Biomedical subjects

C Kalcheim

Publications and source records attributed to C Kalcheim.

At least 19 recordsLinked to original sources

PubMed2001

Differential expression of neuropilin-1 and neuropilin-2 in arteries and veins.

Neuropilin-1 (np1) and neuropilin-2 (np2) are receptors for class-3 semaphorins and for several isoforms of VEGF. We have cloned and characterized two chick isoforms of np2 cDNA. Expression patterns of np1, np2, and ephrin-B2 were compared in the developing vascular system of 24-72 h old chick embryos. We show for the first time that np2 is expressed in blood vessels in vivo from the earliest stages of their formation. In contrast to ephrin-B2, both np1 and np2 are expressed in blood islands of 24 h old chick embryos. At 48-72 h, np1 expression is localized preferentially in arteries with an expression pattern that resembles that of ephrin-B2. In contrast, np2 is expressed preferentially in veins. Thus, neuropilins may play a role in determining the arterial or venous identity of blood vessels.

Amino Acid Sequence↗

PubMed2001

The third wave of myotome colonization by mitotically competent progenitors: regulating the balance between differentiation and proliferation during muscle development.

The myotome is formed by a first wave of pioneer cells originating from the entire dorsomedial region of epithelial somites and a second wave that derives from all four lips of the dermomyotome but generates myofibers from only the rostral and caudal edges. Because the precedent progenitors exit the cell cycle upon myotome colonization, subsequent waves must account for consecutive growth. In this study, double labeling with CM-DiI and BrdU revealed the appearance of a third wave of progenitors that enter the myotome as mitotically active cells from both rostral and caudal dermomyotome edges. These cells express the fibroblast growth factor (FGF) receptor FREK and treatment with FGF4 promotes their proliferation and redistribution towards the center of the myotome. Yet, they are negative for MyoD, Myf5 and FGF4, which are, however, expressed in myofibers. The proliferating progenitors first appear around the 30-somite stage in cervical-level myotomes and their number continuously increases, making up 85% of total muscle nuclei by embryonic day (E)4. By this stage, generation of second-wave myofibers, which also enter from the extreme lips is still under way. Formation of the latter fibers peaks at 30 somites and progressively decreases with age until E4. Thus, cells in these dermomyotome lips generate simultaneously distinct types of muscle progenitors in changing proportions as a function of age. Consistent with a heterogeneity in the cellular composition of the extreme lips, MyoD is normally expressed in only a subset of these epithelial cells. Treatment with Sonic hedgehog drives most of them to become MyoD positive and then to become myofibers, with a concurrent reduction in the proportion of proliferating muscle precursors.

Animals↗

PubMed2001

The sub-lip domain--a distinct pathway for myotome precursors that demonstrate rostral-caudal migration.

We have previously reported that the myotome is formed by a first wave of pioneer cells generated from all along the dorsomedial portion of the epithelial somite and a second wave of cells issued from all four edges of the dermomyotome. Cells from the extreme rostral and caudal edges directly generate myofibers that elongate towards the opposite pole of each segment and along the pre-existing myotomal scaffold. In contrast, cells from the dorsomedial and ventrolateral lips first reach the extreme edges and then contribute to myofiber formation. The mechanism by which these epithelial cells translocate remained unknown and was the goal of the present study. We have found that epithelial cells along the dorsomedial and ventrolateral lips of the dermomyotome first delaminate into the immediate underlayer of the corresponding lips, the sub-lip domain, then migrate longitudinally along this pathway until reaching the extreme edges from which they differentiate into myofibers. Cells of the sub-lip domain are negative for Pax3 and desmin but express MyoD, Myf5 and FREK, suggesting that they are specific myogenic progenitors.

Animals↗

PubMed2000

Mechanisms of early neural crest development: from cell specification to migration.

The neural crest is a group of embryonic progenitors that forms during the process of neurulation by interactions that take place between the prospective epidermis and the specified neuroectoderm. Although initially an integral part of the neuroepithelium, neural crest cells separate from the central nervous system primordium by a process of epitheliomesenchymal transition and become a motile cell population. These mesenchymal cells then migrate through stereotypic pathways, some of which are common and others unique to various vertebrate species. Furthermore, the availability of distinct migratory pathways also differs according to embryonic stage and axial level. Studies have begun to address the molecular basis of neural crest specification, delamination, and migration. The present review summarizes some major advances in our understanding of the nature of the intercellular interactions and the molecules that mediate them during early phases of neural crest ontogeny.

Animals↗

PubMed2000

Inhibition of noggin expression in the dorsal neural tube by somitogenesis: a mechanism for coordinating the timing of neural crest emigration.

We have previously shown that axial-dependent delamination of specified neural crest cells is triggered by BMP4 and negatively regulated by noggin. Increasing activity of BMP4 towards the rostral part of the axis is achieved by graded expression of noggin in the dorsal neural tube, the latter being high opposite unsegmented mesoderm, and progressively downregulated facing epithelial and dissociating somites, coinciding in time and axial level with initial delamination of neural crest cells (Sela-Donenfeld, D. and Kalcheim, C. (1999) Development 126, 4749-4762). Here we report that this gradient-like expression of noggin in the neuroepithelium is controlled by the paraxial mesoderm. Deletion of epithelial somites prevented normal downregulation of noggin in the neural tube. Furthermore, partial ablation of either the dorsal half or only the dorsomedial portion of epithelial somites was sufficient to maintain high noggin expression. In contrast, deletion of the segmental plate had no effect. These data suggest that the dorsomedial region of developing somites produces an inhibitor of noggin transcription in the dorsal neural tube. Consistent with this notion, grafting dissociating somites in the place of the unsegmented mesoderm precociously downregulated the expression of noggin and triggered premature emigration of neural crest progenitors from the caudal neural tube. Thus, opposite the unsegmented mesoderm, where noggin expression is high in the neural tube, BMP4 is inactive and neural crest cells fail to delaminate. Upon somitogenesis and further dissociation, the dorsomedial portion of the somite inhibits noggin transcription. Progressive loss of noggin activity releases BMP4 from inhibition, resulting in crest cell emigration. We propose that this inhibitory crosstalk between paraxial mesoderm and neural primordium controls the timing of neural crest delamination to match the development of a suitable mesodermal substrate for subsequent crest migration.

Animals↗

PubMed1999

Myotome formation: a multistage process.

The epaxial muscles of the body are localized in a dorsomedial position with respect to the axial structures, attach to the vertebral column and are concerned with maintenance of posture and movements of the vertebral column. The epaxial musculature derives from the myotome, a transient embryonic structure whose formation is initiated at the epithelial somite stage and is accomplished following complete dissociation of the epithelial dermomyotome. Recent results suggest that myotome development is a multistage process, characterized by addition of sequential waves of muscle progenitors. A first wave originates along the medial part of the epithelial somite and gives rise to a primary myotomal structure; a second wave arises from the rostral and caudal lips of the epithelial dermomyotome and from the dorsomedial lip, which contributes indirectly through the rostral and caudal edges, and a third wave which is composed of mitotically active resident progenitors accounts for significant growth of the myotomal mass and for its transition into epaxial muscle. In this review we discuss the origin, migration and known cellular and molecular features that characterize each wave of progenitors that colonize the myotome.

Animals↗

PubMed1999

Expression of neurotrophin receptors trkB and trkC and their ligands in rat adrenal gland and the intermediolateral column of the spinal cord.

Neurotrophins and their trk receptors constitute major classes of signaling molecules with important actions in the developing and adult nervous system. With regard to the sympathoadrenal cell lineage, which gives rise to sympathetic neurons and chromaffin cells, neurotrophin-3 (NT-3) and nerve growth factor (NGF) are thought to influence developing sympathetic neurons. Neurotrophin requirements of chromaffin cells of the adrenal medulla are less well understood than those for NGF. In order to provide the bases for understanding of putative functions of neurotrophins for the development and maintenance of chromaffin cells and their preganglionic innervation, in situ hybridization has been used to study the expression of brain-derived neurotrophic factor (BDNF) and NT-3, together with their cognate receptors trkB and trkC, in the adrenal gland and in the intermediolateral column (IML) of the spinal cord. BDNF is highly expressed in the embryonic adrenal cortex and later in cells of the cortical reticularis zone. Adrenal medullary chromaffin cells fail to express detectable levels of mRNAs for BDNF, NT-3, and their cognate receptors trkB and trkC. Neurons in the IML express BDNF and trkB, and low levels of NT-3 and trkC. Our data make it unlikely that BDNF and NT-3 serve as retrograde trophic factors for IML neurons but suggest roles of BDNF and NT-3 locally within the spinal cord and possibly for sensory nerves of the adrenal cortex.

Adrenal Glands↗

PubMed1999

F-Spondin, expressed in somite regions avoided by neural crest cells, mediates inhibition of distinct somite domains to neural crest migration.

Neural crest (NC) cells migrate exclusively into the rostral half of each sclerotome, where they avoid the dermomyotome and the paranotochordal sclerotome. F-spondin is expressed in these inhibitory regions and throughout the caudal halves. In vitro bioassays of NC spreading on substrates of rostral or caudal epithelial-half somites (RS or CS, respectively) revealed that NC cells adopt on RS a fibroblastic morphology, whereas on CS they fail to flatten. F-spondin inhibited flattening of NC cells on RS. Conversely, F-spondin antibodies prevented rounding up of NC cells on CS. Addition of F-spondin to trunk explants inhibited NC migration into the sclerotome, and treatment of embryos with anti-F-spondin antibodies yielded migration into otherwise inhibitory sites. Thus, somite-derived F-spondin is an inhibitory signal involved in patterning the segmental migration of NC cells and their topographical segregation within the RS.

Animals↗

PubMed1999

Characterization of the early development of specific hypaxial muscles from the ventrolateral myotome.

We have previously found that the myotome is formed by a first wave of pioneer cells generated along the medial epithelial somite and a second wave emanating from the dorsomedial lip (DML), rostral and caudal edges of the dermomyotome (Kahane, N., Cinnamon, Y. and Kalcheim, C. (1998a) Mech. Dev. 74, 59-73; Kahane, N., Cinnamon, Y. and Kalcheim, C. (1998b) Development 125, 4259-4271). In this study, we have addressed the development and precise fate of the ventrolateral lip (VLL) in non-limb regions of the axis. To this end, fluorescent vital dyes were iontophoretically injected in the center of the VLL and the translocation of labeled cells was followed by confocal microscopy. VLL-derived cells colonized the ventrolateral portion of the myotome. This occurred following an early longitudinal cell translocation along the medial boundary until reaching the rostral or caudal dermomyotome lips from which fibers emerged into the myotome. Thus, the behavior of VLL cells parallels that of their DML counterparts which colonize the opposite, dorsomedial portion of the myotome. To precisely understand the way the myotome expands, we addressed the early generation of hypaxial intercostal muscles. We found that intercostal muscles were formed by VLL-derived fibers that intermingled with fibers emerging from the ventrolateral aspect of both rostral and caudal edges of the dermomyotome. Notably, hypaxial intercostal muscles also contained pioneer myofibers (first wave) showing for the first time that lateral myotome-derived muscles contain a fundamental component of fibers generated in the medial domain of the somite. In addition, we show that during myotome growth and evolution into muscle, second-wave myofibers progressively intercalate between the pioneer fibers, suggesting a constant mode of myotomal expansion in its dorsomedial to ventrolateral extent. This further suggests that specific hypaxial muscles develop following a consistent ventral expansion of a 'compound myotome' into the somatopleure.

Animals↗

PubMed1999

Regulation of the onset of neural crest migration by coordinated activity of BMP4 and Noggin in the dorsal neural tube.

For neural crest cells to engage in migration, it is necessary that epithelial premigratory crest cells convert into mesenchyme. The mechanisms that trigger cell delamination from the dorsal neural tube remain poorly understood. We find that, in 15- to 40-somite-stage avian embryos, BMP4 mRNA is homogeneously distributed along the longitudinal extent of the dorsal neural tube, whereas its specific inhibitor noggin exists in a gradient of expression that decreases caudorostrally. This rostralward reduction in signal intensity coincides with the onset of emigration of neural crest cells. Hence, we hypothesized that an interplay between Noggin and BMP4 in the dorsal tube generates graded concentrations of the latter that in turn triggers the delamination of neural crest progenitors. Consistent with this suggestion, disruption of the gradient by grafting Noggin-producing cells dorsal to the neural tube at levels opposite the segmental plate or newly formed somites, inhibited emigration of HNK-1-positive crest cells, which instead accumulated within the dorsal tube. Similar results were obtained with explanted neural tubes from the same somitic levels exposed to Noggin. Exposure to Follistatin, however, had no effect. The Noggin-dependent inhibition was overcome by concomitant treatment with BMP4, which when added alone, also accelerated cell emigration compared to untreated controls. Furthermore, the observed inhibition of neural crest emigration in vivo was preceded by a partial or total reduction in the expression of cadherin-6B and rhoB but not in the expression of slug mRNA or protein. Altogether, these results suggest that a coordinated activity of Noggin and BMP4 in the dorsal neural tube triggers delamination of specified, slug-expressing neural crest cells. Thus, BMPs play multiple and discernible roles at sequential stages of neural crest ontogeny, from specification through delamination and later differentiation of specific neural crest derivatives.

Activins↗

PubMed1998

TrkB and neurotrophin-4 are important for development and maintenance of sympathetic preganglionic neurons innervating the adrenal medulla.

The adrenal medulla receives its major presynaptic input from sympathetic preganglionic neurons that are located in the intermediolateral (IML) column of the thoracic spinal cord. The neurotrophic factor concept would predict that these IML neurons receive trophic support from chromaffin cells in the adrenal medulla. We show here that adrenal chromaffin cells in the adult rat store neurotrophin (NT)-4, but do not synthesize or store detectable levels of BDNF or NT-3, respectively. Preganglionic neurons to the adrenal medulla identified by retrograde tracing with fast blue or Fluoro-Gold (FG) express TrkB mRNA. After unilateral destruction of the adrenal medulla, 24% of IML neurons, i.e., all neurons that are preganglionic to the adrenal medulla in spinal cord segments T7-T10, disappear. Administration of NT-4 in gelfoams (6 microgram) implanted into the medullectomized adrenal gland rescued all preganglionic neurons as evidenced by their presence after 4 weeks. NT-3 and cytochrome C were not effective. The action of NT-4 is accompanied by massive sprouting of axons in the vicinity of the NT-4 source as monitored by staining for acetylcholinesterase and synaptophysin immunoreactivity, suggesting that NT-4 may enlarge the terminal field of preganglionic nerves and enhance their access to trophic factors. Analysis of TrkB-deficient mice revealed degenerative changes in axon terminals on chromaffin cells. Furthermore, numbers of FG-labeled IML neurons in spinal cord segments T7-T10 of NT-4-deficient adult mice were significantly reduced. These data are consistent with the notion that NT-4 from chromaffin cells operates through TrkB receptors to regulate development and maintenance of the preganglionic innervation of the adrenal medulla.

Adrenal Medulla↗

PubMed1998

Identification of early postmitotic cells in distinct embryonic sites and their possible roles in morphogenesis.

Differential proliferation within defined embryonic anlage is likely to play a major role in morphogenesis. We have identified cell populations in the avian embryo that begin exiting the cell cycle as early as the 25-somite stage. These include first the floor plate and then the roof plate of the neural tube, cells that constitute the lamina terminalis and the diencephalic-mesencephalic junction of the developing brain. Outside the nervous system, the central portion of the notochord contains early postmitotic cells. In the heart, such cells will populate the epimyocardium at the level of the truncus arteriosus exclusively and the endocardial cushions that serve as an anchor for the growing intracardial septa. Surprisingly, the endoderm at the level of the prospective midgut is composed of post-mitotic progenitors. These cells are later found both in the caudal portion of the duodenum and in derivatives adjacent to the umbilical region of the primitive midgut. The possible implications of this early, localized withdrawal from the cell cycle to morphogenetic events and lineage segregation are discussed.

Animals↗

PubMed1998

The origin and fate of pioneer myotomal cells in the avian embryo.

The ontogeny of the myotome was investigated using [3H]thymidine or Brdu treatment in conjunction with 1,1', di-octadecyl-3, 3, 3', 3',-tetramethylindo-carbocyanine perchlorate (DiI) labeling and expression of specific markers. We have identified a subset of early post-mitotic cells that is present in the dorsomedial aspect of epithelial somites and is homogeneously distributed along their entire rostrocaudal extent. The post-mitotic quality of this cell subset enabled us to trace their fate in time-course experiments. Following initial somite dissociation, this epithelial post-mitotic layer bends underneath the medial portion of the nascent dermomyotome. Then, these cells progressively lose epithelial arrangement and migrate in a rostral direction where they accumulate temporarily. Subsequently, these early post-mitotic precursors extend processes that reach both rostral and caudal edges of each segment. Medial somite-derived myofibers also fill the entire mediolateral extent of the segment and reach the dorsomedial lip of the dermomyotome, thus forming the primary myotome. During this process, their large nuclei localize to a narrow stripe in the middle of the nascent myotome. Consistent with the proliferation studies, DiI labeling of the medial epithelial somite cells gave rise to a primary myotomal structure, and continuous pulsing of the DiI-injected embryos with radioactive thymidine revealed that these fibers indeed developed from post-mitotic progenitors. As these early post-mitotic cells that arise prior to somite dissociation are the first wave of progenitors that constitutes the myotome, we have termed them avian muscle pioneers. We propose that the primary myotome formed by the muscle pioneers constitutes a longitudinal scaffold that serves as a substrate for the addition of subsequent waves of myotomal cells.

Animals↗

PubMed1998

The cellular mechanism by which the dermomyotome contributes to the second wave of myotome development.

We have shown that a subset of early postmitotic progenitors that originates along the medial part of the epithelial somite gives rise to the primary myotome (Kahane, N., Cinnamon, Y. and Kalcheim, C. (1998). Mech. Dev. 74, 59-73). Because of its postmitotic nature, further myotome expansion must be achieved by cell addition from extrinsic sources. Here we investigate the mechanism whereby the dermomyotome contributes to this process. Using several different methods we found that cell addition occurs from both rostral and caudal edges of the dermomyotome, but not directly from its dorsomedial lip (DML). First, labeling of quail embryos with [3H]thymidine revealed a time-dependent entry of radiolabeled nuclei into the myotome from the entire rostral and caudal lips of the dermomyotome, but not from the DML. Second, fluorescent vital dyes were injected at specific sites in the dermomyotome lips and the fate of dye-labeled cells followed by confocal microscopy. Consistent with the nucleotide labeling experiments, dye-labeled myofibers directly emerged from injected epithelial cells from either rostral or caudal lips. In contrast, injected cells from the DML first translocated along the medial boundary, reached the rostral or caudal dermomyotome lips and only then elongated into the myotome. These growing myofibers had always one end attached to either lip from which they elongated in the opposite direction. Third, following establishment of the primary myotome, cells along the extreme dermomyotome edges, but not the DML, expressed QmyoD, supporting the notion that rostral and caudal boundaries generate myofibers. Fourth, ablation of the DML had only a limited effect on myotomal cell number. Thus, cells deriving from the extreme dermomyotome lips contribute to uniform myotome growth in the dorsoventral extent of the myotome. They also account for its expansion in the transverse plane and this is achieved by myoblast addition in a lateral to medial direction (from the dermal to the sclerotomal sides), restricting the pioneer myofibers to the dermal side of the myotome. Taken together, the data suggest that myotome formation is a multistage process. A first wave of pioneers establishes the primary structure. A second wave generated from specific dermomyotome lips contributes to its expansion. Because dermomyotome lip progenitors are mitotically active within the epithelia of origin but exit the cell cycle upon myotome colonization, they can only provide for limited myotome growth and subsequent waves must take over to ensure further muscle development.

Animals↗

PubMed1997

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.

Animals↗

PubMed1996

The role of neurotrophins in development of neural-crest cells that become sensory ganglia.

A fundamental issue of neural-crest ontogeny is understanding how different types of cells are created at the right time and in the correct numbers. Sensory ganglia are among the many derivatives of the vertebrate neural crest. Their proper formation requires the regulation of several processes such as cell fate specification, proliferation, survival, and terminal differentiation. The timescale of the occurrence of processes involved in the regulation of cell number and identity, coincides with key morphogenetic events such as cell migration, homing and gangliogenesis. To gain insight into these processes, we characterized the cellular basis of metameric migration of neural-crest cells and of consequent ganglion organization, which are imposed by intrinsic differences within rostral and caudal sclerotomal compartments. We also established a transient requirement for neural tube-derived factors in regulating the proliferation, survival and differentiation of prospective DRG cells. Additionally, we showed that cooperation between the mesodermal cells and the neural tube is necessary for modulating cell number in the nascent ganglia. BDNF, NT-3 and basic FGF were found to mediate this environmental signalling. All the above factors display neurogenic activity for a subset of early-committed sensory neuron progenitors. This observation raises the possibility of an early redundancy in the response of individual neural-crest progenitors to distinct factors. This overlap in responsiveness progressively disappears upon the colonization of specific ganglionic sites and the subsequent establishment of selective innervation patterns by post-mitotic sensory neurons.

Animals↗

PubMed1996

Expression and regulation of brain-derived neurotrophic factor and neurotrophin-3 mRNAs in distinct avian motoneuron subsets.

We performed a detailed study of the expression of neurotrophin-3 and brain-derived neurotrophic factor transcripts in spinal motoneurons using in situ hybridization of serially sectioned chick embryos aged 3 to 8 days (E3 to E8). Neurotrophin-3 mRNA is detected in motoneuron subsets from E3.5 to E4 only in brachial segments of the neural tube and from E5 in both brachial and lumbar regions. Expression of brain-derived neurotrophic factor mRNA is first evident on E5 in a subset of brachial level motoneurons and from E6 also in motoneurons located in the rostral-most portion of the lateral motor column, as well as in the tail-innervating region of the spinal cord. Analysis along the rostrocaudal extent of the brachial lateral motor column reveals an overlap zone of expression of both neurotrophins of about two segments. In transverse sections of this region, it is observed that neurotrophin-3-positive motoneurons preferentially occupy the lateral part of the column, whereas brain-derived neurotrophic factor-producing motoneurons are localized in a more medial position. These results show that the two factors are synthesized at discrete axial levels of the spinal cord by distinct motoneuron subpopulations. Since brain-derived neurotrophic factor mRNA is expressed within the brachial but not the lumbar lateral motor column, we tested the possibility that brain-derived neurotrophic factor expression is regulated by the type of peripheral target, that is, the wing or the leg. Unilateral transplantation of a wing bud instead of a leg bud and vice versa, prior to the onset of peripheral innervation, failed to alter the original pattern of brain-derived neurotrophic factor mRNA observed in either level of the axis. Thus, the early synthesis of brain-derived neurotrophic factor by subsets of spinal motoneurons is independent of the type of peripheral target and may instead reflect intrinsic differences between motoneuron populations.

Animals↗

PubMed1995

Epithelial-mesenchymal conversion of dermatome progenitors requires neural tube-derived signals: characterization of the role of Neurotrophin-3.

Development of the somite-derived dermatome involves conversion of the epithelial dermatome progenitors into mesenchymal cells of the dermis. In chick embryos, neural tube-derived signals are required for this conversion, as the interposition of a membrane between neural tube and somites results in a failure of the dermatome to lose its epithelial arrangement. However, dermis formation can be completely rescued by coating the membranes with Neurotrophin-3, but not with the related molecule Nerve growth factor. Neurotrophin-3 was also found to be necessary for dermatome dissociation using in vitro explants or partially dissociated dermomyotomes. The functional relevance of these observations was investigated by neutralizing endogenous Neurotrophin-3 using a specific blocking antibody. Antibody-treated embryos revealed the presence of tightly aggregated cells between myotome and ectoderm instead of the loose dermal mesenchyme observed in embryos treated with control antibodies. As previous studies have demonstrated the presence of Neurotrophin-3 in the neural tube, these results suggest that it may be a necessary neural tube-derived signal required for early stages of dermis formation.

Animals↗