EXPRESSION OF THE HOMEOBOX-CONTAINING GENE EN-2 DURING THE DEVELOPMENT OF THE CHICK CENTRAL-NERVOUS-SYSTEM

The expression of the homeobox-containing gene En-2 was analysed with the monoclonal antibody 4D9 in the chick central nervous system throug hout embryogenesis. Confirming previous studies, early expression of t he En-2 protein [beginning at stage 9 of Hamburger and Hamilton (HH9)] is restricted to a portion of the neural tube containing the primordi a of the cerebellum, the isthmic region and the mesencephalic grisea, and forms a double gradient decreasing both caudally and rostrally fro m a high point located around the midbrain-hindbrain constriction. Thi s mes-isthmo-cerebellar region contains all the En-2-positive germinat ive cells and the great majority of the En-2-positive postmitotic neur ons throughout embryogenesis. Nevertheless, as the postmitotic neurons appear, En-2 expression also occurs outside this region: in two colum ns of non-motoneuron cells in rhombomeres two to four (between HH20 an d HH30) and, from HH24 onwards, throughout the grey matter of the lumb ar and thoracic spinal cord, with the exception of the ventral motoneu ron columns. Here, a detailed description of En-2 expression is provid ed for the mes-isthmo-cerebellar region at stages HH30-32 [embryonic d ay (E) 7], HH37 (E11) and HH46 (E21, hatching). This allows the visual ization of cellular groups with heterogeneous patterns of En-2 express ion, which are specific for each group in the intensity of En-2 expres sion, the distribution of the labelled cells and the temporal regulati on of the gene. The use of tyrosine hydroxylase antiserum shows coexpr ession of the tyrosine hydroxylase enzyme and En-2 protein in the caud al part of the nuclei tegmenti pedunculo-pontinus, the area ventralis of Tsai and the substantia grisea centralis, but not in the locus coer uleus. In the cerebellum, the first expression, which is located in th e deep nuclei and parasagittal bands of Purkinje cells, is down-regula ted when the molecular layer interneurons and the granular cells begin to express the gene, at the end of embryogenesis. Finally, at hatchin g, En-2 expression permits the visualization in the cerebellum of a po pulation of small En-2-negative cells located around the Purkinje cell s that may correspond to those described in chick/quail chimaeras as h aving an origin different from that of the bulk of granular neurons.