Gord Fishell

Neuronal birthdate underlies the development of striatal compartments.

The striatum of the mammalian forebrain is composed of two complementary functional compartments, the patches and the matrix. By injecting [3H]thymidine at different embryonic times and sacrificing the rats as young adults, we found that the earliest neurons to leave the mitotic cycle were restricted to the patch compartment. Neurons that became postmitotic at later times preferentially joined the matrix compartment. Distinctive periods of cell proliferation may underlie pattern formation throughout the developing forebrain.

Spatially localized neuronal cell lineages in the developing mammalian forebrain

The role of cell lineage in the organization of the cerebral cortex and striatum of the developing rat forebrain was analysed using retroviral-mediated gene transfer to mark the progeny of individual progenitors. Injections around the onset of neurogenesis (embryonic day 14) produced neuronal- and glial-specific clones in the striatum and cortex. The majority of the neuronal clones were restricted to either the deep or superficial layers of the cortex and to either the striatal patch or matrix compartments of the striatum. Moreover, modeling the distributions of the neuronal clones in various ways revealed that grouping the clones into deep vs superficial cortical compartments and patch vs matrix striatal compartments best accounted for the clone distributions. These results suggest that at the onset of neurogenesis there is a heterogeneity of neuronal progenitors within the proliferative ventricular zone.

Neuronal lineages in chimeric mouse forebrain are segregated between compartments and in the rostrocaudal and radial planes

On the basis of neuronal phenotypes and the mode of development of the mammalian forebrain, the cerebral cortex can be subdivided into deep versus superficial layers, and the striatum into patch versus matrix compartments. Interspecific chimeric Mus musculus----Mus caroli mice were used to determine the contribution of lineage to cellular position within these forebrain compartments. Statistical analysis revealed evidence of both spatial and compartmental lineage segregation. A significant difference in genotype ratio depending on chimeric specimen was observed between areas (regardless of compartment) that were separated by greater than 300 microns in the rostrocaudal plane. Differences were observed between early-born (striatal patch and deep cortex) versus late-born (striatal matrix and superficial cortex) neurons, but not between neurons of cortex as a whole versus neurons of striatum as a whole. The difference between early- and late-born neurons was primarily due to the difference between deep and superficial cortical neurons. On a finer scale of analysis, differences in genotype ratios were seen between radially aligned deep versus superficial cortical compartments, in both the neuronal and glial populations. This evidence is consistent with an early positional and compartmental segregation of forebrain progenitor cells.

Pattern formation in the striatum: developmental changes in the distribution of striatonigral projections

The mammalian striatum (the major subcortical structure in the telencephalon) can be divided into two compartments, the patch and the matrix, on the basis of various neurochemical and hodological markers expressed in the adult. The primary efferent target of striatal neurons is the substantia nigra. We have previously shown that the patch compartment sends projections to the substantia nigra embryonically; whereas the matrix does not form a similar projection until the early postnatal period (Fishell and van der Kooy, J. Neurosci., 7 (1987) 1969-1978). The projection of patch neurons to the substantia nigra is the earliest developmental marker for the patch compartment. Here we ask about the early distribution of patch projections and their possible relation to striatal compartmentalization. Embryonic anterograde axonal tracing of the striatonigral pathway can take advantage of the temporal separation of patch versus matrix projections to reveal the terminal distribution of patch striatonigral neurons independent of the nigral terminal distribution from the striatal matrix. The anterograde tracer rhodamine isothiocyanate was shown in a model system to persist in labeled neurons for more than a week, but to be available for uptake into these neurons for a few days after injection at the most. These properties of rhodamine isothiocyanate were combined experimentally with short and long term survival periods. This allowed assessment of the changing developmental distribution of nigral fibers from specifically the striatal patch compartment. In all experimental cases the anterogradely labeled sections of the substantia nigra were also stained with antibodies to tyrosine hydroxylase, which permitted differentiation of the dopamine cell rich pars compacta from the dopamine cell poor pars reticulata. The results show that in the adult the majority of patch and matrix striatonigral projections are confined to the substantia nigra pars reticulata. Furthermore, their fiber distribution within the pars reticulata is overlapping rather than complementary. Most interestingly, in the late embryonic period (most noticeably at embryonic day 19) there is a marked overlap between patch striatonigral fibers and nigral dopamine perikarya. By early postnatal times, when the matrix compartment begins to form its striatonigral projection, the overlap of patch striatonigral fibers and dopamine cells has largely disappeared. The results suggest that a transient interaction between patch striatonigral fibers and dopamine neurons (which is concomitant with the formation of striatal compartments), may be an important developmental event in the phenotypic maturation of striatal pa

Pattern Formation in the Mammalian Forebrain: Striatal Patch and Matrix Neurons Intermix Prior to Compartment Formation

The striatum of the mammalian forebrain is divided into two compartments: the patches and the matrix. Neurons of the patch compartment in the rat striatum become postmitotic earlier in neurogenesis than neurons of the matrix compartment. The selective adhesion of patch neurons to one another has been suggested previously to be an important developmental mechanism of striatal compartmentation. We asked if the selective adhesion of patch neurons is expressed before or after the migration of the majority of the matrix neurons into the striatum. Patch neurons were labelled in vivo by a fluorescent retrograde tracer injected into the substantia nigra on embryonic day 19, which almost exclusively labelled patch neurons. Matrix neurons were labelled with a maternal injection of bromodeoxyuridine at embryonic day 18. When animals were killed at embryonic day 20, the majority of the retrogradely labelled patch neurons were intermixed with the bromodeoxyuridine‐labelled matrix neurons, although there appeared to be clustering of some of the patch neurons. However, by postnatal day 2 there was a complete segregation of the clusters of the retrogradely labelled patch neurons from the bromodeoxyuridine‐labelled matrix neurons in the striatum. This process was modelled in vitro. The patch and matrix compartments were labelled in vivo at embryonic day 13 and 18 respectively, with different birthdate markers ([3H]thymidine or bromodeoxyuridine). At embryonic day 20 the striatal tissue was removed, dissociated and reaggregated in suspension cultures. After 1 day in vitro, labelled patch and matrix neurons were randomly intermixed within the reaggregates. Examination of the cultures at 2.5 and 4 days in vitro revealed clumping of the labelled patch neurons towards the centres of the reaggregates. Over this same period, the labelled matrix neurons did not clump and were dispersed towards the periphery of the reaggregates. The results suggest that patch neuron adhesiveness may appear relatively soon after these neurons become postmitotic, but that this adhesiveness is unable to overcome the initial force produced by the massive migration of matrix neurons into the striatum. We hypothesize that a migratory phase of embryonic striatal development exists, when fated patch and matrix neurons intermix. After this migratory phase, patch neuron adhesiveness can produce the mature segregation of the striatal compartments.

Embryonic lesions of the substantia nigra prevent the patchy expression of opiate receptors, but not the segregation of patch and matrix compartment neurons, in the developing rat striatum.

Unilateral lesions of the substantia nigra on embryonic day 19 prevent the development of the normal patchy distribution of opiate receptors in the ipsilateral rat striatum. Independent, early and permanent labelling of patch compartment neurons in the same brains on embryonic day 14 with [3H]thymidine revealed that the substantia nigra lesions did not prevent the aggregation of early born neurons into patches, but rather blocked the normal expression of one phenotype (dense opiate receptor binding) of these patches. Thus, early nigrostriatal connections may not be critical for the fundamental patch/matrix compartmentation of the striatum, but may be important in the maturation of phenotypic markers of these compartments.

The Development of Striatal Compartments: From Proliferation to Patches

The conception of the organization of the striatum has changed in recent times from that of a homogeneous structure to that of a distinctly compartmentalized one. Part of this change is really a question of level of analysis. On a single cell level, morphological studies have demonstrated that the vast majority of striatal cells are of a single medium spiny type (Kemp and Powell, 1971). On a multicellular level, the striatum can be divided into two compartments, the patch and the matrix, which can be differentiated on the basis of several neurochemical and hodological markers. In single striatal sections, the small patches appear imbedded into the larger matrix compartment, but it is clear from serial section reconstructions that the patches form a continuous labyrinthian compartment through the striatum (Graybiel and Ragsdale, 1978). The distribution of the patches can be delineated in the adult by high levels of opiate receptors (Kent et al., 1982; Pert et al., 1976), substance P (Gerfen, 1984; Haber and Watson, 1985), neurotensin (Goedart et al., 1984), and afferents from the prefrontal cortex (Donoghue and Herkenham, 1986, Gerfen, 1984). In complementary fashion, the matrix compartment can be identified in the adult by high levels of somatostatin (Gerfen, 1984), neurotensin receptors (Goedart et el., 1984), acetylcholinesterase (Graybiel, 1984), thalamic terminals from the centromedian parafasicular complex (Herkenham and Pert, 1981), and terminations from neurons located in sensorimotor cortex (Donoghue and Herkenham, 1986).

The development of laterality in the forebrain projections of midline thalamic cell groups in the rat

Bilateral forebrain (caudoputamen, nucleus accumbens and frontal cortical areas) injections of two different fluorescent retrograde tracers demonstrated that labeled cells situated in the midline nuclei of the thalamus and midbrain each project only unilaterally to the forebrain, regardless of the laterality of their perikarya. Thus, these intermingling midline perikarya send their axons primarily ipsilaterally and to a lesser degree contralaterally, but never bilaterally to the forebrain. At embryonic day 19, these midline nuclei exist as two bilaterally situated, independent structures, each projecting only ipsilaterally to the forebrain. By postnatal day 2, these perikarya fuse into a single mass on the midline. Upon fusion, many of the perikarya of the two developing subnuclei cross the midline, intermingle with each other, and thus some neurons come to have contralateral forebrain projections. These observations suggest that neurons are able to maintain their axonal projections while migrating short distances.

The Development of Striatal Compartmentalization: The Role of Mitotic and Postmitotic Events

An intriguing question in the evolution of the mammalian brain is how pattern can be conserved despite enormous changes in size and functional capacities. This phenomenon is dramatically demonstrated in the striatum where compartmental pattern is conserved across different mammalian species (Johnston et al., 1990)(Fig.1). Although the majority of the mammalian striatum is comprised of a seemingly homogeneous population of medium spiny neurons (Kemp and Powell, 1970), it can be compartmentalized into two subdivisions, the patch and matrix compartments, on the basis of different distributions of neurotransmitters, receptors and connections (Graybiel et al. 1981; Gerfen et al, 1984; van der Kooy et a1., 1987). Compartmentalization of the striatum into small patches in a matrix background is restricted to mammals (Reiner et al., 1989) and has been shown in rodents (Herkenham and Pert, 1981), cats (Graybiel et al., 1981), monkeys (Goldman-Rakic, 1982) and humans (Haber, 1986). Despite the large differences in total striatal size, the ratio of patch/matrix area and the number of individual patches is maintained in different mammalian species (Johnston et al., 1990; Fig. 1). The maintenance of striatal compartmentalization suggests that common developmental mechanisms are involved in establishing this pattern and raises many questions concerning how this is achieved. We hypothesize, first, that cell lineages are committed to becoming members of compartments while still proliferating in the ventricular zone and that these precursors share common properties with the precursors which produce the compartments of the other major portion of the telencephalon, the cerebral cortex. Second, we suggest that the adult striatal pattern of small patches embedded within a matrix background is produced through cellular interactions in the postmitotic region that position compartmentally committed cells.