Institut dEmbryologie Cellulaire et Moléculaire, CNRS FRE2160, 49bis Avenue de la Belle Gabrielle, 94736 Nogent-sur-Marne Cedex, France
*Author for correspondence (e-mail: teillet{at}infobiogen.fr)
Accepted July 19, 2001
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SUMMARY |
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Charrier, J. B., Teillet, M.-A., Lapointe, F. and Le Douarin, N. M. (1999). Development 126, 4771-4783
Key words: Apoptosis, Avian embryo, Cell death, Cell survival, Floor plate, Notochord, Quail/chick, Shh, Somite, Neural tube, Spinal cord
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INTRODUCTION |
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Although the term programmed cell death (PCD) was initially used to describe cell elimination from certain developing tissues, as part of their developmental program, this term is now used to designate a particular type of cell death (Jacobson et al., 1997). Cell death occurs according to two different paradigms in animal cells (Kerr et al., 1972). These authors made a distinction between the type of cell death occurring during tissue homeostasis and development and pathological cell death observed at the center of acute lesions. In the latter case, the cells and their organelles swell and rupture in a process called necrosis that generally induces an inflammatory response. In the former, the cells shrink and condense, but their organelles and plasma membrane retain their integrity. The dead cells are rapidly phagocytosed by neighboring cells without inducing inflammatory responses. Kerr and his colleagues named this process apoptosis and clearly distinguished it from necrosis.
The fact that animal cells possess a built-in suicide program was first demonstrated by genetic studies carried out in Caenorhabditis elegans (Ellis and Horvitz, 1986). Execution of this program involves a proteolytic cascade controlled intracellularly by a family of specialized regulatory proteins (Martin and Green, 1995; Chinnaiyan and Dixit, 1996; White, 1996; Jacobson et al., 1997). The first known of the proteases involved in this cascade is encoded by a gene called ced-3 that is required for the 131 PCDs that take place regularly during the development of C. elegans (Ellis and Horwitz, 1986; Ellis et al., 1991). This gene encodes a cysteine protease that is homologous to the mammalian interleukin-1ß-converting enzyme (ICE) (Yuan et al., 1993). Mammals possess many members of the ced/ICE family involved in PCD (Chinnaiyan and Dixit, 1996; Kuida et al., 1995; Kuida et al., 1996). They are referred to as caspases (Alnemri et al., 1996). Moreover, there is now evidence that the intracellular death program is constitutively expressed in all nucleated cells (Ishizaki et al., 1995; Weil et al., 1996).
The role of apoptosis in development remained unidentified. Drosophila embryos in which the apoptotic program has been genetically altered die early in development (White et al., 1994). Such is also the case for mice in which CPP32 (caspase 3) is deleted by targeted gene disruption. Interestingly, their central nervous system (CNS) shows a large excess of neuroepithelial cells (Kuida et al., 1996).
The cell death program of animal cells is, however, restrained by inhibitors that block PCD. The C. elegans gene ced-9, or bcl2 in mammals, is one such example (Hengartner and Horwitz, 1994; Korsmeyer, 1995). Evidence from in vitro experiments indicates that, at least in higher vertebrates, developing cells require signals to counteract their intrinsic PCD program (Raff, 1992). These signals come either from themselves, e.g. in lens (Ishizaki et al., 1993) or in cartilage (Ishizaki et al., 1994), or from other cells as an autocrine versus paracrine process. However, the role of the paracrine control of apoptosis in development is not yet fully understood.
We have designed experimental conditions which reveal an unsuspected effect of the floor plate and notochord (here designated as midline structures) on the neural epithelium destined to yield the neurons and glia of the spinal cord. These midline cells are indispensable for the survival of the early neural tube itself, as has been shown previously for the myotomal and sclerotomal lineages of the paraxial mesoderm (Teillet et al., 1998b).
In earlier cell lineage studies, carried out in the avian embryo (Catala et al., 1995; Catala et al., 1996), we demonstrated that the formation of the spinal cord primordium involves the juxtaposition of two populations of cells of distinct embryonic origins. One of them constitutes the neural ectoderm destined to form the lateral walls of the spinal cord (basal and alar plates) and the neural folds (roof plate and neural crest). The other is the presumptive territory of the floor plate, which arises from precursors contained in Hensens node, the avian organizer. During its caudalward movement involved in the posterior elongation of the embryo, the node leaves in its wake a cord of cells that later splits into dorsal and ventral components that become the floor plate and the notochord respectively. The floor plate, as it becomes isolated from the notochord, is inserted into the neural ectoderm. The precursors contained in the node yield these two midline structures along the entire neural axis without additional cells joining them by convergence from lateral regions, thus functioning as stem cells (Catala et al., 1995; Catala et al., 1996; Teillet et al., 1998a; Charrier et al., 1999).
Inhibition of Hensens node regression can be obtained by removal of a region that includes the posterior end of the node and the anterior tip of the primitive streak, known as the axial-paraxial hinge (APH) (Charrier et al., 1999). The operation does not prevent the formation of a neural tube posterior to the excised zone. However, the portion of the embryo that develops caudal to the level of the operation (which corresponds to somites 20-21 when the operation is performed at the 5- to 6-somite stage), is devoid of midline cells (i.e. notochord and floor plate). In this region, the neural tube and the somites only possess dorsal molecular markers (Charrier et al., 1999). Subsequently, all the tissues forming the embryo in the midline cell-deprived area undergo apoptosis. After 3 days, embryos are truncated posterior to the forelimbs (see Fig. 7 of Charrier et al. (Charrier et al., 1999)).
In the present work, we demonstrate that midline cells exert a positive effect on the survival and growth of the neural tube. This effect is mimicked by Sonic hedgehog (SHH) a secreted protein produced by the notochord and floor plate among other locations in vertebrates (Echelard et al., 1993; Marti et al., 1995; and see Teillet et al., 1998a).
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MATERIALS AND METHODS |
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Microsurgery experiments (Fig. 1)
Two types of experiments (A and B) were performed at E2.5 in order to rescue the caudal neural tube deprived of midline cells. In both experiments, the APH (the caudal end of Hensens node together with the rostral tip of the primitive streak) was previously excised at the 5- to 6-somite stage (ss) in chick or quail embryos at E1.5, as described previously (Charrier et al., 1999).
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Experiments B (Fig. 1D)
Fragments of neural tube deprived of midline cells were dissected out after enzymatic dissociation, as above. These tissues prepared from quail or chick donors were transplanted into chick hosts (20-25ss) that had previously had their own neural tube-notochord complex surgically removed as described previously (Teillet et al. 1998b). Excisions and grafts were performed at the level of the last formed somites and in the non-segmented region over the total length of about 10 somites. The graft was generally separated from the host axial tissues by a gap at least one somite long in order to discriminate host and grafted tissues. Three types of transplantations were performed: (1) floor plate-deprived neural tube alone; (2) floor plate-deprived neural tube above a notochord or a floor plate fragment; (3) floor plate-deprived neural tube above a layer of SHH-producing fibroblast clusters.
Experiment C (Fig. 1E)
A longitudinal slit was made in the neural plate immediately lateral to the notochord and floor plate, through the three germ layers. This operation was performed in 10-12ss chick embryos, at the level of the sinus rhomboidalis, i.e. from SI to rostral Hensens node level. The other side of the neural plate, remaining in contact with midline cells, was used as a control.
Experiment D (Fig. 1F)
In order to remove the neural tube from the influence of the paraxial mesoderm, an incision was made lateral to the neural tube, immediately after its closure. This longitudinal incision was at the level of the last formed somites and in the non-segmented region, over a total length of about 10 somites, in 12-15ss chick embryos. The contralateral side served as a control.
Operated embryos were reincubated for periods of time ranging from 1 to 3 days according to the experimental series. Embryos were treated for in situ hybridization either in whole mount (see the following paragraph) or in serial sections. In the latter case, the embryos were immersed in modified Carnoy fluid (absolute ethanol : 37% formaldehyde commercial solution : 100% glacial acetic acid, 6:3:1; v/v/v), dehydrated and embedded in paraplast. Consecutive serial 7 µm sections were mounted on separate slides and treated for in situ hybridization with various RNA probes or for immunohistochemistry.
In situ hybridization
In situ hybridization on whole embryos or on tissue sections were performed as described previously (Charrier et al., 1999). Seven chick riboprobes were used as markers of the different tissues and structures: HNF3ß (Ruiz i Altaba et al., 1995) and Shh (Riddle et al., 1993) for Hensens node, floor plate and notochord; Pax6 and Pax3 (Goulding et al., 1993) and BMP4 (Francis et al., 1994) as dorsoventral markers of the neural tube; Pax3 for the paraxial mesoderm and dermomyotome (Williams and Ordahl, 1994); Pax1 for the sclerotomal part of the somites (Balling et al., 1988); MyoD for the myotome (Charles de la Brousse and Emerson, 1990). Patched (Ptc) was used as an element of SHH signaling pathway (Goodrich et al., 1996).
Immunohistochemistry
Cell death was detected using the TUNEL assay (Gavrieli et al., 1992; Wijsman et al., 1993), using the Roche Kit as previously described (Teillet et al., 1998b).
The QCPN monoclonal antibody (Developmental Studies Hybridoma Bank) was used as a marker of quail cells as previously described (Teillet et al., 1998a).
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RESULTS |
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Another experiment represented in Fig. 1D confirmed the essential role of SHH-producing midline cells on neural tube survival. Fragments of neural tubes devoid of a floor plate, removed from E2.5 embryos that had been subjected to APH excision at E1.5, were grafted into stage-matched chick hosts from which the neural tube and notochord had been excised over a length of about 10 somites. One day later (Fig. 3A-D), the grafted neural tube was still the site of cell death and Pax3 was expressed uniformly in the neural tube. At E4, the continuity of the embryo was maintained. However, the region containing the grafted neural tube deprived of midline cells appeared indented and foreshortened (Fig. 4A). The host neural tube was normal rostral and caudal to the excision. In contrast, the grafted neural tube, although present, was reduced in diameter and only expressed dorsal markers (Fig. 4B-E).
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Various studies in flies and vertebrates show that Hedgehog and Patched are linked during development (Marigo and Tabin, 1996; Goodrich and Scott, 1998). We examined concomitantly the expression patterns of Ptc and Shh in several of our experiments as compared to their normal expression in the neural tube at E3. As already described (Goodrich et al., 1996; Marigo and Tabin, 1996) Ptc was expressed in a dorsoventral gradient, except in the floor plate (Fig. 5E). We observed a low level Ptc expression over the entire surface of midline cell-deprived neural tube in APH-excised embryos (Fig. 5F). This expression was considerably upregulated after the graft of SHH-producing cells, on one side of the neural tube (experiment A3; Fig. 5C,G), or after the transplantation of the tube onto a layer of SHH-producing cells, into a stage-matched host embryo (experiment B3; Fig. 5G,H).
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One day following the operation (at E2.5), a significant growth defect was observed on the operated side (Fig. 6A-C). This defect became progressively more pronounced so that, at E5, the control side became U-shaped at the operated level (Fig. 6D). As early as E2.5, after whole-mount in situ hybridization, the somites isolated from midline cells, were devoid of Pax1 and MyoD expression (Fig. 6A,C,F), while Pax3 was expressed homogeneously in what remained of the somites (Fig. 6B). On serial sections (Fig. 6E-G) the control side, that had remained associated with the notochord and floor plate, showed the normal pattern of Pax6 expression in the ventrolateral neural tube. The other side, separated from the midline cells by the operation, expressed Pax6 in all the remaining neuroepithelium. It was striking to see that, on the operated side, the neural epithelium expressing Pax6 was in continuity with the endoderm in which high levels of Shh transcripts were detected. In spite of the proximity of this tissue, many apoptotic figures were present in the neuroepithelium as compared to the control side (Fig. 6E-G).
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Evaluation of the role of the somites on neural tube survival
To see if the paraxial mesoderm and somites exert a complementary effect on the survival and growth of the neural tube, a slit was made between these tissues at E2 (15-20ss), from SII, over the length of about 10 somites (Fig. 1F). Embryos were examined one day after the operation (at E3; Fig. 7A-D). Cell death, revealed by the TUNEL assay, appeared increased not only in the medial somites on the operated side, as expected (Teillet et al., 1998b), but also to a lesser extent in the neural tube that remained in contact with the floor plate and notochord (Fig. 7D). On the operated side, the ventrodorsal distribution of Shh, Pax3 and Pax6 transcripts in the neural tube was the same as in the side still in contact with the somites (Fig. 7A-C). As shown on sections of embryos sacrificed 4 days after the operation (E5.5; Fig. 8A), on the operated side, the medial somitic derivatives, vertebral cartilage (Fig. 8B) and vertebral muscles (Fig. 8C) were absent. Moreover, the thickness of the mantle of the spinal cord was considerably reduced and haemorrhagic areas invaded the neural tissue. However Pax6 and Pax3 appeared normally distributed at the ventricular level (Fig. 8B,D).
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DISCUSSION |
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Midline cells are necessary for neural tube survival
In this work, two experimental designs have been used to isolate the developing neural epithelium from the midline structures. One consisted of removing the APH in 5-6ss embryos (Charrier et al., 1999). The notochord and floor plate complex was then absent posterior to somite 20 (caudal to the forelimbs). As a consequence, much of the neuraxis developed without any contact, at any time, with the midline cells. In the other paradigm, a slit was made through the three germ layers extending over the length of several somites. In both cases, the deprivation of the influence of the notochord and floor plate from the neural ectoderm resulted in extensive cell death within a few hours and in the complete disappearance of all the embryonic tissues of the region within 48 hours. Insertion of a notochord or a floor plate between the paraxial mesoderm and the neural tube that forms after APH removal rescues these tissues from death and allows their growth and differentiation.
These results suggest that the notochord and floor plate exert an anti-apoptotic effect on the neural ectoderm as it was previously demonstrated for the paraxial mesoderm (Teillet et al., 1998b). This action occurs as soon as the midline structures appear by rostrocaudal displacement of Hensens node and continues for at least 1 day after the formation of the neural tube. Interestingly, either one of these two midline structures is sufficient, in the avian embryo, to restore both survival and early patterning of the neural tube, suggesting that the factor(s) involved is(are) produced by both of them and able to exert a long-range action.
SHH is an anti-apoptotic factor for the neural tube
One candidate for such a factor was SHH, whose role in patterning the neural tube has been amply documented (Tanabe and Jessell, 1996; Goodrich and Scott, 1998). Cells engineered to secrete the SHH glycoprotein were grafted in the same way as for the notochord or floor plate grafts, producing identical survival, growth and early patterning effects on the neural tube and paraxial mesoderm. Pax3 and Pax6, which were uniformly expressed in neural tubes deprived of midline cells, were re-established in their normal dorsoventral patterns (Goulding et al., 1993). Ptc transcripts were strongly and homogeneously upregulated in the ventral neural tube close to the SHH-producing cells, indicating the positive signaling activity of SHH protein on the neural tube. However, it is noteworthy that no floor plate markers (HNF3ß or Shh) were ever detected in this region in the first day of exposure, which is in accordance with the absence of Ptc expression in the normal floor plate (Goodrich and Scott, 1998).
Apoptosis shapes the central nervous system
Several periods of cell death occur during normal nervous system development (Oppenheim et al., 1999). One of them concerns supernumerary neurons competing to establish synaptic contacts with their peripheral targets. Limited amounts of survival factors such as NGF and related neurotrophins are produced in the target fields and ensure a match between the size of the neuronal populations and the territory they have to innervate (Purves, 1988; Oppenheim, 1991; Raff, 1992; Bibel and Barde, 2000).
PCD is also involved in morphogenesis of the neural epithelium at stages of development preceding the neurotrophic loop between axon-projecting neurons and their targets. Observations in the chick embryo revealed the apoptotic nature of this early cell death (Homma et al., 1994). Mice over-expressing the anti-apoptotic gene bcl-2 (Martinou et al., 1994) or lacking the pro-apoptotic gene Bax (White et al., 1998) increased certain neuronal subpopulations, but they did not show large-scale malformations of the nervous system. By contrast, inactivation of the pro-apoptotic genes Casp3, Casp9 and Apaf1 have led to severe malformations due to the reduction of cell death in developing neural tissues (Kuida et al., 1996; Kuida et al., 1998; Hakem et al., 1998; Cecconi et al., 1998; Yoshida et al., 1998).
Therefore, as in other parts of the body, cell death is an important mechanism involved in sculpting nervous tissue by adjusting the initial progenitor pool needed for its proper morphogenesis and functional development.
SHH is a regulator of cell death
PCD is necessarily a highly regulated mechanism both at the intra- and inter-cellular levels. The experimental results reported here support the contention that the extent of the naturally occurring cell death in the neural epithelium is controlled by SHH produced by the notochord and floor plate. This implies that embryonic cells need to receive adequate death-inhibiting signals to avoid the activation of their own ready-to-run death program. It seems that such a signal is needed during a temporal window that goes from the time the ectoderm and mesoderm, already specified as neural epithelium and presomitic primordium respectively, begin their organogenesis and extends at least for 48 hours in the avian embryo. This period corresponds to a phase of active proliferation and patterning of the embryonic tissues.
Whether, in the absence of SHH, the PCD is spontaneously activated in the ectodermal and mesodermal cells or is induced by an extrinsic pro-apoptotic signal remains to be elucidated. One extrinsic pro-apoptotic influence could originate from the dorsal ectoderm or the lateral plate mesoderm that, at this stage, produce BMP4, a secreted protein already implicated in triggering PCD in embryonic cells (Graham et al., 1994). The apoptotic action of BMP4 in hindbrain neural crest is supposed to be antagonized by Noggin (Smith and Graham, 2001). Observation of an increased PCD in the dorsal neural tube (see Fig. 7D) after separation from the somites, which are known to produce Noggin at this stage (Hirsinger et al., 1997), can be explained by a deprivation of Noggin of somitic origin.
SHH has already been considered to be involved in the regulation of cell number in the developing CNS. For example, in transgenic mouse embryos, in which Shh was ectopically expressed in the dorsal neural tube from 10 days post-coitum until birth, an increase in cell number by an elevation in proliferation rates of spinal cord precursors was observed together with an inhibition in differentiation of CNS precursor cells (Rowitch et al., 1999). In contrast, inhibition of SHH signalling in the cephalic region of chick embryos by injecting a blocking anti-SHH antibody induced massive cell death in the cranial neural tube and in the cephalic mesenchyme of neural crest origin, thus reducing significantly the volume of the head (Ahlgren and Bronner-Fraser, 1999). Moreover, increased cell death was observed both in somites and in the neural tube of Shh null mutants (Chiang et al., 1996; Borycki et al., 1999).
SHH may act as a morphogen
A paradoxical effect of SHH has recently been shown in the limb bud of the chick embryo. Apart from its patterning role in this organ, SHH is generally considered to be a proliferation and survival factor for muscle cells (Duprez et al., 1998; Krüger et al., 2001). However, this protein has also been demonstrated to be responsible for cell death in the posterior necrotic zone which thus prevents the zone of polarising activity expanding (Sanz-Ezquerro and Tickle, 2000). In the neural tube of the chick embryo two opposite sites of naturally occurring cell death have been described: one dorsal, in the fusing neural folds, the other ventral, on each side of the floor plate (Homma et al., 1994) (our own unpublished observations). Moreover, cells of the early chick spinal cord respond, either by cell death or by cell survival, to SHH in vivo and in vitro (Oppenheim et al., 1999). One can thus hypothesize that, in the neural tube, a high level of SHH could limit the lateral extension of the floor plate by inducing cell death in the neighboring tissues, while lower levels of the protein would have an anti-apoptotic effect on the cells of the lateral neural tube. In the neural folds, away from the SHH anti-apoptotic influence and submitted to high levels of BMP4, cell death could be naturally increased. Thus, SHH might act as a morphogen by promoting either pro-apoptotic or anti-apoptotic activity in a distance/concentration-dependent manner.
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ACKNOWLEDGMENTS |
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REFERENCES |
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Ahlgren, S. C. and Bronner-Fraser, M. (1999). Inhibition of sonic hedgehog signaling in vivo results in craniofacial neural crest cell death. Curr. Biol. 9, 1304-1314.
Alnemri, E. S., Livingston, D. J., Nicholson, D. W., Salvesen, G., Thornberry, N. A., Wong, W. W. and Yuan, J. (1996). Human ICE/CED-3 protease nomenclature. Cell 87, 171.[Medline]
Balling, R., Deutsch, U. and Gruss, P. (1988). undulated, a mutation affecting the development of the mouse skeleton, has a point mutation in the paired box of Pax-1. Cell 55, 531-555.[Medline]
Bibel, M. and Barde, Y.-A. (2000). Neurotrophins: key regulators of cell fate and cell shape in the vertebrate nervous system. Genes Dev. 14, 2919-2937.[Medline]
Borycki, A. G., Brunk, B., Tajbakhsh, S., Buckingham, M., Chiang, C. and Emerson, C. P. (1999). Sonic hedgehog controls epaxial muscle determination through Myf5 activation. Development 126, 4053-4063.
Catala, M., Teillet, M.-A. and Le Douarin, N. M. (1995). Organization and development of the tail bud analyzed with the quail-chick chimaera system. Mech. Dev. 51, 51-65.
Catala, M., Teillet, M.-A., De Robertis, E. M. and Le Douarin, N. M. (1996). A spinal cord fate map in the avian embryo: while regressing, Hensens node lays down the notochord and floor plate thus joining the spinal cord lateral walls. Development 122, 2599-2610.[Medline]
Cecconi, F., Alvarez-Bolado, G., Meyer, B. I., Roth, K. A. and Gruss, P. (1998). Apaf1 (CED-4 homolog) regulates programmed cell death in mammalian development. Cell 94, 727-737.
Charles de La Brousse, F. and Emerson C. P. (1990). Localized expression of a myogenic regulatory gene, qmf1, in the somite dermatome of avian embryos. Genes Dev. 4, 567-581.[Medline]
Charrier, J. B., Teillet, M.-A., Lapointe, F. and Le Douarin, N. M. (1999). Defining subregions of Hensens node essential for caudalward movement, midline development and cell survival. Development 126, 4771-4783.[Abstract]
Chiang, C., Ying, L. T. T., Lee, E., Young, K. E., Corden, J. L., Westphal, H. and Beachy, P. A. (1996). Cyclopia and defective axial patterning in mice lacking Sonic hedgehog gene function. Nature 383, 407-413.
Chinnaiyan, A. M. and Dixit, V. M. (1996). The cell-death machine. Curr. Biol. 6, 555-562.[Medline]
Christ, B. and Ordahl, C. P. (1995). Early stages of chick somite development. Anat. Embryol. 191, 381-396.[Medline]
Clarke, P. G. and Clarke, S. (1996). Nineteenth century research on naturally occurring cell death and related phenomena. Anat. Embryol. 193, 81-99.[Medline]
Duprez, D., Fournier-Thibault, C. and Le Douarin, N. (1998) SHH induces proliferation of committed skeletal muscle cells in the chick limb. Development 125, 495-505.[Medline]
Echelard, Y., Epstein, D. J., St-Jacques, B., Shen, L., Mohler, J., McMahon, J. A. and McMahon, A. P. (1993). Sonic hedgehog a member of a family of putative signaling molecules, is implicated in the regulation of CNS polarity. Cell 75, 1417-1430.
Ellis, H. M. and Horvitz, H. R. (1986). Genetic control of programmed cell death in the nematode C. elegans. Cell 44, 817-829.[Medline]
Ellis, R. E., Yuan, J. Y. and Horvitz, H. R. (1991). Mechanisms and functions of cell death. Annu. Rev. Cell Biol. 7, 663-698.[Medline]
Francis, P., Richardson, M. K., Brickell, P. M. and Tickle, C. (1994). Bone Morphogenetic proteins and a signalling pathway that controls patterning in the developing chick limb. Development 120, 209-218.
Gavrieli, Y., Sherman, Y. and Ben-Sasson, S. A. (1992). Identification of programmed cell death in situ via specific labeling of nuclear DNA fragmentation. J. Cell. Biol. 119, 493-501.
Glücksmann, A. (1951). Cell deaths in normal vertebrate ontogeny. Biol. Rev. 26, 59-86.[Abstract]
Goodrich, L. V., Johnson, R. L., Milenkovic, L., McMahon, J. and Scott, M. P. (1996). Conservation of the hedgehog/patched signaling pathway from flies to mice: induction of a mouse patched gene by hedgehog. Genes Dev. 10, 301-312.
Goodrich, L. V. and Scott, M. P. (1998). Hedgehog and Patched in neural development and disease. Neuron 21, 1243-1257.[Abstract]
Goulding, M. D., Lumsden, A. and Gruss, P. (1993). Signals from the notochord and floor plate regulate the region-specific expression of two Pax genes in the developing spinal cord. Development 117, 1001-1016.[Medline]
Graham, A., Francis-West, P., Brickell, P. and Lumsden, A. (1994). The signalling molecule BMP4 mediates apoptosis in the rhombencephalic neural crest. Nature 372, 684-686.
Hakem, R., Hakem, A., Duncan, G. S., Henderson, J. T., Woo, M., Soengas, M. S., Elia, A., de La Pompa, J. L., Kagi, D., Khoo, W., Potter, J., Yoshida, R., Kaufman, S. A., Lowe, S. W., Penninger, J. M. and Mak, T. W. (1998). Differential requirement for caspase 9 in apoptotic pathways in vivo. Cell 94, 339-352.[Medline]
Hengartner, M. O. and Horvitz, H. R. (1994). C. elegans cell survival gene ced-9 encodes a functional homolog of the mammalian proto-oncogene bcl-2. Cell 76, 665-676.[Medline]
Hirsinger, E., Duprez, D., Jouve, C., Malapert, P., Cooke, J., Pourquie, O. (1997). Noggin acts downstream of Wnt and Sonic Hedgehog to antagonize BMP4 in avian somite patterning. Development 124, 4605-4614.[Medline]
Homma, S., Yaginuma, H. and Oppenheim, R. W. (1994). Programmed cell death during the earliest stages of spinal cord development in the chick embryo: A possible means of early phenotypic selection. J. Comp. Neurol. 345, 377-395.
Ishizaki, Y., Voyvodic, J. T., Burne, J. F. and Raff, M. C. (1993). Control of lens epithelial cell survival. J. Cell Biol. 121, 899-908.[Medline]
Ishizaki, Y., Burne, J. F. and Raff, M. C. (1994). Autocrine signals enable chondrocytes to survive in culture. J. Cell. Biol. 126, 1069-1077.[Abstract]
Ishizaki, Y., Cheng, L., Mudge, A. W. and Raff, M. C. (1995). Programmed cell death by default in embryonic cells, fibroblasts, and cancer cells. Mol. Biol. Cell 6, 1443-1458.[Abstract]
Jacobson, M. D., Weil, M. and Raff, M. C. (1997). Programmed cell death in animal development. Cell 88, 347-354.[Abstract]
Kerr, J F., Wyllie, A. H. and Currie, A. R. (1972). Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics. Br. J. Cancer 26, 239-257.[Medline]
Korsmeyer, S. J. (1995). Regulators of cell death. Trends Genet. 11, 101-105.[Medline]
Krüger, M., Mennerich, D., Fees, S., Schäfer, R., Mundlos, S. and Braun, T. (2001). Sonic hedgehog is a survival factor for hypaxial muscles during mouse development. Development 128, 743-752.[Medline]
Kuida, K., Lippke, J. A., Ku, G., Harding, M. W., Livingston, D. J., Su, M. S. and Flavell, R. A. (1995). Altered cytokine export and apoptosis in mice deficient in interleukin-1 beta converting enzyme. Science 267, 2000-2003.
Kuida, K., Zheng, T. S., Na, S. Q., Kuan, C. Y., Yang, D., Karasuyama, H., Rakic, P. and Flavell, R. A. (1996). Decreased apoptosis in the brain and premature lethality in CPP32-deficient mice. Nature 384, 368-372.[Medline]
Kuida, K., Haydar, T. F., Kuan, C. Y., Gu, Y., Taya, C., Karasuyama, H., Su, M. S., Rakic, P. and Flavell, R. A. (1998). Reduced apoptosis and cytochrome c-mediated caspase activation in mice lacking caspase 9. Cell 94, 325-337.[Medline]
Marigo, V. and Tabin, C. J. (1996). Regulation of patched by sonic hedgehog in the developing neural tube. Proc. Natl. Acad. Sci. USA 93, 9346-9351.[Medline]
Marti, E., Takada, R., Bumcrot, D. A., Sasaki, H. and McMahon, A. P. (1995). Distribution of Sonic hedgehog peptides in the developing chick and mouse embryo. Development 121, 2537-2547.
Martin, S. J. and Green, D. R. (1995). Protease activation during apoptosis: death by a thousand cuts? Cell 82, 349-352.
Martinou, J. C., Dubois-Dauphin, M., Staple, J. K., Rodriguez, I., Frankowski, H., Missotten, M., Albertini, P., Talabot, D., Catsicas, S., Pietra, C. and Huarte, C. (1994). Overexpression of BCL-2 in transgenic mice protects neurons from naturally occurring cell death and experimental ischemia. Neuron 13, 1017-1030.[Medline]
Olivera-Martinez, I., Thelu, J., Teillet, M.-A. and Dhouailly, D. (2001). Dorsal dermis development depends on a signal from the dorsal neural tube, which can be substituted by Wnt-1. Mech. Dev. 100, 233-244.[Medline]
Oppenheim, R. W. (1991). Cell death during development of the nervous system. Annu. Rev. Neurosci. 14, 453-501.[Medline]
Oppenheim, R. W., Homma, S., Marti, E., Prevette, D., Wang, S., Yaginuma, H. and McMahon, A. P. (1999). Modulation of early but not later stages of programmed cell death in embryonic avian spinal cord by Sonic Hedgehog. Mol. Cell Neurosci. 13, 348-361.[Medline]
Purves, D. (1988). Body and Brain: A Trophic Theory of Neural Connections. Cambridge, MA: Harvard University Press.[Medline]
Raff, M. C. (1992). Social controls on cell survival and cell death. Nature 356, 397-400.[Medline]
Riddle, R. D., Johnson, R. L., Laufer, E. and Tabin, C. (1993). Sonic hedgehog mediates the polarizing activity of the ZPA. Cell 75, 1401-1416.[Medline]
Rowitch, D. H., St-Jacques, B., Lee, S. M., Flax, J. D., Snyder, E. Y. and McMahon, A. P. (1999). Sonic hedgehog regulates proliferation and inhibits differentiation of CNS precursor cells. J. Neurosci. 19, 8954-8965.[Medline]
Ruiz i Altaba, A., Placzek, M., Baldassare, M., Dodd, J. and Jessell, T. M. (1995). Early stages of notochord and floor plate development in the chick embryo defined by normal and induced expression of HNF-3 beta. Dev. Biol. 170, 299-313.
Sanz-Ezquerro, J. J. and Tickle, C. (2000). Autoregulation of Shh expression and Shh induction of cell death suggest a mechanism for modulating polarising activity during chick limb development. Development 127, 4811-4823.[Medline]
Smith, A. and Graham, A. (2001). Restricting Bmp-4 mediated apoptosis in hindbrain neural crest. Developmental Dynamics 220, 276-283.
Tanabe, Y. and Jessell, T. M. (1996). Diversity and pattern in the developing spinal cord. Science 274, 1115-1123.[Medline]
Teillet, M.-A., Lapointe, F. and Le Douarin, N. M. (1998a). The relationships between notochord and floor plate in vertebrate development revisited. Proc. Natl. Acad. Sci. USA 95, 11733-11738.
Teillet, M.-A., Watanabe, Y., Jeffs, P., Duprez, D., Lapointe, F. and Le Douarin, N. M. (1998b). Sonic hedgehog is required for survival of both myogenic and chondrogenic somitic lineages. Development 125, 2019-2030.
Weil, M., Jacobson, M. D., Coles, H. S., Davies, T. J., Gardner, R. L., Raff, K. D. and Raff, M. C. (1996). Constitutive expression of the machinery for programmed cell death. J. Cell. Biol. 133, 1053-1059.
White, E. (1996). Life, death, and the pursuit of apoptosis. Genes Dev. 10, 1-15.[Abstract]
White, F. A., Keller-Peck, C. R., Knudson, C. M., Korsmeyer, S. J. and Snider, W. D. (1998). Widespread elimination of naturally occurring neuronal death in Bax-deficient mice. J. Neurosci. 18, 1428-1439.[Medline]
White, K., Grether, M. E., Abrams, J. M., Young, L., Farrell, K. and Steller, H. (1994). Genetic control of programmed cell death in Drosophila. Science 264, 677-683.
Wijsman, J. H., Jonker, R. R., Keijzer, R., Van de Velde, C. J., Cornelisse, C. J. and Van Dierendonck, J. H. (1993). A new method to detect apoptosis in paraffin sections: in situ end-labeling of fragmented DNA. J. Histochem. Cytochem. 41, 7-12.[Medline]
Williams, B. A. and Ordahl, C. P. (1994). Pax-3 expression in segmental mesoderm marks early stages in myogenic cell specification. Development 120, 785-796.
Yoshida, H., Kong, Y. Y., Yoshida, R., Elia, A. J., Hakem, A., Hakem, R., Penninger, J. M. and Mak, T. W. (1998). Apaf1 is required for mitochondrial pathways of apoptosis and brain development. Cell 94, 739-750.
Yuan, J., Shaham, S., Ledoux, S., Ellis, H. M. and Horvitz, H. R. (1993). The C. elegans cell death gene ced-3 encodes a protein similar to mammalian interleukin-1 beta-converting enzyme. Cell 75, 641-652.[Medline]