 of the chameleon [32]. Immunohistochemical studies performed in the reptile Caiman crocodilus showed that the ventrolateral telencephalon, a homolog of the striatum, was divided into two distinct "small-celled" and "large-celled" fields [33,34]. SP-positive neurons [34], cholinesterase activity, and ascending catecholaminergic axon terminals derived from the midbrain tegmentum [33] were found in the small-cell field. In contrast, the large-cell field had far less catecholamine activity [33]. These studies revealed neurochemical properties preferentially seen in the striatum: elevated acetylcholinesterase activities showing the presence of cholinergic neurons and abundant dopaminergic terminals showing the presence of dopaminergic inputs from the SNc or the ventral tegmental area (VTA). In the turtle, Chrysemys scripta, SP-positive neurons were preferentially found in the small-cell field [34]. However, these papers did not mention if the cell fields were arranged in a mosaic fashion as in mammals. In the lizard, Anolis carolinensis, the gene expression of serotonin (5-HT) receptors was not confined to a mosaic pattern although expression patterns of 5-HT receptor subdivisions in the telencephalon resembled those reported in mammals [35]. While cell populations homologous to the striosome and matrix neurons were present in the reptilian striatum, the mosaic-like compartment organization of two groups of neurons was absent.

Birds
The avian striatum is divided into three components called the medial striatum, lateral striatum, and accumbens [36]. These structures have not been definitively related to any one specific part of the mammalian striatum, although they share some neurochemical expression patterns and connectivity to other parts of the telencephalon [36,37].
The expression of matrix markers was reported in the avian striatum. CB, a calcium-binding protein preferentially expressed in the matrix compartment in mammalian striatum, was found in neurons of the striatum of male zebra finch [38]. A differential distribution of calbindin-positive neurons was shown between the vocal-learner bird Melopsittacus undulates, and the non-vocal learner bird, Colinus virginianus, in which heavily-labeled cells were scattered amidst the weakly-labeled cells [39]. The calbindin-positive and negative neurons appeared to be intermingled. Bruce et al. [37] studied 12 different marker proteins enriched in the basal ganglia of the pigeon, Columbia livia. They showed a gradient of staining intensity of NPY and ENK, both of which are matrix markers, in the rostro-caudal and ventro-dorsal axis of the striatum, and found the globus pallidus "interwoven" into the striatum with finger-like structures in the rostral sections. The low levels of calbindin and high levels of parvalbumin in the medial part of the lateral striatum in pigeons resembled those in the matrix of the mammalian caudo-putamen [40].
The expression of striosome markers was also reported in the literature. The low calbindin and parvalbumin, and high SP levels in the medial part of the striatum raise the possibility that the area corresponded to the striosome compartment [41,42]. The expression of two types of dopamine receptors, D1A and D1B, was studied in the forebrain of the chick, Gallus domesticus [43]. The medial part of the striatum was visualized as a densely-packed and uniformly-stained nucleus positive for D1A receptors. Glutamate receptors are implicated in learned vocalization in the avian brain [44,45]. The mosaic pattern of GluR1 expression was not reported in the striatum of the songbird, Taeniopygia guttata [45] whereas it was one of the markers of the striosome in primates [46]. However, a clear segregation of the striosome within the matrix compartment was not obvious in avian striata, as described in earlier reports [8,[47][48][49]. They suggested that neurons homologous to the striosome are homogenously distributed in the striatum.

Striatal Compartmentalization in the Mammalian Brain
The compartment structure of the striatum is considered to be evolutionarily conserved in mammals including humans [39,50]. Neurons forming these two compartments are generated during partly overlapping stages of striatal development [51][52][53]. In mice, striosomal neurons born earlier than embryonic day 13.5 (E13.5) (E10.5-11.5 in the caudal-and E12.5-13.5 in the rostral part) [53] form the striatal primordium in the basal part of the telencephalon. Then, the massive wave of later-generated matrix neurons migrates into the primordium and divide striosomal neurons into clusters [54]. Consequently, striosomal neurons form patchy cell clusters whereas matrix neurons occupy the space in between the striosomes, resulting in labyrinthine structures in a 3D space [2,55]. TH, an enzyme that generates dihydroxyphenylalanine, is rich in the so-called "dopamine islands", which correspond to the striosomes, during development, whereas TH-immunoreactivity is slightly preferential to the matrix in adulthood. Both D1-and D2-receptor expressing medium spiny neurons are present either in the striosome or matrix [56]. The striatal compartment begins to appear perinatally and exhibits the mature pattern two weeks after birth in rodents [57]. Hagimoto et al. [58] identified differentially labeled progenitor cells in the striosome and matrix and cultured them in vitro. They showed that striosome cells were early-born, stationary, and mutually attractive in migratory behaviors whereas matrix cells were late-born, actively motile, and exhibited repulsive action against striosomal cells in later stages of striatal development. Their findings were consistent with a proposed model of compartmentalization of the striatum [58][59][60][61][62]. Studies using transgenic mice that selectively express enhanced green fluorescent protein (eGFP) either in the striosome or in the matrix, demonstrated distinct functions of these two compartments [56,63,64]. The establishment of mosaic architectures in the striatum is thus an outcome of precise temporal regulation and termination of neuronal migration during development.
In the mammalian striatum, two types of projection neurons can be identified based on different neurochemical and functional properties. Neurons expressing SP project to the GPi and SNc. This is called the direct pathway and is involved in promoting planned movement. Neurons expressing ENK project to the GPe. This is a part of the indirect pathway, which finally projects to the GPi via a projection to the subthalamic nucleus. Striatal projection neurons are derived from the lateral ganglionic eminence (LGE) [65][66][67]. The precursors of interneurons are reported to become postmitotic in the medial ganglionic eminence (MGE), tangentially migrate to a lateral position, and intermingle with projection neuron precursors to generate the striatal primordium, where they differentiate into cholinergic-, calretinin-positive-, or parvalbumin-positive interneurons [68]. Overexpression of Nkx2.1, a transcription factor expressed in the developing MGE, enhances the migration of interneurons to the striatum and reduces migration to the cortex [69], suggesting that Nkx2.1 is involved in the specification of striatal interneurons. Reports show that specific guidance cues consisting of diffusible molecules are involved in the radial migration of projection neurons [70] and tangential migration of interneurons [71][72][73] in the developing striatum. For example, netrin-1, which is secreted from the ventricular zone of the LGE, repels neuronal progenitors derived from the LGE and thereby promotes radial migration and differentiation of striatal projection neurons [70]. The tangential migration of striatal interneurons from the MGE was shown to be regulated by a combination of Nrg1/ErbB4 chemoattraction and EphB/ephrinB chemorepulsion [73]. Recent findings in mice showed that aberrant compartmentalization of the striatum induced by valproic acid administration during striosomal neurogenesis resulted in autism spectrum disorder-like phenotypes [74].
A traditional view is that the striatum can be subdivided into three functionally distinct parts called the sensori-motor, associative, and limbic domains [75], approximately located in the dorso-lateral, dorso-medial, and ventral striatum [76]. In addition to organization in the coronal plane, recent studies analyzing 3-dimentional structures have demonstrated extreme heterogeneity in the rostro-caudal axis [75,77,78]. Hunnicutt et al. [75] generated a comprehensive map to demonstrate cortico-striatal and thalamo-striatal input patterns to the striatum. Patterns of connectivity were consistent with the distribution of the three striatal domains; in addition, the authors showed the presence of another domain located in the most caudal part of the striatum which received strong inputs form the auditory and visual cortices. Gangarossa et al. [77] reported a specific region exclusively distributed with dopamine D1 receptor-expressing striatal projection neurons, which represent striato-nigral neurons forming the direct pathway, in the caudal part of the striatum. This part was shown to be dominated with matrix compartments, since neurochemical studies showed the expression of calbindin, VGluT1 and 2, and the lack of MOB and ENK. Miyamoto et al. [78] constructed a 3-dimensional map of neurochemical markers for the striosome and matrix and found a specific domain in the most caudal part of the striatum.