A Novel Type of Programmed Neuronal Death in the Cervical Spinal Cord of the Chick Embryo

Abstract

We examined the massive early cell death that occurs in the ventral horn of the cervical spinal cord of the chick embryo between embryonic days 4 and 5 (E4 and E5). Studies with immunohistochemical, in situ hybridization, and retrograde-tracing methods revealed that many dying cells express Islet proteins and Lim-3 mRNA (motoneuron markers) and send their axons to the somatic region of the embryo before cell death. Together, these data strongly suggest that the dying cells are somatic motoneurons. Cervical motoneurons die by apoptosis and can be rescued by treatment with cycloheximide and actinomycin D. Counts by motoneuron numbers between E3.5 and E10 revealed that, in addition to cell death between E4 and E5, motoneuron death also occur between E6 and E10 in the cervical cord. Studies with [3H]thymidine autoradiography and morphological techniques revealed that in the early cell-death phase (E4-E5), genesis of motoneurons, axonal elongation, and innervation of muscles is still ongoing. However, studies with [3H]thymidine autoradiography also revealed that the cells dying between E4 and E5 become postmitotic before E3.5. Increased size of peripheral targets, treatment with neuromuscular blockade, and treatment with partially purified muscle or brain extracts and defined neurotropic agents, such as NGF, BDNF, neurotrophin-3, CNTF, bFGF, PDGF, S100-beta, activin, cholinergic differentiation factor/leukemia inhibitory factor, bone morphogenetic protein-2, IGF-I, interleukin-6, and TGF-beta 1, were all ineffective in rescuing motoneurons dying between E4 and E5. By contrast, motoneurons that undergo programmed cell death at later stages (E6-E10) in the cervical cord are target-dependent and respond to activity blockade and trophic factors. Experimental approaches revealed that early cell death also occurs in a notochord-induced ectopic supernumerary motoneuron column in the cervical cord. Transplantation of the cervical neural tube to other segmental regions failed to alter the early death of motoneurons, whereas transplantation of other segments to the cervical region failed to induce early motoneuron death. These results suggest that the mechanisms that regulate motoneuron death in the cervical spinal cord between E4 and E5 are independent of interactions with targets. Rather, this novel type of cell death seems to be determined by signals that either are cell-autonomous or are derived from other cells within the cervical neural tube.

Degenerating Synaptic Boutons in Prion Disease

“…50 Phagocytosis of dying neurons by adjacent neuronal progenitors has been documented in the developing retina and spinal cord. [51][52][53] However, of particular interest are images showing that motor neurons in the developing spinal cord of the cat engulf supernumerary synapses on the motor neuron cell soma. 54 Similarly it has also been shown that injured hypoglossal motor neurons envelop synapses innervating the cell soma.…”

Developmental Delay in Islet-1-Positive Motor Neurons in Chick Spina Bifida

“…Early in the process of neurulation, the cells leave the cell cycle and begin expressing Islet-1; they migrate from the ependymal layer to the ventral horn and then subdivide into Islet-1-positive cells in the LMCm and Islet-1-negative ones in the LMCl. After these events, some motor neurons undergo programmed cell death (PCD), which results in the reduction of the initial number of cells [24,25]. For example, in chick lumbar segments, motor neurons are generated from E2 to E4, and then a steep fall occurs in their number from E6 onward when PCD begins.…”

Distinct susceptibility of developing neurons to death following Bax overexpression in the chicken embryo

“…As a previous study suggested that this PCD is controlled independent of peripheral targets, the fate of individual MNs in the cervical spinal cord may be intrinsically determined. 25 We recently found that dying population appears to be a subgroup of MN that lacks the expression of LIM homeodomain genes Lim-3, and introduction of this transcription factor rescues cervical MN from early PCD (Yaginuma et al, unpublished). Present data show that Bax is inactive in Lim-3-positive MNs (Figure 7).…”

“…32 By contrast, the earlier phase of MN death (E4-E5) occurs independent of target-derived trophic signals. 25 Recent studies demonstrated that this early MN Figure 8 Summary of Bax action in developing neuronal populations in vivo. During early retinal PCD and the period of naturally occurring death of DRG and MNs, the decision to die or survive is regulated by extrinsic factors.…”

“…25 At E4.5 when this early PCD occurs in the VH of the cervical spinal cord, there is no significant difference in the number of TUNEL-labeled cells in control versus Bax-transfected embryos (Figure 5a). By contrast, Bcl-2 overexpression with this same vector clearly reduces the number of TUNELlabeled cells (Figure 5a).…”