Institut für Genetik, Universität Mainz, Saarstraße 21, D-55122 Mainz, Germany ,
The generation of a large number of unique cell types is the basis of the formation and the functionality of the nervous system. Understanding the involved processes requires the knowledge of each cell´s ontogenesis. In the case of the CNS this means one has to clarify I. from which stem cell (neuroblast (NB)) the various cell types originate., II. the time of birth of the particular progeny.
30 NBs can be identified per hemisegment by their time and place of birth and the set of molcular markers they express (1). The DiI labelling technique (2) allows the identification of NBs and their particular lineages in vivo as well as in permanent preparations. We already described the lineages of 17 ventral and medial NBs (3).
Here we present the lineages of the more lateral NBs. All but 3 of the 30 to 32 glial cells per hemineuromere derive from these NBs. Until now we identified at least 7 lineages that include glial cells. Two stem cells, the longitudinal glioblast and another yet unidentified cell exclusively generate glial cells. NB 6-4 is a tagma specific neuroglioblast. In abdominal segments NB 6-4 divides once to generate the medial most cell body glia (MMCBG, (4)) and another cell body glia. In thoracic segments NB 6-4 is a neuroglioblast that produces three glial cells (two of them become MMCBG) and 4 neurons. We identified another four neuroglioblasts.
Interestingly the switch between neuronal and glial fate can occur in both directions and at various points during the lineage. In the case of the thoracic NB 6-4 lineage, glial cells are produced first. This can be concluded from experiments with embryos mutant for cyclinA. In these mutants cells stop dividing after the 14th mitosis (that means NBs divide only once). Stainings with anti repo antibody that labels most glia, mark (beside other cells) two cells per hemineuromere in the typical position of the NB 6-4 glial cells in abdominal segments as well as in thoracic segments. This indicates that the MMCBG and the more lateral cell body glia are the first cells appearing in the NB 6-4 lineage also in thoracic segments.
An example for a switch from neuronal to glial fate occures in the earlier described NB 1-1 lineage (5). Here we present another case: the lineage of NB 7-4. This NB segregates during late stage 8 and expresses engrailed (6). At the end of embryogenesis the lineage consists of 6 to 8 interneurons, that project contralaterally through the posterior commissure, and 5 to 6 glial cells that can be traced in double stained preparations by anti engrailed and anti repo antibodies. The first glial cells born in the 7-4 position migrate from a lateral position towards the midline to form the channel glia. Thus, the channel glia seem to be the first glial cells in the NB 7-4 lineage. Two observations indicate that they are born late in the NB 7-4 lineage. First, one can identify these double labeled cells not before early stage 11. Second, the channel glia can not be found in embryos mutant for cyclinA.
(1) Doe, C.Q. (1992), Development 116, 855863. (2) Bossing, T. and Technau, G.M. (1994), Development 120, 18951906. (3) Bossing, T., Udolph G., Doe, C.Q. and Technau G.M., Devl. Biol., in press. (4) Ito, K., Urban, J. and Technau G.M. (1995), Roux's Arch Dev Biol, 204, 284-307. (5) Udolph,G., Prokop,A., Bossing,T., Technau,G.M. (1993), Development 118, 765775. (6) Broadus, J., Skeath, J.B., Spana, E.S., Bossing, T., Technau, G., and Doe, C.Q. (1995), Mech. of Develop. 53, 393-402.
Supported by the DFG (Te130/7-1)