Formation and migration of neuroblasts in the spinal cord of the chick embryo

Genetic and Molecular Approaches to Study Neuronal Migration in the Developing Cerebral Cortex

The migration of neuronal cells in the developing cerebral cortex is essential for proper development of the brain and brain networks. Disturbances in this process, due to genetic abnormalities or exogenous factors, leads to aberrant brain formation, brain network formation, and brain function. In the last decade, there has been extensive research in the field of neuronal migration. In this review, we describe different methods and approaches to assess and study neuronal migration in the developing cerebral cortex. First, we discuss several genetic methods, techniques and genetic models that have been used to study neuronal migration in the developing cortex. Second, we describe several molecular approaches to study aberrant neuronal migration in the cortex which can be used to elucidate the underlying mechanisms of neuronal migration. Finally, we describe model systems to investigate and assess the potential toxicity effect of prenatal exposure to environmental chemicals on proper brain formation and neuronal migration.

Prox1 regulates a transitory state for interneuron neurogenesis in the spinal cord

Proper central nervous system (CNS) function depends critically on the generation of functionally distinct neuronal types in specific and reproducible positions. The generation of neuronal diversity during CNS development involves a fine balance between dividing neural progenitors and the differentiated neuronal progeny that they produce. However, the molecular mechanisms that regulate these processes are still poorly understood. Here, we show that the Prox1 transcription factor, which is expressed transiently and specifically in spinal interneurons, plays an important role in neurogenesis. Using both gain- and loss-of-function approaches, we find that Prox1 is capable of driving neuronal precursors out of the cell cycle and can initiate limited expression of neuronal proteins. Using RNAi approaches, we show that Prox1 function is required to execute a neurogenic differentiation program downstream of Mash1 and Ngn2. Our studies demonstrate an important, spinal interneuron-specific role for Prox1 in controlling steps required for both cell-cycle withdrawal and differentiation.

Primary sensory afferent innervation of the developing superficial dorsal horn in the South American opossum Monodelphis domestica

The development of the primary sensory innervation of the superficial dorsal horn (SDH) was studied in postnatal opossums Monodelphis domestica by using DiI labelling of primary afferents and with GSA-IB(4) lectin binding and calcitonin gene-related peptide (CGRP) immunoreactivity to label primary afferent subpopulations. We also compared the timing of SDH innervation in the cervical and lumbar regions of the spinal cord. The first primary afferent projections to SDH emerge from the most lateral part of the dorsal root entry zone at postnatal day 5 and project around the lateral edge of the SDH toward lamina V. Innervation of the SDH occurs slowly over the second and third postnatal weeks, with the most dorsal aspect becoming populated by mediolaterally oriented varicose fibers before the rest of the dorsoventral thickness of the SDH becomes innervated by fine branching varicose fibers. Labelling with GSA-IB(4) lectin also labelled fibers at the lateral edge of the dorsal horn and SDH at P5, indicating that the GSA-IB(4) is expressed on SDH/lamina V primary afferents at the time when they are making their projections into the spinal cord. In contrast, CGRP-immunoreactive afferents were not evident until postnatal day 7, when a few short projections into the lateral dorsal horn were observed. These afferents then followed a pattern similar to the development of GSA-IB(4) projects but with a latency of several days. The adult pattern of labelling by GSA-IB(4) is achieved by about postnatal day 20, whereas the adult pattern of CGRP labelling was not seen until postnatal day 30. Electron microscopy revealed a few immature synapses in the region of the developing SDH at postnatal day 10, and processes considered to be precursors of glomerular synapses (and thus of primary afferent origin) were first seen at postnatal day 16 and adopted their definitive appearance between postnatal days 28 and 55. Although structural and functional development of forelimbs of neonatal Monodelphis is more advanced than the hindlimbs, we found little evidence of a significant delay in the invasion of the spinal cord by primary afferents in cervical and lumbar regions. These observations, together with the broadly similar maturational appearance of histological sections of rostral and caudal spinal cord, suggest that, unlike the limbs they innervate, the spinal regions do not exhibit a large rostrocaudal gradient in their maturation.

The generation and diversification of spinal motor neurons: signals and responses

Motor neurons are probably the best characterised neuronal class in the vertebrate central nervous system and have become a paradigm for understanding the mechanisms that control the development of vertebrate neurons. For many investigators working on this problem the chick embryo is the model system of choice and from these studies a picture of the steps involved in motor neuron generation has begun to emerge. These findings suggest that motor neuron generation is shaped by extracellular signals that regulate intrinsic, cell-autonomous determinants at sequential steps during development. The chick embryo has played a prominent role in identifying the sources of these signals, defining their molecular identities and determining the cell intrinsic programs they regulate.

Specific regulation of cyclins D1 and D2 by FGF and Shh signaling coordinates cell cycle progression, patterning, and differentiation during early steps of spinal cord development

In the vertebrate embryo, spinal cord elongation requires FGF signaling that promotes the continuous development of the posterior nervous system by maintaining a stem zone of proliferating neural progenitors. Those escaping the caudal neural stem zone, which is expressed to Shh signal, initiate ventral patterning in the neural groove before starting neuronal differentiation in the neural tube. Here we investigated the integration of D-type cyclins, known to govern cell cycle progression under the control of extracellular signals, in the program of spinal cord maturation. In chicken embryo, we find that cyclin D2 is preferentially expressed in the posterior neural plate, whereas cyclin D1 appears in the neural groove. We demonstrated by loss- and gain-of-function experiments that FGF signaling maintains cyclin D2 in the immature caudal neural epithelium, while Shh activates cyclin D1 in the neural groove. Moreover, forced maintenance of cyclin D1 or D2 in the neural tube favors proliferation at the expense of neuronal differentiation. These results contribute to our understanding of how the cell cycle control can be linked to the patterning programs to influence the balance between proliferation and neuronal differentiation in discrete progenitors domains.

Onset of neuronal differentiation is regulated by paraxial mesoderm and requires attenuation of FGF signalling

While many neuronal differentiation genes have been identified, we know little about what determines when and where neurons will form and how this process is coordinated with the differentiation of neighbouring tissues. In most vertebrates the onset of neuronal differentiation takes place in the spinal cord in a head to tail sequence. Here we demonstrate that the changing signalling properties of the adjacent paraxial mesoderm control the progression of neurogenesis in the chick spinal cord. We find an inverse relationship between the expression of caudal neural genes in the prospective spinal cord, which is maintained by underlying presomitic mesoderm and FGF signalling, and neuronal differentiation, which is repressed by such signals and accelerated by somitic mesoderm. We show that key to this interaction is the ability of somitic mesoderm to repress Fgf8 transcription in the prospective spinal cord. Our findings further indicate that attenuation of FGF signalling in the prospective spinal cord is a prerequisite for the onset of neuronal differentiation and may also help to resolve mesodermal and neural cell fates. However, inhibition of FGF signalling alone does not promote the formation of neurons, which requires still further somite signalling. We propose a model in which signalling from somitic tissue promotes the differentiation of the spinal cord and serves to co-ordinate neural and mesodermal development.

Sonic hedgehog signaling is required during the appearance of spinal cord oligodendrocyte precursors

Spinal cord oligodendrocyte precursors arise in the ventral ventricular zone as a result of local signals. Ectopic oligodendrocyte precursors can be induced by sonic hedgehog (Shh) in explants of chick dorsal spinal cord over an extended developmental period. The role of Shh during normal oligodendrocyte development is, however, unclear. Here we demonstrate that Shh is localized to the ventral spinal cord immediately prior to, and during the appearance of oligodendrocyte precursors. Continued expression of Shh is required for the appearance of spinal cord oligodendrocyte precursors as neutralization of Shh signaling both in vivo and in vitro during a defined developmental period blocked their emergence. The inhibition of oligodendrocyte precursor emergence in the absence of Shh signaling was not the result of inhibiting precursor cell proliferation, and the neutralization of Shh signaling after the emergence of oligodendrocyte precursors had no effect on the appearance of additional cells or their subsequent differentiation. Similar concentrations of Shh induce motor neurons and oligodendrocytes in dorsal spinal cord explants. However, in explants from early embryos the motor neuron lineage is preferentially expanded while in explants from older embryos the oligodendrocyte lineage is preferentially expanded.

Progenitor dispersal and the origin of early neuronal phenotypes in the chick embryo spinal cord

Using DiI fluorescent dextrans, we have created fate maps of the neural plate and early neural tube describing the extent of progenitor cell dispersal and the spatial origin of morphologically distinct neuronal cell types along the dorsoventral axis of the developing chick spinal cord. Nonuniform dispersal and mixing of progenitors occur within the early neuroepithelium, with the degree of dispersal being determined by the initial position of the cells along the mediolateral axis of the neural plate. Dispersal is greatest in the midregions of the ventricular epithelium and decreases toward the dorsal and ventral midlines. Phenotypically diverse classes of neurons are born at specific dorsoventral locations in the neural tube. Motor neurons are the most ventral cell type generated followed, at progressively more dorsal positions, by distinct classes of interneurons. Several genes show dorsoventrally restricted patterns of expression within the neural tube and the fate maps were used to investigate the relationship between one of these genes, Pax3, and progenitor cell dispersal and fate. The results indicate that the dorsoventral pattern of Pax3 expression is not maintained by restrictions to cell mixing and are consistent with a role for this transcription factor in specifying the identity of neurons with contralateral descending axons.

NeuroM, a neural helix-loop-helix transcription factor, defines a new transition stage in neurogenesis

Genes encoding transcription factors of the helix-loop-helix family are essential for the development of the nervous system in Drosophila and vertebrates. Screens of an embryonic chick neural cDNA library have yielded NeuroM, a novel neural-specific helix-loop-helix transcription factor related to the Drosophila proneural gene atonal. The NeuroM protein most closely resembles the vertebrate NeuroD and Nex1/MATH2 factors, and is capable of transactivating an E-box promoter in vivo. In situ hybridization studies have been conducted, in conjunction with pulse-labeling of S-phase nuclei, to compare NeuroM to NeuroD expression in the developing nervous system. In spinal cord and optic tectum, NeuroM expression precedes that of NeuroD. It is transient and restricted to cells lining the ventricular zone that have ceased proliferating but have not yet begun to migrate into the outer layers. In retina, NeuroM is also transiently expressed in cells as they withdraw from the mitotic cycle, but persists in horizontal and bipolar neurons until full differentiation, assuming an expression pattern exactly complementary to NeuroD. In the peripheral nervous system, NeuroM expression closely follows cell proliferation, suggesting that it intervenes at a similar developmental juncture in all parts of the nervous system. We propose that availability of the NeuroM helix-loop-helix factor defines a new stage in neurogenesis, at the transition between undifferentiated, premigratory and differentiating, migratory neural precursors.

Cytokines which signal through the LIF receptor and their actions in the nervous system

A number of different cytokines, each initially characterized on the basis of very different biological activities, all have very similar signalling pathways and share a similar tertiary structure. These cytokines include leukaemia inhibitory factor, ciliary neuronotrophic factor, oncostatin M, growth-promoting activity and cardiotrophin 1. They all have been found to regulate a number of properties of cells of the developing and mature nervous system in vitro and thus are neuroregulatory cytokines. The actions of these cytokines include regulation of neurotransmitter phenotype, differentiation of neuronal precursor cells both in the peripheral nervous system and in the spinal cord, survival of differentiated neurons, and regulation of development of both astrocytes and oligodendrocytes. In addition, studies in animal models show that these factors can rescue sensory and motor neurons from axotomy-induced cell death, which suggests that they can act as trauma factors for injured neurons. Analysis of the expression patterns of the different neuroregulatory cytokines and their receptors reveals that the receptors are expressed throughout nervous system development and following trauma, whereas the cytokines show temporal and spatial specific expression patterns. This is consistent with the idea that specific cytokines have specific roles in neural development and repair, but that their signalling pathways are shared. The phenotypes of the receptor knockouts show clear deficits in nervous system development, indicating a crucial role for LIF receptor signalling. Knockouts of individual cytokines are less dramatic, but LIF and CNTF knockouts do reveal deficits in maintenance of motor neurons or following trauma. Thus, whereas LIF and CNTF have clear roles in maintenance and following trauma, it is unclear which of the cytokines is involved in nervous system development. In clinical terms, these findings add further support to the use of these cytokines in nervous system trauma and disease.

Restriction in cell fates of developing spinal cord cells transplanted to neural crest pathways

At early neural tube stages, individual stem cells can generate neural crest cells as well as dorsal or ventral spinal cord cells. To determine whether this pluripotency is lost as development proceeds, we back-transplanted quail spinal cells from different developmental stages and different spinal locations into the crest migratory pathways of st 16-20 chicken host embryos. The transplanted spinal cells from st 27 dorsal cord and st 18 ventral cord differentiated within the new crest environment into sensory and sympathetic neurons, satellite and Schwann cells, and melanocytes. St 27 ventral cells still generated several crest derivatives but not sensory or sympathetic neurons. This loss in ability to produce neurons correlates with the end of neurogenesis in ventral cord. The end of neurogenesis in the cord, therefore, results from an intrinsic change in the potential of spinal neuroepithelial cells to generate neurons.

Target contact regulates expression of synaptotagmin genes in spinal motor neurons in vivo

During neuromuscular development, neuronal contact with peripheral targets is associated with an increase in synaptic vesicle protein (SVP) gene expression, suggesting that target contact and upregulation of SVP genes are causally related. To test this idea, we analyzed the developmental expression pattern of synaptotagmin (syt) mRNAs in the chick lateral motor column (LMC) using in situ hybridization. Syt I mRNA in the LMC is upregulated from Embryonic Day 4.5 (E4.5) to E5.5, coincident with the time these neurons begin to make contact with their muscle targets. In contrast, levels of mRNA for neurofilament do not change during this time. Extirpation of the limb bud prior to motor axon outgrowth eliminates the increase in syt I mRNA ipsilaterally. Later in development, there is a switch in syt isoform abundance in the LMC, with syt II mRNA being upregulated between E15 and E20 and syt I mRNA being downregulated. Our results suggest that contact with targets upregulates syt I gene expression during neuromuscular synapse formation in vivo, and that a later stage of synaptic maturation involves changes in SVP isoform abundance.

Development of an identified spinal commissural interneuron population in an amniote: neurons of the avian Hofmann nuclei

The commissural interneurons of the Hofmann nuclei (HN) of the avian spinal cord (The axonal projections of the Hofmann nuclei in the spinal cord of the late stage chicken embryo, Anat Embryol (Berl), A.L. Eide, 1996, Vol 193, pp 543-557) provide a unique opportunity to describe the development of an identified spinal commissural axon projection and its terminal collaterals in an amniote vertebrate. Here, we use the lipophilic tracer Dil to label these and other commissural projections anterogradely and retrogradely from the time the HN neurons are born. [3H]thymidine birthdating shows that the final mitoses of HN neurons occur at stages 21-24 [developmental day (d) 4]. By direct comparison, this follows the generation of motoneurons and of large, dorsally located commissural interneurons. The first HN neurons reach the ventrolateral margin of the spinal cord by d6 by a radial migration through the ventral horn. Radial migration occurs after the extension of HN axons across the midline. Thus, HN neurons are determined to be commissural interneurons before attaining their definitive locations. The HN neurons subsequently aggregate into segmentally iterated clusters at the ventrolateral margin of the spinal cord by d8. Also by d8 their logitudinal axons attain mature extent in the ventral funiculus of the contralateral side and begin to sprout collaterals. The collaterals are directed predominantly toward the medial aspect of the ventral horn at all stages, forming by d12 a dense thicket of terminals that thins out over several segments to each side of the HN of origin. The initial direction of collateral outgrowth is largely appropriate for the mature termination pattern of the HN. Terminal arbors, however, are less focused at early developmental stages than at later stages.

Requirement for LIM homeobox gene Isl1 in motor neuron generation reveals a motor neuron-dependent step in interneuron differentiation

Motor neuron differentiation is accompanied by the expression of a LIM homeodomain transcription factor, Islet1 (ISL1). To assess the involvement of ISL1 in the generation of motor neurons, we analyzed cell differentiation in the neural tube of embryos in which ISL1 expression has been eliminated by gene targeting. Motor neurons are not generated without ISL1, although many other aspects of cell differentiation in the neural tube occur normally. A population of interneurons that express Engrailed1 (EN1), however, also fails to differentiate in Isl1 mutant embryos. The differentiation of EN1+ interneurons can be induced in both wild-type and mutant neural tissue by regions of the neural tube that contain motor neurons. These results show that ISL1 is required for the generation of motor neurons and suggest that motor neuron generation is required for the subsequent differentiation of certain interneurons.

Cytoskeleton gradients in three dimensions during neurulation in the rabbit

Morphogenetic movements leading to the formation of the neural tube and cellular differentiation leading to neuronal and glial cell lineages are both part of early development of the vertebrate nervous system. In order to analyze the degree of overlap between these processes, cellular differentiation during the shaping of the neural plate is investigated immunohistochemically by using monoclonal intermediate filament protein antibodies and the 7.5-8.0-day-old rabbit embryo as a model. Western blotting is used to confirm the specificity of the antibodies, which include a new monoclonal vimentin antibody suitable for double-labeling in combination with monoclonal cytokeratin (and fibronectin) antibodies. Starting in the early somite embryo and concomitant with neural plate folding, a gradual loss of cytokeratin 8 (and 18) expression in the neuroepithelium is mirrored by a gain in vimentin expression with partial coexpression of both proteins. At the prospective rhombencephalic and spino-caudal levels, vimentin expression, in particular, changes (i.e., increases) along gradients in three dimensions: along the longitudinal axis of each neuroepithelial cell from basal to apical, in the transverse plane of the embryo from dorsolateral to ventromedial and along the craniocaudal axis from prospective rhombencephalic toward spino-caudal levels of the neural plate. At the prospective mes- and prosencephalic levels, the expression change also proceeds from basal to apical within each neuroepithelial cell, but along the other axes described here, the progress in expression change is more complex. Although the functional meaning of these highly ordered expression changes is at present unclear, the gradients suggest a novel pattern of neuroepithelial differentiation which may be functionally related to the process of interkinetic nuclear migration (Sauer [1935] J. Comp. Neurol. 62:377-402) and which partially coincides with the morphogenetic movements involved in the shaping of the neural plate.

Lineage specification of neuronal precursors in the mouse spinal cord

We have investigated the differentiation potential of precursor cells within the developing spinal cord of mice and have shown that spinal cord cells from embryonic day 10 specifically give rise to neurons when plated onto an astrocytic monolayer, Ast-1. These neurons had the morphology of motor neurons and > 83% expressed the motor neuron markers choline acetyltransferase, peripherin, calcitonin gene-related peptide, and L-14. By comparison, < 10% of the neurons arising on monolayers of other neural cell lines or 3T3 fibroblasts had motor neuron characteristics. Cells derived from dorsal, intermediate, and ventral regions of the spinal cord all behaved similarly and gave rise to motor neuron-like cells when plated onto Ast-1. By using cells that expressed the lacZ reporter gene, it was shown that > 93% of cells present on the Ast-1 monolayers were motor neuron-like. Time-lapse analysis revealed that the precursors on the Ast-1 monolayers gave rise to neurons either directly or following a single cell division. Together, these results indicate that precursors in the murine spinal cord can be induced to differentiate into the motor neuron phenotype by factors produced by Ast-1 cells, suggesting that a similar factor(s) produced by cells akin to Ast-1 may regulate motor neuron differentiation in vivo.

Dividing neuron precursors express neuron-specific tubulin

Neuronal differentiation involves specific molecular and morphological changes in precursors and results in mature, postmitotic neurons. The expression of neuron-specific beta tubulin, as detected by the monoclonal antibody TuJ1, begins during the period of neurogenesis. Indeed, TuJ1 expression precedes that of the 160 kD neurofilament protein in both the central and peripheral nervous systems. In the embryonic rat spinal cord, bipolar cells and some mitotic cells in the ventricular zone were TuJ1 immunoreactive (IR). Sensory ganglia also contained cells with TuJ1-IR mitotic spindles in situ. In embryonic rat sensory and sympathetic ganglion cell cultures pulsed with the thymidine analog bromodeoxyuridine (BrdU), TuJ1 label was detected in the spindle of mitotic cells and in the midbody of cells joined at cytokinesis, indicating that neuron-specific tubulin expression was initiated during or before the final mitosis of neuronal progenitors. Dorsal root ganglion cultures included TuJ1-IR cells with several shapes that may reflect morphological transitions, from flattened stellate neural crest-like cells to differentiated bipolar neurons. Indeed, the presence of flattened TuJ1-IR cells was correlated with neurogenesis. Some sympathetic neuron precursors possessed long TuJ1-IR neurites, as well as TuJ1-IR spindle microtubules and BrdU-labeled chromosomes, indicating that these precursors can possess long processes during metaphase. These results support the hypothesis that neuron-specific tubulin expression represents an early molecular event in neuronal differentiation exhibited by a wide range of neuronal precursors. The cessation of proliferation can occur at different points during neuronal differentiation, as TuJ1-IR was detected in cells undergoing mitosis. Future studies directed toward understanding the molecules that initiate neuron-specific tubulin expression may lead to the factors that control the initial phases of neuronal differentiation.

Expression pattern of the murine LIM class homeobox gene Lhx3 in subsets of neural and neuroendocrine tissues

Murine Lhx3 cDNA isolated from the mouse pituitary cDNA library encodes a LIM-type homeodomain protein that contains two tandemly repeated LIM domains and the homeodomain. The identities of predicted amino acid sequences between the mouse of Lhx3 and Xenopus Xlim-3 genes are 80, 95, and 97% in the LIM domains 1 and 2, and the homeodomain, respectively, and 84% in the entire protein. 5'-RACE procedures and genomic cloning revealed that two distinct N-terminal sequences arise from two different exons 1a and 1b. Exon 1a encodes a sequence similar to that of Xlim-3, whereas exon 1b encodes a different N-terminus. It is likely that there are two transcription initiation sites in the Lhx3 gene. The Lhx3 transcripts were detected by whole mount in situ hybridization as early as day E9.5 post coitum in Rathke's pouch and the closing neural tube. During subsequent development, Lhx3 expression was observed in the anterior and intermediate but not in the posterior lobes of the pituitary, and in the ventral hindbrain and spinal cord. Northern blot analysis of adult tissues showed that Lhx3 mRNA persists in the pituitary. The expression pattern of Lhx3 is well conserved between Xenopus and mouse, underscoring the functional importance of this gene as a regulator of development. A number of established cell lines of pituitary origin express Lhx3 and therefore constitute a useful tool for further study of Lhx3 gene function.

Agrin mRNA variants are differentially regulated in developing chick embryo spinal cord and sensory ganglia

Agrin is a synapse-organizing protein synthesized and externalized by motor neurons in the spinal cord, which organizes the postsynaptic apparatus of the developing neuromuscular junction. Agrin mRNA in the nervous system consists of several alternatively spliced variants. Splicing of agrin gene transcripts at the major site of variability results in four variants: encoding 8 (B8) or 11 (B11) amino acid inserts, both (B19), or predominant variant (B0) without inserts. The insert-containing variants are neuron specific and encode agrin proteins with greater synapse-organizing activity than the B0 variant. Here, we report the localization and developmental regulation of agrin mRNA variants in chick embryo spinal cord and dorsal root ganglia. In situ hybridization using antisense oligodeoxynucleotide (ODN) probes specific for the B8 and B11 sequences shows that the neuron-specific variants are concentrated in ventrolateral cells of the chick embryo spinal cord, presumably motor neurons, beginning at embryonic day 4 (E4). By E14, the insert-containing mRNAs are found almost exclusively in presumptive motor neurons. These variants are also found in dorsal root ganglia and sympathetic ganglia, but not in non-neural tissues. Analysis by polymerase chain reaction showed that the B11 and B19 mRNA variants appeared in spinal cord at E4, whereas the B8 variant was first seen at E14. During development, B11 decreased and disappeared by E20, whereas B8 increased from E14 to E20. A similar time course was seen in dorsal root ganglia. The greatest acetylcholine receptor-aggregating activity in the spinal cord was seen from E6 to E10, coincident with the highest proportion of B11-containing transcripts and with the peak of synaptogenesis in limb muscles. These data provide the first evidence linking appearance of the neuron-specific agrin mRNA variants with expression of the functional protein. The B11 and B19 variants appeared in E2 (stage 15) neural tubes cultured for 2 days with or without notochord and trunk tissues, indicating that there is no peripheral signal required to induce these agrin mRNA variants in developing motor neurons.

Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes

Motor neurons located at different positions in the embryonic spinal cord innervate distinct targets in the periphery, establishing a topographic neural map. The topographic organization of motor projections depends on the generation of subclasses of motor neurons that select specific paths to their targets. We have cloned a family of LIM homeobox genes in chick and show here that the combinatorial expression of four of these genes, Islet-1, Islet-2, Lim-1, and Lim-3, defines subclasses of motor neurons that segregate into columns in the spinal cord and select distinct axonal pathways. These genes are expressed prior to the formation of distinct motor axon pathways and before motor columns appear. Our results suggest that LIM homeobox genes contribute to the generation of motor neuron diversity and may confer subclasses of motor neurons with the ability to select specific axon pathways, thereby initiating the topographic organization of motor projections.

Effects of MR exposure on cell proliferation and migration of chick motoneurons

The effect of magnetic resonance (MR) exposure on the proliferation and migration of motoneurons was examined in chick embryos. Embryos were exposed in ovo to a static magnetic field of 1.5 T for 6 hours and to 64-MHz radio-frequency field pulses and a switched magnetic field gradient with an amplitude of 0.6 G/cm for 4 hours. For cell proliferation studies, embryos were exposed to MR fields during the developmental stage at which motoneuron proliferation is most active. For cell migration studies, embryos were exposed to MR fields at the developmental stage just before lateral motoneuron migration. The results show that the birth dates, migration, and proliferation of lateral motoneurons were unaffected by the MR exposure conditions in this study.

Programmed cell death during the earliest stages of spinal cord development in the chick embryo: a possible means of early phenotypic selection

The spatiotemporal distribution of cell death in the chick embryo neural tube and spinal cord (brachial region) was examined between stage (St.) 12 and 22, in plastic semithin sections. Between St. 12 and 16, the total number of pycnotic cells per segment was low, whereas after St. 16 the number of pycnotic cells was substantially increased. Between St. 17 and 19 three cell death foci or regions could be recognized. One region, the dorsal pycnotic zone, was located in the most dorsal part of the spinal cord, including the neural crest, with the highest number of pycnotic cells observed at St. 18. The second region, or ventral pycnotic zone, was located between motoneurons and the floor plate and had the highest number of dying cells at St. 17. The third region, the floor plate pycnotic zone, was located in the midportion of the floor plate and had the greatest amount of cell death at St. 19. Although low numbers of pycnotic cells were also observed in other regions between St. 17 and 19, no pycnotic cells were found in the ventrolateral region that gives rise to motoneurons. Ultrastructural observations as well as data from in situ nick end labeling indicate that the pycnotic cells observed in the neural tube die by apoptosis and that the debris from the dead cells is phagocytized primarily by adjacent healthy neuroepithelial cells. Although the spatiotemporal distribution of pycnotic cells suggests that cell death at these early stages could play a role in establishing the pioneer axonal pathway for spinal commissural neurons, preliminary observations following perturbations of cell death do not support this notion. Alternatively, early cell death may be involved in the regulation of cellular patterning along the dorsoventral axis of the neural tube by a kind of negative selection of specific progenitor cells.

Ontogenesis and taste bud cell turnover in the chicken. I. Gemmal cell renewal in the hatchling

Taste bud cell turnover rate was examined in oral epithelium of the precocial chick, which at hatching contains the adult complement of taste buds. Forty newly hatched chicks received single or double pulse injections of tritiated thymidine (specific activity was 6.7 Curies/millimole; dosage was 0.5 microCuries/g body weight, intraperitoneally). Anterior mandibular epithelium was processed for light microscopic autoradiography at 2 and 16 hours, as well as 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 18, and 20 days after the initial pulse. In a coded and randomized procedure, the section (7 microns) through the bud's center was selected for counting > or = 6 silver grains over round-clear and gracile-dense gemmal cell nuclei. The mean number of labelled cells/bud varied significantly (P < or = 0.01) during the first four posthatch days, yielding the fastest gemmal cell turnover rates (3.4-4.4 days) yet reported in vertebrates. Average bud diameter also significantly changed during the first four posthatch days, and was reflected in shifts of the distribution of 40-69 microns and > or = 70 microns diameter buds. Both an increase in labelled bud cells and bud diameter during the first two posthatch days may reflect high proliferation rates in initially maturing buds. Subsequent decrease in bud diameter between 2 and 3 days postinjection may indicate splitting of large-diameter (> or = 70 microns) buds and/or normal bud cell death due to failure of sensory afferentation. Bud-splitting alone, however, cannot account for significant decreases in bud cell label which did not occur before 4-6 days postinjection.

Development of specific muscle and cutaneous sensory projections in cultured segments of spinal cord

Development of sensory projections was studied in cultured spinal segments with attached dorsal root ganglia. In spinal segments from stage 30 (E6.5) and older chicken embryos, prelabeled muscle and cutaneous afferents established appropriate projections. Cutaneous afferents terminated solely within the dorsolateral laminae, whereas some muscle afferents (presumably Ia afferents) projected ventrally towards motoneurons. Development of appropriate projections suggests that sufficient cues are preserved in spinal segments to support the formation of modality-specific sensory projections. Further, because these projections developed in the absence of muscle or skin, these results show that the continued presence of peripheral targets is not required for the formation of specific central projections after stage 29 (E6.0). Development of the dorsal horn in cultured spinal segments was assessed using the dorsal midline as a marker. In ovo, this midline structure appears at stage 29. Lack of midline formation in stage 28 and 29 cultured spinal segments suggests that the development of the dorsal horn is arrested in this preparation. This is consistent with earlier reports suggesting that dorsal horn development may be dependent on factors outside the spinal cord. Because dorsal horn development is blocked in cultured spinal segments, this preparation makes it possible to study the consequences of premature ingrowth of sensory axons into the spinal cord. In chicken embryos sensory afferents reach the spinal cord at stage 25 (E4.5) but do not arborize within the gray matter until stage 30. During this period dorsal horn cells are still being generated. In spinal segments, only those segments that have developed a midline at the time of culture support the formation of midline at the time of culture support the formation of specific sensory projections.(ABSTRACT TRUNCATED AT 250 WORDS)

Hu neuronal proteins are expressed in proliferating neurogenic cells

We have utilized immunochemical techniques to investigate the developmental expression of the Hu proteins, a neuron-specific family of RNA binding proteins in vertebrates. Previous work suggests that these proteins may play an important role in neuronal development and maintenance. For the present study, we developed a monoclonal antibody (MAb 16A11) that binds specifically to an epitope present in gene products of all known Hu genes, including HuD, HuC, and Hel-N1. Using brief pulses (1-2 h) of the DNA precursor analog bromodeoxyuridine (BrdU) in conjunction with MAb 16A11, we observed Hu+/BrdU+ cells in nascent sensory and sympathetic ganglia in vivo, and in populations of cultured neural crest cells. In addition, a few Hu+ cells were ambiguously BrdU+ in the neural tube. We conclude that Hu+ cells first appear in avian neurogenic populations immediately before neuronal birthdays in the peripheral nervous system, and at the time of withdrawal from the mitotic cycle in the central nervous system. Consistent with these conclusions, we have also observed neural crest-derived cells that are both Hu+ and in metaphase of the cell cycle. We suggest that Hu proteins function early in neurogenic differentiation.

Activity-dependent interactions between motoneurones and muscles: their role in the development of the motor unit

In this review article we have attempted to provide an overview of the various forms of activity-dependent interactions between motoneurones and muscles and its consequences for the development of the motor unit. During early development the components of the motor unit undergo profound changes. Initially the two cell types develop independently of each other. The mechanisms that regulate their characteristic properties and prepare them for their encounter are poorly understood. However, when motor axons reach their target muscles the interaction between these cells profoundly affects their survival and further development. The earliest interactions between motoneurones and muscle fibres generate a form of activity which is in many ways different from that seen at later stages. This difference may be due to the immature types of ion channels and neurotransmitter receptors present in the membranes of both motoneurones and muscle fibres. For example, spontaneous release of acetylcholine may influence the myotube even before any synaptic specialization appears. This initial form of activity-dependent interaction does not necessarily depend on the generation of action potentials in either the motoneurone or the muscle fibre. Nevertheless, the ionic fluxes and electric fields produced by such interactions are likely to activate second messenger systems and influence the cells. An important step for the development of the motor unit in its final form is the initial distribution of synaptic contacts to primary and secondary myotubes and their later reorganization. Mechanisms that determine these events are proposed. It is argued that the initial layout of the motor unit territory depends on the matching of immature muscle fibres (possibly secondary myotubes) to terminals with relatively weak synaptic strength. Such matching can be the consequence of the properties of the muscle fibre at a particular stage of maturation which will accept only nerve terminals that match their developmental stage. Refinements of the motor unit territory follows later. It is achieved by activity-dependent elimination of nerve terminals from endplates that are innervated by more than one motoneurone. In this way the territory of the motor unit is established, but not necessarily the homogeneity of the physiological and biochemical properties of its muscle fibres. These properties develop gradually, largely as a consequence of the activity pattern that is imposed upon the muscle fibres supplied by a given motoneurone. This occurs when the motor system in the CNS completes its development so that specialized activity patterns are transmitted by particular motoneurones to the muscle fibres they supply.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression patterns of the cell adhesion molecule Nr-CAM during histogenesis of the chick nervous system

Neuron-glia-related cell adhesion molecule (Nr-CAM) is a recently characterized cell adhesion molecule in the family of immunoglobulin-related molecules of which the neural cell adhesion molecule, N-CAM, is the prototype. Nr-CAM shares structural properties with another member of this family (neuron-glia CAM, Ng-CAM) and both molecules exhibit homophilic and heterophilic binding properties. To understand better the role of such molecules in development, we have examined the sites of synthesis and expression of Nr-CAM by means of in situ hybridization and immunohistochemistry. Both methods indicated that Nr-CAM is expressed only in the nervous system. The molecule was observed on neurons in both the peripheral and central nervous systems and on epithelial floor plate cells in the spinal cord, but it was absent in the germinal zones. The protein was present on perikarya, but was found preferentially on axonal tracts. As observed for messenger RNAs specifying other cell adhesion molecules, messenger RNA for Nr-CAM was localized in the perikarya. The temporal expression of Nr-CAM was correlated with various neural morphoregulatory events, including cell proliferation and migration, axonal outgrowth and myelination. The molecule was expressed during the onset of neurogenesis at embryonic day 3 in the floor plate epithelium, and then on postmitotic ventral horn motor neurons of the spinal cord. At later stages, it was expressed throughout the spinal cord but disappeared from the floor plate. In the cerebellum, Nr-CAM was found on granule and Purkinje neurons and afferent fibers. Both local and projection neurons in the optic tectum, as well as axonal pathways throughout the telencephalon, expressed Nr-CAM. In the peripheral nervous system, Nr-CAM was expressed strongly in sensory and autonomic ganglia and in the enteric nervous system. At the onset of myelination, there was a general decrease in staining for Nr-CAM protein in the central nervous system but not in the periphery. Comparison of the expression of Nr-CAM to that of the structurally related Ng-CAM showed considerable overlap in their distributions, although there were differences in the levels at which each CAM was observed in particular structures. For example, sympathetic ganglia stained more intensely for Nr-CAM protein than for Ng-CAM. This differential but co-distributed pattern is consistent with the idea that although similar cell adhesion molecules have independent binding specificities, they may have related functions that act synergistically in the development of the nervous system.

Signals from the notochord and floor plate regulate the region-specific expression of two Pax genes in the developing spinal cord

Members of the paired box (Pax) gene family are expressed in discrete regions of the developing central nervous system, suggesting a role in neural patterning. In this study, we describe the isolation of the chicken homologues of Pax-3 and Pax-6. Both genes are very highly conserved and share extensive homology with the mouse Pax-3 and Pax-6 genes. Pax-3 is expressed in the primitive streak and in two bands of cells at the lateral extremity of the neural plate. In the spinal cord, Pax-6 is expressed later than Pax-3 with the first detectable expression preceding closure of the neural tube. When the neural tube closes, transcripts of both genes become dorsoventrally restricted in the undifferentiated mitotic neuroepithelium. We show that the removal of the notochord, or implantation of an additional notochord, dramatically alter the dorsoventral (DV) expression patterns of Pax-3 and Pax-6. These manipulations suggest that signals from the notochord and floor plate regulate the establishment of the dorsoventrally restricted expression domains of Pax-3 and Pax-6 in the spinal cord. The rapid changes to Pax gene expression that occur in neural progenitor cells following the grafting of an ectopic notochord suggest that changes to Pax gene expression are an early effect of the notochord on spinal cord patterning.

Spinal neurogenesis and axon projection: a correlative study in the rat

The purpose of the present study was to determine the relationship between the duration of a spinal neuron's neurogenic period and the length of its axon or level of projection. Spinal segment L1 was chosen for examination and neurons were divided into four projection groups: 1) supraspinal projection (SSp), 2) long ascending propriospinal (LAPr), 3) short ascending propriospinal (SAPr), and 4) descending propriospinal (DPr). To determine the duration of the neurogenic period for each group, 3H-thymidine was administered to fetal rats during the proliferative period for spinal neuroblasts on one of embryonic (E) days E13 through E16. Between 50 and 100 days after birth neurons in each group were labeled with the retrograde fluorescent tracer Fluoro-Gold. To demonstrate nerve cells with SSp projections, spinal cords were hemisected at spinal segment C3 in one group of animals and Fluoro-Gold was applied to the sectioned surface of the cord. Three additional sets of animals were used to label nerve cells with LAPr, SAPr, and DPr projections by injecting Fluoro-Gold into the gray matter at spinal segments C6, T12, and L5, respectively. Neurons labeled with both Fluoro-Gold and 3H-thymidine and neurons labeled with Fluoro-Gold alone in each animal in each group were counted and the data statistically analyzed. Results showed that within each spinal lamina neurons with different projections were generated, i.e., completed cell division, at significantly different rates. Neurons with the longest axons, those with SSP projections, were generated first. These were followed by those with LAPr projections, and finally those with SAPr and DPr projections. In most laminate there was no significant difference between the neurogenic periods of rostrally projecting short propriospinal (SAPr) neurons versus caudally projecting short propriospinal (DPr) neurons. It was concluded that the duration of the neurogenic period for a given group of neurons within each spinal lamina is inversely related to the distance between the nerve cell and its projection site regardless of the direction of its projection.

Early stages of motor neuron differentiation revealed by expression of homeobox gene Islet-1

Motor neurons in the embryonic chick spinal cord express a homeobox gene, Islet-1, soon after their final mitotic division and before the appearance of other differentiated motor neuron properties. The expression of Islet-1 by neural cells is regulated by inductive signals from the floor plate and notochord. These results establish Islet-1 as the earliest marker of developing motor neurons. The molecular nature of the Islet-1 protein suggests that it may be involved in the establishment of motor neuron fate.

Development of the human pontine nuclei: a morphometric study

The morphometric development of the pontine nuclei in the human fetus from 16 to 40 gestational weeks, in a 2-month-old infant and in a 63-year-old adult were examined employing a serial celloidin section method and computer assisted electronic planimeter. The results of our study can be summarized as follows: (1) the development of the human pontine nuclei accelerated in volume after 32 gestational weeks and continued after birth, (2) neuron numbers remained relatively constant after 27 gestational weeks. It was difficult to clearly distinguish neurons from glia before 27 gestational weeks. The total estimated neuronal numbers were not indicative of the gestational stages in infants 27 gestational weeks and older, (3) individual neurons appeared to continue to develop after 32 gestational weeks in accordance with size, distribution and circularity ratio, (4) many islet-shaped groups of large neurons appeared and were scattered throughout the pontine nuclei after 32 gestational weeks.

Characterization of a cell surface adhesion molecule expressed by a subset of developing chick neurons

We have isolated a 105-kDa membrane glycoprotein expressed by subsets of developing chick neurons. This glycoprotein, identified by the JC7 monoclonal antibody, is present on the surface of axons and cell bodies of developing spinal motor neurons, dorsal root ganglion sensory neurons, sympathetic and parasympathetic neurons, and a small subset of brain neurons. Late in development the JC7 antigen is expressed at high levels on CNS nonneuronal glial-like cells. When attached to latex beads this glycoprotein can mediate homophilic adhesion and when used as a culture substrate stimulates a highly branched pattern of neurite outgrowth from dorsal root ganglion explants. The JC7 antigen appears to be identical to the SC1, BEN, and DM antigens. Its limited distribution, adhesive qualities, and ability to stimulate neurite outgrowth suggest it may play a role in the selective growth of neural processes during development.

Lack of evidence for cell death among avian spinal cord interneurons during normal development and following removal of targets and afferents

Chick embryos and posthatched chicks were examined at several ages for the presence of pyknotic interneurons in the lumbar spinal cord. Because no pyknotic interneurons were found, direct cell counts of healthy interneurons were carried out and a comparison made between early- and late-stage embryos and hatchlings. There was no decrease in the number of interneurons in the ventral intermediate gray matter of the spinal cord between embryonic day (E) 8 and 2 weeks posthatching (PH) or in the dorsal horn between E10 and 2 weeks PH. To study whether interneuron survival is regulated by targets or afferents, a situation known to exist in other developing neural populations, early embryos were subjected to (1) removal of one limb, resulting in the loss of lateral motor column motoneurons and dorsal root ganglion sensory afferents; (2) transection of the thoracic spinal cord, thereby removing both descending afferents and rostral targets of spinal interneurons, or (3) a combination of the two operations. No reductions in interneuron numbers were found as a result of these operations. Furthermore, morphometric analysis also revealed no change in neuronal size following these experimental manipulations. By contrast, there was a slight decrease in the total area of spinal gray matter that was most prominent in the dorsal region following limb bud removal. Our results indicate (1) that spinal interneurons fail to exhibit the massive naturally occurring death of postmitotic neurons that has been observed for several other populations of spinal neurons, and (2) spinal interneurons appear to be relatively resistant to induced cell death following the removal of substantial numbers of afferent inputs and targets.

Neurogenesis of ascending supraspinal projection neurons: ipsi- versus contralateral projections

The present study tests the hypothesis that contralaterally projecting supraspinal projection neurons (SPNs) are generated prior to ipsilaterally projecting SPNs. Neuronal time of origin was determined by injecting pregnant rats with tritiated thymidine on one of embryonic (E) days E12 through E15. In mature offspring of thymidine-treated dams, SPNs in the lumbar cord were retrogradely labelled with True Blue delivered at the site of a hemisection in spinal segment C3. Ipsi and contralaterally projecting SPNs in laminae I, VII and VIII and the lateral spinal nucleus, which are known to give rise to long sensory pathways, were generated simultaneously throughout their neurogenic period (E12-E14), while ipsilaterally projecting SPNs in lamina IV and the nucleus dorsalis, which give rise to short sensory pathways, completed neurogenesis one day later (E15). Results suggest that the projection target and its distance from the nerve cell body of origin are more consistent correlates of the duration of the neurogenic period than the course of the axon.

Generation patterns of immunocytochemically identified cholinergic neurons at autonomic levels of the rat spinal cord

The time at which a neuron is "born" appears to have significant consequences for the cell's subsequent differentiation. As part of a continuing investigation of cholinergic neuronal development, we have combined ChAT immunocytochemistry and [3H]thymidine autoradiography to determine the generation patterns of somatic and autonomic motor neurons at upper thoracic (T1-3), upper lumbar (L1-3), and lumbosacral (L6-S1) levels of the rat spinal cord. Additionally, the generation patterns of two subsets of cholinergic interneurons (partition cells and central canal cluster cells) were compared with those of somatic and autonomic motor neurons. Embryonic day 11 (E11) was the first day of cholinergic neuronal generation at each of the three spinal levels studied, and it also was the peak generation day for somatic and autonomic neurons in the upper thoracic spinal cord. The peak generation of homologous neurons at upper lumbar and lumbosacral spinal levels occurred at E12 and E13, respectively. Somatic and autonomic motor neurons were generated synchronously, and their production at each rostrocaudal level was virtually completed within a 2-day period. Cholinergic interneurons were generated 1 or 2 days later than motor neurons at the same rostrocaudal level. In summary, the birthdays of all spinal cholinergic neurons studied followed the general rostrocaudal spatiotemporal gradient of spinal neurogenesis. In addition, the generation of cholinergic interneurons also followed the general ventrodorsal gradient. In contrast, however, autonomic motor neurons disobeyed the rule of a ventral-to-dorsal progression of spinal neuronal generation, thus adding another example in which autonomic motor neurons display unusual developmental patterns.

CHox E, a chicken homeogene of the H2.0 type exhibits dorso-ventral restriction in the proliferating region of the spinal cord

CHox E is a novel chicken homeogene that belongs to the H2.0 family of homeodomains. Its homeobox sequence is interrupted by an intron between amino acids 44 and 45. Expression of CHox E during embryogenesis is localized to the central nervous system. The anterior boundary of CHox E expression can initially be localized to rhombomere number 1, later in development this boundary reaches up to the rhombencephalic isthmus. CHox E expression in the spinal cord localizes dorso-ventrally to the dorsal half of the basal plate. CHox E expression is always restricted to the proliferating region, the ventricular zone. As the ventricular zone becomes restricted laterally, so does the CHox E expressing region. Once this region of the ventricular zone ceases to exist, CHox E specific transcripts become undetectable. The site and time of CHox E expression suggest a very early function in the differentiation of the cells derived from that region of the ventricular zone.

Control of cell pattern in the developing nervous system: polarizing activity of the floor plate and notochord

Individual classes of neural cells differentiate at distinct locations in the developing vertebrate nervous system. We provide evidence that the pattern of cell differentiation along the dorsoventral axis of the chick neural tube is regulated by signals derived from two ventral midline cell groups, the notochord and floor plate. Grafting an additional notochord or floor plate to ectopic positions, or deleting both cell groups, resulted in changes in the fate and position of neural cell types, defined by expression of specific antigens. These results suggest that the differentiation of neural cells is controlled, in part, by their position with respect to the notochord and floor plate.

Ontological study of calbindin-D28k-like and parvalbumin-like immunoreactivities in rat spinal cord and dorsal root ganglia

The calcium ion plays an important role in some critical developmental events in the nervous system, such as neurulation and neurite elongation. Therefore, as the intracellular calcium-binding proteins calbindin-D28k (CaB) and parvalbumin (PV) may be expressed in these developmental events. Accordingly, the ontological expression of CaB and PV was examined immunocytochemically in the spinal cord and dorsal root ganglia (DRG) of the rat, in order to evaluate the relationship between CaB and PV expression, and other important developmental events. During the ontogenesis of the spinal cord, the CaB-like immunoreactivity was mainly observed in the cell somata. The immunoreactive cells in the ventral horn of the cervical and thoracic, lumbar, and sacral segments first appeared at embryonic day (E)-12, E-13, and E-14, respectively. However, these cells were not detected in the intermediate gray matter of the same segments at E-14, E-15, and E-16, respectively, and in the dorsal horn at E-14-E-15, E-16, and E-17, respectively. The peak of immunoreactive cells, both as to number and intensity, occurred in the perinatal period. However, from postnatal day (P)-14 on, the number and intensity of the positive cells decreased, the adult levels being reached at P-35. The PV-like immunoreactivity was mainly detected in the fibers and punctata during the ontogenesis of the spinal cord. The immunoreactive fibers first appeared on the surface of the dorsal horn in the cervical and thoracic segments at E-14, then entered the dorsal horn at E-15, and reached the intermediate gray matter and ventral horn at E-16. The first appearance of these fibers in the same areas of the lumbar and sacral segments occurred 1 day later than in the cervical and thoracic segments. During the perinatal period, the maximum content of PV-like immunoreactive fibers, together with many punctata, was seen in the gray matter. However, between P-14 and P-17, most of them lost immunoreactivity rapidly, with the exception of the medial region of the intermediate gray matter, where the PV-immunoreactive punctata remained up to the adult stage. In DRG neurons, both CaB and PV was expressed, but in different neurons. Neurons labeled with anti-CaB and anti-PV sera were first detected at E-16 and E-14, respectively. These neurons were large or medium-sized in the prenatal period.(ABSTRACT TRUNCATED AT 400 WORDS)

The murine paired box gene, Pax7, is expressed specifically during the development of the nervous and muscular system

Eight murine paired box-containing (Pax) genes have been isolated so far. The gene described here, Pax7, contains not only a paired box, but also an octapeptide and a paired-type homeobox. As shown by Northern and in situ analysis, Pax7 is expressed from day 8 to 17 p.c. during embryogenesis. At early stages Pax7 transcripts are present in a subset of cells throughout the entire brain, but later in development expression is limited to the mesencephalon. In the developing neural tube Pax7 is restricted to the dorsal ventricular zone along the entire antero-posterior axis, suggesting a role for Pax7 in the formation of certain parts of the CNS. Additionally Pax7 expression can be followed during myogenesis from the dermamyotome of the somites to the skeletal muscle tissues.

Migratory patterns of clonally related cells in the developing central nervous system

Neurons and glioblasts that arise in the ventricular zone migrate to form discrete nuclei and laminae as the central nervous system develops. By stably labeling precursor cells in the ventricular zone, pathways taken by different cells within an individual clone can be described. We have used recombinant retroviruses to label precursor cells with a heritable marker, the E. coli lacZ gene; clones of lacZ-positive cells are later mapped histochemically. Here we review results from three regions of the chicken central nervous system--the optic tectum, spinal cord, and forebrain--and compare them with previous results from mammalian cortex and other regions of the vertebrate CNS. In particular, we consider the relationship between migratory patterns and functional organization, the existence of multiple cellular sources of migratory guidance, and the issue of whether a cell's choice of migratory pathway influences its ultimate phenotype.

The response of cultured trigeminal and spinal cord motoneurons to nerve growth factor

Dissociated neurons from the trigeminal (V) region of the metencephalic basal plate or the ventral spinal cord from chick embryos of Day 4 (V basal plate) or Day 5 (spinal cord) were cultured on a laminin substratum either in the presence of nerve growth factor (NGF) or in control medium. Assessment was made of neuronal survival, the amount of neurite elaborated, and the percentage of neurons initiating neurites. The presence of motoneurons was verified by retrograde labeling with the fluorescent dye diI. NGF was found to significantly increase the quantity of neuritic processes produced by the spinal cord dissociates at both 24 and 48 hr in vitro. The percentage of neurons initiating neuritic processes was significantly increased by NGF in the trigeminal population at 48 hr in vitro. Neuronal survival was not enhanced by NGF in either group. Both trigeminal and spinal cord neurons were also found to specifically bind 125I-NGF in culture. These results provide direct evidence for an influence of NGF on process formation of early embryonic motoneurons in culture.

The expression and posttranslational modification of a neuron-specific beta-tubulin isotype during chick embryogenesis

Five beta-tubulin isotypes are expressed differentially during chicken brain development. One of these isotypes is encoded by the gene c beta 4 and has been assigned to an isotypic family designated as Class III (beta III). In the nervous system of higher vertebrates, beta III is synthesized exclusively by neurons. A beta III-specific monoclonal antibody was used to determine when during chick embryogenesis c beta 4 is expressed, the cellular localization of beta III, and the number of charge variants (isoforms) into which beta III can be resolved by isoelectric focusing. On Western blots, beta III is first detectable at stages 12-13. Thereafter, the relative abundance of beta III in brain increases steadily, apparently in conjunction with the rate of neural differentiation. The isotype was not detectable in non-neural tissue extracts from older embryos (days 10-14) and hatchlings. Western blots of protein separated by two-dimensional gel electrophoresis (2D-PAGE) reveal that the number of beta III isoforms increases from one to three during neural development. This evidence indicates that beta III is a substrate for developmentally regulated, multiple-site posttranslational modification. Immunocytochemical studies reveal that while c beta 4 expression is restricted predominantly to the nervous system, it is transiently expressed in some embryonic structures. More importantly, in the nervous system, immunoreactive cells were located primarily in the non-proliferative marginal zone of the neural epithelia. Regions containing primarily mitotic neuroblasts were virtually unstained. This localization pattern indicates that c beta 4 expression occurs either during or immediately following terminal mitosis, and suggests that beta III may have a unique role during early neuronal differentiation and neurite outgrowth.

Ganglioside composition of normal and mutant mouse embryos

The enrichment of gangliosides in neuronal membranes suggests that they play an important role in CNS development. We recently found a marked tetrasialoganglioside deficiency in twl/twl mutant mouse embryos at embryonic day (E)-11. The recessive twl/twl mutants die at embryonic ages E-9 to E-18 from failed neural differentiation in the ventral portion of the neural tube. In the present study, we examined the composition and distribution of gangliosides in twl/twl mutant mouse embryos at E-12. The total ganglioside sialic acid concentration was significantly lower in the mutants than in normal (+/-) embryos. The mutants also expressed significant deficiencies of gangliosides in the "b" metabolic pathway (GD3, GD1b, GT1b, and GQ1b) and elevations in levels of gangliosides in the "a" metabolic pathway (GM3, GM2, GM1, and GD1a). These findings suggest that the mutants have a partial deficiency in the activity of a specific sialyltransferase in the b pathway. Regional ganglioside distribution was also studied in E-12 normal mouse embryos. The ganglioside composition in heads and bodies was similar to each other and to whole embryos. Total ganglioside concentration and the distribution of b pathway gangliosides were significantly higher in neural tube regions than in nonneural tube regions. These findings suggest that b pathway gangliosides accumulate in differentiating neural cells and that the deficiency of these gangliosides in the twl/twl mutants is closely associated with failed neural differentiation.

Expression and phosphorylation of the mid-sized neurofilament protein NF-M during chick spinal cord neurogenesis

The middle molecular weight polypeptide of neurofilaments (NF-M) is modified posttranslationally by extensive phosphorylation. This modification is slow in mature neurons, requiring approximately 24-48 hr for completion and probably occurs outside of the cell soma (Bennett and DiLullo: J Cell Biol 100:1799, 1985c). Thus, NF-M synthesis and phosphorylation are separate events both temporally and spatially. Although it is known that NF-M is among the earliest neuron-specific gene products to be expressed during nervous system development, it is not known what the temporal relationship is between the initiation of NF-M translation and its phosphorylation. To address this question, we have produced an antiserum against the dephosphorylated form of NF-M (NF-M130) and have used this antiserum, together with a previously characterized antiserum against completely phosphorylated NF-M (NF-M160), in an immunohistochemical examination of neurogenesis and the initial period of neuronal differentiation in chick spinal cord. We found that 1) nonphosphorylated and partially phosphorylated NF-M cannot be detected prior to the completion of the terminal mitosis; 2) most postmitotic neuroblasts begin expressing NF-M as they commence migration, but do not contain the completely phosphorylated polypeptide until some time after completion of migration; and 3) those precursor cells of a subpopulation of neuroblasts that begin expressing completely phosphorylated NF-M during their terminal cell cycle (Bennett and DiLullo: Dev Biol 107:94, 1985a) contain no detectable nonphosphorylated or partially phosphorylated NF-M. These cells probably complete the phosphorylation step more rapidly than do mature neurons.