Cytoarchitectonic subdivisions in the subtectal midbrain of the lizard Gallotia galloti

Contemporary study of molecular patterning in the vertebrate midbrain is ha ndicapped by the lack of a complete topological map of the diverse neuronal complexes differentiated in this domain. The relatively less deformed rept ilian midbrain was chosen for resolving this fundamental issue in a way tha t can be extrapolated to other tetrapods. The organization of midbrain cent ers was mapped topologically in terms of longitudinal columns and cellular strata on transverse, Niss1-stained sections in the lizard Gallotia galloti . Four columns extend along the whole length of the midbrain. in dorsoventr al order: 1) the dorsal band contains the optic tectum, surrounded by three ventricularly prominent subdivisions, named griseum tectale, intermediate area and torus semicircularis, in rostrocaudal order; 2) a subjacent region is named here the lateral band, which forms the ventral margin of the alar plate and also shows three rostrocaudal divisions; 3) the basal band forms the basal plate or tegmentum proper; it appears subdivided into medial and lateral parts: the medial part contains the oculomotor and accessory effer ent neurons and the medial basal part of the reticular formation, which inc ludes the red nucleus rostrally; the lateral part contains the lateral basa l reticular formation, and includes the substantia nigra caudally; 4) the m edian band contains the ventral tegmental area, representing the mesencepha lic floor plate. The alar regions (dorsal and lateral) show an overall cell ular stratification into periventricular, central and superficial strata, w ith characteristic cytoarchitecture for each part. The lateral band contain s two well developed superficial nuclei, one of which is commonly misidenti fied as an isthmic formation. The basal longitudinal subdivisions are simpl er and basically consist of periventricular and central strata.