Separate progenitors for radial and tangential cell dispersion during development of the cerebral neocortex.

Cell lineage analyses suggest that cortical neuroblasts are capable of undertaking both radial and tangential modes of cell movement. However, it is unclear whether distinct progenitors are committed to generating neuroblasts that disperse exclusively in either radial or tangential directions. Using highly unbalanced mouse stem cell chimeras, we have identified certain progenitors that are committed to one mode of cell dispersion only. Radially dispersed neurons expressed glutamate, the neurochemical signature of excitatory pyramidal cells. In contrast, tangential progenitors gave rise to widely scattered neurons that are predominantly GABAergic. These results suggest lineage-based mechanisms for early specification of certain progenitors to distinct dispersion pathways and neuronal phenotypes.

The Course of Fibre Diameter Classes Through the Chiasmatic Region in the Ferret.

The present study has examined the transition of fibre order from the optic nerve through the optic chiasm and into the optic tract in the ferret's retinofugal pathway. Semi-thin sections through the chiasmatic region were examined in normal and in monocularly enucleated ferrets in order to display the segregation of the different axon diameter classes as the fibres pass from the optic nerve into the optic tract, and to determine, for each diameter class, where the crossed and uncrossed components become separated in the chiasmatic region. As demonstrated in the preceding manuscript, fine and coarse optic axons begin to segregate from the medium optic axons rostral to the fusion of the two optic nerves. This segregation continues in the chiasmatic region where the different axon diameter classes decussate at different rostro-caudal levels: the fine and coarse diameter axons decussate rostrally, accumulating along the ventral, superficial surface of the contralateral half-chiasm, while the medium diameter axons continue caudally before crossing the midline. These two populations, a ventral, crossed (fine and coarse) population and a dorsal, yet-to-cross (medium) population are discriminable not only by their size in the chiasmatic region, but also by a thin invagination of hypothalamic neuropil separating them at their lateral extreme. The population of ipsilaterally projecting fibres is composed of both fine and medium optic axons. No coarse optic axons project ipsilaterally in the ferret: these fibres all decussate rostrally in the optic chiasm, intermingled with many of the decussating fine fibres. The fibre ordering in the adult ferret's optic chiasm is interpreted as reflecting a gradient of axonal arrival during development, with successively later arriving optic axons entering one of the two optic tracts at a progressively more superficial, rostral and ventral, location in the chiasmatic region. A fibre's position and time of arrival may influence its subsequent crossed or uncrossed course during the period of axonal ingrowth in development.

Cell Survival in the Uncrossed Projection of the Mammalian Retina is Independent of Birthdate.

Naturally occurring cell loss in the retinal ganglion cell population of one eye can be interrupted by removal of the other eye in newborn rodents. Many of the rescued retinal ganglion cells which project ipsilaterally reside in the nasal retina, that part of the retina normally giving rise to primarily crossed optic axons. Their naturally occurring elimination has been attributed to their hypothesized late neurogenesis and the consequent delayed time of arrival of their axons in the target visual nuclei, thereby placing them at a competitive disadvantage with other, early arriving, optic axons. By combining the technique of tritiated thymidine autoradiography with the retrograde axonal transport of horseradish peroxidase in rats that had been enucleated on the day of birth, we report here that rescued cells in the nasal retina which project ipsilaterally are generated at the same time as their neighbours in the temporal retina. Time of genesis does not distinguish them; consequently, their axons should not differ in their arrival times within the target visual nuclei. Since their only obvious anomaly is one of pathway choice at the optic chiasm, their place of arrival, rather than their time, may ultimately determine their naturally occurring elimination.

The topography of expanded uncrossed retinal projections following neonatal enucleation of one eye: differing effects in dorsal lateral geniculate nucleus and superior colliculus.

The topographic organization of the uncrossed retinal projections to the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) was studied in normal adult hooded rats and in rats subjected to unilateral ocular enucleation on the day of birth. Sections were stained for anterograde degeneration products following discrete retinal lesions at various locations. The projection from the temporal crescent to the dLGN in neonatally enucleated rats had an expanded but topographically normal organization, with the nasotemporal and dorsoventral retinal axes displaying polarities identical to those in normal adults. Neonatal enucleation permits the remaining uncrossed retinogeniculate projection to extend primarily along the "lines of projection" into neuropil normally recipient of binocularly conjugate crossed projections. In the SC, the dorsoventral axis of the temporal crescent showed a normal polarity, but the nasotemporal axis failed to display any topographic organization. Retinal loci in the temporal crescent projected throughout the rostrocaudal extent of the ipsilateral SC. Retinal lesions placed outside the temporal crescent failed to produce any substantial degeneration in ipsilateral dLGN or SC. These topographically distinct effects in dLGN and SC following unilateral eye removal on the day of birth are discussed in the context of differing constraints upon axonal ingrowth and connectivity during early development, which may normally bring about the characteristically distinct features of retinogeniculate and retinocollicular organization.

Fibre organization of the monkey's optic tract: II. Noncongruent representation of the two half-retinae.

The representations of the two half-retinae were examined in the monkey's optic tract. Intravitreal injections of tritiated amino acids were made to reveal the distributions of the crossed and uncrossed populations of optic axons, while localized implants of horseradish peroxidase (HRP) were made into different regions of the optic tract in order to examine the distributions and morphological types of retrogradely labelled cells at corresponding loci in the two half-retinae. Crossed and uncrossed optic axons are intermingled throughout most of the optic tract, but uncrossed axons are very sparse or absent along both the deep and superficial extremes of the tract. Implants of HRP into the deeper regions of the tract demonstrate that the crossed and uncrossed optic axons of the P beta retinal ganglion cells are slightly out of binocular registration, with the uncrossed map being shifted to a slightly superficial location relative to the crossed map. The optic axons for the remaining cell classes, revealed by implants of HRP into the superficial portion of the tract, are much more conspicuously out of binocular registration (in particular, the P alpha optic axons); but in their cases, the uncrossed optic axons are shifted to deeper locations relative to the crossed optic axons. Further evidence that these optic axon classes are markedly out of binocular registration comes from the two optic tracts of a bilaterally destriated monkey, in which most of the P beta optic axons have undergone a transneuronal retrograde degeneration. Following a uni-ocular injection of tritiated amino acids, the distributions of the remaining crossed and uncrossed axonal labelling occupied different positions within the tract rather than being intermingled, with the uncrossed optic axons situated deep to the majority of crossed optic axons. These results demonstrate that the optic chiasm does not combine binocularly corresponding optic axons of similar type. They also demonstrate that noncongruent field defects should be a common consequence of damage to the optic tract in humans. If the fibre order in the mammalian optic tract arises as a consequence of the sequence of axonal addition during development, then differences in the relative times of genesis for nasal and temporal members of any cell class, and/or differences in the relative pathlengths between the eye and two optic tracts, may produce the fibre ordering described herein.

Fibre organization of the monkey's optic tract: I. Segregation of functionally distinct optic axons.

The fibre organization of the monkey's optic tract was examined by implanting pellets of horseradish peroxidase into different locations within the tract, or into the superior colliculus and pretectum. Retinae were examined for the distribution, size, and morphological types of retrogradely labelled ganglion cells; optic tracts were examined for the distribution of anterogradely and retrogradely labelled axonal profiles; and lateral geniculate nuclei were examined for the distribution of anterogradely labelled processes within distinct geniculate laminae. Localized implants in the optic tract produced retrograde labelling of ganglion cells across wide regions of the retinal surface. The maximum density of labelled cells was always substantially less than the total ganglion cell density known to be present at those retinal loci. Distinct retinal ganglion cell types were labelled from differing regions within the optic tract: implants into the deep (dorsal) portion of the tract, far removed from the outer, pial, surface, retrogradely labelled predominantly P beta retinal ganglion cells, whereas implants into the superficial (ventral), subpial, part of the tract retrogradely labelled primarily the other retinal ganglion cell types, i.e., the P alpha, P gamma, and P epsilon cells. Within any given class of axon, there is a mapping of the centroperipheral retinal axis across the deep-to-superficial dimension of the tract, but this retinotopy is extremely coarse. Anterograde labelling of axonal terminations within the lateral geniculate nucleus showed a corresponding specificity for distinct geniculate laminae, the deep implants labelling the parvocellular laminae, superficial implants labelling the magnocellular laminae. Implants into the visual centres of the midbrain produced retrograde axonal labelling rostral to the lateral geniculate nucleus only in the superficial part of the optic tract. These results demonstrate that the monkey's optic tract is not a simple topographic mapping of retinal eccentricity. Rather, the primary organizational principle is that of a segregation of functionally distinct optic axon classes. As fibre order in the mammalian optic tract is also a chronological index of axonal arrival during development, the present results provide specific predictions about the temporal order of ganglion call genesis and axonal addition within the visual pathway. They also provide an anatomical basis for the functionally selective visual impairments that may arise following local damage to the optic tract in humans.

Distribution of axons according to diameter in the monkey's optic tract.

The distribution of axonal diameters in the optic tract of Old World monkeys was examined by light and electron microscopy. Axon diameters were measured in samples of 100 axons taken from several locations in a cross section of the tract about 5 mm behind the optic chiasm. Fine-caliber axons (less than 1.75 micron in diameter) were found in all parts of the tract. Dorsally no coarse axons were present. Further ventrally, coarse axons gradually appeared and increased steadily in proportion. The largest optic axons (greater than 2.5 micron) were found in the most ventral parts of the tract, near the pial surface. This pattern of segregation of axons of differing diameters in the optic tract is a rearrangement of the distribution of axon diameters seen in the nerve rather than a continuation of the same pattern. Examination of axon diameters in the optic nerve has shown that there is a preponderance of fine axons centrally, while coarser axons are found in the periphery, near the pial surface; however, histograms from central parts of the nerve contain a greater proportion of coarse axons than the dorsal parts of the optic tract, while histograms from the periphery of the optic nerve contain a conspicuously greater proportion of fine axons than do histograms from the most ventral parts of the tract. This relatively greater segregation of axons according to diameter in the optic tract demonstrates that the distribution of axons in the tract cannot be formed by the simple combination of two hemiretinal maps contained in each optic nerve, as suggested in classic descriptions.(ABSTRACT TRUNCATED AT 250 WORDS)

Position of growth cones within the retinal nerve fibre layer of fetal ferrets.

Optic axons are added to the retinal nerve fibre layer of fish along its vitreal border in a chronotopic manner. Likewise, the optic tract of all vertebrate species acquires axons preferentially along the superficial surface of the pathway. We have examined the developing retina of fetal ferrets (Mustela putorius furo) aged between embryonic day 27 (E27) and E34 to see whether a similar segregation of growth cones is apparent within the mammalian retinal nerve fibre layer. The distributions of growth cone, "wrist" (thick trailing portion of the growth cone), axonal, and glial profiles were determined from electron micrographs, and expressed as a percentage of neural profiles for several retinal locations. The retinal nerve fibre layer of fetal ferrets contains radially elongated bundles of fibres composed of axonal, wrist, and growth cone profiles. Glial processes of varying density divide the adjacent bundles, occasionally subdividing them in the plane of the retina, and give rise to endfeet lining the basal lamina and separating the optic axons from the latter. Growth cones within the developing fibre layer represented about 2.4% of profiles at E28, while at later developmental stages (E34), this value fell to about 0.6%. During this period of axonal outgrowth, growth cones were not preferentially segregated toward the vitreal basal lamina or the glial endfeet within the nerve fibre layer. Rather, they were found scattered throughout the axon bundles of the fibre layer. While there were differences in the proportion of immature profiles found within the vitreal half compared to the scleral half of the fibre layer, such that more growth cones and wrists were found vitreally, there was no clear accumulation of them in association with features of the vitreal margin. The present results show that young and old optic axons course together throughout the depth of the nerve fibre layer. A chronotopic mode of pathway genesis such as seen in the optic fibre layer of fish or in the optic tract of mammals is not present in the nerve fibre layer of ferrets. Differences in growth cone behaviour in the optic fibre layer and tract indicate that the mechanisms governing pathway formation differ along its course.

Precocious invasion of the optic stalk by transient retinopetal axons.

This study demonstrates that the fetal optic nerve contains a conspicuous population of transient retinopetal axons. Implants of the carbocyanine dye, DiI, were made into the retina or diencephalon of fetal ferrets to label the retinopetal axons retrogradely or anterogradely, respectively, and sections were immunostained for beta-tubulin to label the early differentiating axons in the optic nerve. Dye implants into the optic nerve head, but not the retinal periphery, retrogradely labeled somata in the ventrolateral diencephalon, provided the implants were made before embryonic day (E) 30. When dye implants were made into the ventrolateral diencephalon, these same retinopetal axons were anterogradely labeled, coursing through the optic nerve but never invading the retina. The axons course as 2-5 fascicles from their cells of origin and turn laterally to enter the optic nerve where it joins the future hypothalamus. The retinopetal cells can be retrogradely labeled as early as E20, before optic axons have left the retina. The optic nerve and fiber layer are immunoreactive for beta-tubulin on E24 and thereafter, whereas on E20 and E22, they are immunonegative. Yet at these early embryonic ages, immunopositive fascicles of axons course from the diencephalon into the optic stalk, confirming the precocious nature of the retinopetal projection. Implants of dye made into the future optic nerve head at these very early stages also retrogradely label retinopetal cells in the future chiasmatic region. These cells are distributed primarily on the side ipsilateral to the midline, but a few can be found contralateral to it. Both these, as well as the retinopetal axons arising from the ventrolateral diencephalon, may serve a transient guidance function for later developing optic axons.

Chiasmatic course of temporal retinal axons in the developing ferret.

Recent studies on the distribution of optic axons in the mature visual pathways, as well as on the genesis of their ganglion cells of origin, suggest that the time of axonal arrival at the optic chiasm determines the side of the brain to which a temporal retinal axon will project. The present study has examined this issue directly in fetal ferrets, by determining the projection of the temporal retina at different developmental stages. Fetuses of known gestational age were fixed with paraformaldehyde and subsequently implanted with crystals of the carbocyanine dye, DiI, into either the temporal retina, or into one optic tract. The lipophilic diffusion of the dye within the plasma membrane of the axons revealed the course of temporal retinal fibers through the fetal chiasm, as well as the distribution of ganglion cells across the two retinae projecting to one optic tract. During early fetal stages, the temporal retina extends axons preferentially into the ipsilateral optic tract: the early retinal projection shows a classical partial decussation pattern. During later fetal stages, temporal retinal axons can be traced into both optic tracts, and the distribution of cells with crossed and uncrossed optic axons in the temporal retina is overlapping. These results indicate that the mature decussation patterns of retinal ganglion cell classes are not primarily the consequence of regressive phenomena such as cell death; rather, they are formed as axons navigate the chiasmatic region during development. The differences in decussation pattern between cell classes arise from the fact that the mechanisms producing the segregation of nasal and temporal retinal axons at the chiasm must change as development proceeds.

Time of ganglion cell genesis in relation to the chiasmatic pathway choice of retinofugal axons.

The time of generation of retinal ganglion cells in fetal cats has been related to the course taken later by their axons in the optic chiasm. The ganglion cells were labelled with tritiated thymidine either on embryonic day (E) 26 or on E-30. When the cats were mature, ganglion cells were retrogradely labelled with horseradish peroxidase injected into one optic tract. The distribution of double-labelled cells showed that cells in the temporal retina generated on E-26 all have axons that take an uncrossed course in the chiasm, whereas, of the cells generated on E-30 in the temporal retina, some take a crossed course and others take an uncrossed course. The uncrossed axons of the E-26 cohort come from cells having a central distribution on the retina. For the E-30 cohort, the uncrossed axons come from cells having a relatively peripheral distribution, whereas the crossed axons come from more central cells. The present results suggest that the mechanism which serves to direct temporal retinal axons into the ipsilateral optic tract weakens as development proceeds. In principle, the change may occur in either a chiasmatic signal, read by temporal but not nasal optic axons, or in a retinal label, carried by temporal but not nasal cells and their processes. Since temporal retinal cells born concurrently at different places can project to opposite optic tracts, a retinal signal that deteriorates with time in a centroperipheral fashion is favored by the present results.

Two phases of increased cell death in the inner retina following early elimination of the ganglion cell population.

Neurons in the inner nuclear layer (INL) of the vertebrate retina undergo considerable programmed cell death during development, but the determinants of this cell death remain largely unknown. The present study examines the role of retinal ganglion cells in support of INL neurons in the developing ferret retina. The retinal ganglion cell population was eliminated by optic nerve transection at postnatal day (P) 2, and the incidence of cell death was examined using terminal deoxytransferase dUTP nick-end labelling (TUNEL) at various ages during the first 3 postnatal weeks. Significant increases in TUNEL-positive cells were observed in the neuroblast layer (NBL) as early as P3, prior to synapse formation within the inner plexiform layer (IPL), and again in the INL at P22, the normal peak of naturally occurring cell death within the ferret's INL. A decrease in TUNEL-positive cells was found in the NBL at P8. These results show three phases of response to the loss of retinal ganglion cells and suggest that cells in the NBL/INL are normally dependent on retinal ganglion cells for their survival. Recent studies have shown that certain populations of retinal neurons are reduced in adult animals that had lost the population of ganglion cells during early development, so the present study also examined when this reduction could first be detected. The number of parvalbumin-immunoreactive amacrine cells was decreased significantly in the NBL of the manipulated eye as early as P8, when we could first label this population, and this difference persisted through adulthood. The fact that cell death in the NBL has already increased within 24 hours of ganglion cell elimination, coupled with the specificity of this effect on the adult complement of INL cell types, shows that cell-cell interactions controlling survival are already highly specific for particular types of retinal neuron early in development

Position of axons in the cat's optic tract in relation to their retinal origin and chiasmatic pathway.

The positions of the crossed and uncrossed optic axons of distinct diameter classes has been examined in the optic tract of the adult cat. In addition, the retinal origin of axons occupying different positions within the tract has been studied. Since the position of a fibre within the optic tract reflects its time of arrival during development, we have used axonal position as an indicator of age and have related this to the chiasmatic pathway choice of the axons. Cats were either monocularly enucleated, to reveal the position and diameter of surviving crossed and uncrossed optic axons in semithin and thin sections, or implants of horseradish peroxidase (HRP) were placed so as to retrogradely label the ganglion cells giving rise to axons within the deep (early arriving), or superficial (later arriving) parts of the tract selectively. This was accomplished by either 1) surgically implanting HRP into the superficial portion of the optic tract, via a transbuccal approach, or 2) making such a transbuccal transection of the superficial fibres, followed by intracerebral injections of HRP to retrogradely label the surviving, deeper, optic axons from their target nuclei. The deep parts of the optic tract contain fine and medium, crossed and uncrossed axons arising from mainly medium sized cells in the contralateral nasal and the ipsilateral temporal retina; there is a clear line of decussation. In contrast, the superficial parts of the tract contain mainly fine diameter axons arising from small cells in the whole contralateral retina, and a small proportion of large diameter axons arising from large, alpha cells in the whole contralateral retina and in the ipsilateral temporal retina. The likelihood that axons from the temporal retina will project contralaterally therefore increases as development proceeds, since these axons are found in the superficial parts of the tract only. This suggests that a time-dependent signal that weakens with age is responsible for directing early arriving optic axons from the temporal retina to take an exclusively uncrossed course.

Glial domains and axonal reordering in the chiasmatic region of the developing ferret.

This study has examined the developing glial architecture of the optic pathway and has related this to the changing organization of the constituent axons. Immunocytochemistry was used to reveal the distribution of glial profiles, and DiI was used to label either radial glial profiles or optic axons. Electron microscopy was used to determine the distribution of glial profiles, axons, growth cones, and wrists at different locations along the pathway. Three different glial boundaries were defined: Two of these are revealed as changes in the distribution of vimentin-immunoreactive profiles occurring in the prechiasmatic optic nerve and at the threshold of the optic tract, respectively, and one by the presence of glial fibrillary acidic protein (GFAP)-immunoreactive profiles at the chiasmatic midline. The latter, midline boundary may be related to the segregation of nasal from temporal optic axons. The boundary at the threshold of the optic tract coincides with the segregation of dorsal from ventral optic axons that emerges at this location in the pathway. The segregation of old from young optic axons is shown to occur only gradually along the pathway. Glial profiles are most frequent in the deeper parts of the tract, coursing parallel to the optic axons and orthogonal to their usual radial axis. These are suggested to arise from later-growing radial glial fibers that are diverted to grow amongst the older optic axons. Those glial profiles may subsequently impede axonal invasion, thus creating the chronotopic reordering by forcing the later-arriving axons to accumulate superficially.

Birthdates of neurons in the retinal ganglion cell layer of the ferret.

The present study determined the temporal and spatial patterns of genesis for neurons of different sizes in the retinal ganglion cell layer of the ferret. Fetal ferrets were exposed to tritiated thymidine on embryonic days E-22 through E-36. One to 3 months after birth, they were perfused and their retinae dissected, and autoradiographs were prepared from resin-embedded sections throughout the entire flattened retinal ganglion cell layer. Soma size differences in conjunction with separate retrograde labeling and calbindin immunocytochemical studies were used as criteria for identifying different retinal ganglion cell subtypes in juvenile and adult ferrets. Neurons of different sizes in the ganglion cell layer were generated at different stages during development. Medium sized cells were generated primarily between E-22 and E-26; the largest cells were generated between E-24 and E-29; small cells were generated between E-26 and E-32; and very small cells were generated between E-29 and E-36. The former three groups were interpreted to be three subtypes of retinal ganglion cells, while the latter group was interpreted to be displaced amacrine cells. This temporal order of the genesis of ganglion cell classes is consistent with the spatial ordering of their fibers in the mature optic chiasm and tract, and it is consistent with the developmental change in decussation pattern recently shown in the optic pathway of embryonic ferrets. The spatial pattern of genesis suggests that ganglion cells of a particular class are added to the ganglion cell layer in a centroperipheral fashion initiated in the dorsocentral retina nasal to the area centralis. No evidence was found for a wave of ganglion cell addition that proceeded in a spiralling pattern around the area centralis, as has been reported in the cat.

Maturational gradients in the retina of the ferret.

In the present set of studies, we have examined the site for the initiation of retinal maturation in the ferret. A variety of maturational features across the developing inner and outer retina were examined by using standard immunohistochemical, carbocyanine dye labelling, and Nissl-staining techniques, including 1) two indices of early differentiation of the first-born retinal ganglion cells, the presence of beta-tubulin and of neuron-specific enolase; 2) the receding distribution of chondroitin sulfate proteoglycans within the inner retina; 3) the distribution of the first ganglion cells to grow axons along the optic nerve; 4) the emergence of the inner plexiform layer; 5) the emergence of the outer plexiform layer and 6) the onset of synaptophysin immunoreactivity within it; 7) the differentiation of calbindin-immunoreactive horizontal cells; and 8) the cessation of proliferative activity at the ventricular surface. Although we were able to define distinct maturational gradients that are associated with many of these features of inner and outer retinal development (each considered in detail in this report), with dorsal retina maturing before ventral retina, and with peripheral retina maturing last, none showed a clear initiation in the region of the developing area centralis. Rather, maturation began in the peripapillary retina dorsal to the optic nerve head, which is consistent with previous studies on the topography of ganglion cell genesis in the ferret. These results make clear that the order of retinal maturation and the formation of the area centralis are not linked, at least not in the ferret.

Rods and cones project to the inner plexiform layer during development.

Mature rod and cone photoreceptor cells extend terminals to the outer plexiform layer (OPL), where they form characteristic spherules or pedicles, synapsing with the second-order neurons of the inner nuclear layer (INL). The present study demonstrates that, prior to the formation of this connectivity, immature rods and cones in the ferret extend processes beyond the level of the horizontal cells and future OPL, reaching the inner plexiform layer (IPL). The number of processes extending to the IPL increases steadily as the population of photoreceptor cells expands postnatally, reaching a maximum 2 weeks after birth. These processes are immunopositive for synaptophysin, and they terminate in two strata occupied by the dendrites of amacrine cells and ganglion cells. The frequency of these processes declines rapidly during the third postnatal week, and they are no longer detectable by the fourth postnatal week. Their loss is neither a consequence of photoreceptor cell death nor is it due to selective protein trafficking mechanisms that render them immunonegative. Rather, these processes retract to the level of the OPL during this period, coincident with the maturation of bipolar and horizontal cell processes. These results demonstrate that, despite the clear presence of environmental signals presaging the formation of the OPL, photoreceptor terminals initially ignore them to grow beyond this level of the retina. Rather, they detect and respond to signals within the IPL during this period, terminating in proximity to the processes of other cells in the inner retina, where they may contribute to transient retinal circuitry during early development.

Chronotopic fiber reordering and the distribution of cell adhesion and extracellular matrix molecules in the optic pathway of fetal ferrets.

We have examined the age-related reordering of optic axons as they pass through the chiasmatic region in fetal ferrets. Proportions of young and old optic axons were determined from electron micrographs taken sequentially through the prechiasmatic nerve, chiasm, and tract. This "chronotopic" reordering of axons was shown to emerge gradually, beginning rostral to the fusion of the two optic nerves, but continuing to develop caudal to the chiasmatic midline. Segregation of young from old optic axons was most pronounced within the optic tract. We then compared the emergence of this fiber reorganization to the distribution of cell adhesion and extracellular matrix molecules and to the glial architecture within the pathway. Using immunohistochemistry, the distributions of the cell adhesion molecules L1, NCAM, and TAG-1 and the extracellular matrix molecules laminin-1 and chondroitin sulfate proteoglycans (CSPGs) were determined. Among these, only the distribution of CSPGs was observed to change in a manner that complemented the segregation of young from old optic axons. CSPGs were densest in the deeper parts of the optic tract, coincident with radial glial fibers that turn to course within the region of the oldest optic axons. Both the glial architecture and the CSPG distribution form as a consequence of the invasion of the first optic axons, shown by the developmental sequence of each, and by the fact that these glial and molecular features fail to form in the absence of optic axons. The data suggest a model in which the gradient of CSPGs across the depth of the tract contributes to the formation of the chronotopic fiber reordering by providing a relatively unfavorable environment for subsequent axonal growth. The CSPGs may do so by interfering with adhesion molecules on optic axons that normally promote elongation.

The distribution of axons according to diameter in the optic nerve and optic tract of the rat.

The distribution of axons according to diameter was examined in the optic nerve and optic tract of adult hooded rats. Observations were made on semithin sections, and measurements of axonal diameters were made on electron micrographs taken from various locations across thin sections through the optic nerve and tract. The distribution of axons by size differs markedly in the optic nerve and tract. Coarse (greater than 2 microns) and fine (less than or equal to 2 microns) axons are distributed throughout all regions of the optic nerve. In the optic tract, in contrast, coarse axons are especially dense dorsally, at the deep border of the tract, while they are absent ventrally, subjacent to the pial surface. No regions of the optic nerve contain densities of coarse axons as high as the deep nor as low as the superficial extremes of the optic tract. Nevertheless, even at the deep (dorsal) border of the optic tract, the coarse axons make up only a small minority (roughly 15%) of the total number of axons in that region. The axons 2 microns or smaller may be divisible into two overlapping, fine and intermediate, diameter classes, that are partially segregated within the optic tract, but not in the optic nerve: the distributions of axon diameters smaller than 2 microns are skewed to distinctly smaller diameters at the dorsal and ventral extremes of the optic tract, while in between, at mid-positions along the deep-to-superficial axis of the optic tract, the axon size distributions contain many more axons greater than 1 micron in diameter. These different axon diameter groups may arise from the morphologically distinct retinal ganglion cell types, and may underlie the components of the trimodal compound axon potential seen in the rat's primary optic pathway. Their partial segregation within the tract anticipates the partial segregation of their terminal arborizations within the laminae of the dorsal lateral geniculate nucleus. The rearrangement of axons into a partial segregation by size within the optic tract may indicate a chronology of axonal arrival during early development, proximity to the pial surface being an index of recency of arrival. As axonal outgrowth and neurogenesis appear to be directly related within the retinal ganglion cell population in mammals, the relative birthdates of the retinal ganglion cell types giving rise to the axon diameter classes in the rat may be inferred from the present results.

The position of the crossed and uncrossed optic axons, and the non-optic axons, in the optic tract of the rat.

The position of the crossed and uncrossed optic axons, and of the non-optic axons, within the optic tract was determined in the adult hooded rat. Horseradish peroxidase histochemistry and lesion-induced degeneration of axonal profiles were independently used to study the position of the three relevant populations of axons within the optic tract. The boundaries of the optic tract are distinct at all but its caudomedial border, where it abuts the supra-optic commissures running parallel to the fibres of the optic tract. Labelling the crossed population of optic axons, or inducing their degeneration, both demonstrate a clear caudomedial border of the optic tract, although a number of optic axons stray out of the optic tract and course within the supra-optic commissures immediately caudomedial to the tract. The uncrossed optic axons are, as a population, positioned relatively deep in the optic tract, towards its dorsal border. A few occur at further ventral positions, but their density is greatly reduced there. There is also a very thin region along the dorsalmost edge of the optic tract free of uncrossed optic axons. The relative position of the uncrossed to the crossed optic axons is discussed in the context of the mammalian optic tract as a chronological map: spatial position in the tract may reflect temporal order of axonal arrival during early development. A large population of non-optic axons belonging to Gudden's commissure courses within the boundaries of the optic tract at a relatively ventral position. They are most frequent caudomedially, and are absent rostrolaterally. Hence, axons of the optic tract and Gudden's commissure are substantially intermingled in the caudomedial half of the optic tract. These non-optic axons greatly outnumber the uncrossed optic axons, and will consequently distort counts of uncrossed optic axons based on intact profiles that remain after removal of the opposite eye. However, they are still a minority in comparison to the crossed optic axons in this region.