Cell patterning.

  • Symposium on Cell Patterning (1974 : London, England)
Date:
1975
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Cell patterning. Source: Wellcome Collection.

    347/372 (page 335)
    fuzz; between stages 43 and 49, when you have connections and synapses there is fuzz. Dr Hunt, your statement that the new fibres growing in go to their right place is arguable. They might well go to their right relative place but we doubt whether they go to their right absolute place. Our interpretation of the changes that occur in development in the tadpole map is that one retinal ganglion cell is sequentially connecting with a whole range of tectal cells. Hunt: Yes, but the context in which locus selection occurs changes with time as the retinal and tectal cell populations grow. Within a relatively small time window, does the cell interpolate into the map in the right place? The fact that the tectal cell sitting under it may be different later from what it is now may be important, but your evidence (Gaze etal. 1974) indicates that the new fibre does not miss its target by 10 or 100 fibres or wander around for several weeks before reaching the right place in the map. A simple target theory applied to the system suggests that the one fibre hits the target pretty well every time. Gaze: Yes and no. In the adult retinotectal projection, fibres from nasal retina go only to the back of the tectum; temporal retina goes only to the front of the tectum. With the tadpole this is not so, in that temporal retina goes to the front of the tectum and nasal retina goes to the back, but from the front of the tectum you can also pick up activity originating in the temporal field. We thought at first that we were probably picking up from fibres going past the electrode to end more caudally, but this is unlikely to be the case, since endings have been shown in rostral tectum following localized lesions of the two poles in the retina. The temporal pole gives degeneration confined to the rostral pole of the tectum and the nasal pole gives degeneration both rostrally and caudally in the tectum. Hunt: At what stage do you see this atopic representation? Gaze: At all stages up to late 50s. Rostrally on the tectum one gets a massive response from the nasal field, which should be at the front of the tectum; but also, from the same rostral tectal position, one frequently picks up local ized, small, isolated single unit responses from the far temporal field, 180° removed. The interesting point is that this corresponds to what is found neuroanatomically; the endings are there as well (Longley, unpublished work). Mark: We seem to be forgetting how small and complicated the brain is and on what a fine scale it is built, because getting the retinal fibres to approxi mately the right place in the tectum is really only stage 1. If you have a field which is, say, 200 /rm across and 10 /urn thick for one of these terminal fibres, it has room to make six million synapses, but obviously it does not, so there must be many fine control mechanisms; for example, retinal fibres never terminate on the apical dendritic trees of the pyramidal cells, they always