Morphological diversity of cell types in macaque monkey visual system

The mammalian central nervous system is a dynamically complex network of anatomically and functionally distinct cell types, neatly organized into laminar and columnar functional domains. An excellent example of this is the primate primary visual cortex (V1), which is comprised of 6 layers, each containing various cell types interconnecting to one another to create a dense yet precise network of circuits (Callaway, 1998). The morphologically diverse cell groups in layers 5 and 6 of primate V1 are important in facilitating the transmission of visual information (Callaway and Wiser, 1996 ; Wiser and Callaway, 1996). They participate not only in local, intracortical circuits, but also distant, cortico-cortical feedforward pathways, e.g. the projection to middle temporal area (MT), and cortico-subcortical feedback pathways, e.g. the projections to the superior colliculus (SC), lateral geniculate nucleus (LGN), and pulvinar (Ungerleider et al., 1983 ; Fries, 1984 ; Shipp and Zeki, 1989 ; Fitzpatrick et al., 1994). In addition, certain layer-6 cells are believed to send collateral axons to both MT and SC (Fries et al., 1985; vogt Weisenhorn et al., 1995). However, due to limitations in anatomical tracing techniques, detailed morphology of the cell types participating in each of these corticocortical and corticosubcortical pathways is still largely unknown. In these experiments, we injected a modified rabies virus into the SC and area MT of macaque monkeys and performed detailed characterization of the labeled cells in V1. We found that SC projecting neurons in V1 are located in layers 5 and 6, consisting of layer-5 tall-tufted, layer-5 non-tufted, and layer-6 non-tufted. However, a few MT projecting neurons are located in layer 6 and have a single non-tufted main ascending branch of apical dendrites similar to those of the SC projecting non-tufted cells. The morphology of the different cell types found in my experiments suggest that they receive and process different types of information before sending their outputs to the SC or area MT. Knowing the detailed morphology of different participants in different circuits provides insight into understanding their functional importance. Furthermore, anatomical information is also useful in guiding future physiological and reversible inactivation studie