FURTHER OBSERVATIONS ON THE SUSCEPTIBILITY OF DIENCEPHALIC PROSOMERESTO EN-2 INDUCTION AND ON THE RESULTING HISTOGENETIC CAPABILITIES

It has been previously shown by chick/quail heterotopic grafts that En -2 expression and a mesencephalic phenotype can be induced within the avian primordial prosencephalic vesicle, although the induction appear ed restricted to the caudal forebrain. The present experiments were ai med at further analyzing the competence of the prosencephalic neuroepi thelium. Different types of grafts were performed between chick and qu ail embryos: (i) caudal forebrain grafts positioned in the midbrain/hi ndbrain junction (the En-2-positive domain); (ii) En-2-positive grafts integrated at different levels of the forebrain. In both cases, the g rafts were transplanted either with a normal orientation or after inve rsion of their rostro-caudal axis. The chimeric embryos were analyzed at stages HH19-24 for expression of En-2 and Pax-6 homeobox-containing genes, normally expressed in the meso-isthmo-cerebellar and prosencep halic domains, respectively. A cytoarchitectonic analysis of grafted a nd surrounding host tissue was also performed at later developmental s tages in chimeric embryos with caudal forebrain grafts. Our results sh ow that the caudal diencephalon, including the prospective territories for prosomeres 1 and 2, is competent to express En-2 when in close co ntact to the En-2 polarizing region, whereas the more rostral neuroepi thelium, including the prospective territories for the third prosomere and telencephalon, does not change its fate under similar conditions. The ectopic-induced neuroepithelium can develop mesencephalon, but al so isthmus and cerebellum according to its site of integration rostral ly or caudally to the mesencephalic/isthmo-cerebellar boundary. Our da ta also show that within the competent diencephalon, the induced En-2 expression can be arrested at the P1/P2 interneuromeric boundary. This arrest appears to be directionally oriented as it only takes place wh en the induction is produced within prosomere 1 but not when it comes from prosomere 2. These data can be considered as resulting from eithe r a possible oriented permissiveness of cells which form the boundary separating prosomeres 1 and 2, or of a different permissiveness of the cells composing these two caudal prosomeres.