Properties of antidromically activated callosal neurons and neurons responsive to callosal input in rabbit binocular cortex

Extracellular spikes were recorded from the cell bodies of antidromically activated callosal axons in the binocular visual cortex of unanesthetized, unparalyzed rabbits. Callosal axons were stimulated near their terminals in the contralateral cortex. Recordings were also obtained from neurons which responded synaptically to contralateral cortical stimulation. The primary method for differentiating antidromic from synaptic activation was the test for collision of impulses. Additional tests provided further confirmation of antidromic activation. Units which sent an axon across the corpus callosum (callosal neurons) were thereby distinguished from units which responded synaptically to callosal input. Eighteen percent of units sampled sent an axon across the corpus callosum. The median conduction velocity of callosal axons was less than 2 m/sec. An additional 18% of units encountered were synaptically activated by contralateral cortical stimulation. Callosal neurons were found to differ from synaptically activated units in three distinct ways. Callosal neurons had very low spontaneous firing rates (median =< 1.0 spike/sec), responded with a single spike to contralateral cortical stimulation and never responded to diffuse flash illumination. In contrast, most synaptically activated units demonstrated high spontaneous firing rates (median = 10.2 spikes/sec), responded with a burst of spikes to contralateral cortical stimulation and were also driven by diffuse flash illumination.     

Systematic variations in the conduction velocity of slowly conducting axons in the rabbit corpus callosum

Extracellular spikes were recorded from the cell bodies of antidromically activated callosal axons in rabbit visual cortex. Callosal axons were stimulated near their terminals in the contralateral cortex. The primary method for differentiating antidromic from synaptic activation was the test for collision of impulses. Additional tests provided further confirmation of antidromic activation. A decrease in antidromic latency always occurred when an antidromic volley followed either a spontaneous spike or a preceding antidromically elicited spike at appropriate intervals. The time course and magnitude of the latency decrease coincided with that of a threshold decrease at the site of electrical stimulation. The antidromic latency decrease was primarily due to an increase in axon conduction velocity. These systematic variations in conduction velocity and stimulus threshold strongly suggest that an afterdepolarization follows the activation of callosal axons. While such afterpotentials are known to occur in unmyelinated C fibers, the present evidence suggests that they also occur in the smallest of myelinated axons.     

Serotonin-mediated inhibition from dorsal raphe nucleus of neurons in dorsal lateral geniculate and thalamic reticular nuclei

Electrophysiological studies using rats anesthetized with chloral hydrate were performed to determine whether or not serotonin originating in the dorsal raphe nucleus (DR) acts as an inhibitory transmitter or neuromodulator on neurons of the dorsal lateral geniculate nucleus (LGN) and neurons located in the thalamic reticular nucleus (TRN) immediately rostral to the dorsal LGN. In the LGN, conditioning stimuli applied to the DR preceding test stimulus to the optic tract and visual cortex inhibited orthodromic and antidromic spikes in about one-third of the relay neurons and in more than half of the intrageniculate interneurons. Conditioning stimulation of the DR also produced an inhibition of the spikes elicited by stimulation of the optic tract and visual cortex of at least three-quarters of the TRN neurons. Iontophoretic application of serotonin (25 nA) inhibited the orthodromic spikes of the LGN relay neuron and TRN neuron. A close correlation was observed between the effects of DR conditioning stimulation and iontophoretic serotonin in the same neurons. The inhibition with DR conditioning stimulation and iontophoretically applied serotonin was antagonized during iontophoretic application of methysergide (15-40 nA), a serotonin antagonist. These results strongly suggest that serotonin derived from the DR acts on the LGN and TRN neurons as an inhibitory transmitter or neuromodulator to inhibit transmission in these nuclei.     

Three types of reticular neurons involved in the spinobulbo-spinal reflex of cats

Reticular neuron activity was recorded in 28 chloralosed cats in order to analyze the reflex arc of the spino-bulbo-spinal (SBS) reflex. Three types of reticular neurons, types I (input), II (output) and III (relay), were identified by unit discharges in response to stimulation of the sural nerve.

(1) Type I (input) neurons received spinal ascending volleys monosynaptically and responded to stimulation of the sural nerve with spikes of low amplitude and short latency. Unit spikes, however, were not produced by stimulation of the superficial radial nerve and the sensorimotor cortex. These input neurons were located in the dorsocaudal part of the medial bulbar reticular formation.

(2) Type II (output) neurons were part of the reticulospinal tract, which sends axons to the spinal cord, since these neurons exhibited antidromic spikes following stimulation of the ventrolateral funiculus of the spinal cord. Unit spikes were evoked by stimulation either to the sural or superficial radial nerves. These neurons were located in the ventrocaudal part of the medial bulbar reticular formation.

(3) Type III neurons included relay neurons. Unit spikes were evoked by stimulation of the sural nerve, superficial radial nerve and sensorimotor cortex. However, unit discharges were not obtained by antidromic stimulation to the reticulospinal tract. These neurons were distributed widely in the brain stem, both in the bulb and pons.

(4) Latency difference of unit discharges between input and output neurons was 3.5–5 msec, indicating the presence of interneurons (relays) between input and output neurons. Spikes of output neurons with 3.8–4.2 msec latency were observed following stimulation of the region where input neuron activity was found. We may conclude that three kinds of reticular neurons, input, relay and output, were involved in pathways of the SBS reflex.

Reactions of neurons of the reticular and ventral anterior thalamic nuclei to electrical stimulation of the centrum medianum of the thalamus

Responses of 145 reticular (R) and 158 ventral anterior (VA) thalamic neurons to electrical stimulation of centrum medianum (CM) were studied in cats anaesthetized with thiopental sodium (30-40 mg/kg intraperitoneally) and immobilized with d-tubocurarine (1 mg/kg). 4.1% of R and 4.4% of VA neurons under study responded to CM stimulation by antidromic spike (latency 0.3-2.0 ms). The conduction velocity of antidromic excitation in axons of those neurons was found to be 1.7-7.6 m/s. There were neurons which responded by antidromic spike to the other thalamic nuclei stimulation as well as to CM. This fact is the electrophysiological proof of the axonal branching in these neurons. 53.8% of R and 46.9% of VA neurons responded to CM stimulation with orthodromic excitation. Two groups of cells were separated among neurons excited orthodromically. The first group neurons responded to CM stimulation by discharges composed of 6-12 spikes with frequency of 130-640 per second. The neurons of the second group generated a single spike. Inhibitory reactions were noticed only in 0.7% of R and in 4.4% of VA neurons. It is shown that afferent impulses from relay nuclei, lateral posterior nucleus and motor cortex converged to some R and VA neurons responding to CM.     

Postnatal development of parvalbumin immunoreactivity in the cerebral cortex of the cat

Parvalbumin immunoreactivity in the developing neocortex of the cat progresses following specific laminar, areal, and, in a particular area, roughly anteroposterior gradients. Parvalbumin immunoreactivity first occurs in basket cells and later in chandelier neurons. Pyramid-like immunoreactive neurons are also transitorily observed from the second to the third week in layer V of the auditory association-related areas. Parvalbumin-immunoreactive neurons first appear in the primary somatosensory cortex and primary auditory and visual areas, followed by the primary motor and polysensory association areas and, finally, the auditory association areas and cortical areas related to the limbic system. In addition to cortical neurons, three fiber systems are immunolabeled with antiparvalbumin antibodies: thalamocortical, callosal, and ipsilateral corticocortical. Parvalbumin-immunoreactive thalamocortical fibers appear during the first month of postnatal life. Parvalbumin-immunoreactive callosal and ipsilateral corticocortical fibers are seen from the fourth postnatal week onward. Because all parvalbumin-immunoreactive cortical neurons in adulthood are nonpyramidal inhibitory cells, the present findings suggest that a number of ipsilateral corticocortical and callosal connections may be inhibitory. © 1994 Wiley-Liss, Inc.     

Reactions of medial geniculate body neurons to stimulation of the auditory cortex]

Extra- and intracellular responses of pars principalis neurons in the medial geniculate body to stimulation of the first (AI), second (AII) and third (AIII) auditory cortex were studied in experiments on cats immobilized with d-tubocurarine. In geniculate neurons both antidromic (45-50%) and orthodromic (50-55%) reactions occurred in response to the auditory cortex stimulation. The latencies for antidromic and orthodromic responses were 0.3-2.5 ms and 2.0-ms, respectively. Late responses appeared with a latency of 30-200 ms. 63% of neurons responded antidromically to both AII and AI stimulation, that confirms the suggestion on the projection of a considerable number of the geniculate neurons to both auditory zones. Orthodromic responses of geniculate neurons consisted either of 1-2 spikes or a burst of 8-12 spikes with a frequency of 300-600/sec. The bursts are supposed to be the responses of inhibitory geniculate neurons. Intracellular recording showed the following responses: antidromic spikes, EPSP, EPSP-spike, EPSP-spike-IPSP, EPSP-IPSP and initial IPSP. Above 50% of initial IPSPs had the latency of 2.0-4.0 ms. They are supposed to be produced with the participation of intermediate inhibitory neurons located in the medial geniculate body.     

Variations in conduction velocity and excitability following single and multiple impulses of visual callosal axons in the rabbit

The present study examined the conduction properties of 75 visual callosal axons of the awake rabbit. These axons were studied by measuring latency to antidromic activation of cell bodies following midline callosal and/or contralateral cortical stimulation. Seventy-three of 75 neurons (axon conduction velocities = 0.3 to 12.9 m/sec) demonstrated decreases in antidromic latency and threshold to a test stimulus which followed an antecedent conditioning stimulus at appropriate intervals. Control experiments indicated that (i) the latency and threshold variations resulted from prior impulse conduction along the axon, and (ii) the latency decrease reflected an increase in conduction velocity along the main axon trunk. The maximum magnitude of the latency decrease for different axons ranged from 3 to 22% of control values, and the duration ranged from 18 to 169 msec. The duration of the latency decrease was greater for slowly conducting axons than for fast conducting axons. Latency increases to an antidromic test stimulus occurred for up to several minutes following a train of antidromic conditioning pulses. Antidromic latency and threshold shifts were also observed in somatosensory callosal axons and in some corticotectal axons.     

Axosomatic input to subpopulations of cortically projecting pyramidal neurons in primate prefrontal cortex

Pyramidal cells, the major class of cortical excitatory neurons, can be divided into different subpopulations based upon the target region of their principal axon projection. The activity of pyramidal neurons is regulated in part through inhibitory synaptic inputs to the soma from local circuit neurons. However, little is known about how the density of these axosomatic inputs differs among subpopulations of pyramidal neurons in the prefrontal cortex of primates. In this study, retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was used to identify pyramidal neurons in monkey prefrontal cortex (areas 9 and 46), which were labeled via either associational (ipsilateral hemisphere) or callosal (contralateral hemisphere) principal axon projections. Ultrastructural analysis revealed that the relative number of terminals apposed to the somatic membrane did not differ between associational and callosal neurons. However, neurons in the supragranular layers were apposed by a significantly greater number of axon terminals than were neurons in the infragranular layers. These findings suggest that the laminar environment of a neuron may play a more important role than principal axon projection in determining the amount of axosomatic inhibitory input it receives. Synapse 25:326–334, 1997. © 1997 Wiley-Liss, Inc.     

Antidromic identification of dopaminergic and other output neurons of the rat substantia nigra

In the present study dopamine (DA)-containing and other output neurons of the substantia nigra (SN) wer identified by antidromic stimulation from postulated target nuclei, the caudate-putamen, the thalamus, the cortex and the pontine reticular formation. To guide electrode placements, the topography of the nigrostriatal projection system was determined by retrograde tracing methods. Spontaneously active cells present in the SN were then classified in two groups according to the shape of their action potentials and their firing rate. Type I cells were located mainly in the pars compacta and could be antidromically-activated (AD-activated) from various locations along the course of the nigrostriatal pathway (caudate-putamen, globus pallidus, MFB) but not from other brain areas (ventromedial thalamus, motor cortex, pontine reticular formation). These neurons had a slow bursting pattern of firing, a very slow conduction velocity (0.58 m/sec), and a wide action potential. Their firing rate was dramatically reduced following the intravenous administration of apomorphine (ID 50: 9.3 microgram/kg), or the iontophoretic application of DA and GABA. Type II cells were located predominantly in the pars reticulata; most of them could be AD-activated from the ventromedial thalamus and the MFB but not from the motor cortex. A few of these cells could be AD-activated from the pontine reticular formation and the thalamus. A minority of type II cells, located in or near the pars compacta could be AD-activated from the caudate-putamen. In addition, their conduction velocuty was much higher (2.8 m/sec) and their firing rate far in excess of that exhibited by type I neurons. These neurons were inhibited by the iontophoretic application of GABA but not of DA. The microinjection of 6-hydroxydopamine (a neurotoxin relatively specific against catecholamine-containing neurons) in the vicinity of the MFB blocked selectively the propagation of antidromic spikes in type I but not type II cells. It is concluded that type I cells are the DA neurons of the nigrostriatal pathway. Type II cells are mainly oupput neurons that project to the ventromedial thalamus, the pons and the forebrain. This telencephalic projection most likely constitutes a second, non-DA, fast-conducting nigrostriatal pathway.     

Cat spinoreticular neurons: Locations, responses and changes in responses during repetitive stimulation

Neurons in the lumbosacral spinal cord which project to the medial pontomedulary reticular formation were studied in the chloralose-anesthetized cat. Such neurons, identified by antidromic activation, were found predominantly in lamina VIII and medial lamina VII, and most were found to project to the contralateral reticular formation. Receptive fields for natural stimuli were generally complex, having various combinations of excitatory and inhibitory areas ipsi- and/or contralaterally. Adequate stimuli ranged from innocuous to noxious, with the stimuli required for decreasing a neuron's activity usually more intense than the stimulus required for increasing it. Electrical stimulation of hindlimb nerves indicated the presence of extensive convergence. Responses of spinoreticular neurons were found to decline during periods of repetitive stimulation. The response decrements were found to have many of the parametric features of behavioral habituation and were similar to response decrements previously observed in the medial pontomedullary reticular formation.     

Multiple single units and population responses during inhibitory gating of hippocampal auditory response in freely-moving rats

Paired clicks were presented to awake, freely-moving rats to examine neuronal activity associated with inhibitory gating of responses to repeated auditory stimuli. The rats had bundles of eight microwires implanted into each of four different brain areas: CA3 region of the hippocampus, medial septal nucleus, brainstem reticular nucleus, and the auditory cortex. Single-unit recordings from each wire were made while the local auditory-evoked potential was also recorded. The response to a conditioning stimulus was compared to the response to a test stimulus delivered 500 ms later: the ratio of the test response to the conditioning response provided a measure of inhibitory gating. Auditory-evoked potentials were recorded at all sites. Overall, brainstem reticular nucleus neurons showed the greatest gating of local auditory-evoked potentials, while the auditory cortex showed the least. However, except for the auditory cortex, both gating and non-gating of the evoked response were recorded at various times in all brain regions. Gating of the hippocampal response was significantly correlated with gating in the medial septal nucleus and brainstem reticular nucleus, but not the auditory cortex. Single-unit neuron firing in response to the clicks was most pronounced in the brainstem reticular nucleus and the medial septal nucleus, while relatively few neurons responded in the CA3 region of the hippocampus and the auditory cortex. Taken together, these data support the hypothesis that inhibitory gating of the auditory-evoked response originates in the non-lemniscal pathway and not in cortical areas of the rat brain.     

The distribution of the cells of origin of callosal projections in cat visual cortex

The distribution of neurons projecting through the corpus callosum (callosal neurons) was examined in retinotopically defined areas of cat visual cortex. As many callosal neurons as possible were labeled in a single animal by surgically dividing the posterior two-thirds of the corpus callosum and exposing the cut ends of callosal axons to horseradish peroxidase. The distribution of callosal neurons within a visual field representation was related to standard electrophysiological maps as well as to recording sites marked by electrolytic lesions. Callosal neurons were found in every retinotopically defined cortical area. The portion of the visual field representation that contained callosal neurons increased progressively from the area 17/18 border to area 19, to areas 20 and 21, and to the lateral suprasylvian visual areas. In area 17, the portion of the visual field representation containing callosal neurons extended from the vertical meridian out to a maximum of 10 degrees azimuth. In the posteromedial lateral suprasylvian visual area, callosal neurons were present in a region extending from the vertical meridian representation out to a representation of 60 degrees azimuth. Most callosal neurons were medium to large pyramids at the border of layers III and IV. A few layer IV stellates were among the callosal neurons of areas 17 and 18. In area 19 and even more so in the lateral suprasylvian visual areas, callosal neurons included pyramidal and fusiform-shaped cells in layers V and VI. The laminar distributions of callosal neurons in areas 20 and 21 were similar to those of area 19 and the lateral suprasylvian visual areas. The widespread distribution of callosal neurons in areas 20 and 21 and in the lateral suprasylvian visual areas suggests that the regions of peripheral visual field representation in cat cortex, as well as the representations of the vertical meridian, have access to the opposite cerebral hemisphere. This finding is significant in light of demonstrations of the importance of some of these cortical areas in the interhemispheric transfer of visual learning.     

The morphological characteristics of the callosal neurons of the first auditory area of the cortex (AI) in the cat]

Cortical stratification of callosal neurons in the primary auditory cortex (AI) of cat was studied by means of horseradish peroxidase (HRP). Two main groups of callosal neurons were revealed. The first group comprising 60% of all AI callosal neurons consisted predominantly of layer III large pyramidal neurons. Average area of these pyramidal neuron perikaryon profiles was 261.8 +/- 8.8 microns2. The number of HRP-labelled callosal neurons in layer III was 22% of all cells in this layer. The second group comprising 27% of all AI callosal neurons consisted mainly of large cells of layers V and VI which could not be classified as pyramidal neurons. Average area of these nonpyramidal neuron perikaryon profiles was 250.3 +/- 8.4 microns 2. In layer I callosal neurons were not revealed, in layers II and IV accordingly 6% and 7% of AI callosal neurons were located.     

Some properties of spinal neurons projecting to the medial brain-stem reticular formation

Experiments were done to locate and characterize neurons projecting from the spinal cord to the brain-stem reticular formation. In barbiturate-anesthetized or midcollicular-decerebrate cats, concentric bipolar electrodes were placed in the medial brain-stem reticular formation of the caudal pons and rostral medulla. Neurons were identified as projecting to brain-stem reticular formation using the antidromic collision criterion. Of 30 cells, 20 projected to the ipsilateral and ten to the contralateral brain-stem reticular formation. The mean conduction velocity was 49.6 m/sec. Cell somata were located deep to lamina V of the dorsal horn. Neurons in this sample fell into two categories based on their receptive field. One type of cell was excited by firm pressure over such deep structures as muscles, ligaments and periosteum. The second type had a large cutaneous receptive field. Neurons of this second type responded maximally to noxious cutaneous stimuli. Inhibitory receptive field components were common in neurons with both types of receptive field. In addition to antidromic activation, stimulation of medial brain-stem reticular formation produced both synaptic excitation and marked inhibition of some cells projecting to the brain-stem reticular formation. It is concluded that spinoreticular neurons may relay information concerning noxious stimuli.     

Distribution and properties of commissural and other neurons in cat sensorimotor cortex.

Commissural, cortico-cortical and cortico-caudate neurons have been investigated in the primary sensorimotor cortex of the cat, using antidromic stimulation techniques, and histological identification of recording sites. These neurons are to be found in all cortical laminae except the first; commissural and cortico-cortical neurons were found to be commonest in laminae III and VI, whilst cortico-caudate neurons were most abundant on the border between laminae III and V, in motor areas. In sensory areas topographically identified as representing distal parts of limbs, commissural neurons are very rare, confirming neuroanatomical studies on the origin and termination of callosal fibres. The intracerebral neuronal projections investigated in this study had slow conduction velocities (less than 1 m/sec, up to about 10 m/sec). It was found that projections from area 6, whether commissural, cortico-caudate, or cortico-peduncular have slower conduction velocities than their counterparts from area 4. It is suggested that this is related to the type of motor control in which these two areas are involved (slowly-responding postural movements, as opposed to more rapid distal limb movements). No neurons were found which had both commissural (or cortico-cortical), and cortico-fugal projections.     

Callosal terminals in the rat prefrontal cortex: Synaptic targets and association with GABA-immunoreactive structures

The callosal projections of the cerebral cortex play an important role in the functional integration of the two hemispheres, and the anatomy of these connections has been extensively studied in primary sensory and motor regions. In the present investigation, we examined the synaptic targets of callosal terminals in a limbic association area, the prefrontal cortex (PFC) in the rat. In addition, we examined the relationship of callosal afferents to GABA local circuit neurons within the PFC. Callosal terminals were labeled by either anterograde transport of Phaseolus vulgaris leucoagglutinin from superficial or deep layers or by anterograde degeneration following electrolytic lesion of the contralateral PFC. Callosal terminals in either the superficial or deep layers labeled by either method formed primarily asymmetric axo-spinous synapses (approximately 95%), while the remainder formed axo-dendritic synapses. Some of the dendrites postsynaptic to callosal terminals exhibited a morphology characteristic of local circuit neurons. This observation was confirmed in tissue immunolabeled for GABA, in which degenerating callosal terminals sometimes formed asymmetric synapses on GABA-labeled dendrites. In addition, GABA-labeled terminals and callosal afferents were sometimes observed to converge onto common postsynaptic dendritic shafts or spines within the PFC. These results indicate that callosal terminals in limbic association cortex, consistent with sensory and motor cortices, primarily target the spines of pyramidal neurons. In addition, the results suggest that callosal afferents to the PFC interact with GABA local circuit neurons at multiple levels. Specifically, a proportion of callosal terminals appear to provide excitatory drive to GABA cells, while GABA terminals may modulate the excitation from callosal inputs to the distal dendrites and spines of PFC pyramidal neurons. Synapse 29:193–205, 1998. © 1998 Wiley-Liss, Inc.     

Prenatal specification of callosal connections in rhesus monkey

Anatomical tracing and quantitative techniques were used to examine the tempo and pattern of maturation for callosal projection neurons in the monkey prefrontal cortex (PFC) during fetal and postnatal development. Nineteen monkeys were injected with retrograde tracers (fluorescent dyes, horseradish peroxidase conjugated to wheat germ agglutinin [WGA-HRP] or HRP crystals) at various ages between embryonic day 82 (E82) and adulthood. The size of injection sites was varied in fetal, newborn, and adult cases. In adults, labeled neurons were found in greatest density in the homotopic cortex of the opposite hemisphere and considerable numbers were also observed in a constellation of heterotopic areas including the medial and lateral orbital cortex, the dorsomedial convexity, and the pregenual cortex. The majority of labeled neurons were consistently concentrated in the lower half of layer III in all areas. In cases with large injection sites, callosal neurons of layer III formed a continuous and uninterrupted band that extended over the entire lateral surface of the prefrontal cortex spanning both homotopic and heterotopic areas. In contrast, in cases with small injection sites, the labeling of layer III neurons exhibited discontinuities. Between embryonic ages E82 and E89, injections limited to the cortical layers labeled only a small number of neurons in the opposite hemisphere, indicating that few callosal axons have invaded the cortex by this age. However, by E111 comparable injections labeled a large number of callosal neurons and many features of their distribution were adult-like. The number and constellation of cytoarchitectonic areas that were labeled in the frontal cortex of the opposite hemisphere were the same as in adults and the majority of callosal neurons were found in supragranular layer III. Finally, in fetal animals beyond E111, labeled neurons extended as a nearly unbroken band over a wide expanse of the dorsolateral PFC, resembling the pattern seen in adult monkeys with large injections. The conclusion we draw from these results, together with our earlier findings (Schwartz and Goldman-Rakic: Nature 299:154, 1982), is that callosal neurons whose axons enter the cortical layers of the primate prefrontal cortex achieve their mature laminar and areal distribution prior to birth and do so largely by cumulative processes.     

Guidance of callosal axons by radial glia in the developing cerebral cortex

During development, columns of the mammalian cerebral cortex are formed by migration of neurons along fascicles of radial glia. Subsequently, axons of the corpus callosum connect reciprocal regions of each cerebral hemisphere. To determine whether the radial growth of callosal afferents through the developing cortex may be guided by particular cellular elements, we examined the ultrastructural relationship between callosal afferents and radial fibers in the early postnatal hamster sensorimotor cortex. Developing callosal axons and their growth cones were labeled with HRP injected into the cortex at 3 d postnatal when the growth cones have extended across the callosum and are just entering the contralateral cortex. An EM analysis of 30 HRP-labeled axons and their growth cones revealed that they extended upon fascicles of radial processes associated with migrating neurons. Reconstruction of seven of these growth cones, serially sectioned in their entirety, showed that growth cones were associated with the same radial fascicle as their axon. Growth cones also touched other cellular elements such as axons. However, the finding that callosal afferents, from the point at which they enter the cortex to their growth cones, were apposed to a continuous fascicle of radial fibers suggests that callosal axons are tracking along radial processes. We conclude that the majority of the radial processes within fascicles are likely to be glial, based on their relatively large diameters, electron-lucent cytoplasm with a regular array of microtubules, the presence of glycogen granules, occasional cytoplasmic protrusions lacking microtubules, and their consistent association with migrating neurons. We propose therefore that radial glia may serve a guidance function for growing callosal axons in their radial trajectory through the developing cerebral cortex.