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In the adult cerebral cortex, we described previously a population of spiny neurones as typically (1) unfolding their major dendrite (the apical dendrite) towards the white matter, (2) lying in the cortical layers 5 and 6 and (3) axonally projecting to other cortical areas but not to subcortical centres (Bueno-Lopez et al.,1991; Reblet et al.,1992). The conspicuous inverted morphology of these adult cells was similar to that of many early generated cells to the transient (cortical) subplate (Marln-Padilla,1972; Shatz et aI., 1988). Although most subplate neurones disappear during late development, some subplate cells seem to remain alive at the deepermost portion of layer 6 and the uppermost one of the white-matter in the adult cortex (Shatz at aI., 1988). Wa asked whether the conspicuous inverted neurones of the adult cortex and cortical subplate are cells generated at the same time during ontogenesis and thus whether the inverted cells are part of a sort of spiny, cortico-cortical projection neurones that have characteristically a morphological phenotipe indeed distinct from that of the most common class of projection neurone of the cortex, namely the typically oriented pyramidal cell. To find out the generation time of cortical neurones, pregnant rabbits received a single injection of 5-bromo-2' -deoxyuridine (BrdU) in sterile normal saline (70 mg per Kg of body weight, i.p.) at different gestation days (GoI3, G014, Go15, G016, Go17, Go18 and GI9). The mating day was considered GOO. Offsprings were studied from weaning on. [Only the offspring of a pregnant rabbit that received the BrdU injection at G013 was sacrificed during foetal time (G025)).The rest of the animals were perfused transcardially with a solution of 4% paraformaldehyde in PB 0,1 M pH 7,6. Their brains were then removed from the skulls and postfixed during three hours in the same fixative. After cryoprotection in a solution of PBS-30% sucrose, the brains were frozen and then cut on a cryotome at 30 J.1m. Sections were "free-floating"", immunocytochemically incubated with a monoclonal antibody against BrdU as in Soriano et al. (1991) To verify the development of the morphological phenotype of the cortlco-cortical projection cells, we placed Oil or OiA crystals on the fixed occipital cortices of a number of prenatal rabbits aged Go18, G021, G025, G027 and postnatal (P)O, P1, P3 and adult (P22 on). These brains were fixed as above (except those aged G018, which were fixed by immersion in 10% neutral formalin). Some animals would receive, instead of Oil or OiA crystals, an in vivo injection of 10% Biocitin or 4% Fluorogold in their occipital cortex at PO or P2 and, after 2 days, perfused as above. Biocitin was then revealed prior to BrdU, both by means of an ABC (Vector) kit and DAB but the latter being Nickel-intensified in the case of BrdU. When successful, we used all these experiments with double labelling (with BrdU at the cell nucleus and the retrogradely transported tracer at the cell somata and dendrites) to correlate the cell generation times to the morphologies of the identified corti co-cortical projection cells. As early as G018, two morphological subtypes of cortico-cortical projection cells were retrogradely labelled by the fluorescent dyes at a range of distances from the injection sites. These two projection cell types were either typically or inversely oriented pyramidal neurones (Fig. 1). Cells of both types were labelled at the cortical plate and the uppermost portion of the intermediate zone, the socalled subplate. Some other cells of both types were also labelled deep in the intermediate zone. These findings strongly suggest that both morphological phenotypes of cortico-cortical projection neurones appear during very early development, indeed before these cells go into the cortical plate. Earlier hypothesis favoured instead that the morphologies of the inverted and other atypically oriented cells mature from other cell forms in concordance with extrinsic cues, through a process of cell rotation or differential cell growing (for discussion see Van der Loos, 1977). We have seen no intermediate morphology supporting this morphological continuum. In our study, the typically oriented cells retrogradely labelled as corticocortical projection neurones at any cortical or subcortical depth, had at G018 an apical dendrite that proceed until reaching the marginal zone (presumptive cortical layer 1) (Fig. 1). The inversely oriented cortico-cortical projection cells had in turn a major (apical) dendrite that went deep into the cortical plate or the intermediate zone below it. Some of these inverted cells had additionally a slender dendrite or an axonal branch that proceeded towards the marginal zone and occasionally went into it. Whereas the typically oriented cells either Figure.1. Camera lucida drawings of retrogradely labelled cells at GD18 and PO.The arrowheads point to could kept their contact with layer 1 thecellaxons. for life or loose it after birth (in the '"

Development of inverted cells in infragranular layers of the rabbit visual cortex.