Dendritic growth and the control of neuronal form

Morphological characteristics and electrophysiological responses of visceral nociceptive neurons in somatosensory cerebral cortex of cat

The morphology of intracellular neurobiotin-labeled cortical neurons activated by noxious visceral stimuli in cat was reported. The electrophysiologic properties of 851 neurons and structure of 52 neurons of representative area of the greater splanchnic nerve (GSN) in the primary somatosensory cortex were described. The neurons responding to stimulation of GSN with strong intensity were referred to as viscero-nociceptive neurons (VNNs). Patterns of the responses of VNNs in SI area were excitatory, inhibitory and mixed. According to latency of discharge clusters in response, the VNNs could be classified into two kinds. The neurons only showing long latency were called specific VNNs; those having discharge clusters with both long and short latency induced by stimulus were nonspecific VNNs. Following acquisition of electrophysiological data, neurobiotin was injected into some cells by electrophoresis and their structure and location in the cerebral cortex were studied. The results provided some findings about morphological and electrophysiological characteristics of VNNs in SI area. It is thought that VNNs have more complicated morphology in the dendritic construction and axon distribution than nonvisceral nociceptive neurons (NVNNs). Most of the excitatory VNNs showed pyramidal shape and most of the inhibitory VNNs had stellate shape in camera lucida drawing. The architecture of the nonspecific VNNs might be different from specific VNNs in dendritic distribution.

Stem cells and neurogenesis in the adult brain

A new interest in replacing neurons lost to trauma or disease has been generated by findings that challenge the traditional view of the static (irreparable) adult brain. The discovery of stem cells and neurogenesis in the adult central nervous system is responsible for much of this interest.

Developmental changes in the dendritic architecture of salt-sensitive neurons in the nucleus of the solitary tract

Recent studies have provided evidence that brainstem gustatory neurons undergo substantial dendritic growth during a period of postnatal development that coincides with the maturation of their response to salts, suggesting a relationship (perhaps causal) between the physiology and morphology of developing salt-sensitive neurons. In an initial effort to explore this issue, we used extracellular and intracellular recording and intracellular labeling techniques to examine the structure and function of individual gustatory neurons in the rostral nucleus of the solitary tract (rNST) of young (postnatal day [P] 22-28) and adult rats. We found that P22-28 cells that responded to all three of the salts in our taste array had a greater dendritic length, a greater cell volume, and more dendritic branches than the cells that responded to one salt. As a group, taste-sensitive neurons in P22-28 animals had a higher maximum dendritic branch order and a trend toward more dendritic branch points than gustatory neurons in adult animals. The dendritic arbors of P22-28 taste neurons that responded to all three salts were larger (greater surface area and volume), more extensive in the rostrocaudal axis and exhibited a higher maximum branch order, more branch points and higher swelling density than adult cells that responded to all three salts. These results demonstrate that the morphology of salt-sensitive gustatory neurons in developing animals is closely related to the number of salts that evoke a response. The data also support the postulate that gustatory neurons in the rat brainstem undergo substantial dendritic remodeling between the fourth week of life and adulthood. Dendritic remodeling may play an important role in the maturation of the rNST response to NaCl.

Carbonic anhydrase II and carbonic anhydrase-related protein in the cerebellar cortex of normal and lurcher mice

The developmental profiles of carbonic anhydrase II (CA-II) and a carbonic anhydrase related protein (CARP) were studied in rat and mouse cerebella. Enzyme histochemistry, immunohistochemistry, in situ hybridisation and Western blotting were used to study the synthesis and expression of these enzymes in cerebellar sections from age matched control, CA-II deficient and lurcher mice, the latter being characterised by Purkinje cell degeneration. Both CA-II and CARP were first found to be expressed in the Purkinje cells in the 9 day old mouse, and the immunoreactivity of both peptides increased with time. Immunohistochemistry showed more intense staining of CARP than of CA-II in Purkinje cells throughout the developmental profile of the mouse, and this was mirrored by the mRNA levels determined by in situ hybridisation. Immunohistochemistry of CA-II and CARP also demonstrated the progressive dendritic growth of the mouse and rat Purkinje cells. CA-II and CARP immunoreactivity ceased by the end of cerebellar maturation. The onset of Purkinje cell degeneration was detected at day 10 in the lurcher mouse, with concomitant marked decrease in CA-II level: however CARP expression was found to be unchanged. By postnatal day 16 neither CA-II mRNA, protein, nor activity was detectable in contrast to CARP which remained at a decreased level unit the Purkinje cells population had completely degenerated. Our findings suggest a role of CA-II in the degenerative processes of the lurcher Purkinje cells, with CARP playing an important role in the development and maturation of the cerebellar cortex.

Glycinergic transmission regulates dendrite size in organotypic culture

We previously demonstrated that inhibitory synaptic transmission influences dendrite development in vivo. We now report an analogous finding in an organotypic culture of a glycinergic projection nucleus, the medial nucleus of the trapezoid body (MNTB), and its postsynaptic target, the lateral superior olive (LSO) of gerbils. Cultures were generated at 6-7 days postnatal and grown in serum containing medium with or without the glycine receptor antagonist, strychnine (SN), at 2 microM. LSO neurons were then labeled with biocytin, and the dendritic arbors were analyzed morphometrically. Compared to neurons form age-matched in vivo tissue, the neurons cultured in control media were somewhat atrophic, including decreases in dendritic branching and length. Incubation in strychnine led to a dramatic increase in dendritic branching and total dendritic length. Control neurons averaged 6.3 branches compared to 18 branches/neuron in SN-treated cultures. There was a similar increase in primary dendrites and total dendritic length. The physical elimination of MNTB cells did not mimic SN treatment, presumably because glycinergic LSO neurons generated intrinsic connections. In fact, the LSO soma area was significantly greater following MNTB removal, suggesting that these afferents provide a second signal to postsynaptic neurons. These results suggest that spontaneous glycinergic transmission regulates the growth of postsynaptic processes.

The effects of neonatal dorsal root section on the survival and dendritic development of lumbar motoneurons in the rat

Peripheral nerve crush during the early neonatal period results in the death of a large proportion of affected motoneurons and abnormal dendritic development in those which survive. The present study reports the effects of neonatal dorsal root section on motoneurons supplying the extensor digitorum longus muscle of the rat. This lesion did not result in motoneuron death, but did disrupt subsequent dendritic development. In cells retrogradely labelled with cholera toxin subunit B conjugated to horseradish peroxidase, there was little change in adult dendritic morphology in the transverse plane, where abnormalities associated with loss of efferent contact and cell death have been found. However, there was a caudal expansion of the dendritic field, an effect seen following nerve crush but not after blockade of neuromuscular transmission alone. The results show that disruption of dorsal root sensory inputs alone can affect the dendritic development of motoneurons but does not cause their death. In conjunction with our earlier findings, it is clear that both afferent and efferent connections are required for normal dendritic development, and disruption of either has a characteristic effect on survival and dendritic morphology.

Preserved features of thalamocortical projection neuron dendritic architecture in the somatosensory thalamus of the rat, cat and macaque

A number of studies have shown that the organization of the mammalian somatosensory thalamus varies between species. As differences in cellular and synaptic thalamic organization would be expected to influence neuronal dendritic architecture, we compared somatosensory thalamocortical projection (TCP) neurons from the rat, cat and macaque. The results show that key features of the dendritic branching pattern remain unchanged despite large differences in the size of TCP neurons between the species. The features examined were: (i) ratio of the length of terminal branches to the length of the entire dendritic tree; (ii) the percentage of branch points that gave rise to two daughter branches as opposed to those that gave rise to three or more daughter branches; (iii) the proportional sum of absolute deviations (a measure of branching symmetry), and (iv) the mean branch order of the terminal segments. The present study provides evidence that somatosensory TCP neurons in these species comprise a homogeneous class and share a common dendritic architecture that is conserved across species despite changes in other aspects of thalamic circuitry. This suggests that TCP neuronal form is based on relatively stable genetic blueprint and that epigenetic factors (e.g. synaptic input) resulting from evolutionary changes in thalamic organization have had less influence on dendritic architecture.

Triceps surae motoneuron morphology in the rat: a quantitative light microscopic study

The rat is now the model of choice for many studies of motor function. However, little quantitative information on the structure of rat motoneurons is available. In conjunction with efforts to define the physiologic and anatomic substrates of operantly conditioned plasticity in the spinal cord, 13 physiologically identified triceps surae motoneurons in the rat lumbar spinal cord were labeled intracellularly with horseradish peroxidase and completely reconstructed and measured with a computer-based neuron-tracing system. Somata were all located in the ventral horn of lumbar segments 4-5, had an average diameter of 35 microns, and had 6-12 dendrites. Dendrites ramified throughout the ventral horn and also penetrated the white matter. Their spread was greater in the rostrocaudal and dorsoventral directions (1.53 +/- 0.24 mm and 1.35 +/- 0.23 mm, respectively) than in the mediolateral direction (0.85 +/- 0.14 mm). Regardless of soma location, dendritic fields usually extended throughout the ipsilateral coronal cross-section of the ventral horn. As a result, the ventral or lateral extent of the field was correlated strongly with the soma's distance from the ventral or lateral border, respectively, of the ventral horn. Furthermore, although soma locations in the coronal plane varied widely, the centers of the dendritic fields tended to cluster near the center of the ventral horn. Dendrites constituted 96.2-98.4% (mean +/- SD = 97.3 +/- 0.7%) of the total neuronal surface area. Each of the 104 dendrites studied had an average of 13 branch points and 27 segments. First-order segment diameters ranged from 1.4 to 11.7 microns (mean +/- SD = 5.3 +/- 2.1 microns). Total dendritic length, surface area, volume, number of dendritic segments, and maximum segment order were correlated strongly with diameter of the first-order segment. Proceeding distally between branch points, the mean decrease in dendritic diameter (i.e., tapering) +/- the standard deviation was 22 +/- 8% of the proximal diameter. The average ratio +/- the standard deviation of the sum of the average diameters of each daughter segment raised to the 1.5 power to the average diameter of the parent segment raised to the 1.5 power (i.e., Rall's ratio; Rall, 1959) was 0.87 +/- 0.08. In comparison with cat alpha-motoneurons, rat motoneurons had smaller soma diameters, fewer dendrites, smaller total surface areas, and shorter total dendritic lengths. However, the number of terminations per dendrite was similar in the two species, so that rat motoneurons had more terminations per unit dendritic length.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for age-dependent changes in motoneuron dendritic morphology following neonatal nerve-crush in the rat

Motoneurons supplying the extensor hallucis longus muscle of the rat were temporarily separated from the muscle by sciatic nerve-crush at five days postnatally. Such treatment permanently alters the reflex response of the affected motoneurons without the large-scale cell death associated with nerve-crush at birth. After reinnervation, the motoneurons were retrogradely labelled with cholera toxin subunit-B conjugated to horseradish peroxidase and the dendritic tree of each labelled cell was analysed. When compared to normal data, significantly higher levels of dendritic density were observed in the rostrodorsally orientated parts of the dendritic field. This was similar to that found previously for the same motor pool after nerve-crush at birth. However, in other parts of the field where a lower dendritic density was found after nerve-crush at birth, no change was seen after nerve-crush at five days. These data present evidence for the influence of sensory afferents on the development of motoneuron dendrites. Taken together with the previous findings after nerve-crush at birth, we suggest that the differential dendritic changes caused by neonatal nerve lesion contribute to an imbalance in the pattern of excitatory and inhibitory inputs to the motoneuron, which results either in cell death, or the abnormal activity seen in those motoneurons which survive.

Ageing in the autonomic nervous system: a result of nerve-target interactions? A review

There are few generalisations that can be made regarding the changes that occur in autonomic nerves during ageing. Old age has different effects, including loss of neurones, loss of axon branches and alterations in neurotransmitters and other intracellular features. However, these age-related events are associated with particular and often small groups of neurones and are frequently species specific. Changes occur at different periods during development and maturity without any obvious age-stage at which neurodegenerative changes come to predominate. Some of the observations regarding neuronal changes in old age can be interpreted as the result of altered interactions between neurones and their peripheral target tissues. Recent studies in my laboratory support this contention. The neurotrophic theory has been used to explain such interactions during early development and it seems possible that, for example, alterations in the access of neurones to target-derived growth factors may underlie some of the changes that have been observed in old age. Plasticity in the mature autonomic nervous system may also be governed by similar relationships between nerves and their target tissues.

The influence of inhibitory afferents on the development of postsynaptic dendritic arbors

The growth and maintenance of dendritic form is dependent on normally functioning excitatory afferents. We have now examined the development of dendritic arbors in the gerbil lateral superior olive (LSO), following contralateral cochlear removal at postnatal day 7, a manipulation that substantially eliminates driven inhibitory transmission. Previous studies have demonstrated that the morphology of LSO dendritic arbors varies with tonotopic position and becomes more restricted with age. The presumed decrease of inhibitory transmission in the contralateral LSO resulted in a hypertrophic response. Quantification of Golgi-impregnated neurons revealed that dendrites had a significantly greater number of branch points, and their arbors were more spread out along the frequency axis compared to normal. This was especially apparent in the high frequency projection region where the glycine receptor density is known to be 4-fold higher than in the low frequency projection region. A measure of LSO nucleus size, cross-sectional area, was identical to control values, indicating no overt signs of degenerative phenomena. Cochlear ablation resulted in a significant atrophy of the ipsilateral LSO, with significant effects on dendritic structure. We conclude that decreased inhibitory transmission during development does not lead to a net degenerative response. Rather, the postsynaptic neurons exhibit a hypertrophic phenotype that may be due to the persistence of an immature state. These results indicate that activity-dependent morphogenetic events are a consequence of both excitatory and inhibitory synaptic transmission.

Effect of neonatal degranulation on the morphological development of rat CA3 pyramidal neurons: inductive role of mossy fibers on the formation of thorny excrescences

Intracellular peroxidase injections were made on CA3 pyramidal neurons from slices obtained from 3-60 day old control and experimental rats. Experimental rats had been irradiated at birth with gamma-rays to destroy the granule cells of the fascia dentata. Neonatal gamma-ray irradiation induced a clear cut reduction of the total number of granule cells (80% decrease at P60) and mossy fibers as revealed by Nissl and Timm stains. The number of branching points and segments, total dendritic length, and density of dendritic spines in the stratum radiatum and stratum oriens on CA3 pyramidal cells were not significantly different between control and irradiated rats. To quantify the development of thorny excrescences we measured the number and total area of the thorny excrescences per neuron at different stages of development. gamma-Ray irradiation induced a reduction in the number and total area of thorny excrescences. This effect was readily apparent by postnatal day (P) 12, and a reduction of 70% in the total area was observed at P30. In conclusion, gamma-ray irradiation destroys the majority of granule cells and induces a reduction in the development of thorny excrescences. Our data strongly suggest that this effect is directly caused by the lack of mossy fibers rather than by the irradiation itself since other parameters (i.e., the number of segments and branching points, density of non-mossy spines, and total dendritic length) were not affected. Therefore, we suggest that mossy fibers play an inductive role in the formation of thorny excrescences.

Refinement of dendritic arbors along the tonotopic axis of the gerbil lateral superior olive

We have investigated the development of dendritic arbors in a central auditory nucleus in the Mongolian gerbil, the lateral superior olive (LSO). The morphology of these arbors has been shown to vary with tonotopic position in adults, with high frequency neurons having a more restricted field. In the present study, qualitative observations were made on horseradish peroxidase-filled neurons from animals 1-11 days postnatal, and quantitative results were obtained from Golgi-impregnated material from animals 10 days postnatal and older. The tonotopic position of each cell was computed as a percent of the total distance along the LSO. The dendritic arbors of high frequency neurons became spatially constrained along the frequency axis during the 3rd postnatal week, while those in the low frequency region retained a broader arborization into adulthood. This refinement was correlated with a decrease in total dendritic length and the number of branch points per neuron, particularly in the high frequency projection region. The distribution of octave bandwidths to which single LSO neurons responded in 13-16 day animals showed a similar course of maturation across the tonotopic axis: high frequency neurons responded to a larger number of octaves, and with greater variability, than those in adults. These data suggest that a specific alteration in dendrite morphology, which occurs after the onset of response to airborne sound, may contribute to adult frequency selectivity.

Competitive interactions during dendritic growth: a simple stochastic growth algorithm

A simple growth algorithm is presented that deals with one feature of dendritic growth, the distance between branches. The fundamental assumption of our growth algorithm is that the lengths of dendritic segments are determined by the branching characteristics of the growing neurite. Realistic-appearing dendritic trees are produced by computer simulations in which it is assumed that: (1) growth of individual neurons occurs only at the tips of each growing neurite; (2) the growing neurite can either branch (as a bifurcation) or continue to elongate; (3) events at any one growing tip do not affect the events at any other growing tip; and (4) the probability of branching is a function only of the distance grown either from the cell body (if branching has not occurred) or from the previous branch point. An analytic solution of a differential equation based on these same assumptions produces a distribution of dendritic segment lengths that accurately fits an experimentally determined distribution of dendritic segment lengths of reconstructed neurons, accounting for about 89% of the sample variance. Our analysis indicates that, immediately following branching, the temporary suppression of further branching during dendritic growth may be an important mechanism for regulating the distance between branches.

Fractal dimension of dendritic tree of cerebellar Purkinje cell during onto- and phylogenetic development

The cerebellar Purkinje cell has highly branched dendrites extending in a transverse plane to the cerebellar folium. Branching patterns of the dendrites were subjected to fractal analysis to obtain a quantitative measure of their intricacy. Using a square-covering method, we found that dendrites have a statistical self-similarity so that their fractal dimension can be evaluated. In postnatal mouse, dimensions of Golgi-stained Purkinje cells increased as dendrites developed. The time course of the dimension increase was correlated with development of the width and area of the dendritic fields. After the dimension reached the maximum value at postnatal day 20, the width and area further continued to grow until day 100. In human cerebellum, dimensions of Purkinje cell dendrites increased greatly during pregnancy and reached 97% of the adult value at birth. On the other hand, 41% of the adult dendritic field width and 11% of the adult area had been attained at birth. The physical size of dendrites increased in a more lagged phase than the dimensions, and growth proceeded mainly after birth. These observations indicate that after the maximum complexity and fundamental tree pattern have been attained, the overall tree size further increases. It is observed that phylogenetic development of dendritic trees of Purkinje cells in several species of vertebrates showed a correlation with the increase in dimensions. These studies demonstrated the potency of the fractal analysis in evaluating development of dendritic trees of neurons.

Postnatal development of cat hind limb motoneurons supplying the intrinsic muscles of the foot sole

The postnatal development of dendrite anatomy in alpha-motoneurons intracellularly labeled with horseradish peroxidase (HRP), innervating the intrinsic muscles of the sole of the foot (IFS MNs) in the cat, was investigated. The number of dendrites per neuron was about 11 and did not change from birth to adult. The number of branches per dendrite decreased during the same period by 20-25%. The net elimination of dendritic branches appeared to occur at distal branching points, as revealed by topological analysis. The dendritic branching pattern tended to be asymmetric at birth and the net decrease in dendritic branching postnatally did not alter this pattern. The length of preterminal branches (PTB) increased by a factor of 2, while terminal branch (TB) length increased by a factor of 3.3 postnatally. The large increase in TB length was attributed to both longitudinal growth and an apparent lengthening caused by resorption of distal branches during development. Dendritic length in the transverse spinal cord plane increased in parallel with the overall growth of the parent spinal cord segment, while dendritic growth along the rostro-caudal axis exceeded, by about one order of magnitude, dendritic growth in the transverse plane. Average branch diameter doubled from birth to adult. The decrease in branch diameter across branching points did not obey satisfactorily to the 'power rule' of Rall. However, the 1.5 power ratio of daughters-to-parents branch dropped from 1.18 to 1.08 between 3 weeks of age and adult. Tapering was evident in both PTBs and TBs. The rate of taper did not change postnatally. From birth onwards, 'local' branch diameter correlated closely with amount of membrane area and combined length of the dendritic branches located distal to the 'supporting' parent branch. These relations were similar in all age groups and are suggested to be properties intrinsic to the IFS MNs. The local branch diameter also co-varied with the number of distal dendritic branches, but in this case there was a systematic shift in the relationship with increasing postnatal age. It appears that the local diameter in IFS MN dendrites is a key indicator of the size of the distal dendritic arborization.

Statistical evaluation of dendritic growth models

A mathematical model (Kliemann, W. 1987. Bull. math. Biol. 49, 135-152.) that predicts the quantitative branching pattern of dendritic tree was evaluated using the apical and basal dendrites of rat hippocampal neurons. The Wald statistic for chi 2-test was developed for the branching pattern of dendritic trees and for the distribution of the maximal order of the tree. Using this statistic, we obtained a reasonable, but not excellent, fit of the mathematical model for the dendritic data. The model's predictability of branching pattern was greatly enhanced by replacing one of the assumptions used for the original method "splitting of branches for all dendritic orders is stochastically independent", with a new assumption "branches are more likely to split in areas where there is already a high density of branches". The modified model delivered an excellent fit for basal dendrites and for the apical dendrites of hippocampal neurons from young rats (30-34 days postpartum). This indicates that for these cells the development of dendritic patterns is the result of a purely random and a systematic component, where the latter one depends on the density of dendritic branches in the brain area considered. For apical dendrites there is a trend towards decreasing pattern predictability with increasing age. This appears to reflect the late arrival of afferents and subsequent synaptogenesis proximal on the apical dendritic tree of hippocampal neurons.

A Golgi study of dendritic development in the dorsal lateral geniculate nucleus of normal ferrets

The development of neurons in the dorsal lateral geniculate nucleus (dLGN) of pigmented ferrets was studied by using the Golgi-Hortega technique. In adult ferrets, four dLGN cell classes were defined on the basis of somatic and dendritic morphology. Classes 1 and 2 were divided into stellate and oriented subtypes. Class 1 and 4 cells are characterized by filiform appendages, class 2 cells by club-like appendages, and class 3 cells by stalked appendages. At birth, dLGN neurons have simple dendritic arbors. During the first postnatal week, dendritic length and proximal branching density increase markedly. By postnatal day 21 (P21), dendritic morphology begins to take on mature characteristics and by the time of eye opening (P30-P35), most neurons can be classified. Also by that time, dLGN cells are covered with abundant filiform appendages. Developmental changes in appendage density were quantified for class 1 stellate cells. These data reveal that appendage density reaches a peak at P56, decreases sharply until P90, and then gradually declines to mature levels by P180. Elaboration and elimination of transient appendages occurs centrifugally; at maturity appendage density remains greater distally.

Protein synthesis is required for the initiation of dendritic growth in embryonic rat sympathetic neurons in vitro

We have utilized an experimental paradigm which allows the manipulation of dendritic growth in sympathetic neurons in culture to examine the effects of inhibitors of protein synthesis and RNA synthesis on the development of dendrites. Embryonic rat sympathetic neurons extend only axons when they are grown in serum-free medium on a polylysine substrate. The addition of an extract of basement membrane proteins (BME) to this culture system elicits dendritic growth within 48 h. Both cycloheximide and actinomycin-D inhibited BME-induced dendritic growth in greater than 80% of the neuronal population and reduced the number of dendrites extended by greater than or equal to 97%. In contrast, cycloheximide was found to have minimal effects on axonal growth in short-term (less than or equal to 18 h) cultures as measured with respect to the percentage of the population with axons and the number of axons per neuron. However, this inhibitor did significantly reduce (84%) the length of the axonal plexus extended. These results indicate that dendritic and axonal growth in sympathetic neurons are differentially dependent on protein synthesis such that the formation of dendrites requires protein synthesis whereas the initiation, but not the elongation, of axons is relatively independent of protein synthesis.

Morphometric analysis of developing neuronal geometry in the dorsal cochlear nucleus of the hamster

Since the shape of a cell's dendritic field and the distribution of its input determine the information that a cell receives and transmits, it is important to ascertain how the spatial relations between axonal arbors and dendritic fields develop. This study investigates the development of the dendritic fields of fusiform cells in the dorsal cochlear nucleus. Golgi-impregnated cells from postnatal day 10, 15, 25, 45 and 60+ hamsters were reconstructed. The computerized morphometric system used analyzed cells in the plane of section and also allowed rotation to and analysis in other specified planes, such as that parallel to the terminal arbors of the cochlear nerve fibers. The results suggest that the apical dendritic fields are oriented parallel to the axis of cochleotopic organization and that this orientation develops gradually after birth. Dendritic growth is the result of addition of new dendritic branches. The angles between branches also change and were analyzed by viewing the dendritic fields in different planes. This revealed that some preferential expansion of the apical dendritic field in the plane parallel to the cochlear projection planes is the result of increased angles between branches in that plane. Thus, dendritic growth via the addition of branches and the 'fanning-out' of existing branches underlie the development of oriented dendritic fields.

Identifiable reticulospinal neurons of the adult zebrafish, Brachydanio rerio

Reticulospinal neurons of the larval zebrafish Brachydanio rerio have been categorized into 27 different types (Kimmel et al.: Journal of Comparative Neurology 205:112-127, 1982; Metcalfe et al.: Journal of Comparative Neurology 251:147-159, 1986). Nineteen of these occur as bilateral pairs which are individually identifiable. Since considerable remolding of brain structures (e.g., cell death and modifications of neuronal architecture) occurs during development, we ask if these cells are preserved in the adult zebrafish and the extent to which neuronal morphology of the larva is conserved during ontogeny. In our analysis, we studied reticular neurons from 84 brains retrogradely labelled from the spinal cord with HRP. We show that all reticulospinal types of the larva are retained without considerable change in morphology in the adult. Many neurons, including the Mauthner cell and two of its serial homologues, MiD2cm and MiD3cm, can be individually and unambiguously identified. In addition, the appearance of later developing (tertiary) neurons leads to an increase in the numbers of some neuron types. Although tertiary neurons are often isomorphic with neighboring cells, they can have unique morphologies of their own and, therefore, are also individually identifiable. We suggest that the appearance of tertiary neurons may serve to extend the behavioural repertoire of the embryo. Moreover, morphological repetitions in adjacent segments of the otic region (level of VIIIth nerve entry) may represent the replication of a functional motif, perhaps involving the C-type escape response which is known to involve the Mauthner cell.

Sexual dimorphism in accessory olfactory bulb mitral cells: a quantitative Golgi study

The purpose of the present study was to identify the existence of sexual dimorphism in the dendritic field of accessory olfactory bulb mitral cells in rats and to investigate the effects of male orchidectomy and female androgenization on the day of birth upon this dendritic field. The rapid Golgi method was used to conduct a quantitative study of various characteristics of the dendritic field of accessory olfactory bulb mitral cells. The results indicated greater values for males than females for the following characteristics: (i) somatic area; (ii) degree of branching in the dendritic field; (iii) total dendritic length; and (iv) dendritic density around the neuronal soma. Orchidectomy of males, as well as androgenization of females, on the day of birth inverted these differences.

Differentiation of apical, basal and mixed dendrites of fusiform cells in the cochlear nucleus

Many studies suggest that the details of morphogenesis (e.g. the length and number of dendrites) are determined by factors extrinsic to the cell, while the basic form of the neuron (e.g. the shape of the soma and the placement of the primary dendritic trunks) is determined by intrinsic factors. The following study describes the development of the dendrites of fusiform cells in the dorsal cochlear nucleus of the hamster using Golgi-stained brains from hamsters of various ages. Two basic types of dendrites are described--apical and basal--which emanate from opposite ends of the cell body and differ in their morphology. A third type of dendrite that exits the cell laterally can create a deflection in the perimeter of the cell body altering its shape. The morphology of these dendrites is described and compared to the apical and basal dendrites. Segments of laterally extending dendrites that are near apical dendrites are qualitatively and quantitatively identical to apical dendrites (that is they branch frequently and are spine-laden) and the converse is true of the segments near basal dendrites. The results suggest that during development, whether a dendritic will be apical-like or basal-like is determined by the location of its distal segment. Thus, extrinsic factors influence the overall form of these neurons.

Analysis of binary trees when occasional multifurcations can be considered as aggregates of bifurcations

The geometrical properties of neurons are important for the way they function within neural circuits. The arborescent processes of neurons that are necessary for the transmission of the information are formed by branching and elongation of segments. In studies that model the outgrowth the tree structures have generally been considered as binary. However, multifurcations do occur. It will be shown that if the multifurcations can be considered as aggregates of bifurcations they may be included in the topological analysis of neuronal branching patterns.

HPLC detection of dopamine and noradrenaline in the cochlea of adult and developing rats

The presence and postnatal development of dopamine (DA) and noradrenaline (NA) in the rat cochlea were detected by high-performance liquid chromatography coupled with electrochemical detection. Cochlear DA content rose gradually after birth. Conversely, NA concentrations rose rapidly between postnatal day 1 and 8; then, up to day 30, it increased more slowly. On day 30, both DA and NA levels were around 5 times higher than on day 1. In the adult rat cochlea, NA mean content was 234 +/- 22.2 pg/mg protein, while DA mean content was 23.6 +/- 1.1 pg/mg protein. Adrenaline was always undetectable. The present study is the first report describing directly the presence of DA in the rat cochlea. DA might serve as one of the lateral efferent neurotransmitters, whereas NA probably acts as a neurotransmitter of the sympathetic cochlear innervation. Nevertheless, their influences on the cochlear physiology, either in adulthood or during development are still a matter of discussion.

Phenylethanolamine N-methyltransferase-immunoreactive terminals synapse on adrenal preganglionic neurons in the rat spinal cord

Adrenergic neurons in the C1 region in the ventrolateral medulla oblongata send descending axons into spinal cord which terminate in thoracic and upper lumbar segments, overlapping the distribution of sympathetic preganglionic neurons. The present study was undertaken to determine whether adrenergic fibers synapse directly on preganglionic neurons which innervate the adrenal medulla and to examine the ultrastructure of these fibers during development. The ultrastructure and synaptology of adrenergic axons in the intermediolateral nucleus of mid-thoracic spinal cord were studied in 7-, 9-, 24-, 30-, 60-, and 90-day-old rats using immunocytochemical staining for phenylethanolamine N-methyltransferase, the epinephrine-synthesizing enzyme. Phenylethanolamine N-methyltransferase-immunoreactivity was observed in the cytoplasm of unmyelinated axonal varicosities and intervaricose segments in the neuropil of intermediolateral nucleus. Phenylethanolamine N-methyltransferase-immunoreactive synaptic boutons were filled with spherical electron-lucent vesicles and occasional larger dense-core vesicles. These boutons were observed to form symmetrical synaptic contacts with dendritic processes at all ages examined. Asymmetrical synapses on dendrites were also observed in adult rats. Axosomatic synaptic contacts were frequently observed in immature rats, but were never observed in adult rats. To determine whether adrenergic axons synapse on preganglionic neurons which project to the adrenal medulla, adrenal preganglionic neurons were retrogradely labeled with horseradish peroxidase and adrenergic axons were stained for phenylethanolamine N-methyltransferase-immunoreactivity. In young rats, phenylethanolamine N-methyltransferase-immunoreactive boutons were observed to form symmetrical axosomatic and axodendritic synaptic contacts with adrenal preganglionic neurons in intermediolateral nucleus. These contacts had already formed by postnatal day 7, the youngest age studied. In contrast, it was not possible to verify that adrenal preganglionic neurons receive adrenergic innervation in adult rats, since phenylethanolamine N-methyltransferase-immunoreactive boutons were only observed in contact with small diameter dendrites that were not retrogradely labeled by horseradish peroxidase. These studies demonstrate that adrenal preganglionic neurons receive adrenergic synapses prior to the first postnatal week. The initial synapses which form on preganglionic somata and proximal dendrites appear to reorganize late in development. It is suggested that these become more distally located as the dendritic tree matures. More generally, these observations suggest that adrenergic bulbospinal neurons are involved in central regulation of adrenal development and function.

Postnatal development of cat hind limb motoneurons. II: In vivo morphology of dendritic growth cones and the maturation of dendrite morphology

The maturation of dendrite morphology was studied by light and electron microscopy in cat spinal alpha-motoneurons intracellularly labeled with horseradish peroxidase. Alpha-motoneurons supplying the triceps surae (TS) and the intrinsic foot sole (SP) muscles were investigated in kittens from birth to 44-46 days of postnatal (d.p.n.) age. At birth, a large number of dendritic branches displayed growth cones, filopodia, and fusiform processes. The growth cones were of lamellipodial and filopodial types, but intermediate forms also occurred. The growth cones shared several morphological features with the neuritic growth cones studied in vitro. It was suggested that the occurrence of different types of growth cones--even in the same dendrite--may reflect their transformation from one type to the other and the level of growth activity could be inferred from the number and form of the growth cones. About 50-70% of the terminal branches in the dendrites of newborn kittens possessed growth cones, filopodia, and/or fusiform processes. The corresponding figure for preterminal branches was 20-30%, with a clear decrease in incidence when approaching the soma. During the period under study, most of these growth-associated processes disappeared from the dendrites so that at 44-46 d.p.n. of age only about 10% of the terminal and less than 1% of the preterminal branches had growth-associated processes. Analysis of the three-dimensional distribution of dendritic branches with such processes disclosed that they were relatively more frequent in the medial, rostral, and caudal dendritic territories. It was concluded that the pattern of distribution and disappearance of growth cones, filopodia, and fusiform processes coincided with postnatal longitudinal dendritic growth and the development of the adult dendritic territories described in a preceding paper (Ulfhake et al., '88). Dendritic growth, with respect to length and caliber, also occurred in the absence of growth cones and filopodia. It is suggested that the important role of these processes may be to act as a steering device in establishing the adult distribution and synaptology of the dendrites. Comparison of TS and SP alpha-motoneuron dendrite morphology at birth and at 22-24 d.p.n. age showed that the SP neurons lagged in the maturation process. Light and electron microscopic observations indicated that postnatally direct contacts might exist between dendrites and fine blood vessels in the neuropil without any interposing glial sheath. The number of such suspected contacts diminished during the period under study, indicating that the glial ensheathment of the blood vessel takes place, in part, postnatally.

Postnatal development of cat hind limb motoneurons. I: Changes in length, branching structure, and spatial distribution of dendrites of cat triceps surae motoneurons

The postnatal development of length, branching structure, and spatial distribution of dendrites of triceps surae motoneurons, intracellularly stained with horseradish peroxidase, was studied from birth up to 44-46 days of postnatal (d.p.n.) age in kittens and compared with corresponding data from adult cats. The number of dendrites of a triceps surae motoneuron was about 12, and the arborization of each dendrite generated an average of 12-15 terminal branches. There was no net change in the number of dendrites of a neuron or in the degree of branching of the dendrites despite the occurrence of both a transient remodeling of the dendritic branching structure and changes of the spatial distribution of the dendritic branches during postnatal development. The perisomatic territory in the transverse plane occupied by the dendritic branches of a motoneuron increased in parallel with the overall growth of the spinal cord. Thus, the relative size of the dendritic territory in this plane was kept almost constant, whereas dendritic branches projecting in the rostrocaudal direction grew much faster than the spinal cord and also became more numerous. At birth the rostro-caudal dendritic span of individual motoneurons bridged 1:6 to 1:5 of the L7 spinal cord segment length; this figure was 1:3 at 22-24 d.p.n. Hence, in this direction, the growing dendritic branches invaded novel dendritic territories. The change in dendritic branch length from birth to 6 weeks of age corresponded to an average growth rate of 2 to 4 microns per dendritic branch and day, which implies that the total increase in length of the dendrites of a neuron could amount to 1 mm/day. The increase in branch length did not occur in a uniform or random manner; instead, it followed a spatiotemporal pattern with three phases: From birth to 22-24 d.p.n., growth was particularly prominent in greater than or equal to 3rd order preterminal and 2nd through 6th order terminal branches. From 22-24 to 44-46 d.p.n., a large increase in branch length confined to terminal branches of greater than or equal to 3rd branch orders was observed. As indicated by topological analysis, this length increase was probably due in part to a resorption of peripheral dendritic branches during this stage of development. From 44-46 d.p.n. to maturity, the increase of dendritic branch length was restricted to preterminal branches of low (less than or equal to 4th) branch order.(ABSTRACT TRUNCATED AT 400 WORDS)

Link-Vertex Analysis of Purkinje cell dendritic trees from the murine cerebellum

A method of analysis of dendritic trees is described, called Link-Vertex Analysis, which defines the branching patterns (topology) by means of a discontinuous function, the terminal/link (T/L) vertex ratio, computed at all successive vertex orders within a network, and represented as a graphical plot for a given tree. The T/L vertex ratio is defined as the number (n) of terminal (pendant) vertices (Vp) divided by the total number (n) of all link vertices (both dichotomous, Vd, and trichotomous, Vt) multiplied by the number of segments arising from each vertex--i.e., nVp/(2nVd + 3nVt) per vertex order. Hypotheses of growth are simulated by computer modelling and the distributions of computer generated T/L vertex ratios per order compared with those of actual dendritic trees. The differential distribution of T/L vertex ratios per order in trees grown by either random terminal or random segmental branching discriminates between these two forms of growth. Significant advances of this method include the detection of regional variations in growth and remodelling in a tree and the incorporation of Vt with little perturbation of the Vd data. The analytical powers of this method have been tested using the dendritic fields of 20-day-old mouse cerebellar Purkinje cells defining possible temporal sequences and relative contributions of random and non-random terminal growth, and also the frequency of branching over a growth front relative to that of the rest of the tree.

Neonatal deafening alters nonpyramidal dendrite orientation in auditory cortex: a computer microscope study in the rabbit

In order to examine the influence of afferent input on nonpyramidal dendrite development in the auditory cortex, unilateral deafening was carried out in neonatal rabbits at birth, approximately 6 days prior to the onset of hearing. Deafening was produced by surgical removal of the incus and stapes ossicles, aspiration of the cochlear perilymph, and kanamycin injection into the oval window. At 60 days of age, acoustic stimulation of the deafened ear was unable to evoke auditory brainstem responses. The brains of experimental and littermate control rabbits were processed according to the Golgi-Cox Nissl method. The dendritic systems of lamina III/IV spine-free nonpyramidal cells in the auditory cortex contralateral to the deafened ear were digitized from 340-micron-thick coronal sections with the aid of a computer microscope. Three-dimensional spatial and statistical analyses revealed that nonpyramidal dendrite length in neonatally deafened rabbits increased 27% relative to littermate controls. A fan-in projection analysis revealed that the increased dendrite length in the deafened animals was maximum in the tangential direction and toward the white matter. Computer rotation of digitized neurons from neonatally deafened rabbits also revealed evidence of abnormal dendritic growth in the form of recurved dendrites. We interpret our results to indicate that unilateral cochlear destruction early in development causes a reorganization of the ascending auditory pathway which extends to the contralateral cerebral cortex. Because the auditory cortex contralateral to the deafened ear still receives acoustic input from the undamaged ipsilateral ear, normal nonpyramidal dendritic growth in the auditory cortex is, in part, dependent upon afferent activity arising from both ears.

Differentiation of neuronal growth cones: specialization of filopodial tips for adhesive interactions

Adhesive contacts made by filopodia of developing neurons are important in neurite growth and in the formation of synaptic junctions. In the present work, filopodial interactions of cultured chicken retina neurons were studied by using video-enhanced contrast, differential interference contrast (VEC-DIC) microscopy and the high-voltage electron microscope (HVEM). Use of the HVEM to examine whole mounts of fixed cells showed that filopodia in older cultures developed an appearance that might be expected of nascent synapses, becoming enlarged at their endings and accumulating organelles resembling synaptic vesicles. VEC-DIC microscopy, used to observe the motility and adhesive properties of filopodia in living cells, showed there was a particularly high affinity between filopodia tips. Contacting filopodia typically repositioned themselves so they could attach at a tip-to-tip position, occasionally bending as much as 90 degrees to achieve this preferred orientation. Interacting filopodia frequently remained together as they pushed or pulled on each other, moved laterally together, or stretched tightly and underwent intense vibratory movements. Such linked motility occurred even when apparent gaps existed between the filopodia. Examination of these gaps with the HVEM revealed filamentous structures linking the apposed membranes. The filamentous links were 10-13 nm in diameter and 30-100 nm long. Although it has not yet been established that the filaments reflect the native configuration of the interconnecting materials, the structures seem likely to be associated with the strongly adhesive behavior of the filopodial tips. The possible significance of these structural and functional properties of filopodia tips to axon growth and synapse formation is discussed.

6-OHDA-induced ectopia of external granule cells in the subarachnoid space covering the cerebellum. III. Morphology and synaptic organization of ectopic cerebellar neurons: a scanning and transmission electron microscopic study

The present report describes the ultrastructure, surface morphology, and synaptic connectivity of ectopically placed cerebellar neurons after treatment of newborn rats with 100 micrograms 6-hydroxydopamine (6-OHDA), administered intracisternally. In addition to granule cells which form the majority of ectopic neurons, neurons exhibiting the ultrastructure of basket/stellate cells are found in the subarachnoid space over the cerebellum. The ectopic neurons present an almost complete spectrum of homologous efferent and afferent connections. Parallel fiber synapses are found on thorns of spiny branchlets of Purkinje cell dendrites which also have grown out into the subarachnoid space, and on the somata and dendrites of basket/stellate cells. Many ectopic parallel fibers are seen to pass into the molecular layer of the underlying cerebellar cortex through defects in the pial surface, presumably connecting with intracortical postsynaptic partners. Synapses between ectopic Purkinje cell dendrites and basket/stellate cell axons are also observed; however, the source of these axons remains uncertain. Granule cell dendrites are engaged in glomeruli with mossy fibers. Moreover, ectopic granule cell colonies are densely innervated by noradrenergic fibers. Our results show that the technique of generating ectopia of external granule cells provides an additional model for investigating influences of epigenetic factors on the development of nerve cells.

Vertex analysis of Purkinje cell dendritic trees in the cerebellum of the rat

All networks are made up of vertices (points interconnected by segments), which include terminals interconnected by terminal segments, nodes interconnected by link segments and the root point connected to the tree by the root segment. All nodes may be classified into unique types according to the number of terminal and link segments they drain. For example, there are three distinct dichotomous nodes, a 'primary' node draining two terminal segments, a 'secondary' node draining one terminal segment and a link segment, and a 'tertiary' node draining two link segments. The numbers of primary and tertiary nodes approximate to equality in large networks and thus the ratio of primary to secondary nodes defines topology. All higher order nodes ( trichotomous and beyond) may be resolved into dichotomous forms and incorporated into the analysis. Different forms of growth may thus be analysed by comparing the frequency distributions of nodes with those generated by computer simulated growth models. Moreover, all vertices can be ordered so that metrical parameters are easily incorporated and the hierarchical arrangements of vertices of different order discerned. The dendritic trees of 48 Purkinje cells, taken from folia along the primary fissure, were analysed using vertex analysis. The mean number of segments in Purkinje cell trees was 881 +/- 23 (s.e.) and mean total dendritic length 7959 +/- 233 (s.e.) micrometers. Segment lengths were longest over proximal segments but over most of the tree segment lengths were constant at 10 +/- 0.2 (s.e.) micrometers. Vertex, segment and terminal frequency distributions of equivalent orders were all normal with a slight positive skew. Peak frequencies were recorded at the 12th equivalent order. The mean primary/secondary nodal vertex ratio was 0.93 and the proportion of trichotomous branch points in the tree was 5%. Comparison of the frequency distribution of all vertices with computer generated models showed that growth of the Purkinje cell was most closely simulated by a random terminal growth model, incorporating 5% trichotomy , in which the branching of high order terminals was more likely than low order terminals. It was concluded that growth of the Purkinje cell tree could proceed by random terminal branching with growth occurring preferentially over a front composed of terminals that are ascending through a corridor in the molecular layer whose margins are defined by neighbouring trees.

Glial heterogeneity may define the three-dimensional shape of mouse mesencephalic dopaminergic neurones

The shape of a neurone--the projection and branching pattern of axons and dendrites--appears to be determined by a combination of intrinisic and environmental influences. We have previously shown that striatal target neurones influence the biochemical maturation of ascending mesencephalic dopamine (DA) cells in culture, as well as the elongation rate of DA neurites. Using a similar approach in which the morphology of individual DA cells can be studied after 3H-DA uptake and autoradiography, we now report on in vitro neurone-glia interactions and show that glial cells exert a morphogenetic effect on DA neurones. Dopaminergic neurones from the mesencephalon were plated on glial monolayers prepared either from the striatal or the mesencephalic region of the embryonic brain. On mesencephalic glial cells the majority of DA neurones develop a great number of highly branched and varicose neurites, whereas on striatal glia they only exhibit one long, thin and rather linear neurite. These results demonstrate that glial cells from two different brain regions have distinct properties which could be used to define neuronal polarity observed in vivo.

Hormonal and humoral influences on brain development

This review discusses evidence for neurotransmitters as developmental signals in such ontogenic processes as neural tube formation (neurulation), germinal cell proliferation, and neuronal and glial differentiation during brain organogenesis, as well as evidence for other roles of these neurotransmitters in non-neuronal tissues of vertebrates and invertebrates. Evidence also is presented for hormonal regulation of brain development during postnatal neurogenesis and for interrelationships which may link neurotransmitters and hormones in a humoral milieu, providing a variety of control mechanisms for the central and peripheral nervous system during key phases of their development. Given the evidence for neurotransmitters and hormones as coordinating influence on neural ontogeny, it is possible that drugs, stress, and environmental influences may have the ability to perturb particular aspects of these developmental systems if present during those "critical periods" when such humoral influences are important for normal ontogeny.

Target regulation of synaptic number in the compressed retinotectal projection of goldfish

In order to determine the morphological consequences of the formation of a compressed retinotectal projection, the optic neuropil lamina (stratum fibrosum et griseum superficialis, SFGS) was examined in large goldfish 3 months to 4 years after ablation of the caudal half of the tectum both with crush of the optic nerve (HTX) without (HT). In semithin sections, the SFGS, as delineated with orthograde HRP labeling, shows a persistent hypertrophy of about 25% in HTX and HT groups. Comparison of ultrastructural stereological data with similar data on control and regenerated projections to intact tecta (Murray and Edwards, '82) indicated that this hypertrophy can be attributed largely to an increased number of axons and not to increases in terminal or dendritic compartments. A normal number of synaptic terminals per column through SFGS is conserved in HTX and HT groups. Planimetric analysis and observations using orthograde HRP labeling reveal no group differences in size and shape of terminal profiles. The same number of retinal ganglion cells project to a half-tectum as to an intact tectum, as indicated by estimates of ganglion cell number and of the minimum percentage of them which project to the tectum using retrograde HRP labeling. The results suggest that the regenerating and sprouting optic axons participating in the formation of a compressed retinotopic projection compete for a limited accommodation inthe SFGS and that this capacity to accept synaptic input becomes saturated at the control innervation density. The results are consistent with the formation of a smaller than normal number of terminals per optic axon, numerical estimates for which are given. If the percentage of terminals which are optic does not change, then the number of terminals per axon is reduced by about 40%.

Influence of neonatal cochlear removal on the development of mouse cochlear nucleus: II. Dendritic morphometry of its neurons

Right cochleae were aspirated from 6-day-old mice to determine the influence of cochlear integrity on the dendritic development of neurons within cochlear nucleus (CN). At 45 days of age, cochlear destruction was confirmed histologically and the brains were stained by the Golgi-Cox method to permit dendritic morphometry in CN ipsilateral (deafferented) and contralateral (normal) to the neonatally lesioned cochleae. The dendritic field cross-sectional area of ventral CN bushy cells was reduced on the deafferented side, as was the total dendritic length of stellate cells throughout ventral and dorsal CN. The neonatal deafferentation had no statistically significant effect on the total dendritic length of those dorsal CN fusiform cells that developed. These dendritic changes are interpreted as lack of development due to the loss of auditory nerve afferents during a critical period of development and indicate that any congenital pathology that compromises the cochlear sensorineural structures may lead to central auditory abnormalities as well.