Fig 1.
Axons of cortico-cortical pathways arrive at their targets during development in the order of neuron generation (inside-out).
(A) Section of area 17 of the rat at P1 (Nissl staining) shows immature neurons in the superficial and middle part of the CP (A1); neurons in the deep part of the CP show better cytological differentiation (A2); The VZ and SVZ are dense, and abundant cells are migrating through the IZ (A3). At P1, neurons in area 17 that project to area 18a are found mostly in infragranular layers V–VI (A4). (B) Section of area 17 of the rat at P6 (Nissl staining) shows immature neurons in the superficial part of the CP (B1); neurons in the middle part of the CP (B2) show better cytological differentiation; neurons in the deep part of the CP (B3) are differentiated and separated by neuropil. At P5, neurons in area 17 that project to area 18a are found in infragranular layers V–VI but also in supragranular layers II–III (B4). (C) Section of area 17 of the rat at P40 (Nissl staining) shows mature neurons across all cortical layers (C1, C2, C3). After P28, neurons in area 17 that project to area 18a are found mostly in supragranular layers II–III, the typical pattern of the adult cortex (C4). Roman numerals indicate cortical layers. Calibration bar in Panel C applies to Panels A, B, and C. Calibration bar in Panel C3 applies to Panels A1–A3, B1–B3, C1–C3. (Note: Panels A, B, C and A1–A3, B1–B3, C1–C3 are an examination of material from a gift of Dr. Alan Peters; data shown in Panels A4, B4, and C4 are from [25]). CP, cortical plate; IZ, intermediate zone; MZ, marginal zone; P1, postnatal day 1; P5, postnatal day 5; P6, postnatal day 6; P28, postnatal day 28; P40, postnatal day 40; SP, subplate; SVZ, subventricular zone; VZ, ventricular zone; WM, white matter.
Fig 2.
Pathways connecting limbic areas form earlier in development.
(A) Sketch of the right hemisphere (medial view) showing the major white matter tracts (green lines) in the human brain (redrawn from Fig 751 in [27]). Tracts connecting limbic areas (cingulate bundle and uncinate fasciculus) are curved and tracts connecting eulaminate areas (superior and inferior longitudinal fasciculi) are straight. (B) Formation of the temporal, occipital, and frontal poles in development (redrawn from Fig 11.4 in [1]). Early in development (left panel), there is an early posterior pole (orange). As the neocortex expands (middle panel), the early posterior pole is pushed anteriorly and ventrally to its final location; the temporal pole is composed of areas that are displaced with the early posterior pole (right panel). We propose that the axons of the cingulate bundle (green line in the three panels) and the uncinate fasciculus arrive to the subplate of their target areas early in development and are curved by the displacement of the early posterior pole to its final location. ac, anterior commissure; cc: corpus callosum.
Fig 3.
A mechanism for the expansion of the neocortex in development and evolution.
(A) Sketch of the mammalian neural tube showing the proneuromeric compartments according to [1]. The alar plate is colored in blue and the basal plate is red; the notochord is green. The telencephalic vesicle (highlighted in darker blue) grows out of the alar plate of the secondary prosencephalon. (B) Section through the secondary prosencephalon at the level of the black line in (A) shows the optic vesicles and the telencephalic vesicles (redrawn from Fig 264 in [30]); the telencephalic vesicles consist of pallial and subpallial territories. (C) Two organizers direct the patterning of the pallium: the hem, at the border of the roof plate and the prospective hippocampus, and the antihem, at the corticostriatal junction near the prospective olfactory cortex ([28]; reviewed in [16]). The morphogen molecules segregated by these organizers form overlapping gradients (solid and dashed arrows) that pattern the pallium in 4 sectors: medial, dorsal, lateral, and ventral pallial sectors. We hypothesize the distinction of 2 parts on the MPall sector corresponding to allocortex (hippocampus) and the adjacent mesocortex (marked with “?”) based on architectonic analysis of adult rats and primates [16]. (D, E) Coronal sections of the adult rat brain (D) and the adult human brain (E) at the level of the anterior commissure show two trends of increasing laminar elaboration according to the Dual Origin of the Neocortex hypothesis ([29]; reviewed in [16]). One trend is traced back to the ancestral hippocampal cortex (solid arrow marks the trend) and the other to the ancestral olfactory cortex (Pir in Panel D, AON in Panel E; dashed arrow marks the trend). The hem and the antihem likely directed the expansion of the neocortex from rodents to primates. ac, anterior commissure; accum, nucleus accumbens; AON, anterior olfactory nucleus in the primary olfactory cortex; cc, corpus callosum; Cd, caudate nucleus; Cl, claustrum; DPall, dorsal pallium; Endo, endopiriform nucleus; LOT, lateral olfactory tract; LPall, lateral pallium; MPall, medial pallium; VPall, ventral pallium; Hipp, anterior extension of the hippocampal formation; Pir, piriform cortex in the primary olfactory cortex; Put, putamen; sm, stria medullaris; TOL, olfactory tubercule.