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S E Koester

Publications and source records attributed to S E Koester.

8 recordsLinked to original sources
PubMed1997

Directed growth of early cortical axons is influenced by a chemoattractant released from an intermediate target.

Projection neurons throughout the mature mammalian neocortex extend efferent axons either through the ventrolaterally positioned internal capsule to subcortical targets or through the dorsally located midline corpus callosum to the contralateral cortex. In rats, the internal capsule is pioneered on E14, but the corpus callosum is not pioneered until E17, even though these two types of projection neurons are generated at the same time. Here we use axonal markers to demonstrate that early cortical axon growth is directed toward the nascent internal capsule, which could account for the timing difference in the development of the two efferent pathways. This directed axon growth may be due to a chemoattractant and/or a chemorepellent secreted by intermediate targets of cortical efferent axons, the nascent internal capsule, or the medial wall of the dorsal telencephalon (MDT), respectively. To test for these soluble activities, explants of E15 rat neocortex and intermediate targets were cocultured in collagen gels. Cortical axon outgrowth was directed toward the internal capsule, but outgrowth was nondirected and suppressed when cocultured with MDT, suggesting that the internal capsule releases a chemoattractant for cortical axons, whereas the MDT releases a chemosuppressant. Because the chemoattractant Netrin-1 is expressed in the internal capsule, we cocultured cortical explants with E13 rat floor plate, which expresses Netrin-1, or with Netrin-1-transfected or control-transfected 293T cells. Cortical axon growth was directed toward both floor plate and Netrin-1-transfected 293T cells, as it had been toward the internal capsule, but not toward control-transfected 293T cells. These findings suggest that early events in cortical axon pathfinding may be controlled by a soluble activity which attracts initial axon growth toward the internal capsule and that this activity may be due to Netrin-1.

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PubMed1994

Axons of early generated neurons in cingulate cortex pioneer the corpus callosum.

The internal capsule and corpus callosum are the two major efferent axonal pathways of the mammalian neocortex. Previous studies have shown that the first cortical axons to grow through the internal capsule, the pathway from cortex to its subcortical targets, are extended by subplate neurons, which are the earliest generated neurons in the neocortex. Here, we characterize the origin of the first axons to project through the other major efferent pathway of the cortex, the midline corpus callosum, which connects the two cortical hemispheres. Using anterograde Dil tracing, we show that cortical axons first cross the midline through the nascent corpus callosum at E17. Retrograde Dil labeling from medial cortex at E18 reveals that these axons originate from a discrete group of neurons in medial (presumptive cingulate) cortex. These early callosal cells have complex morphologies with highly branched dendrites and later appear to take on a pyramidal form characteristic of callosal neurons in deep layers of cingulate cortex. 3H-thymidine birthdating demonstrates that these cells are predominantly generated on E14, making them among the earliest generated neurons in this cortical region. Injections of retrograde tracers in one cortical hemisphere at late embryonic or early postnatal ages result in substantial numbers of neurons labeled in the ipsilateral subplate, but only a few neurons labeled in the contralateral subplate. Thus, subplate neurons do not pioneer or ever project in significant numbers through the corpus callosum. We conclude that the two major efferent pathways from cortex, the corpus callosum and the internal capsule, are pioneered by developmentally and spatially distinct populations of early generated cortical neurons.

Aging↗
PubMed1993

Connectional distinction between callosal and subcortically projecting cortical neurons is determined prior to axon extension.

In adult rats, layer 5 cortical neurons send axons through the corpus callosum to contralateral cortex or through the internal capsule to subcortical targets, but individual neurons reportedly do not have both connections. Here we confirm this adult separation and address whether it develops by extension of axon collaterals to both sets of targets with later elimination of one or the other (a phenomenon common in developing cortex) or by initially selective axon outgrowth. Retrograde tracers Fast Blue and Diamidino Yellow were injected in the subcortical path at the pyramidal decussation and in the contralateral cortex, respectively, of adult and newborn rats. In 16 adults, no cortical neurons were double-labeled, indicating that none project to both sites. In 17 neonates, hundreds of thousands of layer 5 neurons were single-labeled in each brain, but only one was double-labeled. In cases in which the injections to one of the two targets was delayed, again, no double-labeled cells were found. These results indicate that the connectional distinction found in adults is not achieved by the elimination of long transient callosal or subcortical collaterals. To determine if shorter transient collaterals are extended by callosal neurons into the internal capsule, i.e., the subcortical pathway, we injected DiI into one cortical hemisphere of aldehyde-fixed Embryonic Day (E)19 and E21 brains. Two types of axons are seen in the white matter of the cortex opposite the injection: those which turn and extend through the callosum and those which branch, sending collaterals to both ipsilateral and contralateral cortex. Rarely were collaterals seen to extend into or toward the internal capsule. [3H]Thymidine birthdating studies show that layer 5 callosal and subcortically projecting neurons are generated at the same stage of corticogenesis. Thus, from early stages of axon extension, callosal and subcortically projecting cells are distinct classes of neurons and, responding differentially to cues present in cortex, initiate growth toward class-specific and nonoverlapping sets of targets. We conclude that the distinction between the two projection classes in inherent to them and is likely to be determined at an early stage of cortical development, prior to neuronal migration from the neuroepithelium.

Animals↗
PubMed1992

Functional classes of cortical projection neurons develop dendritic distinctions by class-specific sculpting of an early common pattern.

We demonstrate in rat neocortex that the distinct laminar arrangements of the apical dendrites of two classes of layer 5 projection neurons, callosal and corticotectal, do not arise de novo, but are generated later in development from a common tall pyramidal morphology. Neurons of each class initially elaborate an apical dendrite in layer 1. Layer 5 callosal neurons later lose the segments of their apical dendrite superficial to layer 4, generating their characteristic short pyramidal morphology. The apical dendrite of layer 5 callosal neurons later lose the segments of their apical dendrite superficial to layer 4, generating their characteristic short pyramidal morphology. The apical dendrite of layer 5 callosal neurons is actively eliminated, rather than passively displaced, as superficial cortical layers expand. Corticotectal neurons and callosal neurons superficial to layer 5 maintain their apical dendrite to layer 1. Therefore, this selective dendritic loss occurs in a neuron class-specific manner and, within the callosal population, in a lamina-specific manner. Based on our additional observations and other studies, this phenomenon can be extended to other types of cortical projection neurons. These findings show that selective dendritic elimination plays a major role in shaping the functional architecture characteristic of the adult cortex.

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PubMed1990

Target selection by cortical axons: alternative mechanisms to establish axonal connections in the developing brain.

We have described our studies of the development of projections from layer 5 of the rat neocortex to subcortical targets in the midbrain and hindbrain. The major points are briefly summarized here. 1. Layer-5 neurons extend a primary axon out of cortex and along a spinally directed trajectory, bypassing all of their targets in the midbrain and hindbrain. These targets are later contacted exclusively by collaterals formed by a delayed interstitial branching of the primary axon, not by growth cone bifurcation. 2. Collateral branches only form at stereotypic positions, not randomly along the length of the axon. Thus, specific cues identify branch points, and the length of the primary axon well behind its growth cone responds to these cues. 3. Layer-5 neurons in diverse areas of cortex initially develop the same basic set of collateral branches, although they will permanently retain different subsets of the initial common set. Therefore, branch cues are recognized by layer-5 neurons independent of whether the collateral projection formed is functionally appropriate for the cortical region in which the neuron resides. 4. In vitro and in vivo evidence indicates that one of the major branches, which forms the corticopontine projection, is induced and directed into its target, the basilar pons, by a diffusible, target-derived, tropic signal. Thus, a chemotropic cue promotes recognition of the basilar pontine target by the primary layer-5 axons. 5. In this system, then, target selection is not the responsibility of the growth cone of the primary axon.(ABSTRACT TRUNCATED AT 250 WORDS)

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