The avian griseum tectale: cytoarchitecture, NOS expression and neurogenesis

The griseum tectale (GT) is a retinorecipient layered formation located in the rostral alar midbrain, just behind the constriction that separates it from the diencephalon (pretectum). Tritiated-thymidine autoradiographic data on neuronal birthdates show that the GT cell population starts to be generated at HH21 and most neurons are born by stage HH25, accumulating within a primordial periventricular layer, which shows strong nitric oxide synthase immunoreactivity at later stages of development. There is a barely noticeable rostrocaudal neurogenetic and differentiation gradient across the GT, which seems to continue into that of the neighboring optic tectum. The GT layering develops gradually by radial migration of its postmitotic neurons between stages HH26 and HH35. The structure of the mature GT can be divided into periventricular, central, and superficial layers, similarly to the adjacent optic tectum, but it shows different layering aspects, particularly in the retinorecipient superficial layer.