Horizontal cells in the retina of the brush-tailed possum.

Most species of eutherian (placental) mammals examined have two types of horizontal cell, one is axonless and the other has a short axon. We have recently shown that a marsupial, the quokka wallaby, also has two types of horizontal cell and that the axonless cell in this species has unusual stubby processes that pass through the inner nuclear layer to reach the inner plexiform layer. In order to discover whether these descending processes are a feature of marsupials in general, I examined the morphology of retinal horizontal cells in the brush-tailed possum, using horseradish peroxidase labelling. There are two types of horizontal cell in the possum. One type is axonless and has long, fine dendrites somewhat similar to that in the quokka; however, there are several marked differences between the axonless cells seen in the two species. The axonless cell in the possum has on average ten secondary dendrites, twice as many as seen in the quokka. These dendrites are arranged in a radial distribution, unlike those in the quokka, which are polarised in a direction often orthogonal to the overlying ganglion cell axons. Axonless horizontal cells in the possum do not have descending processes that reach the inner plexiform layer as has been seen in the quokka. The second horizontal cell type, the short-axon cell, has an axon and an axonal arbor and is similar to the short-axon cell seen in the retina of the quokka. Therefore, the morphology of the axonless horizontal cell appears to be variable, while that of the short-axon cell is conserved in marsupials as in eutherians.