Biomedical subjects

A M Harman

Publications and source records attributed to A M Harman.

34 records · Page 2Linked to original sources

PubMed1995

Development of the visual cortex in a wallaby--phylogenetic implications.

The visual cortex of one of the smallest macropod marsupials, a wallaby, the quokka Setonix brachyurus, was examined at maturity and during development from postnatal day 1 to 150 in Nissl-stained or Golgi-stained sections. Injections of horseradish peroxidase into the primary visual centres in adults identified cortical neurons projecting to these regions. The pattern of cell generation was determined by tritiated thymidine/autoradiography. The adult visual cortex was composed of the usual six layers of cells with dendritic morphologies similar to those seen in eutherian (placental) mammals. The margin of Layer 6 overlapped slightly with the white matter, and pyramidal cells projecting to subcortical regions were located at all depths of Layer 6. Cortical development was similar to that of eutherian mammals, with the formation of transient cellular layers below the developing cortical plate. Autoradiographic studies confirmed that the cortical plate was composed of two layers: a compact zone of densely packed cells located pial to a striated layer containing more loosely packed cells. Postnatal cell generation of Layers 6-2 took place from postnatal day 3 to day 85, with Layer 4 neurons, the main target of incoming visual axons, being generated around postnatal day 40. Cells located at maturity in the white matter, mostly of glial morphology, were generated from postnatal day 40. A subplate zone containing early differentiating cells was not evident, and postnatal 3H-thymidine labelling did not identify a population of early generated neurons below the cortical plate. In the tammar wallaby Macropus eugenii, the majority of target neurons for incoming geniculate axons are generated about 20 days after the first axons enter directly into the cortical plate without a waiting period in a subplate. Geniculate axons in the wallabies have a relatively longer 'wait' for their target neurons than do those in cats or monkeys. Therefore, if geniculate axons in marsupials make temporary synapses, while they wait for their target neurons to be generated, their temporary connections must, unlike those in cats or monkeys, be with cells already in the cortical plate.

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PubMed1994

Morphology and birth dates of horizontal cells in the retina of a marsupial.

Most eutherian (placental) mammals have two horizontal cell types; however, one type only has been seen in rodents. In order to assess whether one type of horizontal cell or two is a basic mammalian feature, we have examined the morphology of horizontal cells in a marsupial, the quokka wallaby, by Golgi staining or horseradish peroxidase labelling. The birth dates of horizontal cells have also been determined by 3H-thymidine/autoradiography. There are two types of horizontal cell in the wallaby retina. One type has no axon and corresponds to the axonless cell in eutherian species; the other has shorter dendrites, an axon, and an axonal arbor, corresponding to the eutherian short-axon cell. As in eutherian mammals, the dendrites of each horizontal cell type lie in the outer plexiform layer (OPL) and contact cones and the axonal arbor of the short-axon cell contacts rods. The dendrites of the axonless cells are long, with an average length of 250 microns, and each cell has one, sometimes two, short, stubby processes, which branch off a dendrite, traverse the inner nuclear layer, and reach the inner plexiform layer. The dendritic field of these cells is elongated, and dendrites show a preferential orientation at right angles to the trajectory of overlying ganglion cell axons. Short-axon cells have a morphology similar to that seen in other species, although the axonal arbor is relatively small. Both types of horizontal cell are generated in the first phase of retinal cell generation.

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PubMed1992

Biphasic retinal neurogenesis in the brush-tailed possum, Trichosurus vulpecula: further evidence for the mechanisms involved in formation of ganglion cell density gradients.

We investigated cell generation in the retina of the brush-tailed possum (Trichosurus vulpecula) by using tritiated (3H)-thymidine labelling of newly generated cells. Animals aged between postnatal day (P) 5 and 85 each received a single injection of 3H-thymidine. Following autoradiographic processing, maps of labelled cells were constructed from retinal sections. Retinal cell generation takes place in two phases, the first is concluding in the retinal periphery at P53 as the second is seen to commence in midtemporal retina. In the first phase, cells in central retina are generated earlier than those in peripheral regions. In the second phase, cells complete their final division in midtemporal retina first and in the periphery last. Cells generated in the first phase comprise virtually all cells in the ganglion cell layer, amacrine cells, horizontal cells, and cones. Ganglion cells are produced at a slightly earlier stage than displaced amacrine cells, horizontal cells, or cones. Amacrine cells in the inner nuclear layer are the final cells produced in the first phase. When ganglion cells and amacrine cells are pooled, their combined rate of production matches that of the other cell types. These data indicate that the ratio of displaced amacrine cells: horizontal cells: cones: combined ganglion cells and amacrine cells does not change throughout development. However, the ratio of ganglion cells:macrines changes steadily as development proceeds to favour amacrine cells. In the second phase, sparse numbers of nonganglion cells in the ganglion cell layer and large numbers of bipolar and Müller cells are produced along with all rods. The two phases in the possum are similar to those seen in the wallaby, the quokka. However, fewer cells are added in central retina in the possum than in the quokka and cell addition continues for a more extended period in the periphery in the possum. We suggest that this difference in cell addition could account for the development of a more pronounced visual streak of retinal ganglion cells in the possum than in the quokka. A comparison of possum retinal cell generation with that of other marsupials adds support for the "homochrony theory."

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PubMed1992

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.

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PubMed1992

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.

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PubMed1991

Generation and death of cells in the dorsal lateral geniculate nucleus and superior colliculus of the wallaby, Setonix brachyurus (quokka).

To study postnatal cell generation in primary visual centres of the quokka, tritiated thymidine was injected into pouch-young aged postnatal day (P)1-P85. Brains were examined at P100, just before eye-opening, when primary visual projections are essentially mature. Neurons in the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) were generated at P1-P10 and P1-P18 respectively. Peak numbers of labelled cells were seen at P3 and P5 in the dLGN and SC. Cell death was assessed in the dLGN and SC of young aged P10-P150. Low numbers of dying cells were seen in the dLGN throughout this period, with a small peak at P85. A more substantial peak of cell death was seen in the SC, also at P85. In the quokka, the time interval between the peaks of cell generation and of cell death in the dLGN and SC is 70-80 days, considerably longer than the interval of 40 days between birth and death of retinal cells.

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PubMed1990

Retinofugal projections in a marsupial, Tarsipes rostratus (honey possum).

We examined the mature retinal projections of the honey possum, Tarsipes rostratus, an Australian, diprotodont marsupial, using uni-ocular injections of horseradish peroxidase and tetramethylbenzidine processing. The suprachiasmatic nucleus, medial, lateral and dorsal terminal nuclei of the accessory optic tract, pretectal nuclei and superior colliculus received bilateral retinal input. Contralateral input only was observed in the lateral posterior nucleus. The pattern of input to these regions was essentially similar to that seen in other marsupials. Cyto-architectural examination of the dorsal lateral geniculate nucleus (dLGN) indicated that 5 laminae were present in the alpha-segment, but the beta-segment appeared to lack lamination. Input to the dLGN was bilateral, overlapping considerably, and was organised in a laminar fashion with 7 and 3 terminal bands in the alpha- and beta-segments, respectively. The monocular segment accounted for 12% of the total volume of the dLGN. In the alpha-segment, 2 terminal bands each received exclusively contralateral or ipsilateral input and 3 bilateral input. In the beta-segment, 2 terminal bands received bilateral and 1 contralateral input. The volumes of the nucleus receiving contralateral and ipsilateral input were 77 and 54% of the total, respectively. A marked overlap of input from the two eyes is an unusual feature for a diprotodont marsupial and has previously been seen only in the feathertail glider. Our findings for the dLGN are of interest in the light of recent serological and taxonomic studies which suggest a close link between the feathertail glider and the honey possum.

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PubMed1989

Cell death in the inner and outer nuclear layers of the developing retina in the wallaby Setonix brachyurus (quokka).

We have examined the number and distribution of dying cells in the developing inner (INL) and outer (ONL) nuclear layers of sectioned quokka retinae (N = 31) from embryonic day (E)24 to postnatal day (P)192. Before birth, dying cells were seen in the optic fissure. Thereafter two major phases of cell death took place in the INL. The first phase was more pronounced within the vitread part with peak numbers of dying cells at P50. By contrast, during the second phase, cell death was more extensive in the sclerad portion; peak numbers of dying cells were recorded at P85 and P100 for the vitread and sclerad parts respectively. At these stages, photoreceptors were seen in the INL suggesting that these ectopic cells contribute to the pool of dying cells. The pattern of cell death broadly followed a central to peripheral sequence in the first phase but, in the second, was seen initially in mid-temporal retina and then became panretinal. Dying cells were seen in the ONL but in smaller numbers than in the INL. There was a peak of cell death at P26 which may represent death of mitotic cells at the ventricular surface. In the quokka, retinal cell genesis takes place in two phases (Harman and Beazley: Neuroscience 28:219-232, '89). The two major phases of cell death described here peak approximately 40 days after episodes of maximal cell genesis. These findings, together with data for the mouse, suggest that a biphasic pattern of cell genesis and cell death may be a feature of eutherian as well as marsupial retinal development.

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PubMed1989

Generation of retinal cells in the wallaby, Setonix brachyurus (quokka).

We have examined the generation of retinal cells in the wallaby, Setonix brachyurus (quokka). Animals received a single injection of tritiated thymidine between postnatal days 1-85 and retinae were examined at postnatal day 100. Retinae were sectioned, processed for autoradiography and stained with Cresyl Violet. Ganglion cells were labelled by injection of horseradish peroxidase into the optic tracts and primary visual centres. Other cells were classified according to their morphology and location. Retinal cell generation takes place in two phases. During the first phase, which concludes by postnatal day 30, cells destined to lie in all three cellular layers of the retina are produced. In the second phase, which starts by postnatal day 50, cell generation is almost entirely restricted to the inner and outer nuclear layers. Cells produced in the first phase are orthotopic and displaced ganglion cells, displaced and orthotopic amacrine cells, horizontal cells and cones. Glia in the ganglion cell layer, orthotopic amacrine cells, bipolar and horizontal cells. Muller glia, and rods are generated in the second phase. Cells became heavily labelled with tritiated thymidine in the central retina before postnatal day 7, over the entire retina (panretinal) by postnatal day 7 and from postnatal day 18, only in the periphery. The second phase of cell generation is initiated at P50, in a region extending from the optic nerve head to mid-temporal retina. Subsequently, cells are generated in annuli, centred on mid-temporal retina, which are seen at progressively more peripheral locations. Therefore, cell addition to the inner and outer nuclear layers continues for longer in peripheral than in mid-temporal retina. We suggest that such later differential cell addition to the inner and outer nuclear layers contributes to an asymmetric increase in retinal area. This non-uniform growth presumably results in more expansion of the ganglion cell layer peripherally than in mid-temporal retina and may play a role in establishing density gradients of ganglion cells.

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PubMed1988

Development of cell density gradients in the retinal ganglion cell layer of marsupials.

An area centralis and visual streak, specializations of high cell density in the mammalian retinal ganglion cell layer, develop from a more uniform cell distribution. In the wallaby and kangaroo, this transition is first seen 60 days postnatally. Total live cell numbers in this layer fall by approximately one-third as these specializations start to appear and dying cells are seen. However, dying cells have already become concentrated at the retinal rim prior to the emergence of live cell density gradients. Our results suggest that cell death may partially sculpt patterns of live cells, particularly by lowering densities around the entire far periphery. Studies of cell division demonstrate that ganglion cells are generated and enter the ganglion cell layer before the area centralis and visual streak are formed. However, cell addition to the inner and outer nuclear layers continues as cell density gradients become apparent in the ganglion cell layer. Furthermore, this late cell generation ceases first in areas adjacent to the presumptive area centralis. The differential distribution of such prolonged cell addition to the inner and outer nuclear layers may result in an asymmetric expansion of the retina. This process would partially explain the changing topography of cells in the ganglion cell layer. Mathematical analysis of patterns of overall retinal growth support this interpretation.

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PubMed1987

Patterns of cytogenesis in the developing retina of the wallaby Setonix brachyurus.

Patterns of mitosis were examined during development from embryonic day (E) 19, 9 days before birth, for retinae of the wallaby Setonix brachyurus, using cresyl violet stained material. For neural retina, mitosis took place at the ventricular surface from the earliest stage of eye formation until postnatal day (P) 100. Numbers of mitotic figures reached a peak of approximately 12,000 by P43. Average densities ranged between 300/mm2 and 600/mm2 up to P12 and then fell to below 50/mm2 by P25 before reaching a second peak of over 400/mm2 at P43. Up to P50, mitoses were present across the entire retina. By P60, a 'cold spot' lacking mitotic activity had formed in temporal retina and progressively extended to reach peripheral regions by P100; no mitoses were seen at P150. The timing and location of the 'cold spot' coincided with our previous description of the appearance of an area centralis in the ganglion cell layer (Dunlop and Beazley 1985). Inner and outer plexiform layers (IPL and OPL) formed between P24-40 and P50-100 respectively and were seen first in temporal retina. Furthermore, the extent of the OPL matched the mitotic 'cold spot'. By contrast to neural retina, mitosis in the pigment epithelium was panretinal and was largely complete by P3. The data suggest that cell addition to the inner or outer nuclear layers contribute to differential retinal expansion and the establishment of cell density gradients in the ganglion cell layer.

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PubMed1987

Displaced retinal ganglion cells in the wallaby Setonix brachyurus.

Displaced ganglion cells have been examined in wholemounted and sectioned retinae following bilateral injection of horseradish peroxidase into optic tracts of the wallaby, Setonix brachyurus, "quokka". Such cells, which lie in the vitread part of the inner nuclear layer, are located mainly in superior retina as a streak-like band dorsal to the area centralis and visual streak of orthotopic ganglion cells. Only between 1 and 2% of the total ganglion cell population were displaced, but an analysis of cell morphology and soma diameter suggested that displaced ganglion cells represented several cell types.

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PubMed1986

Development of visual projections in the marsupial, Setonix brachyurus.

Retinal projections to the primary visual centres were studied following injection of tritiated proline into one eye in the Marsupial, Setonix brachyurus between 10 and 100 days postnatal and in adults. Initially, projections from the two eyes overlapped extensively, particularly between 20 and 50 days. There was a gradual refinement thereafter, including a segregation of inputs from the two eyes within both the lateral geniculate nucleus (LGN) and superior colliculus (SC) by 70 days. Such refinement in visual centres is discussed in relation to the concurrent emergence of retinal ganglion cell density gradients, a decrease in ganglion cell numbers, cell death in the ganglion cell layer and loss of optic axonal profiles.

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PubMed1986

Visual acuity of the northern native cat (Dasyurus hallucatus)--behavioural and anatomical estimates.

Behavioural estimates of the visual acuity of the Northern Native Cat (or Northern Quoll)--Dasyurus hallucatus--were made using the Mitchell jumping stand technique. A value of 2.3-2.8 cycles per degree was obtained. This functional acuity compared well with predictions based on the peak ganglion cell density (2600 cells/mm2) determined from the retinal ganglion cell density map.

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PubMedDate not supplied

Changing topography of the RPE resulting from experimentally induced rapid eye growth.

The retinal pigment epithelium (RPE) of the quokka wallaby. Setonix brachyurus, grows and changes throughout life. To investigate factors that determine changes in the quokka RPE, we have examined topography of this tissue in experimentally enlarged eyes. Unilateral eyelid suture was conducted at the time of normal eye opening, postnatal day (P) 110, and animals were examined at 1 or 1 1/2 years of age. The numbers and densities of RPE cells and the extent of multinucleation were compared with those in normal animals. Eyelid suture resulted in a 9.8% and 17.4% increase in retinal area at 1 and 1 1/2 years, respectively; a significant degree of myopia was associated with this enlargement. Cell density topography in experimental eyes was not the same as in controls. Cells from central retina were disproportionately larger in the experimental than control eyes. However, the RPE cell topography in sutured eyes was not the same as that of aged retinae of a similar size. Notably, in sutured eyes there was no development of the high or highest cell densities seen in equatorial and temporal central RPE in aged retinae, respectively. Furthermore, the degree of cell enlargement in peripheral regions was slight compared with that observed in similar-sized, aged retinae. There was no increase in RPE cell number; rather, average cell area increased accompanied by no change or a slight decrease in RPE thickness. Consequently, overall volume of cells did not change significantly. The large number of multinucleate cells normally seen in aged animals was not observed in experimentally enlarged eyes, implying that an increase in cell volume may be the trigger for multinucleation.

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PubMedDate not supplied

Experimental eye enlargement in mature animals changes the retinal pigment epithelium.

Form deprivation has been shown to result in myopia in a number of species such that the eye enlarges if one eye is permanently closed at the time of eye opening. In the quokka wallaby, the eye grows slowly throughout life. After form deprivation, the eye enlarges by 1-1.5 years of age to the size of that in a 4-6-year-old animal and the number of multinucleated retinal pigment epithelial (RPE) cells in the enlarged retina remains much lower than would be expected in eyes of comparable size. Here we have repeated the experiment but examined animals at 4 years of age. The sutured eye grew significantly larger than did its partner. Numbers of RPE cells were comparable between sutured and partner eyes but were lower than in normal animals of similar age. Reductions in RPE cell density were greater in nasal than in dorsal or ventral retina and were not seen in temporal retina. The distribution of multinucleated cells was quite different in the sutured and open eyes. As in normal eyes, partner eyes had most multinucleated cells in ventral retina, while in the sutured eyes such cells were located mainly in the far periphery. In conclusion, the RPE is significantly changed by the eye enlargement process. However, it is not known whether this change results from an active part played by the RPE in the retinal expansion process or whether the changes are simply a result of a passive increase in area of the RPE.

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