Feline islands of Calleja complex: I. Cytoarchitectural organization and comparative anatomy

Cytoarchitectural analyses demonstrated that the islands of Calleja complex (ICC) is highly developed and discretely organized in the cat. The feline complex is clearly divided into morphological units, each containing a granular Callejal island and a population of satellite neurons. These ICC units change progressively in cytoarchitecture from the lateral to the medial edge of the olfactory tubercle. In particular, the islands flatten, sink into the tubercular molecular layer, and increase in cell density, while their satellite neurons increase in number and decrease in size. The lateromedial transformation was judged to take place in five stages, resulting in the successive appearance of lateral, lateral transitional, central, medial transitional, and medial ICC units. The first two unit types display prominently two additional components of the feline ICC-namely, clusters of dwarf cells and small pyramidal-like neurons constituting the densocellular layer cupping the base of lateral Callejal islands. All of the various types of ICC units contact the tubercular molecular layer via their dwarf and/or granule cell components, raising the possibility of direct olfactory input to the entire Callejal complex (apart from the isla magna). Output from the complex is presumed to arise from the satellite neurons, which are distinguished from adjoining cell populations by their close association with Callejal islands, typical chromophilic character, and relatively large size (15-42 micron in soma length). In the tubercular ICC, these neurons are most numerous immediately above Callejal islands in a fiber-rich zone continuous with the supratubercular zone and hence with the ventral pallidum. In the accumbal ICC, satellite neurons are most conspicuous in granule-cell-poor spaces within the isla magna, where many non-granular neurons are uncharacteristically small and chromophobic. The isla magna itself is unusual not only for its large size but for lateral extensions encircling a group of accumbal neurons far caudally. Such extensions are one of several indications that the isla magna is intimately associated with the nucleus accumbens. A comparative anatomical survey of the ICC in rats, cats, and macaque monkeys demonstrated a number of species differences. Of particular interest is the finding that the complex is unambiguously divided into discrete island-satellite cell units only in cats and macaques. In these species, the complex is also distinguished by a predominance of superficial islands and an especially prominent isla magna. ICC units, however, were most conspicuous in cats.