Autoradiographic study of the development of the medial thalamic nuclei in the rabbit

The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Compartmental organization of the thalamostriatal connection in the cat

The compartmental organization of the thalamostriatal connection in the cat was studied by labelling thalamic fibers in anterograde axonal transport experiments and comparing their striatal distributions with the arrangement of striosomes and matrix tissue identified by histochemical staining methods. When analyzed according to their principal compartmental targets in dorsal striatum, the thalamic deposits indicated the existence of medial and lateral divisions within the thalamostriatal projection. Nuclei of the medial division, which includes parts of the thalamic midline, projected primarily to striosomes. The lateral division, which embraces the anterior and posterior intralaminar groups, the rostral ventral tier nuclei, and parts of the posterior lateral nuclear complex, predominantly innervated matrix tissue. In the dorsal division of the nucleus accumbens, the medial system preferentially terminated in zones that stain heavily in butyrylcholinesterase and substance P preparations, but fibers from both the medial and the lateral systems largely avoided the histochemically marked compartments such as the border islands of the nucleus accumbens that are seen elsewhere in the ventral striatum. Medial division: Thalamic deposits involving the paraventricular and rhomboid nuclei of the thalamic midline elicited labelling of striosomes and, invariably, ventral extrastriosomal matrix, the nucleus accumbens, and the amygdala. This projection was topographically organized: rostral thalamic deposits elicited labelling in the medial caudate nucleus and the medial nucleus accumbens. More caudal injections produced more lateral labelling. Lateral division: The lateral division is composed of at least three projection systems distinguished by their patterns of matrix innervation. Deposits involving the anterior intralaminar nuclei and the striatally projecting cells located lateral to the stria medullaris (anterior intralaminar complex) produced an even, diffuse labelling of the matrix tissue and weak labelling of the striosomes. Injections placed in the ventroanterior, ventrolateral, and ventromedial nuclei (rostral ventral complex) elicited fibrous labelling of matrix tissue that often showed nonstriosomal inhomogeneities. Deposits involving the centromedian and parafascicular nuclei (posterior intralaminar complex) produced a highly variable pattern of matrix labelling that included both homogeneous and decidedly patchy innervations of the extrastriosomal matrix. Each of these lateral thalamostriatal systems showed a similar spatial organization, whereby dorsoventral and mediolateral thalamic axes were roughly preserved in the projection to striatum.

Development of the rat thalamus: I. Mosaic organization of the thalamic neuroepithelium

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses and young pups were analyzed in order to delineate the boundaries of the proliferative thalamic neuroepithelium, describe its early transformations, identify its regional divisions, and, finally, attempt to relate its distinct neuroepithelial components to specific thalamic nuclei that they supply with neurons. On day E13 the thalamic neuroepithelium consists of two divisions, the rostral lobe and the caudal lobe, and interposed between the two is a small transient structure, the reticular protuberance. By day E14 the rostral lobe has become partitioned into the anterior lobule and the reticular lobule, and the caudal lobe into the intermediate lobule and the posterior lobule. By day E15 these four lobules have become further partitioned into sublobules, characterized as regional eversions and inversions (concavities and convexities) of the thalamic neuroepithelium. Several of these sublobules are still recognizable on day E16 but progressively disappear thereafter. In this introductory paper, some evidence is presented in support of the hypothesis that the identified thalamic sublobules represent putative cell lines committed to produce neurons for specific, early-generated thalamic nuclei. Detailed documentation of the evidence on which the identifications are based is provided in subsequent papers of this series which deal with the early development of specific thalamic regions and nuclei. In our attempt to identify these putative cell lines, we sought to meet the following criteria: (1) a good match between the time course of mitotic activity in a neuroepithelial sublobule and the birth days of neurons in the nucleus that it is postulated to supply with neurons, (2) relative proximity between the putative neuroepithelial source and the thalamic target structure, and, where possible, (3) the tracing of migrating cells from the germinal source to its destination. Using these criteria we have made the following tentative identifications. The early derivatives of the anterior thalamic lobules are the sublobules (committed cell lines) of the anterior thalamic nuclei, and of the central lateral and mediodorsal nuclei. The early derivatives of the reticular lobule and reticular protuberance are the sublobules of the reticular nuclear complex. The early derivatives of the intermediate lobule are the sublobules of the ventrolateral and ventrobasal nuclei. Finally, the early derivatives of the posterior lobule are the sublobules of the dorsal geniculate, ventral geniculate, and medial geniculate nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Observation on the development of the striatum in mice and rats

The morphology and development of the striautum (caudate-putamen) have been examined in normal and reeler mice of the C57 BL/6J strain and in Sprague-Dawley rats. The striatum of an adult mouse contains approximately 690,000 neurons on each side and of these about 97% are medium-sized cells and just under 3% are large cells. In adult rats the percentage of large- and medium-sized neurons is about the same, but the total number of neurons is in excess of 1,500,000. Tritiated (3H) thymidine autoradiographic studies indicate that neuronogenesis in the mouse striatum occurs both in the neuroepithelium of the ventricular zone and in the underlying subventricular zone, between the 12th day of gestation and the first few days postnatally, with the peak period of neuronogenesis being on the 14th and 15th embryonic day. In both mice and rats striatal neurons are generated along three distinct gradients: from caudal to rostral, from lateral to medial, and from ventral to dorsal. The large neurons are among the earliest cells generated. A major proportion of the medium-sized neurons is also generated during this period, but proliferation of these neurons continues for at least several days. In the immediate postnatal period there appears to be some turnover of neurons in the striatum, some cells degenerating while others are still being formed. Cell counts indicate that in the rat there may be as many as 2,000,000 neurons in the striatum at postnatal day 4, but that this number is reduced to the adult figure by day 8.

A model of neocortical histogenesis based on Cajal's principles of homotropism and neurocladism

Generation and regeneration of the Nervous System are considered essentially similar processes since the pioneering studies of Cajal (15). The present paper is a hypothetical attempt to explain the puzzling internal-to-external histogenetic pattern of the mammalian neocortex(4), largely on the basis of various observations made by Cajal on the developing and regenerating Central and Peripheral Nervous System. Fundamental to our model are the principles of excitatory neurocladism and of reciprocal homotropism as described by Cajal. By assuming that both latter principles are valid for the early stages of neocortical development, an explanation can be given for most of the main events of neocortical histogenesis that is consistent with known facts. Our main conclusion is that neocortical neurogenesis differs from that of any other area of the neural tube due to the interposition of the corticipetal fiber system in between the cortical matrix layer and the cortical plate. The mechanical obstacle thus posed to neuroblast migration by this fiber system appears responsible - by activating the operation of the aforementioned principles of Cajal - for both the cellular and topographical neurogenetic sequences of neocortical development, as well as the growth predominantly in width, all features that are unique to the neocortex as compared to the rest of the neural tube.

Neurogenesis in the brain stem of the rabbit: an autoradiographic study

With the aid of (H3)-thymidine autoradiography, neurogenesis was documented in the nuclear groups of the medulla oblongata, pons, and mid-brain, as well as in the brain stem reticular formation of the rabbit. Following single injections of (H3)-thymidine, counts were taken of intensely labeled neurons within the nuclei of the functional columns related to the cranial nerves, nuclei of several other functional classifications, and nuclei that did not fit into a functional category. In the brain stem as a whole, neurogenesis was found to occur between days 10.0 and 18.5 of gestation: however, the majority of nuclei studied contained intensely neurons only between days 12.0 and 15.0. Only in the pontine nucleus and the tectum were intensely labeled cells observed as late as day 18.5. Directional gradients of histogenesis were often observed within, as well as between, various nuclei. Within the nuclear columns related to the cranial nerves, a clear mediolateral spread of neurogenesis was observable such that nuclei of the motor columns reached a peak in neurogenesis before those in the sensory columns. Likewise, a mediolateral proliferation pattern was seen in the brain stem reticular formation. Other individual directional gradients were discernible; however, in the brain stem as a whole, distinct overall gradients were not observable. In many individual nuclei, gradients in neuron size were observed such that large neurons preferentially arose prior to smaller neurons. Information pertaining to gradients in neurogenesis, as well as to relationships among functionally related nuclei, are discussed.

Development of the diencephalon in the rat. IV. Quantitative study of the time of origin of neurons and the internuclear chronological gradients in the thalamus

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 13 and 14 (E13 + 14) until the day before birth (E21 + 22). With this progressively delayed comprehensive labelling procedure we determined the time of origin of neurons in the nuclei of the epithalamus, thalamus, and ventral thalamus. The zona incerta, subthalamic nucleus, reticular nucleus, posterior nucleus, and ventral lateral geniculate nucleus are composed of the earliest arising neurons (E13, or before, to E15). The neurons of the lateral habenular nucleus are produced between days E13--16. The neurons of the medial geniculate and lateral geniculate nuclei, the ventrobasal and ventrolateral complexes, and the nucleus lateralis, pars posterior, arise rapidly on days E14--15; the medial geniculate nucleus with a peak on day E14, the others with a peak on day E15. Neurons of a group of nuclei, with ill-defined boundaries medial to the sensory relax nuclei, arise apparently on days E15--16, with a peak on day E15; these may represent the intralaminar nuclei. The next group is generated on days E15--16 but with peak formation time on day E16; this includes the anteroventral, anterodorsal, anteromedial and mediodorsal nuclei. The rhomboid, reuniens and paratenial nuclei, and the paraventricular nucleus, pars anterior, arise next (E16--17). The medial habenular nucleus forms last and over a protracted period (E15--19). With their lengthy generation time the lateral and medial habenular nuclei resemble more the nuclei of the hypothalamus than the nuclei of the dorsal thalamus.

Neurogenesis in the epithalamus, dorsal thalamus and ventral thalamus of the rat: an autoradiographic and cytological study

Times of final mitotic division for neurons of the epithalamic, dorsal thalamic and subthalamic nuclei of the rat were determined with the aid of thymidine-H3 autoradiography. Intensely labelled neurons were observed in the brains of animals injected with radiochemical from days 13 to 19 of gestation. The pattern of distribution of the labelled neurons indicated that neurogenesis in the regions followed caudorostral, lateromedial and ventrodorsal neurogenetic gradients, all of which were found to operate simultaneously. Since neurogenesis in the epithalamus, subthalamus and caudolateral thalamic regions began on days 13 and 14 of gestation, the ventrodorsal and lateromedial proliferative gradients were clearly discerned only within the ventral and dorsal thalamus exclusive of the epithalamus. These directional neurogenetic gradients were apparent throughout the entire thalamus and within individual thalamic nuclei. No neurogenetic pattern based upon neuronal size was observed, i.e., large neurons were not preferentially formed earlier than smaller ones. Detailed information has also been provided on the cytological character of each thalamic nucleus.