On the development of the cerebellum of the trout, Salmo gairdneri. II. Early development

Development of the cerebellum in turbot (Psetta maxima): Analysis of cell proliferation and distribution of calcium binding proteins

The morphogenesis, cell proliferation and neuronal differentiation of the turbot (Psetta maxima) cerebellum has been studied using conventional histological techniques and immunohistochemical methods for proliferating cell nuclear antigen and calcium binding proteins. As in other vertebrates, the cerebellar anlage emerges as proliferative plates of neural tissue during the embryonic period. The anlage of the cerebellum persists without morphological changes until the end of the larval life when the mantle zone is differentiated. The major ontogenetic changesthat drive the formation of the cerebellar subdivisions begin in late premetamorphic larvae when cerebellar plates growth and merge medially. This transformation is accomplished by the reorganization of proliferative zones as well as by the onset of cell differentiation. The cerebellum becomes fully differentiated during metamorphosis when parvalbumin and calretinin were detected in Purkinje and eurydendroid cells. Sustained proliferation is maintained in all subdivisions of the cerebellum and this support the robust growth of this part of the brain that takes place during the metamorphic and juvenile periods.The location and histological organization of the proliferative activity in the turbot mature cerebellum are described and their functional significance was analyzed in light of the information available for other teleosts.

Afferent and efferent connections of the cerebellum of a salmonid, the rainbow trout (Oncorhynchus mykiss): A tract-tracing study

The connections of the cerebellum of the rainbow trout were studied by experimental methods. The pretectal paracommissural nucleus has reciprocal connections with the cerebellum. Three additional pretectal nuclei project to both the corpus and valvula cerebelli, and seem to receive cerebellar afferents. A large number of cells of the lateral nucleus of the valvula project to wide regions of the cerebellum, including the valvula, the corpus, the granular eminences, and the caudal lobe, whereas the contralateral inferior olive and scattered reticular cells project only to the corpus and valvula cerebelli. Afferents to the corpus were also observed from the ventral tegmental nucleus, the "paraisthmic nucleus," the perilemniscal nucleus, the central gray, and the octavolateral area. Valvular afferents were also observed from the torus semicircularis and the midbrain tegmental areas. In most cases of cerebellar application, labeled fibers were seen in the thalamus, the pretectum, the torus longitudinalis and torus semicircularis, the nucleus of the medial longitudinal fascicle, and midbrain and rhombencephalic reticular areas. From the corpus cerebelli some fibers also project to the posterior tubercle and the hypothalamus. Moreover, the granular eminences project to the cerebellar crest. DiI application to most of the areas showing labeled fibers after cerebellar tracer application led to the labeling of characteristic eurydendroid cells, mainly in the valvula cerebelli and the caudal lobe. A few putative eurydendroid cells were labeled from the octavolateralis regions. These results in a teleost with a generalized brain indicate several differences with respect to the cerebellar connections reported in other teleost fishes that have specialized brains.

Zebrin II immunoreactivity in the rat and in the weakly electric teleost Eigenmannia (gymnotiformes) reveals three modes of Purkinje cell development

Monoclonal antibody (mab) anti-zebrin II recognizes a single 36-kD polypeptide in Purkinje cells in the rat and fish cerebellum. In the adult rat, zebrin II+ Purkinje cells form, in each hemicerebellum, seven parasagittal bands interposed by zebrin II- bands. We show that, in rats, immunoreactivity first appears caudally at postnatal day 5 and spreads; all Purkinje cells are labelled by postnatal day 12. Subsequently, immunoreactivity is selectively lost so that by day 18 the adult pattern of zebrin II+/-immunoreactive bands is created. This pattern indicates two types of Purkinje cells according to developmental trajectory, zebrin II-/+/-. In the adult gymnotiform teleost Eigenmannia, Purkinje cells in the corpus cerebelli (CCb), lateral valvula cerebelli (VCbl), and eminentia granularis anterior (EGa) are zebrin II+. Purkinje cells in the eminentia granularis posterior (EGp) and medialis (EGm) and the medial valvula cerebelli (VCbm) are zebrin II-. Zebrin II antigenicity is first present at 6 days postspawning (P6) in the EGa and at P8 in the CCb. In the valvula, labelling does not appear until P29. Immunoreactivity in the CCb, VCBl, and the EGa persists in the adult, whereas in the VCbm Purkinje cells become zebrin II- before reaching adulthood. These developmental histories (zebrin II-/+ and zebrin II-/+/-) correspond to the patterns of Purkinje cell development in mammals. Additionally, Eigenmannia has a third class of Purkinje cells, in the EGp and EGm, that never express zebrin II immunoreactivity, indicating that zebrin II expression is not an obligatory feature of Purkinje cell development in all vertebrates.

Autoradiographic studies of cerebellar histogenesis in the premetamorphic bullfrog tadpole: I. Generation of the external granular layer

This study examines the time of origin of cells in the external granular layer (EGL) in the frog cerebellum during early stages of development. Premetamorphic bullfrog tadpoles were given multiple intraperitoneal injections of 3H-thymidine (10 microCi/g body weight per injection) at developmental stages ranging from 4 weeks to 1 year and were killed at either 6 or 12 months of age. Autoradiograms were analyzed to determine the time when cells of the EGL were generated by an examination of the labeling pattern in the neuroepithelial cap where EGL cells were presumably formed and in the EGL into which they migrated. The developmental stage of the cerebellum in the 6-month-old tadpole was essentially the same as that of the 12-month-old animal except for an increased size in the older tadpole. The cerebellum in both age groups contained a distinct neuroepithelial cap and an EGL, which was somewhat better formed in the 12-month-old tadpole. Some heavily labeled cells were found in the neuroepithelial caps of 6-month-old tadpoles from injection times of 6 weeks to 6 months. In the cerebella of 12-month-old tadpoles, however, heavily labeled cells were found in the neuroepithelial cap only with the injection time of 12 months; with injection times from 7 to 11 months, the cells were labeled lightly. Labeled EGL cells were found in the cerebella of 6-month-old tadpoles from an injection time of 6 weeks on; with injection times from 10 weeks to 6 months some EGL cells contained heavy amounts of label. In the cerebella of 12-month-old tadpoles, labeling of EGL cells was not detectable with injection times of 7-9 months; they contained light to medium labeling with injection times of 10 and 11 months and heavy labeling when injected at 12 months. These results indicate that EGL cells are generated continuously in premetamorphic tadpoles from the age of 6 weeks to 12 months. Furthermore, these results suggest that the rate of EGL cell formation is faster during the second half-year of development than during the first.

Granule cell development in the frog cerebellum during spontaneous and thyroxine-induced metamorphosis

Granule cell maturation in the cerebellum of bullfrog tadpoles was studied during both spontaneous and thyroxine-induced metamorphosis by using electron microscopy and Golgi-impregnated preparations. The production of cerebellar microneurons, a majority of which are granule cell precursors, was quantitatively compared during spontaneous and thyroxine-induced metamorphosis by using stereological methods and biochemical measurements of DNA. Granule cell migration and differentiation appeared morphologically similar during spontaneous and thyroxine-induced metamorphosis. In both instances, granule cells migrated tangentially along the pial surface, migrated into the internal granular layer, developed dendritic arbors, and formed synaptic contacts with the processes of Golgi cells and with mossy fibers. These events are similar to developmental processes that have been described in detail in other animals. Quantitative stereological measurements demonstrated similar overall patterns of change during spontaneous and thyroxine-induced metamorphosis. Most notably, increases in the volume of the external granule layer correlated with increases in the relative and total amounts of DNA. However, measurements of total DNA were consistently reduced during the period of accelerated change that occurs in thyroxine-induced metamorphosis, although external granular layer volume was greater in these tadpoles after 2 and 3 weeks of thyroxine treatment than in spontaneously metamorphosing tadpoles. While granule cell development in the frog is largely dependent on thyroid hormone, differences between thyroid-hormone-induced and spontaneously metamorphosing tadpoles suggest that normal patterns of cerebellar development are also dependent on events that occur in premetamorphic tadpoles in the absence of thyroid hormone.

Early stages in the formation of the cerebellum in the frog

The formation of the cerebellum was studied during the first 6 months of the tadpole stage of the bullfrog by using standard histological methods and reconstructions from serial horizontal sections. Three major developmental phases were noted in the formation of the cerebellum. (1) During the first 5 weeks of development, the neuroepithelium proliferated and the dorsal mesencephalic plates increased in size. (2) Starting in the sixth week, a patch of neuroepithelium began to differentiate and gave rise to a small population of Purkinje cells. In subsequent weeks, the area of differentiation continued to spread and a Purkinje cell layer became established along the dorsal margin of the cerebellar plate. (3) In the 12th week, the ventrolateral part of the cerebellar plate began to increase in size and generate two populations of small cells. The lateralmost part of the neuroepithelium in this area generated a group of cells that formed an external granular layer that was one cell deep. Cells of this external granular layer migrated inward into the primitive molecular layer, and by the 26th week only a remnant of an external granular layer remained in the cerebellum. The more medially situated part of the neuroepithelium gave rise to another population of small cells that formed a column, which appeared to be continuous with the Purkinje cells, but differed from them in size. It should be noted that full maturation of the cerebellum occurs during metamorphosis, which in this species remains some 2 years away.

Mitochondria in the postnatally developing rat cerebellar cortex: a morphological and biochemical study

Ultrastructural changes during development in the proliferating cells of the external granular layer and the granule cells of the internal granular layer of the rat cerebellar cortex were examined. Changes in the number of free ribosomes, development of membrane systems, nuclear appearance and the number of cristae and matrix density of mitochondria suggested that both the round and elongated cells of the external granular layer and the granule cells of the internal granular layer were at same stage of maturation at any given age. Quantitatively, the relative volumes of cytoplasm and of mitochondria in the granule cells of internal granular layer were significantly higher than in the proliferating external granular layer cells; however, mitochondrial density and its increase during development were not significantly different in the two cell populations. The activity of the mitochondrial enzyme, succinate dehydrogenase, was also similar in ultrastructurally preserved and metabolically competent perikaryal fractions enriched in replicating external granular layer cells, granule cells and Purkinje cells. These findings emphasize the similarities between proliferating external granular layer cells and granule cells of internal granular layer. They lend support to the view that these cell types at the ages examined, represent a continuum of maturation towards full neuronal differentiation.

On the development of the cerebellum of the trout, Salmo gairdneri. IV. Development of the pattern of connectivity

Synaptogenesis has been studied in the corpus cerebelli of the trout Salmo gairdneri, Richardson, 1836. The first synapses are observed in hatchlings and occur between parallel fibres and the shafts of Purkinje dendrites. Subsequently the axosomatic synapses of Purkinje axon collaterals on the neurons of the ganglionic layer appear, and finally the synapses made by climbing fibres and mossy fibres, and by stellate cell axons develop. Young synapses in the cerebellum of the trout resemble the mature structures so closely that the criteria for the identification of the latter can also be applied to the former. The number of parallel fibre synapses and of Purkinje axon collateral synapses increases considerably during development. Eurydendroid cells, the axons of which leave the cerebellum, receive an abundance of Purkinje axon collaterals on their somata and main dendritic trunks. Mossy fibre synapses are numerous in the granular layer. Climbing fibre contacts and synapses of stellate cell axons, both with Purkinje cells, are found occasionally. The following pattern of connectivity is proposed. The main input-output system is formed by the mossy fibres, the granule cells, the Purkinje cells and the eurydendroid cells. Additional pathways are formed by (1) the mossy fibres, granule cells and eurydendroid cells, and (2) the climbing fibres, Purkinje cells and eurydendroid cells. The afferent-efferent systems, mentioned above, are influenced by a number of internuncial elements: (1) The Golgi cells receive their input from the parallel fibres and contact with their axon collaterals the dendrites of granule cells. (2) Axon collaterals of Purkinje cells are in synaptic relation with Golgi cells. (3) Axon collaterals of Purkinje cells impinge upon the somata and main dendrites of other Purkinje cells. (4) Stellate cells, which derive their input from the parallel fibres, synapse with dendrites and somata of Purkinje cells. The possible functional roles of all of these neuronal elements are discussed.

On the development of the cerebellum of the trout, Salmo gairdneri. V. Neuroglial cells and their development

The neuroglia of the cerebellum of Salmo gairdneri Richardson, 1836, has been studied in mature and developing specimens with light and electron microscopy. The light microscopic observations were largely carried out on Golgi material. The cerebellum of the trout contains all of the neurologlial cell types described for the mammalian cerebellum, viz. ependymal cells, Golgi epithelial cells, velate protoplasmic astrocytes, smooth protoplasmic astrocytes and oligodendrocytes. In addition two types of glial elements, which combine characteristics of ependymal cells and of velate astrocytes, are found. These elements are designated as ependymoid astrocytes and astrocytoid ependymal cells. Smooth astrocytes and oligodendrocytes were observed only in later stages of development and possibly arise from the secondary matrix. The other glial cell types, as well as transitional forms between these types, are present in rather early stages, and show a similar ultrastructure. It is plausible that all these types develop from the glioblasts produced by the ventricular matrix layer. Many glial cells are radially oriented and keep in contact with the meningeal surface throughout development. The lattice formed by matrix cells in the earliest stages, and by glial cells and the axons of granule cells later on, plays a role in directing the migration of cells. Other functions of the glia, such as dividing the cerebellar cortex in synaptic compartments, are suggested. It may be concluded that the high degree of differentiation of the teleostean cerebellum is also reflected by the morphology of the neuroglia.