A surface protein expressed by avian myelinating and nonmyelinating Schwann cells but not by satellite or enteric glial cells

Ontogeny of the trigeminal system and associated structures in Alligator mississippiensis

From the appearance of the vertebrate head, the trigeminal system has played a role in behavioral and ecological adaptation. The trigeminal nerve is the primary cranial somatosensory nerve, also innervating the jaw muscles. In crocodylians, the trigeminal nerve plays a role in modulating the high bite force and unique integumentary sensation. In association with these behaviors, crocodylians are known for large trigeminal nerves, a high volume of trigeminal-innervated musculature, and densely packed, specialized sensory receptors. These innovations also occurred in concert with a restructuring of the lateral braincase wall. These morphologies have previously been investigated in phylogenetic and evolutionary contexts, but an ontogenetic, whole-system investigation of trigeminal tissue and associated musculature, cartilage, and bone is lacking, as is an understanding of developmental timing of morphologies significant to hypotheses of homology. Here, we use contrast-enhanced computed tomography imaging to provide description and analysis of the trigeminal system in an ontogenetic series of Alligator mississippiensis from embryonic to adult form. We explore growth rates and allometric relationships of structures and discuss the significance to hypotheses of homology. We find a high growth rate and allometric trajectory of the trigeminal nerve in comparison to other cranial nerves, likely associated with the large volume of trigeminal musculature and high densities of sensory receptors. We identify a similar trend in the pterygoideus dorsalis muscle, the highest contributor to bite force. We narrow ontogenetic timing of features related to the trigeminal topological paradigm and the undeveloped epipterygoid. Overall, we provide a basis for understanding trigeminal development in crocodylians, which upon comparison across reptiles will reveal ontogenetic origins of morphological variation.

FGF2 Stimulates the Growth and Improves the Melanocytic Commitment of Trunk Neural Crest Cells

Neural crest cells (NCCs) comprise a population of multipotent progenitors and stem cells at the origin of the peripheral nervous system (PNS) and melanocytes of skin, which are profoundly influenced by microenvironmental factors, among which is basic fibroblast growth factor 2 (FGF2). In this work, we further investigated the role of this growth factor in quail trunk NC morphogenesis and demonstrated its huge effect in NCC growth mainly by stimulating cell proliferation but also reducing cell death, despite that NCC migration from the neural tube explant was not affected. Moreover, following FGF2 treatment, reduced expression of the early NC markers Sox10 and FoxD3 and improved proliferation of HNK1-positive NCC were observed. Since these markers are involved in the regulation of glial and melanocytic fate of NC, the effect of FGF2 on NCC differentiation was investigated. Therefore, in the presence of FGF2, increased proportions of NCCs positives to the melanoblast marker Mitf as well as NCCs double stained to Mitf and BrdU were recorded. In addition, treatment with FGF2, followed by differentiation medium, resulted in increased expression of melanin and improved proportion of melanin-pigmented melanocytes without alteration in the glial marker Schwann myelin protein (SMP). Taken together, these data further reveal the important role of FGF2 in NCC proliferation, survival, and differentiation, particularly in melanocyte development. This is the first demonstration of FGF2 effects in melanocyte commitment of NC and in the proliferation of Mitf-positive melanoblasts. Elucidating the differentiation process of embryonic NCCs brings us a step closer to understanding the development of the PNS and then undertaking the search for advanced technologies to prevent, or treat, injuries caused by NC-related disorders, also known as neurocristopathies.

Concise Review: Cellular and Molecular Mechanisms of Postnatal Injury-Induced Enteric Neurogenesis

Although still controversial, there is increasing agreement that postnatal neurogenesis occurs in the enteric nervous system (ENS) in response to injury. Following acute colitis, there is significant cell death of enteric neurons and evidence suggests that subsequent neural regeneration follows. An enteric neural stem/progenitor cell population with neurogenic potential has been identified in culture; in vivo, compensatory neurogenesis is driven by enteric glia and may also include de-differentiated Schwann cells. Recent evidence suggests that changes in the enteric microenvironment due to injury-associated increases in glial cell-derived neurotrophic factor (GDNF), serotonin (5-hydroxytryptamine [HT]), products from the gut microbiome, and possibly endocannabinoids may lead to the transdifferentiation of mature enteric glia and may reprogram recruited Schwann cells. Targeting neurogenic pathways presents a promising avenue toward the development of new and innovative treatments for acquired damage to the ENS. In this review, we discuss potential sources of newly generated adult enteric neurons, the involvement of GDNF, 5-HT, endocannabinoids, and lipopolysaccharide, as well as therapeutic applications of this evolving work. Stem Cells 2019;37:1136-1143.

From proliferation to target innervation: signaling molecules that direct sympathetic nervous system development

The sympathetic division of the autonomic nervous system includes a variety of cells including neurons, endocrine cells and glial cells. A recent study (Furlan et al. 2017) has revised thinking about the developmental origin of these cells. It now appears that sympathetic neurons and chromaffin cells of the adrenal medulla do not have an immediate common ancestor in the form a "sympathoadrenal cell", as has been long believed. Instead, chromaffin cells arise from Schwann cell precursors. This review integrates the new findings with the expanding body of knowledge on the signalling pathways and transcription factors that regulate the origin of cells of the sympathetic division of the autonomic nervous system.

Satellite glial cells in sympathetic and parasympathetic ganglia: in search of function

Glial cells are established as essential for many functions of the central nervous system, and this seems to hold also for glial cells in the peripheral nervous system. The main type of glial cells in most types of peripheral ganglia - sensory, sympathetic, and parasympathetic - is satellite glial cells (SGCs). These cells usually form envelopes around single neurons, which create a distinct functional unit consisting of a neuron and its attending SGCs. This review presents the knowledge on the morphology of SGCs in sympathetic and parasympathetic ganglia, and the (limited) available information on their physiology and pharmacology. It appears that SGCs carry receptors for ATP and can thus respond to the release of this neurotransmitter by the neurons. There is evidence that SGCs have an uptake mechanism for GABA, and possibly other neurotransmitters, which enables them to control the neuronal microenvironment. Damage to post- or preganglionic nerve fibers influences both the ganglionic neurons and the SGCs. One major consequence of postganglionic nerve section is the detachment of preganglionic nerve terminals, resulting in decline of synaptic transmission. It appears that, at least in sympathetic ganglia, SGCs participate in the detachment process, and possibly in the subsequent recovery of the synaptic connections. Unlike sensory neurons, neurons in autonomic ganglia receive synaptic inputs, and SGCs are in very close contact with synaptic boutons. This places the SGCs in a position to influence synaptic transmission and information processing in autonomic ganglia, but this topic requires much further work.

Epidermal growth factor (EGF) promotes the in vitro differentiation of neural crest cells to neurons and melanocytes

Proliferation of neural crest (NC) stem cells and their subsequent differentiation into different neural cell types are key early events in the development of the peripheral nervous system. Soluble growth factors present at the sites where NC cells migrate are critical to the development of NC derivatives in each part of the body. In the present study, we further investigate the effect of microenvironmental factors on quail trunk NC development. We show for the first time that EGF induces differentiation of NC to the neuronal and melanocytic phenotypes, while fibroblast growth factor 2 (FGF2) promotes NC differentiation to Schwann cells. In the presence of both EGF and FGF2, the neuronal differentiation predominates. Our results suggest that FGF2 stimulates gliogenesis, while EGF promotes melanogenesis and neurogenesis. The combination of both growth factors stimulates neurogenesis. These findings suggest that these two growth factors may play an important role in the fate decision of NC progenitors and in the development of the peripheral nervous system.

Embryonic development of glial cells and myelin in the shark, Chiloscyllium punctatum

Glial cells are responsible for a wide range of functions in the nervous system of vertebrates. The myelinated nervous systems of extant elasmobranchs have the longest independent history of all gnathostomes. Much is known about the development of glia in other jawed vertebrates, but research in elasmobranchs is just beginning to reveal the mechanisms guiding neurodevelopment. This study examines the development of glial cells in the bamboo shark, Chiloscyllium punctatum, by identifying the expression pattern of several classic glial and myelin proteins. We show for the first time that glial development in the bamboo shark (C. punctamum) embryo follows closely the one observed in other vertebrates and that neural development seems to proceed at a faster rate in the PNS than in the CNS. In addition, we observed more myelinated tracts in the PNS than in the CNS, and as early as stage 32, suggesting that the ontogeny of myelin in sharks is closer to osteichthyans than agnathans.

Fibronectin promotes differentiation of neural crest progenitors endowed with smooth muscle cell potential

The neural crest (NC) is a model system used to investigate multipotency during vertebrate development. Environmental factors control NC cell fate decisions. Despite the well-known influence of extracellular matrix molecules in NC cell migration, the issue of whether they also influence NC cell differentiation has not been addressed at the single cell level. By analyzing mass and clonal cultures of mouse cephalic and quail trunk NC cells, we show for the first time that fibronectin (FN) promotes differentiation into the smooth muscle cell phenotype without affecting differentiation into glia, neurons, and melanocytes. Time course analysis indicated that the FN-induced effect was not related to massive cell death or proliferation of smooth muscle cells. Finally, by comparing clonal cultures of quail trunk NC cells grown on FN and collagen type IV (CLIV), we found that FN strongly increased both NC cell survival and the proportion of unipotent and oligopotent NC progenitors endowed with smooth muscle potential. In contrast, melanocytic progenitors were prominent in clonogenic NC cells grown on CLIV. Taken together, these results show that FN promotes NC cell differentiation along the smooth muscle lineage, and therefore plays an important role in fate decisions of NC progenitor cells.

The generation of adipocytes by the neural crest

Fat cells (adipocytes) develop from adipocyte precursor cells (preadipocytes) that themselves derive from mesenchymal progenitors. Although the events controlling preadipocyte differentiation into mature adipocytes have been largely explored, the mechanisms that direct mesenchymal progenitors down the adipocyte pathway remain unknown. Similarly, although adipocytes are generally thought to derive from mesoderm, key information is lacking regarding the origin and the development of the adipose tissue during embryogenesis. The aim of this study was to gain insight into the ontogeny of fat cells, both in mouse embryonic stem (mES) cell-derived cultures and during normal development. We first used genetically engineered mES cells to produce and select ES cell-derived neuroepithelial progenitors and showed that neuroectoderm, rather than mesoderm, may be a source of adipocytes in mES cell-derived cultures. We then used primary and secondary cultures of developing quail neural crest (NC) cells to demonstrate that NC cells are able, upon stimulation with defined factors, to differentiate into adipocytes, thus providing a powerful system to study the earliest stages of adipocyte differentiation. Finally, we mapped NC derivatives in vivo using Cre-mediated recombination in transgenic mice and demonstrated that a subset of adipocytes originates from the NC during normal development.

Clonally cultured differentiated pigment cells can dedifferentiate and generate multipotent progenitors with self-renewing potential

The differentiation of a given cell should be irreversible in order to ensure cell-type-specific function and stability of resident tissue. However, under stimulation in vitro or during regeneration, differentiated cells may recover properties of immature cells. Yet the mechanisms whereby differentiated cells can change fate or reverse to precursor cells are poorly understood. We show here that neural crest (NC)-derived pigment cells that have differentiated in quail embryo, when isolated from the skin and clonally cultured in vitro, are able to generate glial and myofibroblastic cells. The phenotypic reprogramming involves dedifferentiation of dividing pigment cells into cells that re-express NC early marker genes Sox10, FoxD3, Pax3 and Slug. Single melanocytes generate multipotent progenitors able to self-renew along serial subcloning, thus exhibiting stem cell properties. The presence of endothelin 3 promotes the emergence and maintenance of multipotent progenitors in melanocyte progeny. These multipotent cells are heterogeneous with respect to marker identity, including pigmented cells and dedifferentiated cells that have reacquired expression of the early NC marker HNK1. These data provide evidence that, when removed from their niche and subjected to appropriate culture conditions, pigment cells are phenotypically unstable and can reverse to their NC-like ancestors endowed with self-renewal capacity.

The neurobiology of Schwann cells

This selective review of Schwann cell biology focuses on questions relating to the origins, development and differentiation of Schwann cells and the signals that control these processes. The importance of neuregulins and their receptors in controlling Schwann cell precursor survival and generation of Schwann cells, and the role of these molecules in Schwann cell biology is addressed. The reciprocal signalling between peripheral glial cells and neurons in development and adult life revealed in recent years is highlighted, and the profound change in survival regulation from neuron-dependent Schwann cell precursors to adult Schwann cells that depend on autocrine survival signals is discussed. Besides providing neuronal and autocrine signals, Schwann cells signal to mesenchymal cells and influence the development of the connective tissue sheaths of peripheral nerves. The importance of Desert Hedgehog in this process is described. The control of gene expression during Schwann cell development and differentiation by transcription factors is reviewed. Knockout of Oct-6 and Krox-20 leads to delay or absence of myelination, and these results are related to morphological or physiological observations on knockout or mutation of myelin-related genes. Finally, the relationship between selected extracellular matrix components, integrins and the cytoskeleton is explored and related to disease.

The avian embryo as a model to study the development of the neural crest: a long and still ongoing story

The aim of this review is to evoke briefly the progress that has been made in our knowledge about the contribution of the neural crest to the vertebrate body since it was discovered by Wilhelm His in 1868. Although first studied essentially in amphibian embryos, a large amount of what is known on this very special structure was gained by experimental work carried out on the avian embryo. The making of chimeras between quail and chick has permitted not only to analyse the normal course of neural crest cell migration and differentiation but also to reveal some of the cellular interactions that regulate these events. Looking to the future, we can foresee that the novel methods, which now allow to manipulate gene activities in definite groups of cells and at elected times in the developing embryo, will make the avian model even more instrumental than ever to approach the developmental problems raised by neural crest cell differentiation.

Neural crest cell plasticity and its limits

The neural crest (NC) yields pluripotent cells endowed with migratory properties. They give rise to neurons, glia, melanocytes and endocrine cells, and to diverse 'mesenchymal' derivatives. Experiments in avian embryos have revealed that the differentiation of the NC 'neural' precursors is strongly influenced by environmental cues. The reversibility of differentiated cells (such as melanocytes or glia) to a pluripotent precursor state can even be induced in vitro by a cytokine, endothelin 3. The fate of 'mesenchymal' NC precursors is strongly restricted by Hox gene expression. In this context, however, facial skeleton morphogenesis is under the control of a multistep crosstalk between the epithelia (endoderm and ectoderm) and NC cells.

Self-renewal capacity is a widespread property of various types of neural crest precursor cells

In vertebrates, trunk neural crest (NC) generates glia, neurons, and melanocytes. In addition, it yields mesectodermal derivatives (connective tissues, chondrocytes, and myofibroblasts lining the blood vessels) in the head. Previous in vitro clonal analyses of avian NC cells unraveled a hierarchical succession of highly pluripotent, followed by various intermediate, progenitors, suggesting a model of progressive restrictions in the multiple potentialities of a totipotent stem cell, as prevails in the hematopoietic system. However, which progenitors are able to self-renew within the hierarchy of the NC lineages is still undetermined. Here, we explored further the stem cell properties of quail NC cells by means of in vitro serial subcloning. We identified types of multipotent and oligopotent NC progenitors that differ in their developmental repertoire, ability to self-maintain, and response to exogenous endothelin 3 according to their truncal or cephalic origin. The most striking result is that bipotent progenitors are endowed with self-renewal properties. Thus glia-melanocyte and glia-myofibroblast progenitors behave like stem cells in that they are able both to self-renew and generate a restricted progeny. In our culture conditions, glia-myofibroblast precursors display a modest capacity to self-renew, whereas glia-melanocyte precursors respond to endothelin 3 by extensive self-renewal. These findings may explain the etiology of certain multiphenotypic NC-derived tumors in humans. Moreover, the presence of multiple stem cell phenotypes along the NC-derived lineages may account for the rarity of the "totipotent NC stem cell" and may be related to the large variety and widespread dispersion of NC derivatives throughout the body.

Reversal of developmental restrictions in neural crest lineages: transition from Schwann cells to glial-melanocytic precursors in vitro

In vertebrate embryos, diversification of the lineages arising from the neural crest (NC) is controlled to a large extent by environmental factors. In previous work, we showed that endothelin 3 (ET3) peptide favors the development of glial and melanocytic NC precursors in vitro. This factor is also capable of inducing proliferation of cultured epidermal pigment cells and their conversion to glia. ET3 therefore strongly promotes the emergence of melanocytic and glial phenotypes from precursors and acts on the maintenance of these phenotypes. In the present work, we explored the capacity of ET3 to reprogram glial cells into melanocytes. Schwann cells expressing glial-specific markers [such as the Schwann cell myelin protein (SMP)] were isolated from sciatic nerves of quail embryos and cultured in vitro. We found that ET3 promotes cell growth and sequential expression of melanocyte differentiation markers in cultures of purified SMP-expressing cells, whereas it had no significant effect on SMP-negative cells from the same nerves. Moreover, we provide evidence for the transition of differentiated Schwann cells to melanocytes in clonal cultures. This transition involves the production of a mixed progeny of melanoblasts/melanocytes, glia, and cells bearing differentiation markers of both phenotypes. Therefore, Schwann cells exposed to ET3 transdifferentiate to melanocytes through reversion to the stage of bipotent glial-melanocytic NC precursors. These findings show that NC-derived pigment and glial cells are phenotypically unstable in vitro and may undergo reversal of precursor hierarchy to function as bipotent stem cells.

Role of enteric glial cells in inflammatory bowel disease

Enteric glial cells (EGCs) represent an extensive but relatively poorly described cell population within the gastrointestinal tract. Accumulating data suggest that EGCs represent the morphological and functional equivalent of CNS astrocytes within the enteric nervous system (ENS). The EGC network has trophic and protective functions toward enteric neurons and is fully implicated in the integration and the modulation of neuronal activities. Moreover, EGCs seem to be active elements of the ENS during intestinal inflammatory and immune responses, sharing with astrocytes the ability to act as antigen-presenting cells and interacting with the mucosal immune system via the expression of cytokines and cytokine receptors. Transgenic mouse systems have demonstrated that specific ablation of EGC by chemical ablation or autoimmune T-cell targeting induces an intestinal pathology that shows similarities to the early intestinal immunopathology of Crohn's disease. EGCs may also share with astrocytes the ability to regulate tissue integrity, thereby postulating that similar interactions to those observed for the blood-brain barrier may also be partly responsible for regulating mucosal and vascular permeability in the gastrointestinal tract. Disruption of the EGC network in Crohn's disease patients may represent one possible cause for the enhanced mucosal permeability state and vascular dysfunction that are thought to favor mucosal inflammation.

A Monoclonal Antibody Defining a Carbohydrate Epitope Restricted to Glial Cells

To follow cell lineage segregation of glial cell precursors in the central and peripheral nervous systems (CNS and PNS), we made monoclonal antibodies against purified avian myelin glycoproteins and made detailed studies of antibodies reacting with cellular antigens expressed early in development. We describe here the cellular and molecular characterization of the 4B3 epitope, which is first expressed at day 2.5 in quail embryos and appears strictly specific to glial cell membranes in the PNS and CNS. We demonstrate that, not only oligodendrocytes and astrocytes but also myelinating and non-myelinating Schwann cells, satellite cells of sensory and autonomic ganglia and enteric glial cells are 4B3-positive. This epitope was identified as an N-linked carbohydrate determinant shared by several unrelated molecules, one of them already described as the SMP glycoprotein.

Isolation of enteric glia and establishment of transformed enteroglial cell lines from the myenteric plexus of adult rat

Although enteroglial cells (EGCs) may play a key role in the inflammatory response of the enteric nervous system, little is known about their immunophysiological properties. To facilitate further characterization of enteric glia, we have developed a novel method to isolate and purify EGCs from the myenteric plexus. Myenteric plexus preparations were enzymatically dissociated and EGCs purified by complement-mediated cytolysis of contaminating cells and transformed by retroviral gene transfer. Primary and transformed cells were characterized immunohistochemically and by dot-blot analysis. Functionally, c-fos mRNA expression was assessed in primary and transformed enteroglial cells. All cells displayed robust glial fibrillary acidic protein, S-100 and vimentin immunoreactivities, but no Thy-1.1, desmin, smooth muscle alpha-actin or C3 complement receptor immunoreactivity. This confirmed their enteroglial lineage and excluded contamination with other cell types. Both primary and transformed EGCs displayed little constitutive c-fos mRNA expression. This, however, could be upregulated by various stimuli, including proinflammatory cytokines. In summary, we present a novel method to purify EGCs from rat myenteric plexus for tissue culture and to establish transformed EGC lines that retain their glial nature and functional properties. Such cell lines are now available for physiological studies of the functional properties of enteric glia in vitro.

Endothelin 3 induces the reversion of melanocytes to glia through a neural crest-derived glial-melanocytic progenitor

Functional signaling of endothelin 3 (ET3) and its receptor B (ETRB) has been shown to be required for the development of neural crest (NC)-derived pigment cells in mouse, but the precise role of ET3 is not completely understood. Using the avian embryo as a model, we previously reported that ET3 promotes the survival and proliferation of unipotent melanocyte and bipotent glia-melanocyte precursors in trunk NC cultures. Here we investigated whether, at later stages, embryonic pigment cells respond to ET3. Such a possibility is supported by the previous finding that, in vivo, avian melanocytes express endothelin receptor B2 (ETRB2) during migration and after their differentiation in the skin. We found that in vitro ET3 exerts a dose-dependent stimulation of proliferation and melanogenesis in NC cells that had homed to the epidermis of embryonic quail dorsal skin. Moreover, in clonal cultures of skin-derived pigment cells, ET3 induces rapid cell divisions of clonogenic melanocytes that generate a mixed progeny of melanocytes and cells devoid of pigment granules and expressing glial markers in more than 40% of the colonies. It can therefore be concluded that ET3 is strongly mitogenic to embryonic pigment cells and able to alter their differentiation program, leading them to recapitulate the glial-melanocyte bipotentiality of their NC ancestors.

The developmental potentials of the caudalmost part of the neural crest are restricted to melanocytes and glia

The avian spinal cord is characterized by an absence of motor nerves and sensory nerves and ganglia at its caudalmost part. Since peripheral sensory neurons derive from neural crest cells, three basic mechanisms could account for this feature: (i) the caudalmost neural tube does not generate any neural crest cells; (ii) neural crest cells originating from the caudal part of the neural tube cannot give rise to dorsal root ganglia or (iii) the caudal environment is not permissive for the formation of dorsal root ganglia. To solve this problem, we have first studied the pattern of expression of ventral (HNF3beta) and dorsal (slug) marker genes in the caudal region of the neural tube; in a second approach, we have recorded the emergence of neural crest cells using the HNK1 monoclonal antibody; and finally, we have analyzed the developmental potentials of neural crest cells arising from the caudalmost part of the neural tube in avian embryo in in vitro culture and by means of heterotopic transplantations in vivo. We show here that neural crest cells arising from the neural tube located at the level of somites 47-53 can differentiate both in vitro and in vivo into melanocytes and Schwann cells but not into neurons. Furthermore, the neural tube located caudally to the last pair of somites (i.e. the 53rd pair) does not give rise to neural crest cells in any of the situations tested. The specific anatomical aspect of the avian spinal cord can thus be accounted for by limited developmental potentials of neural crest cells arising from the most caudal part of the neural tube.

The Ets domain transcription factor Erm distinguishes rat satellite glia from Schwann cells and is regulated in satellite cells by neuregulin signaling

Distinct glial cell types of the vertebrate peripheral nervous system (PNS) are derived from the neural crest. Here we show that the expression of the Ets domain transcription factor Erm distinguishes satellite glia from Schwann cells beginning early in rat PNS development. In developing dorsal root ganglia (DRG), Erm is present both in presumptive satellite glia and in neurons. In contrast, Erm is not detectable at any developmental stage in Schwann cells in peripheral nerves. In addition, Erm is downregulated in DRG-derived glia adopting Schwann cell traits in culture. Thus, Erm is the first described transcription factor expressed in satellite glia but not in Schwann cells. In culture, the Neuregulin1 (NRG1) isoform GGF2 maintains Erm expression in presumptive satellite cells and reinduces Erm expression in DRG-derived glia but not in Schwann cells from sciatic nerve. These data demonstrate that there are intrinsic differences between these glial subtypes in their response to NRG1 signaling. In neural crest cultures, Erm-positive progenitor cells give rise to two distinct glial subtypes: Erm-positive, Oct-6-negative satellite glia in response to GGF2, and Erm-negative, Oct-6-positive Schwann cells in the presence of serum and the adenylate cyclase activator forskolin. Thus, Erm-positive neural crest-derived progenitor cells and presumptive satellite glia are able to acquire Schwann cell features. Given the in vivo expression of Erm in peripheral ganglia, we suggest that ganglionic Erm-positive cells may be precursors of Schwann cells.

I-type lectins in the nervous system

The number of animal lectins, basically defined upon their interaction with specific carbohydrate structures, is growing considerably during the last few years. Among these proteins the recently identified subfamily of I-type lectins consists of mainly transmembranous glycoproteins belonging to the immunoglobulin superfamily. Most of the I-type lectins participate in cell adhesion events, as are the different sialoadhesins recognizing sialylated glycan structures, which represent the best characterized subgroup. I-type lectins are abundant in the nervous system and have been implicated in a number of morphogenetic processes as fundamental as axon growth, myelin formation and growth factor signaling. In the present review, we summarize the structural and functional properties of I-type lectins expressed in neural tissues with a main focus on the sialoadhesin myelin-associated glycoprotein, the neural cell adhesion molecule and the fibroblast growth factor receptors.

Endothelin 3 selectively promotes survival and proliferation of neural crest-derived glial and melanocytic precursors in vitro

Genetic data in the mouse have shown that endothelin 3 (ET3) and its receptor B (ETRB) are essential for the development of two neural crest (NC) derivatives, the melanocytes and the enteric nervous system. We report here the effects of ET3 in vitro on the differentiation of quail trunk NC cells (NCC) in mass and clonal cultures. Treatment with ET3 is highly mitogenic to the undifferentiated NCC population, which leads to expansion of the population of cells in the melanocytic, and to a lesser extent, the glial lineages. The effect of ET3 on these two NC derivatives was confirmed by the quantitative analysis of clones derived from individual NCC subjected to ET3: we found a large increase in the survival and proliferation of unipotent and bipotent precursors for glial cells and melanocytes, with no significant effect on multipotent cells generating neurons. ET3 first stimulates expression of both ETRB and ETRB2 by cultured NCC. Then, under prolonged exposure to ET3, ETRB expression decreases and switches toward an ETRB2-positive melanogenic cell population. We therefore propose that the present in vitro experiments (long-lasting exposure to a high concentration of ET3) mimic the environment encountered by NCC in vivo when they migrate to the skin under the ectoderm that expresses ET3.

Dorsal spinal cord neuroepithelium generates astrocytes but not oligodendrocytes

There is evidence that oligodendrocytes in the spinal cord are derived from a restricted part of the ventricular zone near the floor plate. An alternative view is that oligodendrocytes are generated from all parts of the ventricular zone. We reinvestigated glial origins by constructing chick-quail chimeras in which dorsal or ventral segments of the embryonic chick neural tube were replaced with equivalent segments of quail neural tube. Ventral grafts gave rise to both oligodendrocytes and astrocytes. In contrast, dorsal grafts produced astrocytes but not oligodendrocytes. In mixed cultures of ventral and dorsal cells, only ventral cells generated oligodendrocytes, whereas both ventral and dorsal cells generated astrocytes. Therefore, oligodendrocytes are derived specifically from ventral neuroepithelium, and astrocytes from both dorsal and ventral.

The avian embryo as a model in developmental studies: chimeras and in vitro clonal analysis

The avian embryo is a model in which techniques of experimental embryology and cellular and molecular biology can converge to address fundamental questions of development biology. The first part of the chapter describes two examples of transplantation and cell labeling experiments performed in ovo. Thanks to the distinctive histologic and immunocytochemical characteristics of quail and chick cells, the migration and development of definite cells are followed in suitably constructed chimeric quail-chick embryos. Isotopic transplantations of neural tube portions between quail and chick, combined with in situ hybridization with a nucleic probe specific for a quail oligodendrocyte marker, allowed study of the origin and migration of oligodendroblasts in the spinal cord. Heterotopic transplantations of rhombomeres were performed to establish the degree of plasticity of these segments of the hindbrain regarding Hox gene expression, which was revealed by labeling with chick-specific nucleic probes. The second part describes in vitro cell cloning experiments devised to investigate cell lineage segregation and diversification during development of the NC. An original cloning procedure and optimal culture conditions permitted analysis of the developmental potentials of individual NC cells taken at definite migration stages. The results revealed a striking heterogeneity of the crest cell population, which appeared to be composed of precursors at different states of determination. Clonal cultures also provide a means to identify subsets of cells that are the target of environmental factors and to understand how extrinsic signals influence the development of responsive cells.

Identification and characterization of differentiation-dependent Schwann cell surface antigens by novel monoclonal antibodies: introduction of a marker common to the non-myelin-forming phenotype

In an attempt to identify and characterize novel Schwann cell surface molecules with putative functions during development, maintenance, and regeneration of the peripheral nervous system (PNS), we have produced monoclonal antibodies against viable neonatal rat Schwann cells. Using a sensitive live cell ELISA protocol, three monoclonal antibodies reactive with cultured Schwann cells, designated 27B10, 26F2, and 27C7 were isolated. The 27B10 and 26F2 antibodies specifically labelled forskolin-stimulated secondary Schwann cells in vitro as determined by live cell ELISA implying that the expression of the antigens in situ is regulated by axonal contact. The observation that the antigens seemed to be associated with both Schwann cell phenotypes clearly discriminated them from the well characterized myelin proteins as well as from molecules known to be confined to the non-myelin-forming phenotype. Interestingly, both antigens were found to be concentrated at the nodes of Ranvier. Further studies therefore have to show whether the identified antigens share structural or functional homology with adhesion or channel molecules, which display a similar distribution. Following transection of the adult sciatic nerve, the 26F2 antigen was rapidly down-regulated in the distal nerve stump. The 27C7 antibody reacted with an 80 kDa cell surface molecule common to non-myelin-forming Schwann cells. No differences in expression of the antigen between forskolin-treated and untreated Schwann cells in vitro were found, suggesting that the antigen is expressed independently from axonal contact. Two weeks after nerve transection in the absence of myelinating Schwann cells, the antigen was associated with S-100-positive Schwann cells of the distal nerve stump. The antigen was found to be expressed also by non-neuronal tissues, the level of the protein declined towards the adult stage. Comparison of the 27C7 antigen with previously described marker molecules suggests that we have identified a novel Schwann cell surface antigen of the non-myelin-forming phenotype.

Glycoproteins of myelin sheaths

A growing number of glycoproteins have been identified and characterized in myelin and myelin-forming cells. In addition to the major P0 glycoprotein of compact PNS myelin and the myelin-associated glycoprotein (MAG) in the periaxonal membranes of myelin-forming oligodendrocytes and Schwann cells, the list now includes peripheral myelin protein-22 (PMP-22), a 170 kDa glycoprotein associated with PNS myelin and Schwann cells (P170k/SAG), Schwann cell myelin protein (SMP), myelin/oligodendrocyte glycoprotein (MOG), and oligodendrocyte-myelin glycoprotein (OMgp). Many of these glycoproteins are members of the immunoglobulin superfamily and express the adhesion-related HNK-1 carbohydrate epitope. This review summarizes recent findings concerning the structure and function of these glycoproteins of myelin sheaths with emphasis on the physiological roles of oligosaccharide moieties.

Embryonic neural chimeras in the study of vertebrate brain and head development

Construction of neural chimeras between quail and chick embryos has been employed since 1969 when the unique nucleolar structure of the quail nucleus and its use to devise a cell marking technique by associating quail and chick cells in ovo were described in the "Bulletin Biologique de la France et de la Belgique." This method was first applied to the ontogeny of the neural crest, a structure whose development involves extensive cell migration, and, since 1984, to that of the central nervous system (CNS). This chapter highlights some of the most significant findings provided by this approach concerning the CNS, such as (i) demonstration of the common origin of the floor plate and notochord from a group of cells localized in the "organizer", i.e., Hensen's node, and the way in which these two structures become positioned respectively within and under the neural tube during gastrulation and neurulation in Amniotes; (ii) the neural crest origin of the skull vault and the facial and hypobranchial skeleton. This means that the mesodermal contribution to the skull is limited to the occipital and otic regions and extends only to the rostral limit of the notochord. A correlation can be drawn between the development of the telencephalon and the mesectodermally derived skull in the vertebrate phylum; (iii) demonstration that the midbrain-hindbrain junction, at the stage of the encephalic vesicles, acts as an organizing center for tectal and cerebellar structures. This function was correlated with the activity of several developmental genes, thus providing insight into their function during neurogenesis; (iv) the pattern of morphogenetic movements and cell migration taking place in defined brain-to-be areas, as well as the origin of various cell types of nervous tissues; and (v) a new avenue for studying brain localization of either behavioral traits or genetically encoded brain disorders.

A paracrine effect for neuron-derived BDNF in development of dorsal root ganglia: stimulation of Schwann cell myelin protein expression by glial cells

Addition of neurons to cultures of non-neuronal cells derived from quail embryonic dorsal root ganglia causes a 2.5-fold increase in the proportion of cells that express the glial marker Schwann cell myelin protein (SMP) when compared to cultures devoid of neurons. This effect is mediated by BDNF because incubation with a trkB immunoadhesin that sequesters BDNF, but not with trkA or trkC immunoadhesins, abolishes this stimulation. This neuronal activity can be mimicked by treatment with soluble BDNF that stimulates specifically the conversion of SMP-negative glial cells into cells that express this phenotype. That BDNF is the endogenous neuron-derived factor affecting glial development is further supported by the observation that BDNF is extensively expressed in developing sensory neurons of the avian ganglia both in vivo and in vitro, but not by the satellite cells. These results show for the first time a paracrine role for neuronal BDNF on differentiation of peripheral glial cells. This effect of BDNF is likely to be mediated by the p75 neurotrophin receptor because: (1) p75 immunoreactive protein is expressed by a subset of satellite cells; (2) neutralization of p75 abolishes the BDNF-induced stimulation; (3) a treatment of non-neuronal cell cultures with equimolar concentrations of either soluble NGF or NT-3 also affects the proportion of cells that become SMP-positive. Whereas NGF stimulates the acquisition of this glial antigen to a similar extent as BDNF, NT-3 inhibits its expression, suggesting that distinct neurotrophins signal differentially through p75. These findings also suggest that the definitive phenotype of peripheral glia is determined by a balance between positive and inhibitory signals arising in adjacent neurons.

The regulation of Krox-20 expression reveals important steps in the control of peripheral glial cell development

The zinc finger transcription factor gene Krox-20 is expressed in Schwann cells and is required for the myelination of peripheral nerves. We show that the regulation of Krox-20 expression in peripheral glial cells reveals three important steps in the development and differentiation of these cells. (i) Expression of Krox-20 in Schwann cells requires continuous neuronal signalling via direct axonal contact. Therefore Krox-20 appears to be a key component of the transduction cascade linking axonal signalling to myelination. (ii) Krox-20 inducibility is acquired by Schwann cells at the time that they are formed from their precursors. Diffusible factor(s) synthesised by the neural tube can mediate this transition and can be mimicked by NDFbeta or a combination of CNTF and bFGF. Furthermore, the neural tube activity is blocked by a hybrid protein containing the NDF-binding domain of the ErbB4 receptor, strongly implicating NDF in the physiological transition. (iii) In sensory ganglia, the microenvironment is capable of negatively regulating Krox-20, presumably by preventing the conversion of satellite glial cells toward a Schwann cell-like phenotype.

Oligodendrocyte precursors originate from both the dorsal and the ventral parts of the spinal cord

Oligodendrocytes have been recently claimed to originate from bilateral columns of precursors whose extension is limited to the ventral region of the neuroepithelium. We designed an experiment in which the developmental capabilities of the different regions of the ventricular epithelium could be tested. We exchanged isotopically and isochronically defined sectors of the E2 spinal cord between quail and chick embryos and followed the production and migration of oligodendrocytes by using a quail-specific cDNA probe encoding the oligodendrocyte marker Schwann cell myelin protein. We showed that oligodendrocytes are generated in vivo from both ventral and dorsal halves of the neural tube. Moreover, extensive ventrodorsal, as well as dorsoventral, migrations of oligodendrogenic cells take place during spinal cord differentiation.

Genetic and epigenetic control in neural crest development

The neural crest is a fascinating structure of the vertebrate embryo; its ontogeny includes a transient period during which its component cells undergo an epithelio-mesenchymal transition and become migratory. This phase was shown recently to be controlled by the 'Slug' gene which belongs to the 'Snail' family of Drosophila transcription factors. After homing to specific sites in the embryo, the crest-derived cells produce a large variety of phenotypes. Recent advances have shown that during migration most crest cells exhibit various degrees of pluripotentiality, some being already committed to a single and definite fate. Moreover, several lines of evidence point to the existence of totipotent stem cells in the neural crest, the progeny of which become progressively diversified through a combination of intrinsic and extrinsic influences. The latter have been documented by the disruption of several neurotrophin genes, which results in severe deficiencies of selected subsets of neural crest derivatives. The neural crest has also been shown to play an important role in the development of the vertebrate head and hypobranchial region. The genetic control of this process depends on the activity of developmental genes, among which the vertebrate Hox genes are essential, particularly at the rhombencephalic level.

Plasticity in neural crest cell differentiation

The neural crest is a pluripotent population of cells that are endowed with migratory capacities. It has long been known that the differentiation pathway taken by cells derived from the neural crest is largely controlled by the microenvironment to which they home after their migration phase, indicating a high degree of plasticity in their developmental fate. Recent progress has been made concerning the factors which influence survival, growth and differentiation of selected sets of precursors in each embryonic site colonised by derivatives of the neural crest.

Structural abnormalities associated with congenital megacolon in transgenic mice that overexpress the Hoxa-4 gene

Congenital megacolon develops in transgenic mice that overexpress the homeobox-containing gene, Hoxa-4. The current study was done to identify abnormalities of the terminal colon that might account for the phenotype. The terminal bowel of transgenic mice was compared with that of control and lethal spotted (ls/ls) mice, a strain in which megacolon also develops. The terminal colon of the transgenic mice contained fewer ganglia than that of controls, but was hypoganglionic, rather than aganglionic like that of ls/ls mice. The neurons present in the adult transgenic colon were significantly increased in size and a subset of very large neurons (> 40 microns in maximum diameter) were observed. Electron microscopic studies of young adult transgenic mice revealed that the ganglia and nerves of the myenteric plexus had the ultrastructure of extraenteric peripheral nerve rather than that of the enteric nervous system (ENS). The myenteric ganglia in the transgenic animals contained Schwann cells associated with a basal lamina that enveloped axons completely and individually, instead of glia. Although collagen is excluded from the ganglia and thin nerve fibers of the normal ENS, a collagen-containing endoneurium surrounded each of the axon-Schwann cell units of the abnormal nerve fibers of the transgenic colon. Some of the neurons of the transgenic mice were located in these nerve bundles rather than in ganglia. There were also smooth muscle abnormalities in the terminal bowel of the transgenic mice. Wide gaps were present in the longitudinal muscle of the transgenic mice; these gaps contained ganglia that were in contact with the adventitia. These longitudinal smooth muscle cells were more irregular than those of controls and they contained fewer puncta adherens; moreover, a larger proportion of the volume of the cytoplasm of transgenic smooth muscle cells was occupied by organelles. Finally, an extensive thickening and reduplication of the basal lamina surrounding the smooth muscle cells of the muscularis mucosa was observed in the transgenic colon and resembled that found in ls/ls mice. These data suggest that both smooth muscle and the innervation of the terminal bowel of neonatal Hoxa-4 transgenic mice are structurally abnormal. Although some of the abnormalities seen in Hoxa-4 transgenic mice are similar to those which arise in ls/ls mice, the two conditions are not identical. In both animals, the data are consistent with the hypothesis that the defects arise as a result of a defective interaction between the precursors of enteric neurons and smooth muscle.

Negative regulation of Schwann cell myelin protein gene expression by the dorsal root ganglionic microenvironment

In vivo, the surface glycoprotein Schwann cell myelin protein (SMP) is expressed in the quail peripheral nervous system exclusively by Schwann cells. It is not detectable at any developmental stage either in enteric glia or in ganglionic satellite cells. We demonstrate here that the satellite glial cells of the dorsal root ganglia start to express SMP on their surface when they are dissociated into single cells and cultivated in vitro. Activation of SMP synthesis is a rapid event observed in mass cultures of dorsal root ganglia dissociated cells as soon as 4 h after the onset of the culture. Confocal microscope analysis revealed that satellite cells may acquire the Schwann cell marker when still in close contact with the neuronal soma. Clonal cultures of satellite cells from E8 dorsal root ganglia demonstrated that the progeny of these SMP-negative cells steadily express SMP. This, together with similar results previously obtained with enteric glia, suggests that the SMP-positive phenotype is a constitutive trait of the peripheral glial cell lineage which is inhibited in satellite cells in vivo by the microenvironment prevailing in the peripheral nervous system ganglia.

Neuron-Schwann cell signals are conserved across species: purification and characterization of embryonic chicken Schwann cells

A monoclonal antibody, 1E8, which recognizes the peripheral myelin protein, P0, specific for chicken Schwann cells and their precursors (Bhattacharyya et al., Neuron 7:831-844, 1991), was used to immunoselect Schwann cells from embryonic day 14 (E14) chicken sciatic nerve. When cultured, these immunoselected cells displayed properties characteristic of perinatal rodent Schwann cells, including S100-immunoreactivity and O4 antigen-immunoreactivity. In addition, the purified chicken Schwann cells divided slowly when cultured alone, but when co-cultured with chicken or rat sensory neurons, they bound to axons and proliferated. Proliferation was also stimulated by the addition of bovine brain membrane extracts or chicken brain membranes. The 1E8 monoclonal antibody was also used to test the effect of axonal contact on P0 expression. Chicken Schwann cells purified using the 1E8 monoclonal antibody gradually lost P0 when cultured alone. These cells remained 1E8-negative even after prolonged co-culture with embryonic rat dorsal root ganglion neurons or chicken sensory ganglia. These results demonstrate that chicken Schwann cells behave like rodent Schwann cells in their expression of specific antigens, interactions with axons, and regulation of P0 expression. In addition, chicken Schwann cells respond to neuronal signals from the rat and cow, illustrating the cross-species conservation of these signals.

Cell lineage analysis in neural crest ontogeny

The neural crest is a transitory and pluripotent structure of the vertebrate embryo composed of cells endowed with developmentally regulated migratory properties. We review here a series of studies carried out both in vivo and in vitro on the ontogeny of the neural crest in the avian embryo. Through in vivo studies we established the fate map of the neural crest along the neuraxis prior to the onset of the migration and we demonstrated the crucial role played by the tissue environment in which the crest cells migrate in determining their fate. Moreover, the pathways of neural crest cell migration could also be traced by the quail-chick marker system and the use of the HNK1/NC1 monoclonal antibody (Mab). A large series of clonal cultures of isolated neural crest cells showed that, at migration time, most crest cells are pluripotent. Some, however, are already committed to a particular pathway of differentiation. The differentiation capacities of the pluripotent progenitors are highly variable from one to the other cell. Rare totipotent progenitors able to give rise to representatives of all the phenotypes (neuronal, glial, melanocytic, and mesectodermal) encountered in neural crest derivatives were also found. As a whole we propose a model according to which totipotent neural crest cells become progressively restricted (according to a stochastic rather than a sequentially ordered mechanism) in their potentialities, while they actively divide during the migration process. At the sites of gangliogenesis, selective forces allow only certain crest cells potentialities to be expressed in each type of peripheral nervous system (PNS) ganglia.

From neural crest to bowel: development of the enteric nervous system

The ENS resembles the brain and differs both physiologically and structurally from any other region of the PNS. Recent experiments in which crest cell migration has been studied with DiI, a replication-deficient retrovirus, or antibodies that label cells of neural crest origin, have confirmed that both the avian and mammalian bowel are colonized by émigrés from the sacral as well as the vagal level of the neural crest. Components of the extracellular matrix, such as laminin, may play roles in enteric neural and glial development. The observation that an overabundance of laminin develops in the presumptive aganglionic region of the gut in ls/ls mutant mice and is associated with the inability of crest-derived cells to colonize this region of the bowel has led to the hypothesis that laminin promotes the development of crest-derived cells as enteric neurons. Premature expression of a neuronal phenotype would cause crest-derived cells to cease migrating before they complete the colonization of the gut. The acquisition by crest-derived cells of a nonintegrin, nerve-specific, 110 kD laminin-binding protein when they enter the bowel may enable these cells to respond to laminin differently from their pre-enteric migrating predecessors. Crest-derived cells migrating along the vagal pathway to the mammalian gut are transiently catecholaminergic (TC). This phenotype appears to be lost rapidly as the cells enter the bowel and begin to follow their program of terminal differentiation. The appearance and disappearance of TC cells may thus be an example of the effects of the enteric microenvironment on the differentiation of crest-derived cells in situ. Crest-derived cells can be isolated from the enteric microenvironment by immunoselection, a method that takes advantage of the selective expression on the surfaces of crest-derived cells of certain antigens. One neurotrophin, NT-3, promotes the development of enteric neurons and glia in vitro. Because trkC is expressed in the developing and mature gut, it seems likely that NT-3 plays a critical role in the development of the ENS in situ. Although the factors that are responsible for the development of the unique properties of the ENS remain unknown, progress made in understanding enteric neuronal development has recently accelerated. The application of new techniques and recently developed probes suggest that the accelerated pace of discovery in this area can be expected to continue.

Statistical evidence for a random commitment of pluripotent cephalic neural crest cells

The neural crest (NC) of vertebrate embryos yields cell types belonging to the neural, melanocytic and mesectodermal lineages. To test the possibility that the precursors of these lineages segregate from pluripotent cells by a process involving stochastic determinants, we have analyzed with statistical methods the associations between six differentiated cell types in 201 clones obtained in vitro from migrating cephalic NC cells. Our analysis suggests that neuronal, adrenergic and Schwann cells are not randomly associated, whereas these neural cell types differentiate in the clones independently of both melanocytes and cartilage. These results raise the possibility that pluripotent NC progenitors give rise to the precursors of the major NC-derived lineages (neural, melanocytic and mesectodermal) by a process involving stochastic restrictions of their developmental potentialities.

Division and migration of satellite glia in the embryonic rat superior cervical ganglion

While distinct precursors committed to a neuronal or glial cell fate are generated from neural crest cells early in peripheral gangliogenesis, little is known about the subsequent generation and maturation of young satellite glia from restricted glial precursor cells. To examine the division and migration of glial precursor cells and their satellite cell progeny, morphological, immunocytochemical and culture techniques were applied to the developing rat superior cervical ganglion. At embryonic day (E)18.5, numerous clusters of nonneuronal cells appeared transiently in the ganglion. Individual cells with a similar morphology were present in E16.5 ganglia, and are likely to represent the precursor cells which generate these clusters. The clustered cells were distinguishable from neighbouring neurons as well as from endothelial cells and fibroblasts. Morphologically similar cells were present in nerve bundles at E18.5 and surrounding principal neurons and nerve bundles in the adult ganglion. Double-label studies of the E18.5 ganglion with tyrosine hydroxylase to identify noradrenergic neurons and propidium iodide counterstaining to visualize all cell nuclei revealed that the cells in clusters stained with propidium iodide but lacked tyrosine hydroxylase immunoreactivity. To determine if cell clusters arose from division, bromodeoxy-uridine, a thymidine analogue, was administered to pregnant mothers between E16.5-E18.5, and ganglionic cells examined at E18.5 both in vivo and in vitro. Numerous non-neuronal cells divided during this period in situ and composed portions of clusters. When dissociated, superior cervical ganglion satellite glia reacted with an NGF-receptor antibody (MAb 217c) and possessed a flattened shape, in contrast to bipolar Schwann cells. Over half of the 217c-immunoreactive glia at E18.5 had incorporated bromodeoxyuridine during E16.5-18.5 in vivo. At birth, non-neuronal cells were no longer grouped in clusters, but were associated with neuronal cell bodies and processes. These findings suggest that, between E16.5-E18.5, glial precursors divide rapidly to form clusters, and that, after the peak of neurogenesis, daughter cells migrate within the ganglion to associate with nerve cell bodies and processes where proliferation continues at a slower rate. Distinct cellular and molecular interactions are likely to trigger the initial rapid division of glial precursors, initiate their migration and association with neuron cell bodies, and control their subsequent slower division.

Developmental potentialities of cells derived from the truncal neural crest in clonal cultures

The developmental potentialities of single truncal neural crest derived cells were analysed in clonal cultures. The clone-forming ability and differentiation potential of crest cells migrating through the somitic mesoderm of 3-day-old embryos (E3) and of non-neuronal cells of dorsal root ganglia taken at E6-14 were compared. Since most of the cells present in the sclerotomal and rostral parts of the somite at E3 become later on incorporated into the spinal ganglia, one can consider that these two cell populations represent the same derivatives of the trunk neural crest at different developmental stages. After 10 days in vitro, the size of clones and their phenotypic composition varied noticeably, revealing a certain heterogeneity in the founder cell populations in terms of developmental potencies. Clones obtained from migrating neural crest cells at E3 were often large (greater than 1000 cells) and many of them contained neuronal and non-neuronal cells. Dorsal root ganglion cells produced mostly small clones (less than 100 cells) in which only non-neuronal (i.e. glial) phenotypes were expressed. Therefore, both the capacity for proliferation and the differentiation ability of cloned neural crest derived cells decrease considerably with increasing embryonic age. This is even more striking if these results are compared with those obtained previously in our laboratory with single cells cultures of E2 cephalic neural crest. In the latter case, both clone sizes and cellular diversity within the colonies were much higher than with E3 truncal crest and dorsal root ganglia (DRG) non-neuronal cells. The second result of the present work concerns the differentiation of the dormant autonomic neuronal precursors of the DRG. It has been established previously that the non-neuronal cells of the DRG include adrenergic precursors than can differentiate in mass culture of dissociated DRG cells. We show that these cells never differentiate in clonal cultures but depend upon the cell density of the culture. This suggests that cell to cell interaction between crest derived cells are critical in eliciting the differentiation of the adrenergic phenotype.

Molecular characterization of the Schwann cell myelin protein, SMP: structural similarities within the immunoglobulin superfamily

The Schwann cell myelin protein (SMP), previously defined in quail and chick by a monoclonal antibody, is in vivo exclusively expressed by myelinating and nonmyelinating Schwann cells and oligodendrocytes. The isolation of the complete nucleotide sequence of SMP is reported here. The predicted polypeptide chain reveals that SMP is a transmembrane molecule of the immunoglobulin superfamily showing sequence similarities with several surface glycoproteins expressed in the nervous and immune systems. In spite of a 43.5% overall sequence identity between rat myelin-associated glycoprotein (MAG) and quail SMP, SMP does not seem to be the avian homolog of MAG, since their expression, regulation, and functions are significantly different. Unusual sequence arrangements shared by SMP, MAG, and two lymphoid antigens suggest the existence of a particular subgroup in the immunoglobulin superfamily.