Trends in Neurosciences
Forebrain gene expression domains and the evolving prosomeric model
Section snippets
Reasons for changing the model
The initial model we proposed 2, 3, 4 was tentative in several aspects; we subsequently modified some details, particularly of the ventral thalamus and mammillary areas, as our knowledge of developing structures increased 5, 6, 12, 27. The secondary prosencephalon and the adjoining part of the diencephalon represented the most difficult areas, owing to their deformation during telencephalic evagination and morphogenesis. Moreover, patterning effects apparently occur here that are not found
The modified model
Our present proposal, which also includes convenient terminological simplifications, reduces the number of postulated prosomeres to the three caudal diencephalic ones, p1–p3. These contain in their alar regions the pretectum (p1), the thalamus (previously known as the dorsal thalamus) plus the habenula or epithalamus (p2), and the prethalamus (previously known as the ventral thalamus) plus the eminentia thalami (p3) (Fig. 1, Fig. 2, Fig. 3). The present proposal represents just two major
Concluding remarks
Comparison of the final schema of Larsen et al. [26] with our model suggests fundamental agreement with our p1–p3 prosomeres and a non-segmented secondary prosencephalon; we distinctly converge in our views on fundamental forebrain subdivisions. The proposal to reduce terminological confusion by calling the classically defined dorsal thalamus simply ‘thalamus’ and the classically defined ventral thalamus ‘prethalamus’ seems apropos at this point in time [52]. Finally, the present model should
Acknowledgements
The histological material shown was prepared by G. González (laboratory of L.P.). Our work is supported by grants PI-64/00862/FS-01 and BFI2000–03668 (L.P.) and from Nina Ireland and K02MH01046 (J.L.R.R.). Valuable discussions took place during C. Larsen's short stay in Murcia to learn the prosomeric model.
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