The connectivity of cones and cone horizontal cells in a mosaic-type teleost retina

Summary

A total of 20 Golgi-impregnated cone horizontal cells of Nannacara anomala (Cichlidae) were studied in alternating semi- and ultrathin sections in order to examine their connections with the overlying square mosaic of equal double and central single cones. Cone horizontal cells exhibit three types of processes: (a) the long horizontal “axon”, (b) short horizontal dendrites with a terminal swelling, and (c) cone contacting processes ascending towards the outer plexiform layer. As seen in tangential sections, the latter processes are arranged in the form of two concentric circles including a central “spot”. The processes of the inner circle contact the eight double cone pedicles of one square unit: processes of the outer circle contact eight more double cone pedicles which are directly adjacent to the square unit. The central “spot” represents a process which contacts the central single cone. Processes of the inner circle most often terminate in a dichotomous branching which represents the lateral elements to one ribbon synapse, whereas in the outer circle only a single terminal swelling is observed. Because of the mosaic of the cones and the constancy of this pattern of connectivity a model can be constructed where the dendritic fields of the cone horizontal cells overlap to a considerable extent. From this model, it follows that each double cone pedicle is contacted by four different horizontal cells. The functional significance of these findings for color vision is discussed in the light of recent work with the microspectrophotometer characterizing the cone system of this species as bichromatic. The mosaiclike arrangement of the horizontal cell dendrites supports the conclusion that the parallels between the patterns of receptor and horizontal cells are no coincidence but play an important role in lateral inhibition and neural adaptation of the retina.