Embryonic development of four different subsets of cholinergic neurons in rat cervical spinal cord

Abstract

The developmental stage at which a neuron becomes committed to a neurotransmitter phenotype is an important time in its ontogenetic history. The present study examines when choline acetyltransferase (ChAT) is first detected within each of four different subsets of cholinergic neurons previously identified in the cervical enlargement of the spinal cord: namely, motor neurons, partition cells, central canal cluster cells, and dorsal horn neurons. By examining the temporal sequence of embryonic development of these cholinergic neurons, we can infer the relationships between ChAT expression and other important developmental events. ChAT was first detected reliably on embryonic day 13 (E13) by both biochemical and immunocytochemical methods, and it was localized predominantly within motor neurons. A second group of primitive-appearing ChAT-positive cells was detected adjacent to the ventricular zone on E14. These neurons seemed to disperse laterally into the intermediate zone by E15, and, on the basis of their location, were tentatively identified as partition cells. A third group of primitive ChAT-immunoreactive cells was detected on E16, both within and around the ventral half of the ventricular zone. By E17, some members of this "U"-shaped group appeared to have dispersed dorsally and laterally, probably giving rise to dorsal horn neurons as well as dorsal central canal cluster cells. Other members of this group remained near the ventral ventricular zone, most likely differentiating into ventral central canal cluster cells. Combined findings from the present study and a previous investigation of neurogenesis (Phelps et al.: J. Comp. Neurol. 273:459-472, '88), suggest that premitotic precursor cells have not yet acquired the cholinergic phenotype because ChAT is not detectable until after the onset of neuronal generation for each of the respective subsets of cholinergic neurons. However, ChAT is expressed in primitive bipolar neurons located within or adjacent to the germinal epithelium. Transitional stages of embryonic development suggest that these primitive ChAT-positive cells migrate to different locations within the intermediate zone to differentiate into the various subsets of mature cholinergic neurons. Therefore, it seems likely that spinal cholinergic neurons are committed to the cholinergic phenotype at pre- or early migratory stages of their development. Our results also hint that the subsets of cholinergic cells may follow different migration routes. For example, presumptive partition cells may use radial glial processes for guidance, whereas dorsal horn neurons may migrate along nerve fibers of the commissural pathway. Cell-cell interactions along such diverse migratory pathways could play a role in determining the different morphological, and presumably functional, phenotypes expressed by spinal cholinergic neurons.

Transient and continuous expression of NADPH diaphorase in different neuronal populations of developing rat spinal cord

“…Our double-labeling experiments indicated that these are the same cells that have been described previously after immunocytochemical labeling for ChAT (Phelps et al, 1990). The bipolar morphology of these cells suggested that they are in the process of migrating, and consistent with this hypothesis, fewer cells were seen at the ventral midline after E18.…”

“…The bipolar morphology of these cells suggested that they are in the process of migrating, and consistent with this hypothesis, fewer cells were seen at the ventral midline after E18. By following the commissural fibers in this area (Phelps and Vaughn, 1995;Vaughn et al, 19921, these cells could migrate laterally and dorsally (Phelps et al, 1990). Such a migration would position the cells around the central canal, a location in which diaphorase-positive cells are observed by E18.…”

Differential expression of nerve terminal protein isoforms in VAChT-containing varicosities of the spinal cord ventral horn

“…Thus, the origin of the C-terminals has been considered to be located in the immediate vicinity of the motoneurons. In the adult rat, four pools of cholinergic neurons have been defined in the lumbar enlargement, i.e., motoneurons, small neurons clustered around the central canal, medium to large multipolar cells in the intermediate gray matter (partition neurons), and dorsal horn neurons (Barber et Phelps et al, 1984Phelps et al, , 1990. Of these, an origin from the motoneurons is less likely because those monosynaptic interconnections that have been demonstrated among motoneurons have in no case been of the C-type (Cullheim et al, 1977;Lagerbä ck et al, 1981).…”

Expression of synaptophysin during the prenatal development of the rat spinal cord: Correlation with basic differentiation processes of neurons

“…Moreover, choline acetyltransferase, an important enzyme for transmitter biosynthesis, is present within motor neurons when their axons have projected from the spinal cord. 25 Thus, the biosynthesis of two prerequisites for future synaptic transmission at the neuromuscular junctions starts as early as ED 12.…”