Gene transfer in utero biologically engineers a patent ductus arteriosus in lambs by arresting fibronectin-dependent neointimal formation.

Closure of the ductus arteriosus requires prenatal formation of intimal cushions, which occlude the vessel lumen at birth. Survival of newborns with severe congenital heart defects, however, depends on ductal patency. We used a gene transfer approach to create a patent ductus arteriosus by targeting the fibronectin-dependent smooth muscle cell migration required for intimal cushion formation. Fetal lamb ductus arteriosus was transfected in utero with hemagglutinating virus of Japan liposomes containing plasmid encoding 'decoy' RNA to sequester the fibronectin mRNA binding protein. Fibronectin translation was inhibited and intimal cushion formation was prevented. We thus established the essential role of fibronectin-dependent smooth muscle cell migration in intimal cushion formation in the intact animal and the feasibility of incorporating biological engineering in the management of congenital heart disease.

Two forms of cerebellar glial cells interact differently with neurons in vitro.

Specific interactions between neurons and glia dissociated from early postnatal mouse cerebellar tissue were studied in vitro by indirect immunocytochemical staining with antisera raised against purified glial filament protein, galactocerebroside, and the NILE glycoprotein. Two forms of cells were stained with antisera raised against purified glial filament protein. The first, characterized by a cell body 9 microns diam and processes 130-150 microns long, usually had two to three neurons associated with them and resembled Bergmann glia. The second had a slightly larger cell body with markedly shorter arms among which were nestled several dozen neuronal cells, and resembled astrocytes of the granular layer. Staining with monoclonal antisera raised against purified galactocerebroside revealed the presence of immature oligodendroglia in the cultures. These glial cells constituted approximately 2% of the total cell population in the cultures and, in contrast to astroglia, did not form specific contacts with neurons. Staining with two neuronal markers, antisera raised against purified NILE glycoprotein and tetanus toxin, revealed that most cells associated with presumed astroglia were small neurons (5-8 microns). After 1-2 d in culture, some stained neurons had very fine, short processes. Nearly all of the processes greater than 10-20 micron long were glial in origin. Electron microscopy also demonstrated the presence of two forms of astroglia in the cultures, each with a different organizing influence on cerebellar neurons. Most neurons associated with astroglia were granule neurons, although a few larger neurons sometimes associated with them. Time-lapse video microscopy revealed extensive cell migration (approximately 10 microns/h) along the arms of Bergmann-like astroglia. In contrast, cells did not migrate along the arms of astrocyte-like astroglia, but remained stationary at or near branch points. Growth cone activity, pulsating movements of cell perikarya, and ruffling of the membranes of glial and neuronal processes were also seen.

Overlapping functions of the cell adhesion molecules Nr-CAM and L1 in cerebellar granule cell development.

The structurally related cell adhesion molecules L1 and Nr-CAM have overlapping expression patterns in cerebellar granule cells. Here we analyzed their involvement in granule cell development using mutant mice. Nr-CAM-deficient cerebellar granule cells failed to extend neurites in vitro on contactin, a known ligand for Nr-CAM expressed in the cerebellum, confirming that these mice are functionally null for Nr-CAM. In vivo, Nr-CAM-null cerebella did not exhibit obvious histological defects, although a mild size reduction of several lobes was observed, most notably lobes IV and V in the vermis. Mice deficient for both L1 and Nr-CAM exhibited severe cerebellar folial defects and a reduction in the thickness of the inner granule cell layer. Additionally, anti-L1 antibodies specifically disrupted survival and maintenance of Nr-CAM-deficient granule cells in cerebellar cultures treated with antibodies. The combined results indicate that Nr-CAM and L1 play a role in cerebellar granule cell development, and suggest that closely related molecules in the L1 family have overlapping functions.

Terminal arbors of axons that have formed abnormal connections.

Ther terminal arbors of individual retinogeniculate axons that have been induced to grow into an inappropriate geniculate layer have been revealed for light and electron microscopic study by being filled with horseradish peroxidase. After a unilateral ocular enucleation in kittens, single axons from the surviving eye show terminal arbors not only within their own geniculate layers but also in the denervated layers. The new, abnormal arbors arise from the terminal segments of arbors that lie within the nondenervated layer and make patterns of synaptic contacts that appear normal.

Retinal axon pathfinding in the optic chiasm: divergence of crossed and uncrossed fibers.

In the developing mammalian visual system, retinal fibers grow through the optic chiasm, where one population crosses to the opposite side of the brain and the other does not. Evidence from labeling growing retinal axons with the carbocyanine dye Dil in mouse embryos indicates that the two subpopulations diverge at a zone along the midline of the optic chiasm. At the border of this zone, crossed fibers grow directly across, whereas uncrossed fibers turn back, developing highly complex terminations with bifurcating and wide-ranging growth cones. When one eye is removed at early stages, uncrossed fibers from the remaining eye stall at the chiasm midline. These results suggest that crossed and uncrossed retinal fibers respond differently to cues along the midline of the chiasm and that the uncrossed fibers from one eye grow along crossed fibers from the other eye, both guidance mechanisms contributing to the establishment of the bilateral pattern of visual projections in mammalian brain.

Cell-cell interactions influence survival and differentiation of purified Purkinje cells in vitro.

To determine the role of cell-cell interactions in Purkinje cell survival and dendritic differentiation, perinatal mouse Purkinje cells were purified, and their development was analyzed in vitro. In isolation at low density, Purkinje cell survival was poor, improved by neuronal contacts, either with purified granule neurons or with Purkinje cells themselves. Moreover, coculture with specific cell populations led to widely different degrees of Purkinje cell differentiation. Purified Purkinje cells cultured alone or with an inappropriate afferent, the mossy fibers, did not progress beyond immature forms. With astroglia, Purkinje cells had thin smooth processes. Proper Purkinje cell differentiation was driven only by coculture with granule cells, resulting in dendrites with spines receiving synapses. These results suggest that Purkinje cell differentiation is regulated by local epigenetic factors, provided in large part by the granule neuron.

Crossed and uncrossed retinal axons respond differently to cells of the optic chiasm midline in vitro.

In mouse, retinal axon divergence takes place within a cellular specialization localized at the midline of the optic chiasm. To test whether the cells in this locus present cues for differential retinal axon growth, retinal explants were cocultured with cells dissociated from the chiasmatic midline, both taken from day 14-15 embryos, during the principal period of retinal axon divergence. Compared with crossed axons from other retinal regions, axons from ventrotemporal retina, the sole source of uncrossed axons, were shorter, more fasciculated, and fewer in number when growing on chiasm cells. Furthermore, uncrossed axons avoided clusters of chiasm neurons and glia having the composition and arrangement of the midline specialization, but crossed axons readily grew over them. In contrast to the clusters of chiasm cells, however, individual neurons and glia did not elicit differential retinal axon growth. These data demonstrate that cues for divergence derive from cells resident to the chiasm and suggest that cellular interactions among resident midline cells are required to produce these cues.

Chemosuppression of retinal axon growth by the mouse optic chiasm.

To determine whether diffusible guidance cues direct retinal axon growth and divergence at the optic chiasm, we cocultured mouse retinal and chiasm explants in collagen gels. The chiasm reduced retinal neurite lengths and numbers, but did not affect commissural or pontine neurite growth. This reduction in growth was equal for all retinal quadrants and occurred without reorienting the direction of neurite extension. The floor plate, another midline guidance locus, also suppressed retinal neurite outgrowth, whereas cortex or cerebellum explants did not. Growth suppression was not mediated by netrin-1, which instead enhanced retinal neurite extension. We propose that chemosuppression may be a general guidance mechanism that acts in intermediate targets to prime growth cones to perceive other, more specific cues.

Glia, neurons, and axon pathfinding during optic chiasm development.

The importance of vision in the behavior of animals, from invertebrates to primates, has led to a good deal of interest in how projection neurons in the retina make specific connections with targets in the brain. Recent research has focused on the cellular interactions occurring between retinal ganglion cell (RGC) axons and specific glial and neuronal populations in the embryonic brain during formation of the mouse optic chiasm. These interactions appear to be involved both in determining the position of the optic chiasm on the ventral diencephalon (presumptive hypothalamus) and in ipsilateral and contralateral RGC axon pathfinding, development events fundamental to binocular vision in the adult animal.

Retinal ganglion cell axon guidance in the mouse optic chiasm: expression and function of robos and slits.

The ventral midline of the nervous system is an important choice point at which growing axons decide whether to cross and project contralaterally or remain on the same side of the brain. In Drosophila, the decision to cross or avoid the CNS midline is controlled, at least in part, by the Roundabout (Robo) receptor on the axons and its ligand, Slit, an inhibitory extracellular matrix molecule secreted by the midline glia. Vertebrate homologs of these molecules have been cloned and have also been implicated in regulating axon guidance. Using in situ hybridization, we have determined the expression patterns of robo1,2 and slit1,2,3 in the mouse retina and in the region of the developing optic chiasm, a ventral midline structure in which retinal ganglion cell (RGC) axons diverge to either side of the brain. The receptors and ligands are expressed at the appropriate time and place, in both the retina and the ventral diencephalon, to be able to influence RGC axon guidance. In vitro, slit2 is inhibitory to RGC axons, with outgrowth of both ipsilaterally and contralaterally projecting axons being strongly affected. Overall, these results indicate that Robos and Slits alone do not directly control RGC axon divergence at the optic chiasm and may additionally function as a general inhibitory guidance system involved in determining the relative position of the optic chiasm at the ventral midline of the developing hypothalamus.

Acute nonlymphoblastic leukemia in infants: clinical presentation and outcome.

The presenting features and clinical outcome of acute nonlymphoblastic leukemia (ANLL) in infants and older children were compared to identify any differences that might suggest methods to improve therapy. Twelve of the 29 infants were boys and 17 were girls, with ages ranging from two days to 12 months (median, 7 months). By comparison with 222 patients greater than 1 year of age, infants were significantly more likely to have monoblastic or myelomonoblastic leukemia (P less than .0001), chloroma (P less than .0001), marked hepatomegaly (P = .001), and high leukocyte count (P = .005) and were less likely to have Auer rods (P less than .001). Each of these features except leukocyte count showed an association with infant ANLL in a multivariate analysis. Twenty-four (83%) of the infants attained a complete remission, a rate that was not significantly different from that of the older children. Even though infants had a significantly higher CNS relapse rate (P = .003), their event-free survival times were no different than those of older children (P = .74). Ten of the infants remain in initial complete remission for 5+ to 112+ months (median, 52+ months). Infants with ANLL did not have a poorer prognosis than older patients in our study; future protocols for this age group should emphasize more effective systemic therapy, preferably including an epipodophyllotoxin, as well as improved treatment for subclinical CNS leukemia.

Isolated ductus arteriosus aneurysm in the fetus and infant: a multi-institutional experience.

OBJECTIVES: The purpose of this study was to describe the clinical characteristics and outcome and to elucidate the pathogenesis of ductus arteriosus aneurysm (DAA). BACKGROUND: Ductus arteriosus aneurysm is a rare lesion that can be associated with severe complications including thromboembolism, rupture and death. METHOD: We reviewed the clinical records, diagnostic imaging studies and available histology of 24 cases of DAA, diagnosed postnatally (PD) in 15 and antenatally (AD) in 9 encountered in five institutions. RESULTS: Of PD cases, 13 presented at <2 months, and all AD cases were detected incidentally after 33 weeks of gestation during a late trimester fetal ultrasound study. Of the 24, only 4 had DAA-related symptoms and 6 had associated syndromes: Marfan, Smith-Lemli-Opitz, trisomies 21 and 13 and one possible Ehlers-Danlos. Three had complications related to the DAA: thrombus extension into the pulmonary artery, spontaneous rupture, and asymptomatic cerebral infarction. Six underwent uncomplicated DAA resection for ductal patency, DAA size or extension of thrombus. In the four examined, there was histologic evidence of reduced intimal cushions in two and abnormal elastin expression in two. Five of the 24 died, with only one death due to DAA. Of 19 survivors, all but one remain clinically asymptomatic at a median follow-up of 35 months; however, two have developed other cardiac lesions that suggest Marfan syndrome. A review of 200 consecutive third trimester fetal ultrasounds suggests an incidence of DAA of 1.5%. CONCLUSIONS: Ductus arteriosus aneurysm likely develops in the third trimester perhaps due to abnormal intimal cushion formation or elastin expression. Although it can be associated with syndromes and severe complications, many affected infants have a benign course. Given the potential for development of other cardiac lesions associated with connective tissue disease, follow-up is warranted.

Childhood acute promyelocytic leukemia: a rare variant of nonlymphoid leukemia with distinctive clinical and biologic features.

Of 251 consecutive cases of childhood acute nonlymphocytic leukemia (ANLL) seen at St. Jude Children's Research Hospital over a 12-year period, 16 (6.4%) were classified as promyelocytic according to the French-American-British definition. Patients with this form of leukemia were older at diagnosis than the group representing all other ANLL subtypes (median age, 14.8 vs. 9.0 years); they had lower leukocyte counts (median, 4.5 vs. 25.9 x 10(9)/liter), and a higher percentage were girls (68% vs. 44%). They also were much more likely to have a coagulation abnormality (75% vs. 13%). Only 44% of the promyelocytic group achieved complete remission, compared with 79% of the remaining patients (p = 0.001); however, after a median follow-up of 3.5 years, all but two of the responding patients with promyelocytic leukemia remain in complete remission. The majority of induction failures in the promyelocytic group (six of nine) resulted from complications that developed during periods of marrow hypoplasia or before hypoplasia was induced; whereas in the comparison group, more than half of the patients who failed had evidence of absolute or relative drug resistance. It is concluded that acute promyelocytic leukemia in children differs sufficiently from other subtypes of childhood ANLL to justify clinical trials of selective therapy. Recommendations for the use of heparin and blood component support in these patients are given.

Enterobacter cloacae pericardial effusion in a frail elderly patient.

We report a case of a frail 82-year-old man with seronegative rheumatoid arthritis and a recent pacemaker insertion, admitted with pulmonary oedema and a symptomatic pericardial effusion. He was treated with diuretics and an urgent pericardiocentesis, a sample from which cultured Enterobacter cloacae. A subsequent abdominal CT scan revealed faecal loading, an abnormal anorectal canal and sigmoid colon and a bowel perforation. Endoscopy, biopsies and histopathology confirmed a diagnosis of cytomegalovirus (CMV) colitis with coexistent fungal infection. The E. cloacae infection was successfully treated with 6 weeks of intravenous meropenem, while the CMV and fungal infections were treated with a combination of valganciclovir and fluconazole. We postulate that the bowel perforation resulted from a combination of CMV colitis, faecal loading and steroid therapy and led to bacterial translocation of E. cloacae and the development of the pericardial effusion. This case represents an unusual pathophysiology for the development of an E. cloacae pericardial effusion.

Postnatal maturation of neurons in the cat's lateral geniculate nucleus.

The maturation of dendrites in the cat's dorsal lateral geniculate nucleus was studied in Golgi Kopsch preparations of kittens from 3 days to 8 weeks postnatal. During the first postnatal week, more than a month after their birthdate, cells are immature and lack dendrites, bearing only multiple somatic processes or a few short thick extensions. Cells enter an active phase of dendritic extension during the second postnatal week. Growth cone-like structures and filopodia occur at the ends of dendrites and also at dendritic branch points. Assignment to general cell classes based on dendritic disposition is possible only after this period, and characteristic grapelike appendages are obvious after the third week. Mature cells in the lateral geniculate nucleus are not considered spiny, yet spines and hairs are ubiquitous on most cells once dendrites elongate and remain numerous on peripheral dendrites even after the soma and proximal dendrites become smooth, by 4-6 weeks. The decline of spine levels continues after this period. All cells go through a similar but nonsynchronous sequence of maturation. Large cells may mature first, but no correlation was noted between rate of maturation and laminar location or retinal representation. In the second and third postnatal weeks, although the terminal arbors of retinal axons presynaptic to geniculate cells have already attained their final topography and laminar placement, the shape and synaptic relations of axon terminal swellings remain immature (Mason, '82a,b) through the most active phase of dendritic outgrowth. After 3 weeks, both retinal axons and target geniculate cell dendrites finalize the shapes of characteristics appendages and synaptic relations in tandem. Potential interactions between immature axon terminal arbors and dendrite-bare geniculate cells during dendrite outgrowth and subsequent remodeling of structural details are discussed.

Early climbing fiber interactions with Purkinje cells in the postnatal mouse cerebellum.

The time and place of initial contacts between afferent axons and their target cells are not known for most regions of the mammalian CNS. To address this issue, we have selectively visualized afferent climbing fiber axons together with their synaptic targets, Purkinje cells, in postnatal mouse cerebellum. Climbing fibers were orthogradely labeled by injection of rhodamine isothiocyanate into their brainstem source, the inferior olivary nucleus. Purkinje cells were localized with an antibody to a calcium-binding protein, calbindin D-28k (CaBP), in the same section or in adjacent sections. A novel view of the olivocerebellar projection and the morphology of climbing fiber arbors prior to the well-known "nest" stage has emerged from this analysis. At birth, climbing fibers project into the zone of Purkinje cells, before these cells have aligned into a monolayer. During this phase, climbing fibers have simple morphologies consisting of relatively unbranched terminal arbors and small tapered growing tips. Purkinje cells are arranged 3-6 cells deep and have tufted dendrites and relatively smooth somata. By postnatal days 3-4, climbing fibers branch over several adjacent Purkinje cell perikarya, which are still organized in a band several cells thick. From postnatal days 5-7, when climbing fibers subsequently make focused nests on individual cells, Purkinje somata are smoother and form a more distinct monolayer. Up to this time, however, climbing fibers continue to associate with Purkinje perikarya, even though Purkinje cell dendrites have emerged and branched extensively. By postnatal days 8-10, climbing fiber terminals climb onto the trunk of the relatively mature Purkinje dendritic tree. At birth, mossy fibers originating from the pontine nuclei resemble immature climbing fibers in that they also have a simple unbranched morphology and growing tips, but project only so far as the internal granule cell layer. Occasional individual fibers reach into the Purkinje zone both at postnatal day 0 and postnatal day 4, confirming that the fibers formerly described as "combination fibers" (Mason and Gregory, S4. J. Neurosci, 4:1715-1735) can be mossy in origin. These data demonstrate that climbing fibers project among Purkinje cells earlier than suspected, before these afferents begin to arborize and form pericellular nests. Our observations are not in accord with the view derived from autoradiographic tracing studies that as in other cortical areas, climbing afferents wait in the vicinity of Purkinje cells in the early neonatal period, then advance onto these cells in synchrony with Purkinje cell alignment into a monolayer and dendritic maturation.(ABSTRACT TRUNCATED AT 400 WORDS)

Nr-CAM expression in the developing mouse nervous system: ventral midline structures, specific fiber tracts, and neuropilar regions.

Nr-CAM is a member of the L1 subfamily of cell adhesion molecules (CAMs) that belong to the immunoglobulin superfamily. To explore the role of Nr-CAM in the developing nervous system, we prepared specific antibodies against both chick and mouse Nr-CAM using recombinant Fc fusion proteins of chick Nr-CAM and mouse Nr-CAM, respectively. First, we show the specificity of the new anti-chick Nr-CAM antibody compared with a previously employed antibody using the expression patterns of Nr-CAM in the chick spinal cord and floor plate and on commissural axons, where Nr-CAM has been implicated in axon guidance. Using the anti-mouse Nr-CAM antibody, we then studied the expression patterns of Nr-CAM in the developing mouse nervous system along with the patterns of two related CAMs, L1, which labels most growing axons, and TAG-1, which binds to Nr-CAM and has a more restricted distribution. Major sites that are positive for Nr-CAM are specialized glial formations in the ventral midline, including the floor plate in the spinal cord, the hindbrain and midbrain, the optic chiasm, and the median eminence in the forebrain. Similar to what is seen in the chick spinal cord, Nr-CAM is expressed on crossing fibers as they course through these areas. In addition, Nr-CAM is found in crossing fiber pathways, including the anterior commissure, corpus callosum, and posterior commissure, and in nondecussating pathways, such as the lateral olfactory tract and the habenulointerpeduncular tract. Nr-CAM, for the most part, is colocalized with TAG-1 in all of these systems. Based on in vitro studies indicating that the Nr-CAM-axonin-1/TAG-1 interaction is involved in peripheral axonal growth and guidance in the spinal cord [Lustig et al. (1999) Dev Biol 209:340-351; Fitzli et al. (2000) J Cell Biol 149:951-968], the expression patterns described herein implicate a role for this interaction in central nervous system axon growth and guidance, especially at points of decussation. Nr-CAM also is expressed in cortical regions, such as the olfactory bulb. In the hippocampus, however, TAG-1-positive areas are segregated from Nr-CAM-positive areas, suggesting that, in neuropilar regions, Nr-CAM interacts with molecules other than TAG-1.

Studies of retinal representations within the cat's optic tract.

The manner in which each retina can be mapped onto a single cross section of the optic tract of the cat has been defined by neuroanatomical methods. It has been found that the contralateral nasal hemi-retina and both temporal hemi-retinae are represented in each tract by multiple, rough maps which partially overlap one another. All maps show the same general orientation, with area centralis represented dorsomedially, lower retina represented dorsolaterally, and upper retina represented ventromedially. The peripheral part of the horizontal meridian is represented ventrolaterally. Labeling all of the fibers from one eye by axonal degeneration or autoradiographic methods shows that the crossed map is displaced dorsally and medially relative to the uncrossed map, leaving a dorsomedial crescent of pure crossed fibers. Localized retinal lesions or injections of 3H-amino acid show the general orientation of the maps. Lesions within the dorsomedial pure crossed crescent show that fibers in this crescent arise from retinal areas close to the optic disc, near the site of the early fetal fissure. Localized injections of horseradish peroxidase into the optic tract show the relationships of the several maps in terms of the retinal distribution of retrogradely labeled retinal ganglion cells. They show that axons of large and small cells map ventrolaterally in the tract while intermediate sizes map dorsomedially. They confirm that the crossed map is displaced relative to the uncrossed maps. It is suggested that the optic tract develops by fibers taking a position in the tract in accordance with their time of arrival at the chiasm. The several maps are displaced because they develop sequentially and the optic tract can be read as a developmental record, the most dorsomedial axons being the oldest.

An analysis of the retinal afferents to the cat's medial interlaminar nucleus and to its rostral thalamic extension, the "geniculate wing".

The retinal afferents to the medial interlaminar nucleus and to its rostro-dorsal extension at the edge of the pulvinar have been studied in cats by fiber degeneration and autoradiographic methods. Fiber degeneration following section of one optic nerve shows two distinct retinal inputs: One is coarse-fibered and goes to the medial interlaminar nucleus itself; the second, which is fine-fibered, goes to the rostral and medial borders of the medial interlaminar nucleus and continues into the pulvinar. The regions in receipt of these fine fibers have been called the "geniculate wing". The topography of retinal representations and the degree of binocular overlap within the medial interlaminar nucleus and the wing have been studied by combining intraocular injections of 3H proline with local lesions of the injected eye, or with removal of the non-injected eye. In the medial interlaminar nucleus three distinct laminae are recognizable and are particularly clearly shown in horizontal sections. Rostrally and medially, lamina 1 maps the contralateral nasal retina as a mirror reversal of lamina A. Posterior and lateral to this, lamina 2 maps the ipsilateral temporal retina as a mirror reversal of lamina A1. Lamina 3 lies closest to the optic tract and receives crossed afferents from the temporal retina. In this lamina, which is the smallest, vertical retinal dimensions map as in the other layers, but we were unable to determine the mapping of the horizontal dimensions. In the geniculate wing, as in the other geniculate layers, vertical lines of the visual field are mapped as corresponding vertical diencephalic dimensions; horizontal retinal dimensions are mapped as horizontal lines in the wing, with central retinal areas represented furthest from the optic tract. In the geniculate wing the contralateral nasal and ipsilateral temporal retina are mapped with considerable binocular overlap. The crossed temporal retina has no demonstrable representation in the wing.

Domains of regulatory gene expression and the developing optic chiasm: correspondence with retinal axon paths and candidate signaling cells.

In mammals, some axons from each retina cross at the optic chiasm, whereas others do not. Although several loci have been identified within the chiasmatic region that appear to provide guidance cues to the retinal axons, the underlying molecular mechanisms that regulate this process are poorly understood. Here we investigate whether the earliest retinal axon trajectories and a cellular population (CD44 and stage-specific embryonic antigen 1 [SSEA] neurons), previously implicated in directing axon growth in the developing chiasm (reviewed in Mason and Sretavan [1997] Curr. Op. Neurobiol. 7:647-653), correlate with the expression patterns of several regulatory genes (BF-1, BF-2, Dlx-2, Nkx-2.1, Nkx-2.2, and Shh). These studies demonstrate that gene expression patterns in the chiasmatic region reflect the longitudinal subdivisions of the forebrain in that axon tracts in this region generally are aligned parallel to these subdivisions. Moreover, zones defined by overlapping domains of regulatory gene expression coincide with sites implicated in providing guidance information for retinal axon growth in the developing optic chiasm. Together, these data support the hypothesis that molecularly distinct, longitudinally aligned domains in the forebrain regulate the pattern of retinal axon projections in the developing hypothalamus.