While overall the striatum is a relatively homogenous structure, striatal neurons do form clusters, referred to as cell islands.53 Moreover, histochemical and tracing studies show a patchy organization of transmitter-related molecules and afferent terminals.54,55 For example, acetylcholinesterase (AChE)-poor striatal regions (termed striosomes or patches) are surrounded by a densely stained “matrix.” The striosomes, embedded in the AChE-rich matrix, correspond to opiate-receptor patches as well as to several other transmitter-related compounds.55 While several hypotheses have been put forth concerning the significance of the compartmental organization in the striatum, it continues to present a challenge in understanding the functional significance of this complex arrangement.
