Although the development of the nervous system is continuous, the neurones that
constitute the nervous system proceed through specific developmental stages. Such
stages included: birth, differentiation, migration, the formation of efferent and afferent
connections, and the organisation of these connections based on exogenous and
endogenous cues. Alternatively, a neuron may undergo "natural occurring" cell death at
various points during development.
When considering the complexity of the nervous system, it is not surprising that our
knowledge, as to the factors that control these development stages, is limited One
significant obstacle for experimentally addressing these questions, for practical and/or
ethical reasons, has been the animal itself. Thus, the advent of tissue culture techniques
has greatly facilitated research in this area.
In addition to in vivo Dil tract-tracing experiments, I have used three different in vitro
techniques: (i) co-cultures of organotypic explants on a two-dimensional collagen
substrate, (ii) organotypic explants co-cultured within a three-dimensional collagen gel,
and (iii) co-cultures of dissociated cells. Because serum is known to contain many trophic
factors, only serum-free medium was used for all of these experiments. Using the above
in vitro techniques, I investigated questions concerning the development of connections
between the thalamus and cortex.
These experiments yielded the following results, (i) When thalamocortical and
corticothalamic efferent fibres began to grow in vivo at embryonic day 15 (El5),
conserved diffusible target-derived factors enhanced their neurite elongation in vitro, (ii)
The survival of El 5 thalamic and cortical neurones was independent from target-derived
iii
trophic support, since both the thalamus and cortex promoted their own survival in vitro.
possibly through the production of endogenous trophic factors. However (iii), between
E17 and postnatal day 2, a time coincident with the arrival of their fibres to their target,
the survival of thalamocortical and corticothalamic neurones was dependent on factors
released from their target, (iv) The production of growth promoting factors within the
cortex increased with postnatal age and was at least partially regulated by afferent
activity. And finally (v), it seems that the recognition/stop-signalling molecules expressed
within these targets are also highly conserved since they were recognised in inter-species
co-culture experiments. Thus, in general, the development of both thalamocortical and
corticothalamic efferents are highly influenced by factors produced by their target cells in
vitro. In vivo, these factors may play various roles during the different developmental
stages from promoting neurite outgrowth and cell survival, to regulating the
reorganisation of connections
