A POU domain transcription factor-dependent program regulates axon pathfinding in the vertebrate visual system.

Axon pathfinding relies on the ability of the growth cone to detect and interpret guidance cues and to modulate cytoskeletal changes in response to these signals. We report that the murine POU domain transcription factor Brn-3.2 regulates pathfinding in retinal ganglion cell (RGC) axons at multiple points along their pathways and the establishment of topographic order in the superior colliculus. Using representational difference analysis, we identified Brn-3.2 gene targets likely to act on axon guidance at the levels of transcription, cell-cell interaction, and signal transduction, including the actin-binding LIM domain protein abLIM. We present evidence that abLIM plays a crucial role in RGC axon pathfinding, sharing functional similarity with its C. elegans homolog, UNC-115. Our findings provide insights into a Brn-3.2-directed hierarchical program linking signaling events to cytoskeletal changes required for axon pathfinding.

Serum immunoreactive erythropoietin levels and associated factors amongst HIV-infected children.

OBJECTIVES: To determine the spectrum of serum immunoreactive erythropoietin (SIE) levels amongst HIV-infected children aged < 13 years in relation to the levels among healthy children as well as those with renal failure; to examine the relationship between clinical and laboratory parameters and SIE levels. DESIGN: A cross-sectional study with a descriptive non-interventional format. HIV-infected Canadian subjects were recruited through four tertiary Canadian and one Bahamian centre. Children with renal failure and healthy children were recruited from one of the Canadian centres. METHODS: Study subjects had clinical and laboratory profiles determined at baseline and at each of five follow-up periods over 1 year. SIE levels were measured by radioimmunoassay with a normal range of 12-28 IU/I. Data handling and statistical functions were performed by the Canadian HIV Trials Network. RESULTS: The study enrolled 133 HIV-infected subjects and 38 controls. Of these, 117 HIV-infected subjects, 24 healthy controls, and 11 controls with renal failure were eligible for analysis. The median age of infected subjects was 44 months, whereas that of healthy controls was 56 months, and 95 months for controls with renal failure. The median SIE levels were 14 and 11 IU/I for subjects with renal failure and healthy subjects, respectively. The median SIE level was 61 IU/I among zidovudine (ZDV)-treated subjects and 22 IU/I among ZDV-naive HIV-infected subjects. HIV-infected children almost invariably had SIE levels < 200 IU/I. The median SIE levels amongst HIV-infected subjects whose hemoglobin levels were < 100 g/l were 98 and 31 IU/I for ZDV-treated and ZDV-naive subjects, respectively (P = 0.002). This difference in median SIE levels between ZDV-treated subjects and ZDV-naive subjects was also observed among subjects whose hemoglobin levels were > 100 g/l (median, 58 and 15 IU/l, respectively; P < 0.001). Hemoglobin level was the most important predictor of log10 SIE (P < 0.01 for ZDV-treated and ZDV-naive subjects). CONCLUSIONS: SIE levels amongst HIV-infected children were affected by HIV infection, use of ZDV, and presence or absence of anemia. SIE levels amongst HIV-infected children were generally lower than 200 IU/I. This characterization of SIE levels will facilitate clinical trials of exogenous recombinant human erythropoietin in HIV-infected children with anemia.

Role of intravenous immunoglobulin G in autoimmune hematologic disorders.

The data presented in this review established IVIG therapy as an important treatment modality in the autoantibody-mediated cytopenias and coagulation disorders of both children and adults. The immediate response to therapy is thought to be related to nonspecific Fc-receptor blockade of mononuclear phagocytes in the reticuloendothelial system (autoimmune cytopenias), or to idiotypic antibody interaction with pathologic autoantibodies (in acquired coagulation disorders). Although less frequent, long-term responses to IVIG therapy are reported. Such responses must involve an immunomodulating effect of IgG that influences T- and B-cell function, with inhibition of pathologic autoantibody formation. It is possible that idiotypic antibody interactions play a part in long-term responses. The experience with IVIG therapy in the decade following Imbach's important observations reported in 1981 has provided a sound data base regarding the use of this important therapy in patients with autoimmune hematologic disorders. The challenge of the next decade should be to further investigate mechanisms of action of this important therapy and to conduct carefully controlled studies to answer specific clinical questions. Examples of such questions include the following: (1) Can IVIG therapy, administered early in the course of illness in selected children with acute ITP, decrease the incidence of chronic ITP?; (2) Is maintenance, high-dose IVIG therapy a cost-effective method for the management of patients with chronic ITP refractory to corticosteroid therapy and splenectomy?; and (3) Can IVIG therapy administered to selected pregnant women with ITP significantly reduce the incidence of serious thrombocytopenia in their offspring? In conducting these studies, consideration should be given to the type of IVIG preparation used and to the treatment protocol implemented. It is evident that responses to unmodified IgG preparations (with the Fc-receptor part of the molecule left largely intact) are superior to preparations that have been modified during preparation. Responses are also likely to be dose-related. The data reported for IVIG therapy in patients with acquired factor VIII deficiency suggests that the idiotypic antibody content of IgG preparations is also of importance; if so, preparations selected from specific donor pools (for example, multiparous women) known to contain higher levels of circulating autoantibodies (than those from primiparous women or untransfused males) may provide a degree of benefit not seen with standard IVIG preparations. It is therefore important that clinicians and laboratory specialists work closely together in the design and conduct of future clinical trials initiated to answer those important clinical questions raised by the first decade of observations with IVIG therapy for autoimmune hematologic disorders.

Retinal crowding alters the morphology of alpha ganglion cells.

It has been hypothesized that the dendritic field size of individual retinal ganglion cells is regulated early in development by interactions among neighboring cells (Wässle and Reiman, Proc. R. Soc. Lond. B. 200:441-461, 1978). An opportunity to test this hypothesis is provided by the consequences of prenatal enucleation that results in an increased density of ganglion cells in the remaining retina (Chalupa et al., Neuroscience 12:1139-1146, 1984). In the present study we compared dendritic field diameters of alpha ganglion cells in normal retinas to those of adult cats that were monocularly enucleated before birth (on embryonic days 42 and 51) and 6 days after birth. In each animal ganglion cells were labeled by central injections of horseradish peroxidase. The retinas were incubated according to a modified Hanker-Yates procedure and whole-mounted. All cells were sampled from the temporal retina along a corridor established by a line drawn through the optic disk and the area centralis. Alpha cells were differentiated into ON and OFF subtypes on the basis of their level of stratification in the inner plexiform layer (Peichl and Wässle, Proc. R. Soc. Lond. B. 212:139-156, 1981). Dendritic field and soma diameters were determined from complete cell drawings by calculating the mean of the two longest orthogonal diameters. Our main findings are: Early monocular enucleation does not disrupt the mosaics of ON and OFF alpha ganglion cells in the remaining retina of adult animals. Within the retinal corridor sampled, the density of alpha cells was substantially greater than normal in the remaining retina of the prenatal enucleates at all eccentricities except the far periphery. Except at the far periphery, the dendritic field diameters of the prenatally enucleated animals were significantly smaller than normal when compared at equivalent eccentricities. However, when compared at equivalent cell densities, dendritic field dimensions in the prenatally enucleated and normal animals were found to be similar. In the prenatal enucleates the somas of these neurons were also significantly smaller than normal. If the monocular enucleation was postnatal, however, the density and the dendritic field diameters of alpha cells did not differ appreciably from normal. These results indicate that the morphology of retinal ganglion cells can be influenced by increasing the density of developing ganglion cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographic organization in the retinocollicular pathway of the fetal cat demonstrated by retrograde labeling of ganglion cells.

The topographic organization of the developing retinocollicular pathway was assessed by making focal deposits of a retrograde tracer (usually rhodamine latex beads) into the superficial layers of the superior colliculus of fetal cats at known gestational ages. Subsequently, the distributions of labeled cells in the contralateral and ipsilateral retinas were examined. At all stages of development, a high density of labeled cells was found in a delimited area (core region) of both retinas. The locations of the retinal regions containing the high density of labeled cells varied with the locus of the tracer deposit in the superior colliculus in a manner consistent with the topographic organization of the mature cat's retinocollicular pathway. Additionally, some labeled ganglion cells, considered to be ectopic, were found to be scattered throughout the contralateral and ipsilateral fetal retinas. Such ectopic cells were few in number throughout prenatal development. For every 100 cells projecting to the appropriate region of the colliculus, we estimate that less than one ganglion cell makes a gross projection error. The incidence of ectopic cells did not differ between the contralateral and ipsilateral retina, even though the overall density of crossed labeled cells was always greater than that of uncrossed labeled cells. In the youngest fetal animals, tracer deposits into the caudal portion of the superior colliculus resulted in a core region of labeled cells in the contralateral nasal retina as well as in the nasal ipsilateral retina. Such uncrossed nasal cells, not seen in more mature animals, appear to innervate the appropriate topographic location of the superior colliculus, but on the wrong side of the brain. Most likely, these uncrossed nasal ganglion cells contribute to the widespread distribution of the ipsilateral retinocollicular pathway observed in fetal cats after intraocular injections of anterograde tracers (Williams and Chalupa, 1982). Collectively, our findings demonstrate that the developing retinocollicular pathway of the fetal cat is characterized by a remarkable degree of topographic precision.

Quantitative analysis of the optic nerve of the North American opossum (Didelphis virginiana): an electron microscopic study.

On the basis of an electron microscopic examination of the optic nerve of the North American opossum it was estimated that the nerve has approximately 100,000 axons, of which 98% are myelinated. The myelinated axons ranged from 0.3 to 6.7 micrometers in diamter (mean 1.6 micrometers), while unmyelinated axons were 0.2 to 1.6 micrometers in diameter (mean 0.6 micrometers). Axoplasm and axon (axoplasm plus myelin) diameter spectrums were unimodal and positively skewed. The mean of the ratio of axoplasm to axon diameter was 0.69. However, this ratio varied widely across axons and was nonlinearly distributed, decreasing with axon diameter. An inverse relationship between axon density and a high proportion of large-caliber axons was located dorsally in a cross section obtained near the eye. However, in sections obtained near the optic chiasm, regions having the highest proportion of large-diameter axons were in the ventral periphery of the nerve. It is suggested that within the 40-mm length of the nerve, there may be a change from a retinotopic organization of axons according to their diameter and central targets.

Studies of physiology and the morphology of the cat LGN following proton irradiation.

PURPOSE: We have examined the effects of proton irradiation on the histologic and receptive field properties of thalamic relay cells in the cat visual system. The cat lateral geniculate nucleus (LGN) is a large structure with well-defined anatomical boundaries, and well-described afferent, efferent, and receptive field properties. METHODS AND MATERIALS: A 1.0-mm proton microbeam was used on the cat LGN to determine short-term (3 months) and long-term (9 months) receptive field effects of irradiation on LGN relay cells. The doses used were 16-, 40-, and 60-gray (Gy). RESULTS: Following irradiation, abnormalities in receptive field organization were found in 40- and 60-Gy short-term animals, and in all of the long-term animals. The abnormalities included "silent" areas of the LGN where a visual response could not be evoked and other regions that had unusually large or small compound receptive fields. Histologic analysis failed to identify cellular necrosis or vascular damage in the irradiated LGN, but revealed a disruption in retinal afferents to areas of the LGN. CONCLUSIONS: These results indicate that microbeam proton irradiation can disrupt cellular function in the absence of obvious cellular necrosis. Moreover, the area and extent of this disruption increased with time, having larger affect with longer post-irradiation periods.

Early axon outgrowth of retinal ganglion cells in the fetal rhesus macaque.

Employing retinal explants and retrograde transport techniques, we studied the formation of the arcuate fascicles by examining the growth of the central retina, the emergence of the adult fiber layer pattern, and the projections of retinal ganglion cells in the central and peripheral retina. Sixty days prior to foveal pit formation, the distance from the incipient fovea to the optic disk was equal to the adult, even though the retinal area was only 8% of the adult. Arcuate fibers, at this age, were observed to avoid the incipient fovea, with no fascicles and few axons projecting over this region. A small population of 15.2% of the ganglion cells located within 2 mm of the incipient fovea possessed an axon with an aberrant trajectory that wound around and projected 50 to several hundred microns away from the optic disk, compared to only 3% at other retinal locations. The incidence of disorder decreased with increasing fetal age, establishing mature values in late fetal periods. These findings suggest that the area of the central retina does not increase after embryonic day 60 and that guidance factors are present that allow outgrowing ganglion cell axons to distinguish and avoid that portion of the retina that will become the fovea.

Development of the optic nerve of the opossum (Didelphis virginiana).

The development of the optic nerve of a marsupial, the North American opossum, was examined in 24 animals from postnatal days 5 to 78 (P5-P78): gestation is 13 days. The estimated number of axons increased from 24,000 at P5, to 267,000 at P27, approximately 2.7 times the mean number in the adult. Following P27, axon numbers decreased rapidly to 140,000 at P40, then decreased more slowly, attaining adult values between P50 and P59. Thus, the opossum is similar to placental mammals examined in evidencing an overproduction and later attenuation to adult values in the number of axons in the optic nerve during development. Monocular enucleation of 3 animals at P17, 10 days before peak axon counts, resulted in a mean population increase of 24,000 (range 19,000-30,000) above the normal adult mean. Additionally, a 4th animal monocularly enucleated on P7, 3 days prior to the arrival of migrating fibers to central target sites, had a similar value of 26,500 supernumerary axons. Our findings in the opposum, when coupled with previous reports in other mammals, suggest that binocular interactions during development account only for optic nerve axon loss approximately equal in magnitude to the ipsilateral projection from one eye.

Treatment of macular degeneration with proton beams.

Subfoveal neovascular membranes (SNVMs) are a leading cause of severe visual loss in the elderly in the United States. Previously, the only treatment that could halt progression of this disease was laser photocoagulation, which was, however, accompanied by immediate reduction in visual acuity. A single narrow proton beam was used to irradiate 45 patients to either 8 or 14 Cobalt Gray Equivalent. The alignment technique and dosimetry of these treatments are described. The proton beam direction, range, and modulation were planned with the assistance of an eye-specific planning program. A single anterior beam was used, with patients looking nasally toward a blinking fixation light at an angle of 30 degrees. Patients were aligned using a light field projected through a slit collimator. Patients' positions were monitored during treatment with a short-focal-length camera. Depth dose in a flat phantom was measured with a small-diameter parallel plate ionization chamber. Lateral profiles were measured at several depths with silver halide film. Each treatment session lasted 15 min, of which 1 min consisted of beam delivery. The proton beam stopped in the orbital cavity, delivering no primary proton dose to the brain. Dose to the center of the lens of the involved eye was less than 0.5% of the dose delivered to the macula. Treatments of SNVMs with proton beams require only a short visit to the hospital, little immobilization effort, and a minimal amount of treatment room and beam time. Compared to previous treatment trials using x-ray beams, the dose to nonocular tissues is reduced significantly.

Retinal input to the suprachiasmatic nucleus before and after puberty in Djungarian hamsters.

Retinal projections to the suprachiasmatic nucleus (SCN) mediate the effect of photoperiod to entrain circadian rhythms and to control reproductive maturation in the Djungarian hamster. To determine whether the retinal innervation of the SCN had fully developed by the onset of puberty in this hamster species, prepubertal and postpubertal hamsters received an intraocular unilateral injection of horseradish peroxidase (HRP), and after 24 h, the anterograde transport of HRP to the SCN was studied. In prepubertal hamsters, the retinohypothalamic tract (RHT) was found to project to the medial and caudal SCN, principally the ventrolateral regions and, to an extent, the dorsomedial portion of the nucleus. RHT innervation was asymmetric; the SCN contralateral to the monocular injection received the dominant projection. A similar pattern of retinal projections was found postpubertally; however, the ipsilateral SCN was less extensively labelled with HRP and smaller as determined by Nissl counterstain compared to that in prepubertal hamsters. These findings indicate that modifications in the retinal innervation of the SCN occur as late as puberty, and may be part of a developmental change in the mechanism which processes photoperiodic information during sexual maturation.

The role of leukocyte traffic and activation in parturition.

OBJECTIVE: This review focuses on the contribution of immune cell trafficking and activities during the initial phase of activation in the process of parturition. Although uterine contractile activity has been the predominant focus for the mechanism that initiates labor, significant cellular and biochemical changes cause remodeling of the cervix well before term. A convergence of evidence suggests that inflammatory processes that involve prostaglandins, nitric oxide, cytokines, as well as systemic and paracrine endocrine mediators may enhance uterine contractility, promote ripening of the cervix, and thus constitute an integrative hypothesis for the initiation of labor. METHODS: Techniques to study the uterus and cervix of pregnant and virgin C3H/HeN mice included light and fluorescence microscopy. Tissues were processed by histochemistry and immunofluorescence. Analytic approaches to enumerate macrophages and assess activation included quantitative stereologic morphometry and laser scanning cytometry. RESULTS: The transition between relative quiescence of the uterus and enhanced contractility involved migration of macrophages from the uterine endometrium and activation of macrophages in the cervix. Before birth, macrophages migrate into the cervix and are activated in the myometrium. CONCLUSION: Immune cell trafficking and activation are part of the initial mechanism that promotes ripening of the cervix, enhances uterine contractility, and initiates parturition. Markers for the conclusion of pregnancy may have diagnostic or therapeutic value to assess the normal progress of labor or identify women at risk of preterm labor.

Deferoxamine toxicity in hepatoma and primary rat cortical brain cultures.

Deferoxamine is commonly used for treatment of iron intoxication. Because the usual dose is unable to chelate sufficient iron before severe injury occurs, "high-dose" deferoxamine treatment has been proposed. However, several authors have reported severe toxicity after deferoxamine therapy. Although the hemodynamic effects are well described, the cellular toxicity of deferoxamine is unknown. Accordingly, we investigated the cellular toxicity of deferoxamine using in vitro techniques in two cell lines. Brain cells were harvested from fetal rats and cultured for 14-21 days before deferoxamine exposure. Using similar techniques, rat hepatoma cells were grown until confluent. Deferoxamine was added to the cultures to achieve final concentrations of 200-800 microg/ml, corresponding to in vivo infusion rates of 15-60 mg/kg/h. Deferoxamine was removed after 3 or 6 days by changing the medium. Subtoxic FeCl3 (500 mg/dl) was concurrently added to identical cultures to determine if deferoxamine potentiated iron toxicity. Cell viability was measured by a colorimetric assay. The addition of deferoxamine (0.2, 0.4, 0.8 mg/ml) significantly decreased cell viability in both cell groups. The effect of deferoxamine on primary cortical brain cultures was similar for the three concentrations used, and was similar when examined either 72 h or 6 days later. In contrast, hepatoma cell cultures evidenced a dose- dependent cell loss that increased with the length of exposure. The addition ofsubtoxic amounts of FeCl3 (500 microg/dl) in the presence of deferoxamine was protective in all cultures, and abolished deferoxamine-induced cell loss. Interestingly, the addition of serum albumin significantly reduced the amount of iron present in cells, suggesting its potential use to treat iron toxicity. These results suggest that deferoxamine, in the absence of iron, is toxic to cortical brain and hepatoma cells in vitro.

Number and distribution of retinal ganglion cells in anubis baboons (Papio anubis).

The number of retinal ganglion cells in Papio anubis was determined from light microscopic observations of wholemounted and vertically sectioned retinal tissue and electron microscopic examination of optic nerve cross sections. The total number of ganglion cells ranged from 1.41 to 1.81 million (mean 1.58 million, n = 6, SD = 169,927) per retina. The distribution of ganglion cells in cresyl violet stained wholemounts was also examined. Isodensity contours were almost circular perifoveally, but became horizontally elongated outside of the central retina, providing strong evidence for a visual streak. Ganglion cell somata within the streak were found to be significantly smaller than those outside of the streak in comparing regions of equal density. Finally, the distribution of blood vessels within the retina formed a watershed pattern with its crux centered on the ridge of this horizontally oriented high-density zone. Combined, these features indicate that anubis baboons possess a visual streak specialization as reported for lagomorphs, felines, and several primate species. Further, the visual streak appears more pronounced in anubis baboons than in any other primate species studied to date, with the possible exception of Homo sapiens, a similarly ground-dwelling/foraging and secondarily terrestrial species.

Receptive field properties and latencies of cells in the lateral geniculate nucleus of the North American opossum (Didelphis virginiana).

Relay cells in the lateral geniculate nucleus (LGN) of the North American opossum were classified as types 1, 2, and 3 on the basis of their receptive field properties and afferent latencies to optic nerve (ON) and optic chiasm (OX) stimulation and antidromic latencies to stimulation of cortex. Type 1 and type 3 cells gave transient responses and type 2 cells gave sustained responses to appropriate standing contrast in the receptive field center. The differences in response pattern were quantified with a phasic-tonic index (PTI); the PTI values for type 2 cells (PTI less than 63) did not overlap those for type 1 cells (PTI greater than 68) or type 3 cells (PTI greater than 80). With a homogeneous field (1.3 cd/m2), the maintained discharge rates (spikes/s) of type 3 cells (less than 1-11) were significantly lower than those of type 1 cells (3-23) and of type 2 cells (1-22). For all type 2 cells tested with a counterphased sine-wave grating, a null position of the grating was found and the cells were classified as linear. The type 1 and type 3 cells tested were nonlinear (i.e., exhibited excitatory doubling and did not have a null grating position). The maximum velocity of movement that reliably elicited responses (cut-off velocity) was low for type 3 cells (mean = 29.1 degrees/s) and relatively high for type 1 cells (mean = 70.3 degrees/s). Cut-off velocities for type 2 cells (mean = 61.2 degrees/s) were slightly lower than for type 1 cells. Type 1 cells had relatively short afferent (ON and OX) latencies, fast afferent conduction velocities, and short antidromic (cortex) latencies; type 2 cells had intermediate afferent and antidromic latencies and intermediate afferent conduction velocities; and type 3 cells had relatively long afferent and antidromic latencies and slow afferent conduction velocities. The receptive field center diameters were similar for type 1 (4.2-25.5 degrees; mean = 11.4 degrees) and type 3 cells (2.8-23.7 degrees; mean = 11.0 degrees), whereas the receptive field centers for type 2 cells (2.9-15.1 degrees; mean = 6.9 degrees) were significantly smaller. The majority of type 1 (66.7%) and type 3 cells (63.3%) had on-center receptive fields, whereas the proportion of on-center fields was even greater for type 2 cells (83.0%). Only a few of the cells encountered in the opossum LGN (6%) had on-off receptive fields, and a portion of these could not be shown to be relay cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphogenesis of retinal ganglion cells: a model of dendritic, mosaic, and foveal development.

Much is known about the morphological development of mammalian retinal ganglion cells. However, relatively little is understood about the mechanisms that direct dendritic development and determine ganglion cell morphology, mosaic organization, and foveal development. In the following, we review current data on primate retinal ganglion cell development and integrate it with information from other species into a general model of dendritic development. Furthermore, we propose that this model not only explains ganglion cell dendritic development, but also accounts for the establishment of retinal ganglion cell mosaics and the timing of foveal formation.

Early dendritic outgrowth of primate retinal ganglion cells.

The pattern of dendritic stratification of retinal ganglion cells in the fetal monkey (Macaca mulatta) was examined using horseradish peroxidase and retinal explants. Ganglion cells in the rhesus monkey are born between embryonic day (E) 30-70 (LaVail et al., 1983). At E60, E67, and E68, approximately 50% of all ganglion cells within the central 3.0 mm of the retina had dendritic arbors that were unistratified within the inner plexiform layer (IPL), while the remaining 50% had bistratified arbors. Unistratified cells had relatively flat arbors that ramified within a restricted portion of the IPL. In contrast, bistratified cells had one portion of the arbor that branched in the inner half of the IPL and a second portion that branched in the outer half of the IPL. Relatively few bistratified cells were encountered in the central 1.0 mm of the retina but were more numerous with increasing eccentricity. At E81, E90, and E110, the dendritic arbors of ganglion cells increased in both area and complexity, but occupied a relatively small percentage of the total depth of the IPL. The bistratified cells encountered at these fetal ages were typically located in the far retinal periphery. Between E125-E140, the dendritic arbors of individual ganglion cells increased in area and depth to occupy a greater proportion of the total IPL than at earlier fetal ages. These observations suggest that ganglion cells in the macaque undergo at least three stages of dendritic stratification: (1) an initial period of dendritic growth during which the cells have either unistratified or bistratified dendritic arbors; (2) a loss of the majority of bistratified cells through cell death or remodeling of the arbor; and (3) growth or expansion of the arbor to occupy a greater percentage of the total depth of the IPL. The first two stages are similar to recent observations in the fetal cat (Maslim & Stone, 1988) with the exception that dendritic development in the primate lacks an initial diffuse ingrowth to the IPL. Additionally, primate ganglion cells undergo a third stage of dendritic growth in late fetal development during which the arbor occupies a greater proportion of the depth of the IPL.

Transience of astrocytes in the newborn macaque monkey retina.

We investigated the morphology and spatial distribution of retinal astrocytes in newborn and early postnatal macaque monkeys. As in adults, retinal astrocytes in neonatal animals were closely associated with ganglion cell axons and blood vessels. However, in contrast with adults, astrocytes transiently occupied the fovea and perifoveal region in newborns and, to a lesser degree, also in early postnatal animals. The density of the perifoveal astrocytes rapidly declined during the first 2-3 months of life. The results are discussed in relationship to foveal development.

Topographic organization of the retinocollicular projection in the neonatal rat.

The topographic order of the retinocollicular projection in the rat was examined from birth until maturity. Small, localized deposits of rhodamine-filled latex microspheres were placed into the superior colliculus at different locations. To minimize labeling fibers of passage deposit sites were typically, although not exclusively, placed into the caudal-lateral pole of the colliculus. Examination of the area and density of labeled cells in the retinae of these animals led to the following conclusions: (1) At each age examined, the location of the majority of labeled cells was observed to be in appropriate topographic register with the deposit site in the superior colliculus. (2) Confirming the work of previous investigators, errors in topographic projection were observed. These were present in both the contralateral and ipsilateral retinae and decreased with increasing postnatal age. The mature pattern was present by P10. (3) Quantitatively, the number of retinal ganglion cells terminating nontopographically within the colliculus constituted a relatively minor proportion of the total number of labeled cells in both retinae. It is concluded that the majority of the retinal ganglion cells make topographically appropriate terminations within the superior colliculus during development.

Morphogenesis of retinal ganglion cells during formation of the fovea in the Rhesus macaque.

The morphology of retinal ganglion cells within the central retina during formation of the fovea was examined in retinal explants with horseradish-peroxidase histochemistry. A foveal depression was first apparent in retinal wholemounts at embryonic day 112 (E112; gestational term is approximately 165 days). At earlier fetal ages, the site of the future fovea was identified by several criteria that included peak density of ganglion cells, lack of blood vessels in the inner retinal layers, arcuate fiber bundles, and the absence of rod outer segments in the photoreceptor layer. Prior to E112, the terminal dendritic arbor of retinal ganglion cells within the central retina extended into the inner plexiform layer and were located directly beneath their somas of origin or at most were slightly displaced from it. For example, at E90 the mean horizontal displacement of the geometric center of the dendritic arbor from the somas of cells within 600 microns of the estimated center of the future fovea was 4.1 microns (S.D. 2.7, range 1.0-10.0, n = 97). Following formation of the foveal depression the dendritic arbors of cells were significantly displaced from their somas. For example, at E138 the mean displacement was 41.2 microns (S.D. 12.2, range 12.0-56.0, n = 97). The displacement of the dendritic arbor which occurred during this period was not accounted for by areal growth of the dendritic arbor, the somas, or the retina, but was produced by the lengthening of the primary dendritic trunk. Moreover, no significant displacement was observed within the remaining 1.5-6.5 mm of the central retina. These observations provide evidence supporting early speculations that the formation of the foveal pit occurs, in part, by the radial migration of ganglion cells from the center of the fovea during its formation. Our analyses suggest that this migration occurs by the lengthening of the primary dendrite presumably by the addition of membrane. This migration is in a direction opposite to the inward movement of photoreceptors that occurs during late fetal and early postnatal periods (Packer et al., 1990, Journal of Comparative Neurology 298, 472-493).