We determined the neurogenesis characteristics of a distinct subclass of rat striatum gamma-aminobutyric acidergic (GABAergic) interneurons expressing the calcium-binding protein calretinin (CR). Timed-pregnant rats were given an intraperitoneal injection of 5-bromo-2'-deoxyuridine (BrdU), a marker of cell proliferation, on designated days between embryonic day 12 (E12) and E21. CR-immunoreactive (-IR) neurons and BrdU-positive nuclei were labeled in the adult neostriatum by double immunohistochemistry, and the proportion of double-labeled cells was quantified. CR-IR interneurons of the neostriatum show maximum birth rates (>10% double labeling) between E14 and E17, with a peak at E15. CR-IR interneurons occupying the lateral half of the neostriatum become postmitotic prior to medial neurons. In the precomissural neostriatum, the earliest-born neurons occupy the lateral quadrants and the latest-born neurons occupy the dorsomedial sector. No significant rostrocaudal neurogenesis gradient is observed. CR-IR neurons make up 0.5% of the striatal population and are localized in both the patch and the matrix compartments. CR-IR neurons of the patch compartment are born early (E13-15), with later-born neurons (E16-18) populating mainly the matrix compartment. CR-IR cells of the neostriatum are a distinct subclass of interneurons that are born at an intermediate time during striatal development and share common neurogenesis characteristics with other interneurons and projection neurons produced in the ventral telencephalon.

The Purkinje cells of the cerebellar cortex and the cortical afferent and efferent projections are organized into parallel parasagittal zones. The parasagittal organization is clearly revealed by immunocytochemistry with a monoclonal antibody, mabQ113. The mabQ113 antigen is confined to a subset of Purkinje cells that are clustered together to form an elaborate, highly reproducible pattern of bands and patches, interspersed with similar mabQ113- regions. The mabQ113+ territories have been classified into seven parasagittal bands (P1+-P7+) in each hemicerebellum. The degree of correspondence between the compartments revealed by the anterograde labeling of the olivocerebellar projection and by mabQ113 immunocytochemistry has been explored in the adult rat. Horseradish peroxide-wheat germ agglutinin conjugate was injected as an anterograde tracer into the inferior olivary complex. When the injection site did not encompass all the olive, an incomplete, patchy labeling of the molecular layer was seen in the cerebellar cortex. Labeled zones of the molecular layer were interrupted by unlabeled regions to give a pattern of parasagittal cortical bands. The positions of these bands were compared with the distribution of the mabQ113+ antigenic bands as seen on the two adjacent sections. Labeled climbing fibers were found to terminate on both mabQ113+ and mabQ113- Purkinje cell zones. The mabQ113+/mabQ113- boundaries and the bands of climbing fibers seen by using the anterograde tracer typically coincide. The one consistent exception is the midline band of mabQ113+ Purkinje cells, P1+. The normal olivocerebellar projection is exclusively contralateral and the climbing fiber projection to the paramedian vermis splits P1+ down the middle, implying that it consists of two adjacent mabQ113+ bands not separated by mabQ113-territory. It is likely that the climbing fiber projection to the cerebellar cortex and the distribution of the two Purkinje cell phenotypes share a common compartmental organization.

Quantitative electron microscopy has been used to study the number of callosal axons in the corpus callosum of normal and hypothyroid rats during postnatal development. At birth, the normal corpus callosum contains 4.4 x 10(6) axons. This number increases to 11.4 x 10(6) by 5 days of age (P5) and then, in contrast to cats and primates, remains constant until at least P60, the oldest age examined. The number of axons in the corpus callosum of hypothyroid animals is not significantly different from the values observed in normal rats at all ages studied, although the callosal axons of hypothyroid rats remain structurally immature. As extensive elimination of callosal axons has been shown to occur in normal rats past P5, we conclude that new callosal processes grow through the corpus callosum past this age that compensate numerically for the loss. Moreover, as the number of callosally projecting neurons seems to be higher in hypothyroid rats than in normal controls, it seems that the constant axon number derives from more parent neurons, and thus that there are more axon collaterals per callosal neuron in a normal animal than in a hypothyroid one. Taken together, these data indicate that although hypothyroidism does not alter the total number of callosally projecting axons, it interferes with the normal processes that define or sculpt the projection fields, thereby leading to a numerically normal projection with abnormal topography.

We study the neurogenesis of a distinct subclass of rat striatum gamma-aminobutyric acid (GABA)ergic interneurons marked by the calcium-binding protein parvalbumin (PV). Timed pregnant rats are given an intraperitoneal injection of bromodeoxyuridine (BrdU), a marker of cell proliferation, on designated days between embryonic day (E) 11 and E22. Birthdate of PV neurons is determined in the adult neostriatum and nucleus accumbens by using a BrdU-PV double-labeling immunohistochemical technique. PV-immunoreactive interneurons of the neostriatum show maximum birthrates (>10% double-labeling) between E14-E17, whereas PV-immunoreactive interneurons of the nucleus accumbens show maximum double-labeling between E16-E19. In the neostriatum, caudal PV-immunoreactive neurons are born before those at rostral levels, and lateral PV-immunoreactive neurons become postmitotic before medial neurons. In the postcommissural striatum, ventral PV-immunoreactive neurons become postmitotic before dorsal neurons. In the precommissural striatum, ventral neurons are born before dorsal neurons laterally, but a dorsoventral gradient is seen medially. At corresponding coronal levels, PV-immunoreactive neurons of the nucleus accumbens are born shortly after PV neurons of the neostriatum. Analysis of BrdU labeling intensity in the nucleus accumbens shows that medium spiny projection neurons of the shell become postmitotic before neurons of the core. Similarly, PV-immunoreactive interneurons of the nucleus accumbens shell are born before PV interneurons of the core. Compared with cholinergic interneurons of the neostriatum, PV-immunoreactive interneurons are born later, but neurogenetic gradients are similar. The period of striatum PV interneuron genesis encompasses the period for somatostatin interneurons, although the latter neurons do not show neurogenetic gradients, possibly due to heterogeneous subtypes. Consideration of basal telencephalon neurogenesis suggests that subpopulations of striatum interneurons may share common neurogenetic features with phenotypically similar populations in the basal forebrain, with final morphology and connectivity depending on local cues provided by the host environment.