Unique distribution of somatostatin-immunoreactive cells in the retina of the tree shrew (Tupaia belangeri)

Somatostatin-like immunoreactive cells in the tree shrew retina were studied with the monoclonal antibody S8 against the neuropeptide somatostatin 14. As in some other mammals, immunoreactive somata are exclusively found in the ganglion cell layer. Immunoreactive processes form a sparse main plexus in the inner plexiform layer near the border of the inner nuclear layer; fewer additional processes are found closer to the ganglion cell layer. With retrograde labelling of retinal ganglion cells by injections of the tracer Fast Blue into the superior colliculus and lateral geniculate body and counterstaining of the retinae with S8, similar to 5% of the immunoreactive somata were double-labelled at any retinal location. The vast majority of somatostatin-like immunoreactive cells are thus displaced amacrine cells. Their somata are distributed over the entire retina. Their population density is highest in the temporal retina, with peak densities of similar to 5000 cells/mm(2) near the central area and a steep density gradient. In the remaining retina densities are 200-400 cells/mm(2), falling to similar to 100 cells/mm(2) at the retinal margins. This is in stark contrast to the somatostatin-like immunoreactive cells in other mammalian retinae which have densities of 10-40 cells/mm(2) and are confined to restricted retinal regions (inferior retina and/or retinal margin).