Golgi studies on Purkinje cell development in the frog during spontaneous metamorphosis. I. General pattern of development

The dendrite arbor of Purkinje cells is altered following to tail regeneration in the leopard gecko

Purkinje cells of the cerebellum have a complex arborized arrangement of dendrites and are amongst the most distinctive cell types of the nervous system. Although the neuromorphology of Purkinje cells has been well described for some mammals and teleost fish, for most vertebrates less is known. Here we used a modified Golgi-Cox method to investigate the neuromorphology of Purkinje cells from the lizard Eublepharis macularius, the leopard gecko. Using Sholl and Branch Structure Analyses, we sought to investigate whether the neuromorphology of gecko Purkinje cells was altered is response to tail loss and regeneration. Tail loss is an evolved mechanism commonly used by geckos to escape predation. Loss of the tail represents a significant and sudden change in body length and mass, which is only partially recovered as the tail is regenerated. We predicted that tail loss and regeneration would induce a quantifiable change in Purkinje cell dendrite arborization. Post hoc comparisons of Sholl analyses data showed that geckos with regenerated tails have significant changes in dendrite diameter and the number of dendrite intersections in regions corresponding to the position of parallel fiber synapses. We propose that the neuromorphological alterations observed in gecko Purkinje cells represent a compensatory response to tail regrowth, and perhaps a role in motor learning.

LIM-homeodomain genes as territory markers in the brainstem of adult and developing Xenopus laevis

We investigated expression patterns of the LIM-homeodomain (LIM-hd) genes x-Lhx1, x-Lhx2, x-Lhx5, and x-Lhx9 in the brainstem of Xenopus laevis during larval development and in the adult. The two groups of paralogous genes, x-Lhx1/x-Lhx5 and x-Lhx2/x-Lhx9, showed fundamentally different expression patterns, being expressed in ventral versus dorsal territories of the midbrain and hindbrain, respectively. Indeed, prominent expression of x-Lhx1/5 was found in the mesencephalic tegmentum and the hindbrain reticular formation, whereas conspicuous x-Lhx2/9 expression was observed in the torus semicircularis and isthmic nucleus. A few shared expression domains for the two pairs of paralogs included the optic tectum and the anterodorsal and pedunculopontine nuclei. In each structure, expression of the two paralogs was almost identical, indicating that the regulation of their expression in this part of the brain has evolved slightly since gene duplication occurred. Notable exceptions included the expression of x-Lhx1 but not x-Lhx5 in the Purkinje cells and the expression of x-Lhx9 but not x-Lhx2 in the lateral line nucleus. The analysis of LIM-hd expression patterns throughout development allowed the origin of given structures in early embryos to be traced back. x-Lhx1 and x-Lhx5 were relevant to locate the cerebellar anlage and to follow morphogenesis of the cerebellar plate and cerebellar nuclei. They also highlighted the rhombomeric organization of the hindbrain. On the other hand, x-Lhx2 and x-Lhx9 showed a dynamic spatiotemporal pattern relative to tectal development and layering, and x-Lhx9 was useful to trace back the origin of the isthmus in early development.

Postnatal maturation of spiral ganglion neurons: a horseradish peroxidase study

Using an in vitro cochlear preparation from postnatal hamsters, spiral ganglion cells (SGCs) were labeled retrogradely following extracellular injections of HRP into the cochlear nerve. In 24 cochleae from hamsters between postnatal days (P) 0 and 10, the neuronal morphology of 201 SGCs and their peripheral axons were analyzed. From P 0 to 3, labeled SGCs had few distinguishable features. Although SGCs could be traced separately to inner hair cells (IHCs) and outer hair cells (OHCs), they all had roughly bipolar-shaped cell bodies. Approximately half of the labeled SGCs had peripheral axons that spiraled some distance before entering radial fiber bundles. From P 3 to 7, SGCs increased in size by nearly 30% and the number of SGCs with spiraling peripheral axons decreased to near zero. At P 10, the central axon diameter to peripheral axon diameter ratios distinguished two populations of SGCs. The hair-cell innervation patterns of SGCs also changed morphologically as a function of postnatal age. At P 0, radial fiber (RF) terminals of peripheral axons contacted as many as 8 IHCs; by P 3, RFs contacted typically one or two IHCs. The terminal portions of peripheral axons contacting OHCs did not show any appreciable spiral until P 2. By P 5, individual outer spiral fibers (OSFs) had greater spiral lengths underneath row-3 OHCs and the number of OHC contacts was also greatest for row-3 OSFs. These data suggest that SGCs undergo a systematic maturational process. Furthermore, the morphological differentiation of SGCs occurs after they have established separate inner and outer hair cell innervations.

Autoradiographic studies of cerebellar histogenesis in the premetamorphic bullfrog tadpole: II. Formation of the interauricular granular band

This study examines the origin of cells in the interauricular granular band (iagb) in the cerebellum of the frog tadpole during early stages of development by means of histological and autoradiographic methods. Premetamorphic bullfrog tadpoles were exposed to multiple doses of 3H-thymidine (10 microCi/g body weight per exposure) at developmental stages ranging from 1 week to 1 year and were killed at either 6 or 12 months of age. The autoradiographic data were examined to determine the time when cells of the iagb were generated. Our findings show that initial generation of iagb cells begins at week 3 and that a peak in the formation of postmitotic neurons is reached at the age of 10 weeks. This is followed by other peaks of cell generation at the ages of 16 weeks, 10 months, and 11.5 months. The generation cycles of iagb cells are interrupted by periods of quiescence when label cannot be detected in any of the cells. These quiescent periods occur at the ages of 20-26 weeks, 7 months, and 12 months. These findings indicate that cells of the iagb are generated in a cyclical manner over the entire 1-year period which was studied. Comparison of our present data on iagb cell formation with the generation of cells in the EGL shows that the production of these two groups of cells is overlapping, but cells of the iagb begin and cease production before those of the EGL. On the basis of our findings we propose that the cells of the iagb and the EGL belong in separate cell groups which are generated by distinct subpopulations of germinal cells in the neuroepithelial cap.

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.

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

Stellate cell development was studied in the bullfrog cerebellum during spontaneous and thyroxine-induced metamorphosis using the Golgi-Kopsch method and electron microscopy. Cells that possessed axosomatic synapses and resembled stellate cells were present even in the incipient molecular layer of the cerebellum in the premetamorphic tadpole. These cells may have originated from the early, transient wave of external granule cells that have been reported in the cerebellum of premetamorphic tadpoles in the first 6 months of development, and may constitute the variant population of stellate cells that are present later during development or the degenerating cells that have been observed during metamorphosis as scattered dying cells in the molecular layer. Typical stellate cells, whose development resembled the genesis and differentiation of stellate cells in birds and mammals, were initially observed at the outer border of the molecular layer that is adjacent to the external granular layer during the onset of metamorphosis. These stellate cells were bipolar with processes extending in a plane perpendicular to elongating parallel fibers, and with progressive development, became multipolar with dendrites oriented in various directions with respect to the pia. Stellate cell axons innervate the dendrites and somata of Purkinje cells and other stellate cells, and can be categorized into two types: (1) axons with extensive branching near the soma of origin, and (2) long axons with few branches that occasionally terminate in the Purkinje cell layer. Atypical neurons that did not resemble typical stellate cells were also present in the molecular layer; these might be classified as a stellate cell variant. The generation and differentiation of stellate cells can be induced 1 to 2 years prematurely by administering thyroid hormone to premetamorphic tadpoles. Like most events of cerebellar histogenesis in the frog, stellate cell development also appears to be largely a thyroid-dependent phenomenon.

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.

Purkinje cell maturation in the frog cerebellum during thyroxine-induced metamorphosis

Purkinje cell maturation during thyroxine-induced metamorphosis in premetamorphic bullfrog tadpoles was studied using electron microscopy and Golgi (silver-impregnated) preparations. Cerebella from tadpoles were examined following 1, 2, or 3 weeks of thyroxine treatment. Particular attention was paid to possible differences between the two populations of Purkinje cells previously described, i.e. (i) the smaller population located in the dorsal part of the cerebellum, where the Purkinje cells show dendritic arborization long before the appearance of the external granular layer, and (ii) the larger population located in the middle and ventral regions of the cerebellum, where the Purkinje cells begin to undergo maturation during metamorphosis when the external granular layer is established. Following thyroxine treatment, both populations of Purkinje cells showed rapid maturational change. In the mature (dorsal) group, dendritic growth resumed in the presence of an external granular layer increasing the complexity of their dendritic arbors. Moreover, climbing fiber synapses translocated from contacts on the soma to the thorns of growing dendrites, and somatic processes often disappeared. The immature (ventral) group showed dramatic differentiation of the perikaryon including polarization of cytoplasm with subsequent dendritic outgrowth and formation of somatic processes in the presence of climbing fibers. Stellate cell contacts appeared on the smooth portion of the soma of many Purkinje cells. Dendritic growth during thyroxine-induced metamorphosis was characterized by growth (elongation) with minimal branching, which is initially observed during spontaneous metamorphosis. Typically, these growing dendrites ended in growth cones, some with one or several filopodia. Developing Purkinje cell dendritic spines formed synapses with parallel fibers. The present study has provided an example of the dramatic nature of thyroxine's action in inducing the complex series of detailed maturational changes in the cerebellum 1-2 yr ahead of schedule. In addition, the results show that thyroxine-induced Purkinje cell maturation is more rapid and synchronous than that seen during spontaneous metamorphosis. It is concluded that Purkinje cell maturation during metamorphosis is largely dependent on thyroid hormone.

Golgi studies on Purkinje cell development in the frog during spontaneous metamorphosis. III. Axonal development

The development and organization of Purkinje cell axons and their collaterals was studied in the bullfrog using the Golgi-Kopsch method. In the tadpole, axonal collaterals are few and usually unbranched. In the adult, however, intracortical axonal collaterals of Purkinje cells are more numerous, and they form a meager supraganglionic plexus and a more extensive infraganglionic plexus. In contrast to the pattern seen in higher vertebrates, these plexuses have a tendency to be distributed along the length of the cerebellar plate in both tadpoles and froglets. In addition, collateral branches that form intracortical plexuses apparently increase throughout the course of cerebellar development in this species.

Golgi studies on Purkinje cell development in the frog during spontaneous metamorphosis. II. Details of dendritic development

The development of Purkinje cell dendrites was studied in the bullfrog from premetamorphic tadpoles to 10-week-old postmetamorphic frog-lets by the Golgi-Kopsch method. In this species two distinct patterns of arbor formation may be seen, which appear to be related to differences in the timing of initial dendritic development. In Purkinje cells that begin development in early tadpole stages, the dendritic tree is elaborated by continuous and concomitant growth and branching, a process by which the developing arbor expands in both height and width. Arbor formation in Purkinje cells that begin development in metamorphosing tadpoles proceeds in two separate steps. Initially, dendrites of such cells elongate, but form only a few poorly developed branches; only when the arbor reaches near-adult height does branching become extensive. Additional differences present in Purkinje cells are reflected in the paucity of growth cones and filopodia in the tadpole, and numerous filopodia and growth cones in the metamorphic period. An interesting feature of dendritic development in this species is a tendency to alter the arboreal domain by the formation of extra-arboreal dendrites, and possibly by the occasional resorbtion of other partially formed dendrites. The pattern of dendritic development in the frog is different than in mammals and is difficult to interpret. Such unusual development may be due to disturbances in the timing of the formation of Purkinje cell dendrites and of the establishment of the external granular layer (EGL).