Uncrossed Projection Research Articles

Chapter 1 Clonal architecture of the mouse retina

The study of chimeric retinas has yielded insight on the early development of retina. The close match in chimerism ratios between right and left retinas is significant and supports the idea that both retinas originate from a common population of progenitors. We are able to estimate numbers of progenitor cells that contribute to the formation of the retina and the approximate time at which this small group is isolated from surrounding prosencephalic cell fields. These cells undergo at least five rounds of division before the first retinal neurons are generated. The mouse retina is not build from the center outward. There is simultaneous expansion and differentiation in all parts of the retina and as a result clones are not arranged in wedges. Instead the mouse retina is a patchwork of clones that do not differ greatly in size from center to periphery. The most consistent radial feature in mouse retina is a raphe left at the line of fusion of the margins of the ventral fissure. Processes that shape the clonal patchwork are both passive and active, intrinsic and extrinsic. Certain features of the clonal architecture of the retina, such as the size differences of clones are primarily passive responses to extrinsic forces on progenitor cells and their progeny. The fifteen-fold range in the size of cohorts is not due to intrinsic differences in the proliferative capacity of individual progenitor cells, but is due to the extent of cell movement and mixing at early stages of development. In contrast, active or intrinsic processes are illustrated by the partial (and still controversial) restriction of retinal progenitors, the possible clonal differences between ganglion cells with crossed and uncrossed projections, and the consistent differences in ratios of albino and pigmented genotypes in peripheral and central retina.

On the distribution of gamma cells in the cat retina.

Ganglion cells of the cat retina that are neither alpha nor beta cells are often lumped for convenience into a single anatomical group--the gamma cells (Boycott & Wässle, 1974; Stone, 1983; Wässle & Boycott, 1991). Defined in this way, gamma cells are the morphological counterpart to the physiological W-cell class, which includes all ganglion cells that are neither Y (alpha) nor X (beta) cells. We have estimated the retinal distribution of gamma cells by using retrograde transport to label ganglion cells innervating the superior colliculus and by assuming that these included virtually all gamma cells and no beta cells. We excluded labeled alpha cells on the basis of soma size. Our data suggest that gamma cells represent just under half of the ganglion cells in most of the nasal retina, but only about a third of those in the area centralis and temporal retina. Gamma cells do not appear to be more highly concentrated in the nasal visual streak than are other ganglion cells. In the temporal retina, gamma cells with crossed projections to the brain are apparently at least twice as common as those with uncrossed projections.

Lateral superior olive projections to the inferior colliculus in normal and unilaterally deafened ferrets

We have examined the projection from the lateral superior olive (LSO) to the inferior colliculus (IC) in the ferret, with particular interest in the laterality of the projection and in the effects of unilateral cochlear removal in infancy. Large or small deposits of the retrograde tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made in the IC of anesthetized adult ferrets that either were normally hearing or had been unilaterally deafened in infancy (P5 or P25). After 2 days, the ferrets were perfused, and frontal sections of the brainstem were treated with tetramethyl benzidine. For large deposits of WGA-HRP, equal numbers of labelled neurons were found evenly spread through both LSOs. Smaller deposits of WGA-HRP produced four results that contrasted with previous reports on the cat. First, many more labelled neurons were found in the contralateral than in the ipsilateral LSO. Second, the relative number of labelled neurons in each LSO was independent of whether the deposits were in the ventral or in the dorsal IC. Third, the total number of labelled LSO neurons was independent of whether the deposits were in the ventral or in the dorsal IC. Fourth, the proportion of ipsilateral to contralateral labelled neurons was slightly higher in the medial LSO than in the lateral LSO. Ventral IC deposits resulted in more labelled neurons in the medial LSO, and dorsal IC deposits resulted in more labelled neurons in the lateral LSO, as expected. Neonatal cochlear removal did not change any of these results. We conclude that, in the ferret, the organization of the crossed and uncrossed projections from the LSO to the IC differs from that of the cat, and any similarity with the optic chiasm is not obvious.

Effect of a very early monocular enucleation upon the development of the uncrossed retinofugal pathway in ferrets

Monocular enucleations were done in ferret embryos before or during the earliest stages of development of the retinofugal pathway (E23-E26). The effects on the development of the uncrossed pathway from the surviving eye were assessed on embryonic day 30. This stage was chosen for two reasons: (1) we show that in normal development a substantial uncrossed component from the temporal crescent has developed by E30; and (2) the pathway cannot yet have been affected by the cell death that normally occurs in the retina in the perinatal period. Using DiI labelling from either the temporal crescent or the optic nerve head, we have shown that such early enucleations prevent the formation of the uncrossed pathway from the temporal crescent of the surviving eye. Enucleation at E23/24, before or during the period when the first axons reach the chiasm, prevents the formation of the uncrossed projection. The axons that would normally take an uncrossed course stall lateral to the midline of the optic chiasm. At E26, when many axons have reached the optic chiasm, but none yet come from the temporal crescent, enucleation causes a dramatic reduction in the uncrossed projection, and the complete abolition of the normal uncrossed pathway from the temporal crescent. This demonstrates that there is a requirement for an interaction between the axons of the two eyes at the optic chiasm to establish the normal formation of the uncrossed pathway at the optic chiasm.

Development of abnormal lamination and binocular segregation in the retinotectal pathways of the rat

The uncrossed retinotectal pathway of pigmented rats originates from a small fraction of the retinal ganglion cell population. This projection terminates deeply in discrete patches within the upper grey layers where crossed and uncrossed inputs overlap. However, after the experimental enlargement of the uncrossed pathway, the ipsilateral fibers are also found in a superficial tier of the upper grey layers where binocular inputs segregate [36]. We studied the development of the retinotectal projections in rats after the enlargement of the uncrossed pathway as a result of a contralateral (left) optic tract lesion (OTL) made at birth. Horseradish peroxidase (HRP) was used as an anterograde tracer. An abnormal uncrossed projection from the right eye to the collicular surface appeared at postnatal day 3 (P3). Between P5 and P10, this projection developed the bilaminar pattern seen in similarly operated adults. The laminar arrangement of the aberrant terminal fields did not change significantly after an ipsilateral visual cortex ablation on the day of birth. Despite the early development of the aberrant uncrossed pathway, binocular segregation was incipient at P10. At P14, 46% of the operated rats presented gaps in the terminal labeling at the tectal surface. This figure increased to 55.5% at 6 weeks, a proportion still smaller than in adult animals of the same group (69%). Eyelid suture had no effect on segregation. This projection remains plastic for at least 3 weeks, since the removal of the ipsilateral input at either P14 or P21 resulted in the absence of gaps in the contralateral projection. We conclude that the laminar selection of retinotectal projections depends on binocular interactions and that the abnormal segregation of retinal inputs to the superior colliculus has an unusually protracted development which can be reversed long after the previously defined critical period in this system.

Does early enucleation affect the decussation pattern of alpha cells in the ferret?

Naturally occurring cell death has been hypothesized to sculpt various features of the organization of the mature visual pathways, including the recent proposal that the selective elimination of ganglion cells in the temporal retina shapes the formation of decussation patterns. Through a class-specific interocular competition, ganglion cells in the two temporal hemiretinae are selectively lost to produce the decussation patterns characteristic of each individual cell class (Leventhal et al., 1988). The present study has tested this hypothesis by asking whether the removal of one retina in newborn ferrets, which should disrupt binocular interactions at the level of the terminals, alters the decussation pattern of the alpha cells, a cell class that is entirely decussating in the normal adult ferret. Enucleation on the day of birth was found to increase the uncrossed projection by approximately 50%, but not a single uncrossed alpha cell was found in the temporal retina. Either alpha cells never project ipsilaterally during development, or if they do, they cannot be rescued by early enucleation. While naturally occurring cell death plays many roles during development, creating the decussation pattern of the ferret's alpha cell class via a binocular competition at the level of the targets is unlikely to be one of them.

Rapid plastic response following early retinal lesions in rats

Following an early retinal lesion, aberrant uncrossed projections from the opposite, undamaged, retina form in the target visual nuclei. The present study has examined the development of such aberrant projections by making retinal lesions in newborn rat pups, and then examining the nature of the uncrossed retinocollicular projection at different ages following the lesion. Intravitreal injections of horseradish peroxidase were made into the intact eye, and the uncrossed projection was subsequently revealed histochemically. A mature aberrant projection forms as early as postnatal day 9. On postnatal days 5 and 2, aberrant projections are discernable amongst the exuberant uncrossed terminals of normal developing rats, although the former have not matured to form the dense terminal fields characteristic of older projections. Aberrant projections were also detectable as early as 12 h following the lesion, revealed as a relative increase in the density of uncrossed label. These results indicate that lesion-induced plastic responses by intact retinal arbors are initiated shortly after the insult, and they caution the use of retinal lesions in studies of normal retinotopic connectivity during development.

The development of retinal ganglion cell decussation patterns in postnatal pigmented and albino ferrets.

The decussation patterns of retinal ganglion cells in postnatal pigmented and albino ferrets were examined by using retrograde axonal tracers. Following unilateral injections into the optic pathway of newborn pigmented ferrets, approximately 13,000 cells were labelled in the ipsilateral retina. The majority (11,500) of these were located in temporal retina. Postnatally, the numbers of cells projecting ipsilaterally from temporal retina fell by 49%. High rates of loss were observed in both the smaller uncrossed projection from nasal retina (92%) and also in the crossed projection from temporal retina (84%). After injections on the day of birth, a decussation line was not obvious in the crossed projection: > or = 14,000 labelled cells were found in temporal retina. Double tracer studies showed that very few of these cells had axons which projected bilaterally. The numbers of ipsilaterally projecting cells labelled in neonatal albino ferrets was dramatically reduced. Only approximately 2500 were labelled in temporal retina following injections at birth. As in pigmented ferrets, about half of these cells subsequently died. The reduced uncrossed projection in albino neonates was associated with an increase in the crossed projection from temporal retina, in which approximately 21,000 cells were labelled following injections at birth. These results suggest that differential postnatal ganglion cell death establishes the adult decussation pattern in the contralateral retinal projection but merely refines the pattern already established in the uncrossed projection. Postnatal ganglion cell death plays no significant role in generating the abnormal projections found in albino ferrets.

The effects of monocular enucleation on ganglion cell number and terminal distribution in the ferret's retinal pathway.

Anterograde and retrograde tracing techniques were used to examine the effects of removing one eye at birth on the remaining uncrossed retinal pathway in adult ferrets. After enucleation, the adult number of labelled ganglion cells projecting ipsilaterally changed from an average of 6068 in normal pigmented ferrets to an average of 7813 (29% increase) in pigmented enucleates. The change in albino ferrets was from 1455 in normals to 2319 in enucleates (59% increase). Labelled cells scattered across nasal retina accounted for over half the increase in the uncrossed population. After neonatal enucleation, the volume of lateral geniculate nucleus occupied by the uncrossed projection increased substantially: five-fold in pigmented animals and 20-fold in albinos. These results suggest that neonatal removal of one eye has a greater effect on the distribution of uncrossed terminals than on the survival of uncrossed ganglion cells. There was also an increase in the total number of axons in the surviving optic nerve of both pigmented and albino ferrets (93,000 in enucleates compared with 79,000 in normal animals), which cannot be simply explained as a disruption of binocular competition.

The segregation of ON- and OFF-center responses in the lateral geniculate nucleus of normal and monocularly enucleated ferrets.

We have investigated the distribution of ON- and OFF-center responses in the lateral geniculate nucleus of ferrets with normal and abnormal retinal projections. Electrophysiological recordings in normal pigmented animals confirm previous studies on mustelids showing that ON-center responses are found in the anterior, inner parts of laminae A and A1 and OFF-center responses in posterior, outer leaflets. In albino animals, lamina A displays normal patterns of ON/OFF segregation but in lamina A1', which receives an abnormal crossed retinal projection, no consistent patterns of segregation are found. Following monocular enucleation on the day of birth, the uncrossed projection in pigmented ferrets remains expanded across the LGN. Anatomically and physiologically, this projection is segregated into two leaflets: an anterior, inner ON-center leaflet and a posterior outer OFF-center leaflet. We conclude that the persistence of ON/OFF segregation, independent of geniculate location, suggests that self-sorting of retinal input is an important factor in generating the segregation.

Distinctive pattern of organisation in the retinofugal pathway of a marsupial: I. Retina and optic nerve.

The nasotemporal division in the retina and the pattern of crossed and uncrossed axons in the optic nerve were determined in an Australian marsupial, a wallaby, Setonix brachyurus (the quokka), following unilateral horseradish peroxidase injections into primary visual centres. The gross morphology of the nerve was also examined. Ipsilaterally projecting ganglion cells were restricted to the temporal retina, whereas those that project contralaterally were located in all retinal regions. The morphological study of the nerve showed that fasciculation patterns, evident along much of the length of the nerve, became indistinct centrally and were replaced in the prechiasmatic region by dorsoventrally oriented fissures. In this prechiasmatic region, axons were oriented in two directions. Whereas the majority were aligned centroperipherally with the long axis of the nerve, a proportion were aligned dorsoventrally in the fissures. Labelling with HRP revealed that uncrossed axons were restricted to the lateral region of the optic nerve and possibly to discrete fascicles, whereas those destined to cross at the chiasm occupied all regions of the nerve but were less dense on the lateral side. This spatial distribution of crossed and uncrossed projections did not change along the length of the nerve. These results demonstrate that fibre organisation in the marsupial optic nerve is different than that found in eutherian mammals.

Distinctive pattern of organisation in the retinofugal pathway of a marsupial: II. Optic chiasm.

In the mammalian optic chiasm retinal axons from each eye divide into two populations, those that decussate and those that remain uncrossed. In eutherian (placental) mammals, the separation of these pathways is not reflected in the structure of the chiasm. The two populations from each eye are mixed through each hemichiasm, segregating only at the midline, where the uncrossed projection turns back. In this study the optic chiasm of a marsupial, the wallaby, Setonix brachyurus (quokka) has been investigated with staining and neuronal tracing techniques. The chiasm of this mammal is quite different from that of eutherian mammals. In coronal section it can be morphologically subdivided into three regions, a central body in which fasciculated groups of axons from each eye interdigitate across the midline, and two distinct lateral regions, one on each side, which contain the uncrossed retinal projections. In the rostral chiasm the lateral regions are separated from the main body of the chiasm by vertically oriented fibre-free regions. Caudally, the lateral regions increase in size and become less distinct as increasing numbers of contralaterally projecting axons that have crossed the midline project into them. However, the two populations remain predominantly segregated in this region. As the lateral regions develop, the central body of the chiasm becomes thinner and finally detaches at the midline to form the two optic tracts. The routes taken by retinal axons through the eutherian and marsupial chiasm appear to be fundamentally different. Therefore, the developmental factors that determine the laterality of retinal projections are likely to show significant differences in the two mammalian groups.

Time-dependent neuroplasticity in mesostriatal projections after unilateral removal of vibrissae in the adult rat: Compartment-specific effects on horseradish peroxidase transport and cell size

In adult rats the mystacial vibrissae were clipped on one side of the snout, and the influence of this sensory deprivation on crossed and uncrossed striatal afferents from the substantia nigra, ventral tegmental area, and retrorubral area was examined with the horseradish peroxidase tract tracing technique. Unilateral removal of vibrissae was found to affect crossed and uncrossed nigrostriatal projections in a time-dependent manner. One to three days after hemivibrissotomy an apparent neuronal asymmetry was found in the crossed nigrostriatal projection arising in the rostral part of the substantia nigra, with more labeled neurons in the projection to the caudate-putamen on the side of vibrissae removal. This asymmetry resulted mainly from an asymmetry in the subset of nigrostriatal neurons reported to project to the striatal matrix (“dorsal cell type”). In contrast, 4–20 days after hemivibrissotomy reversed asymmetries were found in crossed and uncrossed nigrostriatal projections, with more labeled neurons in the projections to the caudate-putamen of the hemisphere opposite to vibrissae removal (the sensory deprived hemisphere). The asymmetry in the uncrossed projection was found throughout the substantia nigra, but was also most substantial in the projection from its rostral part. The asymmetry in the crossed projection was again restricted to the rostral substantia nigra; interestingly, however, it was limited to the subset of neurons reported to terminate in the striosomes (“ventral cell type”). Evidence was also found for time-dependent changes in size of neurons of the crossed nigrostriatal projections, as well as for changes in striatal afferents from the ventral tegmental area. The time course of these apparent changes in strength of mesostriatal projections is similar to the known time course of recovery from behavioral asymmetries induced by hemivibrissotomy, which is suggestive of a functional relationship between neuronal and behavioral changes. Moreover, the finding of a differential influence of hemivibrissotomy on nigrostriatal afferents to striosomes and matrix is indicative of a functional dissociation of these two systems.

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.

Horizontal optokinetic nystagmus in unilaterally enucleated pigmented rats: Role of the pretectal commissural fibers

Monocular enucleation reduces the asymmetry of horizontal optokinetic nystagmus (H-OKN) in afoveate mammals by increasing responses to naso-temporal visual stimulation. The origin of these larger responses was investigated in adult pigmented rats monocularly enucleated as neonates or as adults by analyzing retinal and commissural projections to the deafferented nucleus of the optic tract (NOT) and the functional role of this nucleus before and after section of the posterior commissure. Anatomically, monocular enucleation reduces the volume of the contralateral deafferented NOT. Anterograde tracers injected in the intact eye reveal a crossed projection of the retina to the NOT and to the dorsal (DTN) and medial (MTN) terminal nuclei of the accessory optic system as in normal rats. In addition, there is an uncrossed projection to the MTN in the rats enucleated as neonates. Retrograde tracer injected in the deafferented NOT confirms the absence of an uncrossed retinal projection but reveals connections between both NOT via the posterior commissure as in normal rats. Electrophysiologically, the larger naso-temporal optokinetic responses in monocularly enucleated rats return to normal after posterior commissurotomy. This study demonstrates that no anatomical remodelling takes place to increase naso-temporal responses in monocularly enucleated rats. The larger responses must then result from functional changes. The role of exclusive contralateral projections of the retina to the NOT and of the commissural connections in mediating the asymmetry of the optokinetic nystagmus in afoveate mammals is discussed.

Early development of the retinal line of decussation in normal and albino ferrets

This work describes the retinal origin of the crossed and uncrossed projections in newborn, 9-day-old and adult normally pigmented and albino ferrets. Horseradish peroxidase (HRP) was injected unilaterally into the thalamic and midbrain visual centers of ferrets to label retinal ganglion cells retrogradely. In normally pigmented adults, the retinal line of decussation was sharp and passed through the area centralis. Ganglion cells with uncrossed axons occupied the entire temporal retina. In albino adults, ganglion cells with uncrossed axons were distributed in the periphery of the temporal retina away from the area centralis. In the normally pigmented adults, about 11% of the retinal ganglion cells had uncrossed axons compared to about 4% in the albinos. At birth, normally pigmented ferrets had a sharp line of decussation with most (about 98%) uncrossed ganglion cells found in the temporal retina. In the newborn albinos, most uncrossed ganglion cells were in the temporal retina (about 89%), but there were many fewer than in the normal neonates and, as in the albino adults, the uncrossed ganglion cells were distributed along the temporal most margin of the retina. In the normal neonates, about 11% of the ganglion cells had uncrossed axons, compared to about 3% in the albino neonates. The area centralis and visual streak were not evident until 9 days after birth. From these results we conclude that the retinal line of decussation is essentially mature by birth in the ferret, and the degree of the albino's abnormality is as extreme in neonates as in adults. The retinal decussation is virtually mature at a stage of development when the crossed and uncrossed retinal afferent axons are completely intermingled in their target nuclei and prior to the onset of significant retinal ganglion cell loss.

Evidence that the relative densities of afferents from both eyes control laminar distribution and binocular segregation of retinotectal projections in rats

In the superior colliculus of normal rodents the crossed retinal projection overlaps the uncrossed projection. The present study describes an abnormal laminar distribution and binocular segregation of the retinotectal afferents induced after the experimental enlargement of the uncrossed retinotectal pathway in pigmented rats. Intraocular injections of anterograde tracers were used to investigate the topographic and laminar organization of retinotectal projections in adult rats given unilateral optic tract lesions at birth. These lesions are known to increase the number of ipsilaterally projecting ganglion cells in the opposite retina. The uncrossed retinal projection to the remaining superior colliculus forms an abnormal band of terminal labeling at the superficial half of the stratum griseum superficiale, markedly different from the laminar distribution of this pathway in unoperated controls. This abnormal uncrossed projection has its maximum density at the rostrolateral quadrant of the tectum. Within this region, the crossed retinotectal projection retracts from the surface of the superior colliculus, leading to partial binocular segregation. The results suggest that both the laminar distribution and the experimental binocular segregation of retinotectal afferents depend on the balance of the densities of the converging pathways from both eyes in the superior colliculus.

Evaluation of the influence of optic stalk melanin on the chiasmatic pathways in the developing rodent visual system

In a number of mammalian species, fibre outgrowth in the developing retinofugal pathway is coincident with the presence of melanin in the retinal part of the optic stalk. The presence of melanin is transient in this developing system and has been proposed to play a role in the guidance of retinofugal fibres. Further, it has been suggested that this stalk melanin accounts for the differences between the size of the uncrossed retinal component in pigmented and nonpigmented strains. However, a recent study showed that there is no melanin in the optic stalk of Manchester rats during fibre outgrowth. Since such rats supposedly have a normal pigment distribution and a normal pattern of decussation at the optic chiasm, this finding appears to undermine the suggested role played by stalk melanin in establishing the laterality of retinal fibre projections in other mammalian species. The aim of this study was to re-evaluate the relationship between melanin in the stalk and the development of the retinofugal pathway in three strains of rat: the Wild type, Long Evans Hooded, and the Albino. The Albino rat, which lacks melanin-bearing cells entirely, was shown to have the smallest uncrossed projection, approximately 1,340 ipsilaterally projecting cells (ipc), whereas the Long Evans (2,760 ipc) and the Wild-type strain (2425 ipc) were found to have a larger uncrossed retinal component. In both pigmented strains, melanin was restricted to the eye cup and absent from the optic stalk throughout all stages of development.(ABSTRACT TRUNCATED AT 250 WORDS)

Crossed and uncrossed projections to cat sacrocaudal spinal cord: II. Axons from muscle spindle primary endings

The terminal fields of primary afferent fibers from tail muscle spindle primary endings were mapped within cat sacrocaudal spinal cord (S3-Ca7), using intra-axonal recording and horseradish peroxidase staining techniques. We sought to determine the ipsilateral and contralateral projection patterns and to relate these to the fibers' muscles of origin. Fifty-three group Ia fibers were successfully stained. Segmental collaterals originated from either the ascending or descending branch within the dorsal columns. Collaterals coursed rostromedially within the dorsal columns and traversed the medial aspect of the dorsal horn. Ipsilateral terminations were similar for all fibers. Within the ventral horn, boutons were consistently observed in the medial or central portions of lamina VII. In lamina VIII, a variable number of boutons was seen on fine branches emerging from larger fibers coursing ventrally. Clusters of terminals were plentiful in the regions of motoneurons, i.e., lamina IX and the nucleus commissuralis. Terminals were found in the adjacent white matter. In addition to ipsilateral terminations, some group Ia fibers (20 of 53) had collateral branches that crossed ventrally to the central canal, terminating within the midline ventral gray commissure and/or the contralateral ventral horn. Crossed projections always originated in medial (dorsal or ventral), but not lateral, muscles of the tail. These data suggest that ipsilateral projections of group Ia fibers make connections on sacrocaudal motoneurons, on neurons mediating segmental reflex functions and on neurons conveying ascending information. It is speculated that crossed and uncrossed connections between group Ia fibers from medial muscles and bilateral dendritic trees of motoneurons subserve synchronized co-contraction of synergistic muscles located on the two sides of the body, such as with dorsal or ventral flexion of the tail. Group Ia projections from lateral muscles, that are entirely ipsilateral, would be involved with lateral movements of the tail.

The nasotemporal division of retinal ganglion cells with crossed and uncrossed projections in the fetal rhesus monkey

The development of the partial decussation pattern in the primate retina was studied in fetal rhesus monkeys of known gestational ages. Retinal ganglion cells with either crossed or uncrossed projections were identified by labeling with HRP following unilateral injections of this tracer into the optic tract. At all fetal ages, very few cells (less than 0.5% of the total ganglion cell population) were found to project to the inappropriate hemisphere. The nasotemporal overlap zone, defined as the retinal region along the vertical meridian containing cells with either crossed or uncrossed projections, also appeared equivalent to that described for the adult animal. A temporal offset in the decussation pattern of large ganglion cells, similar to that of the mature retina, could be recognized as early as 50 d before birth. These results indicate that an adultlike retinal decussation pattern is evident in the fetal primate at a stage when projections from the 2 eyes are completely intermingled within retinorecipient nuclei, and prior to the onset of retinal ganglion cell loss. Moreover, the primate visual system exhibits a degree of precision in the specification of the nasotemporal division unrivaled among the mammalian species studied to date. The developmental specificity evident in the decussation pattern of the fetal rhesus monkey appears to reflect the specialized organization of this primate's retina for binocular focal vision.