Development of palecortical projections through the anterior commissure of hamsters adopts progressive, not regressive, strategies

Lent, Roberto; Guimarães, Ricardo Z. P.

doi:10.1002/neu.480220505

Cited by 19 publications

(15 citation statements)

References 81 publications

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“…The distribution of labeled AC neurons in B6D2F 2 and F 3 animals was compatible with previous descriptions in other rodents (16,27). In short, labeled neurons were found in the anterior olfactory nucleus, anterior and posterior piriform cortex, olfactory tubercle, perirhinal cortex, agranular insular area and temporal cortex, and amygdaloid nuclei.…”

Section: Experiments 3: Distribution Of Ac Commissural Neuronssupporting

confidence: 90%

“…Similarly, a developmental elimination of AC axons has been reported in monkeys (21) and opossums (3). However, in both rats (11)and hamsters (27)this elimination of AC axons was not apparent, which may have been due to a simultaneous production and elimination of axons resulting in no net change in axonal number. Alternatively, the AC in primates and marsupials is mainly of neocortical origin, while in rodents the AC is mainly of paleocortical origin, and therefore the occurrence of axon elimination may be a developmental strategy that is more typically employed by the neocortex (26,27).…”

Section: Discussionmentioning

confidence: 98%

“…However, in both rats (11)and hamsters (27)this elimination of AC axons was not apparent, which may have been due to a simultaneous production and elimination of axons resulting in no net change in axonal number. Alternatively, the AC in primates and marsupials is mainly of neocortical origin, while in rodents the AC is mainly of paleocortical origin, and therefore the occurrence of axon elimination may be a developmental strategy that is more typically employed by the neocortex (26,27). In fact, a small number of transient, bicommissural neurons has been detected by double labeling in developing hamsters (12), encompassing a transitional region of the lateral cortex whose axons bifurcate and project both through the CC and through the AC.…”

Section: Discussionmentioning

confidence: 98%

“…Each hemisphere was transilluminated with a fiberoptic light positioned in order to differentiate the AC by its refringency from the surrounding tissue. With the aid of a stereomicroscope, a small crystal of DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate, Molecular Probes) was inserted in the middle of the AC using the tip of a fine dissecting pin (27). After dye insertion, the hemispheres were placed in fresh fixative and stored in the dark at room temperature for 4 to 16 weeks, depending on the age of the pup.…”

Section: Experiments 3:distribution Of Ac Commissural Neuronsmentioning

confidence: 99%

“…Callosal projections are typically diffuse in the early postnatal period and then gradually are pruned to yield the patchy topographical pattern seen in the adult (7). On the other hand, the AC forms prior to the CC in the embryo (48) and its transcortical projections are confined to specific sites from the outset (27). For this reason, it would not be surprising to find that the surplus AC axons in CC agenesis represent a greater density at existing sites rather than occupancy of novel sites.…”

Section: Introductionmentioning

confidence: 99%

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Increased Axon Number in the Anterior Commissure of Mice Lacking a Corpus Callosum

Livy

Schalomon

Roy

et al. 1997

Experimental Neurology

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Abstract:Relatively few behavioral deficits are apparent in subjects with hereditary absence of the corpus callosum (CC). The anterior commissure (AC) has been suggested to provide an extracallosal route for the transfer of interhemispheric information in subjects with this congenital defect. Anterior commissure size, axon number, axon diameter, and neuronal distribution were compared between normal mice and those with complete CC absence. No difference in midsagittal AC area was found between normals and acallosals, nor were differences found in the numbers or diameters of myelinated axons. However, axon counts indicated an 17%increase or about 70,000 more unmyelinated axons in the AC of acallosal mice, and the mean diameter of unmyelinated axons was slightly less than in normal mice (0.24 vs 0.26 μm). This decrease in axon diameter enabled more axons to pass through the AC without increasing its midsagittal area. The topographical distribution of neurons sending axons through the AC, assessed with lipophilic dyes, was qualitatively similar for almost all the known regions of origin of the anterior commissure in normal and acallosal mice. There was a pronounced deficit of AC cells in the anterior piriform cortex of BALB/c mice, but this occurred whether or not the mouse suffered absent CC. Although the increase in AC axon number is far smaller than the number of CC axons that fail to reach the opposite hemisphere, the higher number of axons present in the AC of acallosal mice may contribute to the functional compensation for the loss of the CC.

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Section: Experiments 3: Distribution Of Ac Commissural Neuronssupporting

confidence: 90%

Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 98%

Section: Experiments 3:distribution Of Ac Commissural Neuronsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Increased Axon Number in the Anterior Commissure of Mice Lacking a Corpus Callosum

Livy

Schalomon

Roy

et al. 1997

Experimental Neurology

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Anterior commissure of the wallaby (Macropus eugenii): Adult morphology and development

Ashwell¹,

Marotte

Li³

et al. 1996

J. Comp. Neurol.

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In metatheria, all neo- and paleo-cortical commissural connections are made by the anterior commissure. We have examined the adult morphology of this commissure and its development in a diprotodontid metatherian, the wallaby (Macropus eugenii), at both the light and electron microscope level. The total number of axons in the adult anterior commissure was 21.7 million, of which 55-62% were myelinated. The dorsal two thirds of the commissure, containing neocortical commissural axons, showed a higher percentage of larger, myelinated axons than the ventral one third, which contains paleocortical commissural axons. The commissure also showed a topographical gradient, with cells in the dorsal cortex projecting through the dorsal region of the commissure, the fasciculus aberrans. In the rostrocaudal axis, axons from the frontal cortex tended to pass more anteriorly through the commissure and those from the occipital more posteriorly, but there was extensive overlap of projections from different areas. The gestation length of this wallaby is 28.3 days, and all commissural development occurs postnatally. The anterior commissure first appeared at P (postnatal day) 14, at which time commissural fibres were apparently derived from the external capsule exclusively. Commissural fibres passing through the internal capsule, and joining the anterior commissure via the fasciculus aberrans, were first noted at P18. By that age there were 94,000 to 161,000 axons. Peak axon counts of 50 to 63 million occurred between P100 and P150. The number of growth cones in a single midline section peaked at approximately P114 (480,000) and dropped to 0 by P170. The distribution of growth cones was analysed during the early stages of anterior commissure development (P18, P30, P82). At P18, growth cones were concentrated in the dorsal parts of the commissural bundle, suggesting a ventrodorsal sequence of addition of axons. There was no apparent preferential association of growth cones with the periphery of the commissure or glial structures at any of the three ages examined. The results show that axonal overproduction and regression in cortical commissural connections are features of development in diprotodontid metatheria, as in eutheria.

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Development of commissural neurons in the wallaby (Macropus eugenii)

et al. 1997

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We have examined the development of the laminar and areal distribution of cortical commissural neurons in a marsupial mammal, the wallaby Macropus eugenii. In this species, commissural axons approach the major cerebral commissure, the anterior commissure, via either the internal capsule or the external capsule and first cross the midline at postnatal day 14 (P14). By retrogradely labelling these axons with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI) at P15, we show here that the cell bodies of these neurons are restricted to a region of cortex adjacent to the rhinal fissure. Most of these labelled neurons are located in the compact cell zone of the cortical plate, with only a few labelled cells found in the zone of loosely packed cells deep to this layer. Over the subsequent 66 days, commissural neurons are found progressively more dorsally, rostrally, and caudally, so that, by P80, they are present throughout the extent of the neocortex. At this age, they are mainly pyramidal in morphology and form a single band within the deeper part of layer 5 of the developing cortex. From P80 to adulthood, the distribution of commissural neurons has been assessed in the visual cortex by using retrograde transport of horseradish peroxidase. At P80, labelled neurons with immature pyramidal morphology are present throughout the occipital cortex; as in DiI material, somata are located in deep layer 5. At P165, previously shown to be the age when commissural axon numbers peak, widespread labelling is present in the occipital region, with labelled cells now found in two bands corresponding to layers 3 and 5. After this age, neurons become more restricted in distribution, so that, by adulthood, commissural neurons are no longer apparent throughout area 17 but are restricted to a localised region around the area 17/18 boundary. Within this region, labelling is still present in layers 3 and 5 but is more dense in layer 3. The gradual restriction of commissural fields seen here in the wallaby is similar to that reported in the neocortex in many eutherians. These findings also support studies in eutheria, suggesting that subplate neurons do not appear to play a major role in commissural development.

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Development of palecortical projections through the anterior commissure of hamsters adopts progressive, not regressive, strategies

Cited by 19 publications

References 81 publications

Increased Axon Number in the Anterior Commissure of Mice Lacking a Corpus Callosum

Increased Axon Number in the Anterior Commissure of Mice Lacking a Corpus Callosum

Anterior commissure of the wallaby (Macropus eugenii): Adult morphology and development

Development of commissural neurons in the wallaby (Macropus eugenii)

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