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C J Shatz

Publications and source records attributed to C J Shatz.

At least 55 records · Page 3Linked to original sources
PubMed1994

Regulation of neurotrophin receptors during the maturation of the mammalian visual system.

Cell division, cell death, and remodeling of connections are major features of the construction of the mammalian CNS. We have begun to address the role of neurotrophins in these events through characterization of the expression of their receptors in the developing ferret visual system. By use of chemical cross-linking of iodinated neurotrophins, proteins corresponding to trkB, trkC, and p75 were identified as receptors for brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) throughout development. BDNF was also cross-linked to a truncated form of trkB that lacks the tyrosine kinase domain (trkB. T1) in retinal target tissues and in cortex. At the earliest developmental age examined (E24), the ratio of full-length to truncated trkB is > > 1 in the retinal target tissues, LGN and superior colliculus. During the ensuing period of retinal ganglion cell death and segregation into eye-specific layers, the amount of truncated trkB increases markedly relative to full-length trkB. By P27, truncated trkB is the predominant receptor for BDNF in the retinal target tissues and this pattern is maintained into adulthood. Within all subdivisions of visual cortex including the ventricular zone (VZ), intermediate zone (IZ), and cortical plate (CP), similar profiles of bands are observed. The developmental increase in abundance of truncated trkB relative to full-length occurs earliest in the VZ, with a major increase between E30 and P3. In the IZ, this shift to a predominance of truncated trkB occurs between P15 and P30, while in the CP the shift is even further delayed, not occurring until after P30. Within each subdivision of cortex, the shift to a predominance of truncated trkB occurs at times that correlate with the onset of cell death and maturation of axonal connections. This study demonstrates that members of the trk family, previously identified in the CNS on the basis of mRNA transcripts, are present as receptors with specific binding affinities for BDNF and NT-3. Moreover, the correspondence between the developmental shift from full-length to truncated trkB and the critical periods for cell fate determination, cell death, and axonal remodeling suggests an important role for neurotrophic factors in the development of the visual system.

Aging↗
PubMed1994

Independent control of dendritic and axonal form in the developing lateral geniculate nucleus.

To identify mechanisms that regulate neuronal form in the mammalian CNS, we have examined dendritic development in the lateral geniculate nucleus (LGN) during the period of segregation of retinal ganglion cell axons. The tracer Dil was used to label retrogradely LGN neurons that send their axons to primary visual cortex at different ages between embryonic day 36 (E36) and E60 in the cat. LGN neurons grow extensively during this period, in concert with the progressive restriction of ganglion cell axons from the two eyes to their appropriate eye-specific layers. At E36 neurons have simple bipolar morphology; by E60 all have acquired complex multipolar dendritic trees. During this period, soma size increases by 190% and total dendritic length increases 240%. Dendritic complexity, as measured by dendritic branch points, also increases. As dendrites grow, the number of spines increases, but their density remains constant at 0.015/micron throughout this period. Since it is known that blockade of action potential activity significantly alters the branching pattern and extent of retinal ganglion cell axonal arbors within the LGN, we also investigated whether the dendritic development of the postsynaptic LGN neurons is similarly susceptible. Following 2 weeks of the intracranial minipump infusion of TTX between E42 and E56, the morphology of LGN neurons was examined. Surprisingly in view of the striking effect of the treatment on the morphology of retinal ganglion cell axons, dendritic growth and development were essentially normal. However, the density of dendritic spines increased almost threefold, suggesting that this specific feature of dendritic morphology is highly regulated by action potential activity. These observations indicate that normally during this period of development, the previously described changes that occur in the morphology of the presynaptic inputs to LGN neurons are accompanied by a progressive growth of post-synaptic dendrites. Because the intracranial TTX infusions have almost certainly blocked all sodium action potentials, our results suggest that the basic dendritic framework of LGN neurons can be achieved even in the absence of this form of neural activity. Moreover, since the same treatment causes a profound change in the morphology of the presynaptic axons, at least some aspects of axonal and dendritic form must be controlled independently during this prenatal period of development.

Animals↗
PubMed1994

Neuronal coupling in the developing mammalian retina.

During the first 3 weeks of postnatal development in the ferret retina, cells in the ganglion cell layer spontaneously generate waves of electrical activity that travel across the retina in the absence of mature photoreceptors (Meister et al., 1991; Wong et al., 1993). Since few chemical synapses are present at the earliest stages when waves are present, we have explored whether gap junctions could act to correlate the activity of cells in the immature ganglion cell layer. Retinal ganglion cells in a living in vitro preparation from postnatal day 1 (P1) to P45 were intracellularly injected with the tracer Neurobiotin and the fluorescent dye Lucifer yellow, molecules that are known to pass through gap junctions. Lucifer yellow consistently filled only the injected cell, whereas Neurobiotin filled not only the injected cell but also passed to a constellation of neighboring cells. Coupling revealed by Neurobiotin is seen as early as P1, but, at this stage, it was not possible to identify the various morphological types of cells that were coupled. Thereafter, alpha ganglion cells showed homologous coupling to other alpha cells and to both conventionally placed and displaced amacrine cells. Likewise, gamma ganglion cells appeared coupled to other gamma cells and to amacrine cells. However, beta ganglion cells never showed tracer coupling in the neonatal or in adult retinas. The percentage of alpha and gamma cells that were coupled to other cells increased progressively with age. By the end of the third postnatal week, the pattern of Neurobiotin coupling in the ferret retina was adult-like, with virtually every injected alpha cell showing tracer coupling. Our observations suggest that intercellular junctions able to pass Neurobiotin are present in the inner plexiform layer during the period when the firing of retinal ganglion cells is highly correlated. Such junctions could contribute to synchronization of the activity of subsets of neighboring ganglion cells during development, but it cannot be the sole mediator of this activity because beta cells, which also participate in the correlated activity, showed no coupling at any stage. In addition, the continued presence of coupling in the adult retina implies that other changes in retinal circuitry are likely to contribute to the disappearance of the waves.

Aging↗
PubMed1994

Segregation of geniculocortical afferents during the critical period: a role for subplate neurons.

To investigate the cellular interactions within the mammalian visual cortex that are important in ocular dominance column formation, we have examined the role of subplate neurons in this process. LGN axons segregate in layer 4 of the cat's visual cortex between the third and sixth postnatal weeks to give rise to the adult pattern of ocular dominance columns. Subplate neurons are a transient population of neurons that sit in the white matter but have extensive projections into the overlying cortex, particularly layer 4, during neonatal life. Many subplate neurons are present at birth, but most are gone by the end of the period of LGN axon segregation. To examine whether these neurons are required for the segregation of LGN axons, we deleted them by intracortical injections of kainic acid either just after LGN axons had grown into layer 4 (first postnatal week) or later, just before the onset of segregation (third postnatal week). The consequences for the patterning of geniculocortical terminals were evaluated by transneuronal transport of 3H-proline injected into one eye at times when segregation would normally be complete. Following deletion of subplate neurons at either age, LGN axons failed to segregate into ocular dominance columns. Following the late deletions only, geniculocortical axons lost their laminar restriction to layer 4 and projected to layers 2 and 3 as well. Deletion of subplate neurons also resulted in long-term changes in the cytoarchitecture of layer 4. These observations suggest that the interactions that mediate segregation of LGN axons within layer 4 of visual cortex are susceptible to influences from subplate neurons. Although the mechanisms by which subplate neurons exert their effect are not yet clear, these experiments strongly suggest that interactions between LGN axons and layer 4 neurons are not sufficient for column formation, and that subplate neurons most likely play a critical role in interactions leading to ocular segregation.

Aging↗
PubMed1993

Transient period of correlated bursting activity during development of the mammalian retina.

The refinement of early connections in the visual pathway requires electrical activity in the retina before the onset of vision. Using a multielectrode array, we have shown that the spontaneous activity of cells in the neonatal ferret retina is correlated by patterns of periodically generated traveling waves. Here, we examine developmental changes in the characteristics of the waves and show that retinal ganglion cells participate in these patterns of activity, which are seen during the same period as synaptic modification in the lateral geniculate nucleus; that the waves subside gradually as the connectivity in the lateral geniculate nucleus stabilizes; and that their spatial structure allows for refinement of the retinotopic map, as well as for eye-specific segregation in the lateral geniculate nucleus.

Aging↗
PubMed1993

Enhancement of transmission at the developing retinogeniculate synapse.

Axon terminals from retinal ganglion cells in the left and right eyes initially overlap with each other in the lateral geniculate nucleus of the neonatal ferret, then segregate into eye-specific layers via an activity-dependent process. Brain slices were used to show that, during this period of reorganization, retinal terminals within the lateral geniculate nucleus evoke excitatory postsynaptic currents composed of both NMDA and non-NMDA receptor-mediated currents. The amplitude of these currents could be enhanced for several tens of minutes to more than an hour by several bursts of high frequency synaptic stimulation, and the induction of enhancement appears to depend on NMDA receptor activation. Synaptic enhancement such as this could provide one of the physiological mechanisms by which retinal terminals segregate into eye-specific layers during development.

Animals↗
PubMed1993

A role for subplate neurons in the patterning of connections from thalamus to neocortex.

During cerebral cortical development, ingrowing axons from different thalamic nuclei select and invade their cortical targets. The selection of an appropriate target is first evident even before thalamic axons grow into the cortical plate: initially axons accumulate and wait below their cortical target area in a zone called the subplate. This zone also contains the first postmitotic neurons of the cerebral cortex, the subplate neurons. Here we have investigated whether subplate neurons are involved in the process of target selection by thalamic axons by ablating them from specific cortical regions at the onset of the waiting period and examining the subsequent thalamocortical axon projection patterns. Subplate neurons were ablated at the onset of the waiting period by intracortical injections of kainic acid. The effect of the ablation on the thalamocortical projection from visual thalamus was examined by DiI-labeling of the LGN days to weeks following the lesion. At two to four weeks post-lesion, times when LGN axons would have normally invaded the cortical plate, the axons remained below the cortical plate and grew past their appropriate cortical target in an anomalous pathway. Moreover, examination of LGN axons at one week post-lesion, a time when they would normally be waiting and branching within the visual subplate, indicated that the axons had already grown past their correct destination. These observations suggest that visual subplate neurons are involved in the process by which LGN axons select and subsequently grow into visual cortex. In contrast, subplate neurons do not appear to play a major role in the initial morphological development of the LGN itself. Subplate ablations did not alter dendritic growth or shapes of LGN projection neurons during the period under study, nor did it prevent the segregation of retinal ganglion cell axons into eye-specific layers. However, the overall size of the LGN was reduced, suggesting that there may be increased cell death of LGN neurons in the absence of subplate neurons. To examine whether subplate neurons beneath other neocortical areas play a similar role in the formation of thalamocortical connections, subplate neurons were deleted beneath auditory cortex at the onset of the waiting period for auditory thalamic axons. Subsequent DiI labeling revealed that in these animals the majority of MGN axons had grown past auditory cortex instead of innervating it. Taken together these observations underscore a general requirement for subplate neurons throughout neocortex in the process of cortical target selection and ingrowth by thalamic axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Involvement of subplate neurons in the formation of ocular dominance columns.

During development of the mammalian visual system, axon terminals of lateral geniculate nucleus (LGN) neurons, initially intermixed within layer 4 of the visual cortex, gradually segregate according to eye preference to form ocular dominance columns. In addition to LGN axons and layer 4 neurons, subplate neurons may also participate in interactions leading to column formation. Deletion of subplate neurons before the formation of ocular dominance columns prevents the segregation of LGN axons within layer 4. Thus, interactions between LGN axons and layer 4 neurons are not sufficient; subplate neurons are also required for formation of ocular dominance columns in the visual cortex.

Animals↗
PubMed1992

How are specific connections formed between thalamus and cortex?

During development precise thalamocortical connections are established, with reciprocal connections forming correctly in a laminar pattern as well as between the correct thalamic and cortical areas. Recent evidence suggests that both spatial and temporal cues may account for this specificity.

Animals↗
PubMed1992

Synapses formed by identified retinogeniculate axons during the segregation of eye input.

The synaptic organization of identified retinogeniculate axons was studied during the prenatal development of eye-specific layers in the LGN of the cat. During this period, retinogeniculate axons undergo stereotyped morphological changes. Retinogeniculate axons originating from one eye and passing through LGN territory destined to be solely innervated by the other eye (inappropriate territory) initially give rise to many side branches. As the eye-specific layers emerge, these axons elaborate extensive terminal arbors within territory appropriate to their eye of origin and concurrently retract their side branches from inappropriate territory (Sretavan and Shatz, 1986). These transient side branches may therefore represent a morphological substrate for the observed functional convergence of inputs from the two eyes onto common LGN neurons during prenatal development (Shatz and Kirkwood, 1984). This possibility was investigated by examining whether identified axons and their side branches form synapses in inappropriate territory. Three retinogeniculate axons from two fetuses aged embryonic day 53 (E53) and E57 were filled with HRP in an in vitro preparation, prior to being processed for electron microscopy (EM). The HRP-filled axons, originating from the contralateral eye, were first reconstructed at the light microscope level. The portion of axon passing through the center of ipsilaterally innervated layer A1 was then serially sectioned and reconstructed by EM. Two sets of 450 serial EM sections revealed that all three contralateral axons established synaptic contacts in ipsilateral territory. Many of these synapses were made by side branches and a few were even formed by the main axon trunks. Both side branches and trunks formed mainly en passant asymmetrical contacts that were associated with spherical synaptic vesicles and that were apposed to immature dendritic elements and dendritic shafts. For comparison, a portion of the same E53 axon within the future contralateral layer A was also serially sectioned and reconstructed for EM. Within this contralateral zone, the E53 axon formed synaptic contacts similar to those established in the ipsilateral region, except that in the appropriate zone they contained significantly more synaptic vesicles. These results demonstrate that axons from the contralateral eye can establish synapses in territory simultaneously innervated by the ipsilateral eye, both via side branches and by means of contacts along the main axon trunk. Thus, the development of eye-specific layers is accompanied by the formation and subsequent elimination of synapses that almost certainly represent a morphological substrate for the known transient functional convergence of inputs from the two eyes.

Animals↗
PubMed1992

Pathfinding and target selection by developing geniculocortical axons.

During development of the mammalian cerebral cortex, thalamic axons must grow into the telencephalon and select appropriate cortical targets. In order to begin to understand the cellular interactions that are important in cortical target selection by thalamic axons, we have examined the morphology of axons from the lateral geniculate nucleus (LGN) as they navigate their way to the primary visual cortex. The morphology of geniculocortical axons was revealed by placing the lipophilic tracer Dil into the LGN of paraformaldehyde-fixed brains from fetal and neonatal cats between embryonic day 26 (E26; gestation is 65 d) and postnatal day 7 (P7). This morphological approach has led to three major observations. (1) As LGN axons grow within the intermediate zone of the telencephalon toward future visual cortex (E30-40), many give off distinct interstitial axon collaterals that penetrate the subplate of nonvisual cortical areas. These collaterals are transient and are not seen postnatally. (2) There is a prolonged period during which LGN axons are restricted to the visual subplate prior to their ingrowth into the cortical plate; the first LGN axons arrive within visual subplate by E36 but are not detected in layer 6 of visual cortex until about E50. (3) Within the visual subplate, LGN axons extend widespread terminal branches. This represents a marked change in their morphology from the simple growth cones present earlier as LGN axons navigate en route to visual cortex. The presence of interstitial collaterals suggests that there may be ongoing interactions between LGN axons and subplate neurons along the entire intracortical route traversed by the axons. From the extensive branching of LGN axons within the visual subplate during the waiting period, it appears that they are not simply "waiting." Rather, LGN axons may participate in dynamic cellular interactions within the subplate long before they contact their ultimate target neurons in layer 4. These observations confirm the existence of a prolonged waiting period in the development of thalamocortical connections and provide important morphological evidence in support of the previous suggestion that interactions between thalamic axons and subplate neurons are necessary for cortical target selection.

Aging↗
PubMed1991

Synchronous bursts of action potentials in ganglion cells of the developing mammalian retina.

The development of orderly connections in the mammalian visual system depends on action potentials in the optic nerve fibers, even before the retina receives visual input. In particular, it has been suggested that correlated firing of retinal ganglion cells in the same eye directs the segregation of their synaptic terminals into eye-specific layers within the lateral geniculate nucleus. Such correlations in electrical activity were found by simultaneous recording of the extracellular action potentials of up to 100 ganglion cells in the isolated retina of the newborn ferret and the fetal cat. These neurons fired spikes in nearly synchronous bursts lasting a few seconds and separated by 1 to 2 minutes of silence. Individual bursts consisted of a wave of excitation, several hundred micrometers wide, sweeping across the retina at about 100 micrometers per second. These concerted firing patterns have the appropriate spatial and temporal properties to guide the refinement of connections between the retina and the lateral geniculate nucleus.

Action Potentials↗
PubMed1991

Morphology of pioneer and follower growth cones in the developing cerebral cortex.

In the developing nervous systems of both invertebrates and vertebrates, neurons must develop precise sets of axonal connections. One strategy used by both orders of animals is to generate a special class of neurons whose axons "pioneer" the first pathways between these cells and their targets. In the developing mammalian telencephalon, the subplate neurons (which are among the first neurons to be generated in development) extend axons to long-distance subcortical targets before the neurons of the deep cortical layers 5 and 6 have been generated. The axons of layer 5 and 6 neurons later follow a similar pathway to form permanent subcortical projections to the thalamus and tectum, and thereafter the vast majority of subplate neurons die. These results have generated the hypothesis that subplate axons may actually be required for the axons of layer 5 and 6 neurons to innervate their appropriate subcortical targets. The complexity of growth cones has previously been correlated with axonal decision making: differences in growth cone morphologies have been noted in comparisons of leading versus following axons (LoPresti, Macagno, and Levinthal, 1973; Nordlander, 1987; Yaginuma, Homma, Kunzi, and Oppenheim, 1991), and at choice points along axon pathways (Raper, Bastiani, and Goodman, 1983; Tosney and Landmesser, 1985; Caudy and Bentley, 1986a,b; Bovolenta and Mason, 1987; Holt, 1989; Bovolenta and Dodd, 1990; Yaginuma et al., 1991). Thus, as a first step toward addressing the question of whether the axons of deep-layer neurons simply follow subplate axons to their targets, we have studied the morphology of cortical growth cones at various points along the corticothalamic pathway and at different stages of development. We examined the brains of fetal ferrets and cats at ages ranging from embryonic days (E) 24 to E50, using the fluorescent lipophilic tracer 1,1-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI) to reveal the axons and growth cones of cortical neurons. Growth cones were drawn, and quantitative measurements of their complexity were made by counting filopodia and calculating their surface area. No morphological differences were found among growth cones at different points along the corticothalamic pathway at a given age. However, growth cones belonging to early-generated cells (likely to be subplate neurons) are significantly larger and more complex than are the growth cones of later-generated cortical neurons. This evidence is consistent with the suggestion that subplate growth cones actively pioneer the corticothalamic pathway, and that the axons of layer 5 and 6 neurons follow it.

Animals↗