griseum periventriculare of the optic tectum lend further support to the hypothesis that the neurons of the mesencephalic nucleus are derived from neural crest of the developing mesencephalon in birds. In interspecific transplantation experiments of cranial neural crest of the mesencephalon, the quail cells occur in the form of clusters and appear to migrate towards the ventricular surface independent of the migration of the cells of the tectum which takes place simultaneously. Three stages in the development of the grafted neural crest material of the quail are observed based on the perikaryal diameter of the cells, and the accumulation of Nissl material in the cytoplasm. The smallest cells are located more superficially near the pial surface, and the larger mature neurons are observed in the stratum griseum periventriculare of the optic tectum. All the layers of the optic tectum including the neuroepithelium on the experimental side are comprised entirely of duck cells. The data presented in this study provide direct evidence that the precursor neurons of the mesencephalic nucleus of the trigeminal nerve are derivatives of the cranial neural crest of the mesencephalon, and that the layers of the optic tectum are formed through the proliferative activity in the neuroepithelium.