Biomedical subjects

B Dreher

Publications and source records attributed to B Dreher.

At least 37 records · Page 2Linked to original sources

PubMed1996

Binocular interactions and disparity coding in area 21a of cat extrastriate visual cortex.

We have examined, using both qualitative and quantitative techniques, binocular interactions of extracellularly recorded single neurons in the extrastriate cortical area 21a of anaesthetized and paralysed cats. Consistent with previous reports we have found that: (a) all area 21a neurons were orientation-selective, with about 65% of them preferring orientations within 30 degrees of the vertical; and (b) over 75% of area 21a cells could be activated through either eye. Furthermore, a significant minority (4 cells; about 10%) of a subpopulation of 39 neurons in which binocular interactions were examined quantitatively, were "obligatory binocular neurons", that is, they responded very weakly, if at all, to the monocular stimuli presented through either eye but responded vigorously to simultaneous stimulation through both eyes. Almost 70% (27/39) of neurons tested quantitatively for binocular interaction have shown significant modulation (over 50%) of their peak responses in relation to binocular positional retinal disparities. The majority of neurons sensitive to binocular positional disparities resembled either "tuned excitatory" (22 cells; 56.5% of the sample) or "tuned inhibitory" (2 cells; 5% of our sample) cells. In particular, they gave, respectively, maximal or minimal responses to optimally oriented, moving photic stimuli when the receptive fields plotted through each eye completely or partially overlapped. Although neurons recorded in area 21a have relatively large receptive fields (mean width 3.3 +/- 1.1 degrees; range 2.0-5.6 degrees), the mean width of the disparity tuning curve (2.8 +/- 1.0 degrees; range 1.3-4.8 degrees) for our sample of area 21a neurons was similar to those of neurons with significantly smaller receptive fields, recorded in areas 17 and 18 of cat's primary visual cortex. We conclude that area 21a of the cat, like areas 17 and 18 of primary visual cortex, is likely to play an important role in binocular depth discrimination and might constitute a "higher order" area for stereoscopic binocular vision.

Animals↗

PubMed1996

Areas PMLS and 21a of cat visual cortex are not only functionally but also hodologically distinct.

In several cats, paired visuotopically matched injections of retrogradely transported fluorescent dyes, diamidino yellow (DY) and fast blue (FB), were made into two visuotopically organized, functionally distinct extrastriate cortical areas, the posteromedial lateral suprasylvian area (PMLS area) and area 21a respectively. After an appropriate survival time, the numbers of thalamic, claustral and cortical cells which were single-labelled with each dye as well as the numbers of cells in these structures labelled with both dyes (double-labelled cells) were assessed. The clear majorities of thalamic cells projecting to PMLS area (DY labelled cells) and to area 21a (FB labelled cells) were located in the ipsilateral lateral posterior-pulvinar complex with smaller proportions located in the laminae C and the medial intralaminar nucleus of the ipsilateral dorsal lateral geniculate nucleus and several nuclei of the rostral intralaminar thalamic group. Despite the fact that DY labelled (PMLS-projecting) and FB labelled (area 21 a-projecting) cells in all thalamic nuclei were well intermingled, only 1-5% of retrogradely labelled thalamic cells projected to both areas (cells double-labelled with both dyes). Small proportions of retrogradely labelled cells were located in the ipsilateral and to a lesser extent the contralateral dorsocaudal claustra. The proportions of claustral neurons retrogradely labelled with both dyes varied from 4 to 9%. Over half of the cortical neurons labelled retrogradely from area 21a or PMLS area were located in the supragranular layers of the ipsilateral area 17, with smaller proportions located in the supragranular layers of the ipsilateral areas 18 and 19 and even smaller proportions located in mainly but not exclusively, the infragranular layers of the ipsilateral areas 21b and 20a. Again despite strong spatial intermingling of neurons labelled with DY and these labelled with FB, the proportions of associational cortical neurons double-labelled with both dyes were small (2 to 5.5%). Finally, small proportions of neurons retrogradely labelled with DY or FB were located, mainly but not exclusively, in the supragranular layers of the contralateral areas 17, 18, 19 and 21a. Again, the proportions of the double-labelled neurons in the contralateral cortices were small (1-4.5%). Thus, the present study indicates that despite the fact that the diencephalic and telencephalic inputs to the visuotopically corresponding parts of area 21a and PMLS area originate from the same nuclei, areas and layers, the two areas receive their afferents from the largely separate populations of neurons.

Animals↗

PubMed1995

Effect of collicular proteoglycan on the survival of adult rat retinal ganglion cells following axotomy.

Consistent with numerous previous studies, we have found that in adult rats 29% of cells retrogradely prelabelled by injections into retino-recipient nuclei are lost 1 week after intraorbital section of the optic nerve. This figure increases to 76% 2 weeks after axotomy. Intraocular injections of 150 ng of 480 kDa chondroitin sulphate proteoglycan purified from the superior colliculi of neonatal rats were performed every third day after axotomy. This procedure resulted in the loss of only 3 and 28% of the axotomized retinal ganglion cells 7 and 14 days respectively after optic nerve section. Intraocular injections of chondroitin sulphate type C, one of the sugar types present on the collicular proteoglycan, also resulted in a significant saving of axotomized ganglion cells (with the loss of only 48% 14 days after optic nerve lesion). These findings suggest that the collicular proteoglycan, and to a lesser extent its sugar moieties, substantially slows down the degeneration of adult retinal ganglion cells following axotomy.

Animals↗

PubMed1995

Glial reactivity in the retina of adult rats.

The present study aimed to characterize the reaction of mammalian (rat) retinal macroglia (Müller cells and astrocytes) to disturbances of their environment in the form of intraorbital section of the optic nerve, intraocular insertion of a thin glass capillary (without damage to the retina) or a combination of both. Glial reactivity was assessed through the use of a battery of antibodies which recognise four different proteins--glial fibrillary protein (GFAP) and three other proteins designated respectively MA1, 4D6 and 4H11. Retinal astrocytes did not exhibit any changes in normally expressed GFAP or MA1. By contrast, the expression of GFAP and MA1 in Müller cells increased 14 days following section of the optic nerve and/or intravitreal insertions of a glass capillary. Three days postoperatively, the expression of GFAP, but not MA1, had already increased significantly in Müller cells. 4D6 and 4H11 proteins were not expressed in astrocytes. In Müller cells, the levels of these proteins increased significantly following combined optic nerve section and intraocular insertion of a glass capillary. Thus, a mechanical disturbance of the intraocular environment constitutes a more effective stimulus in increasing the expression of some Müllerian proteins than damage to the axons of retinal ganglion cells. Such changes have important implications for various ocular treatments that involve intraocular administration of drugs, as well as for the survival/regeneration potential of retinal ganglion cells undergoing Wallerian degeneration.

Animals↗

PubMed1994

Vitread proliferation of filamentous processes in avian Müller cells and its putative functional correlates.

In order to examine to what extent the neuronal and metabolic activities of avascular vertebrate retinae are reflected in the morphology of their Müller cells we have studied (by using several monoclonal antibodies) the morphology of Müller cells in two species of diurnal birds (chicken, Gallus domesticus, and pigeon, Columba livia) and one species of nocturnal saltwater crocodiles (Crocodylus porosi). In all three species, the outer nuclear layer is relatively thin and the Müller cell trunks divide into rootlets that wrap around the photoreceptors. In both diurnal birds studied, the trunks of Müller cells in the inner plexiform layers invariably divide into numerous fine filamentous processes that terminate in small expansions covering most of the vitreal surface of the retina. Furthermore, the networks of filamentous processes of birds' Müller cells exhibit conspicuous horizontal lamination in the inner plexiform layer. In contrast, the filamentous processes arising from the individual Müller cell trunks of the crocodile, if present, are much less numerous and less widely spread than those of diurnal birds. It is proposed that the splitting of the Müller cell trunks into numerous filamentous processes terminating in small vitreal expansions represents a morphological adaptation for: 1) effective spatial buffering of K+ ions in thick and presumably metabolically highly active, yet avascular, avian retinae, and 2) effective absorption and distribution of nutrients leaking from the vitreally located supplemental nutritive organ, the pecten.

Alligators and Crocodiles↗

PubMed1994

The pattern of expression of a 13-kDa protein (CS-32 antigen) in the retina and its target nuclei in developing and mature rats.

A polyclonal antibody (designated CS-32) immunopurifies a 13-kDa protein from the superior colliculus (SC) of neonatal rat. There is a strikingly inverse temporal relationship in expression of the 13-kDa protein in the developing retina and its principal target nuclei, SC and the dorsal lateral geniculate nucleus. The 13-kDa is a cell-surface protein expressed exclusively by neurones and is clearly associated with synapses.

Animals↗

PubMed1994

Non-dominant suppression in the dorsal lateral geniculate nucleus of the cat: laminar differences and class specificity.

Binocular non-dominant suppression (NDS) in the dorsal lateral geniculate nucleus (LGNd) of the cat was studied by recording from single neurons in the LGNd of anaesthetized, paralysed cats while stimulating the non-dominant eye with a moving light bar. The maintained discharge rate of LGNd neurons was varied by stimulating the dominant eye in various ways: by varying the size or contrast of a flashed spot, by varying the inner diameter of a flashed annulus of large outer diameter, by varying the velocity of a moving light bar, and by covering the eye. Non-dominant suppression was quantified either as the decrease in the maintained discharge rate (the "dip"), expressed as spikes per second, or as the ratio of the dip to the maintained discharge rate (the "dip ratio"). At low maintained discharge rates the dip, although low in value, frequently approached the maintained rate, i.e. the dip ratio approached unity. As the maintained discharge rate increased the dip value also increased, but more slowly than the maintained discharge rate, i.e. the dip ratio decreased. At maintained discharge rates above about 30 spikes/s, in many neurons the dip appeared to be approaching a constant value. This strong dependence of NDS on the maintained discharge rate of the LGNd neuron suggests that the inhibitory input to the cell arises from a region of the brain that receives an input both from the non-dominant eye and from the LGNd cell. Reasons are given for thinking that this region is the perigeniculate nucleus. Because of the strong dependence of dip and dip ratio on the maintained discharge rate, it was necessary to adopt stringent criteria when comparing NDS in two different sets of neurons or of the same set of neurons in different conditions. We recognized a significant difference in NDS between two classes of neurons or between two states only if: (1) there was no significant difference between the maintained discharge rates, and (2) there was a significant difference for both dip and dip ratio between the two classes or states. Using these criteria we found: (1) no difference between non-lagged X (XNL) and non-lagged Y (YNL) cells, (2) no difference between on-centre and off-centre cells for either XNL or YNL cells, (3) no difference between XNL cells and lagged X (XL) cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Anesthesia↗

PubMed1994

Extent of bilateral collateralization among pontomesencephalic tegmental afferents to dorsal lateral geniculate nuclei of pigmented and albino rats.

In adult pigmented and albino rats, small amounts of different fluorescent dyes (Fast Blue and Fluoro-Gold) were pressure-injected into the dorsal lateral geniculate nuclei, each nucleus (right or left) being injected with one dye only. After postinjection survival of three days, the distribution of neurons retrogradely labelled by each dye was analysed. Consistent with previous studies, in each strain each dye labelled a large number of neurons in the several ipsilateral visuotopically or retinotopically organized structures--visual cortices, retino-recipient layers of the superior colliculi and the pretectal nuclei. A substantial number of retrogradely labelled neurons was also found in the contralateral parabigeminal nucleus. A few retrogradely labelled neurons were found in the ipsilateral and (to a lesser extent) contralateral dorsolateral divisions of the periaqueductal gray matter, as well as in the ipsilateral parabigeminal nucleus and the caudal part of the lateral hypothalamus. However, in all the above structures there was a paucity of cells retrogradely labelled with both dyes (double-labelled cells). By contrast, in each strain, several "modulatory" nuclei (containing cholinergic and aminergic cells) of the pontomesencephalic tegmentum--dorsal raphe, pedunculopontine tegmental nucleus, parabrachial nucleus, laterodorsal tegmental nucleus and locus coeruleus--contained significant numbers of cells projecting to both ipsilateral and contralateral dorsal lateral geniculate nuclei. In each nucleus, ipsilaterally and contralaterally projecting cells constituted, respectively, about 65-70% and about 30-35% of retrogradely labelled cells. About 25% of the contralaterally projecting cells (i.e. about 5-10% of all retrogradely labelled tegmental neurons) were double-labelled with both dyes. Double-labelled cells were intermingled with single-labelled cells projecting ipsilaterally or contralaterally. The proportions of the ipsilaterally, contralaterally and bilaterally projecting neurons in the modulatory components of the pontomesencephalic tegmentum were virtually identical in pigmented and albino strains. It appears that in both strains the visuotopically organized structures convey to the dorsal lateral geniculate nuclei information related mainly to the contralateral visual field. The projections from these structures might play an important role in regulating transmission of visual information in the retinotopically distinct parts of each dorsal lateral geniculate nucleus. By contrast, the projections from the modulatory nuclei of the pontomesencephalic tegmentum are likely to contribute to the functional synchronization of both dorsal lateral geniculate nuclei during the sleep-wakefulness cycle and saccadic eye movements.

Afferent Pathways↗

PubMed1993

Parabigeminal, pretectal and hypothalamic afferents to rat's dorsal lateral geniculate nucleus. Comparison between albino and pigmented strains.

In adult pigmented and albino rats different fluorescent dyes were injected into the dorsal lateral geniculate nuclei of opposite sides. Differences between the strains occur mainly in parabigemino-geniculate and pretecto-geniculate projections. Both the major contralateral and the minor ipsilateral parabigemino-geniculate projections in albinos were clearly smaller then those in pigmented rats. In pigmented rats but not in albinos the parabigemino-geniculate projections originated mainly from the region where the vertical meridian is represented and contained a small number of neurones projecting bilaterally. In each strain, a small number of retrogradely labelled neurones was found in the ipsilateral and contralateral lateral hypothalami.

Afferent Pathways↗

PubMed1992

Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in the striate cortex of the cat.

In an aseptic operation under surgical anesthesia, one optic nerve of a cat was exposed and subjected to pressure by means of a special cuff. The conduction of impulses through the pressurized region was monitored by means of electrodes which remained in the animal after the operation. The pressure was adjusted to selectively eliminate conduction in the largest fibers (Y-type) but not in the medium-size fibers (X-type). The conduction block is probably due to a demyelination and remains complete for about 3 weeks. Within 2 weeks after the pressure-block operation, recordings were made from single neurons in the striate cortex (area 17, area V1) of the cat anesthetized with N2O/O2 mixture supplemented by continuous intravenous infusion of barbiturate. Neurons were activated visually via the normal eye and via the eye with the pressure-blocked optic nerve ("Y-blocked eye"). Several properties of the receptive fields of single neurons in area 17 such as S (simple) or C (complex) type of receptive-field organization, size of discharge fields, orientation tuning, direction-selectivity indices, and end-zone inhibition appear to be unaffected by removal of the Y-type input. On the other hand, the peak discharge rates to stimuli presented via the Y-blocked eye were significantly lower than those to stimuli presented via the normal eye. As a result, the eye-dominance histogram was shifted markedly towards the normal eye implying that there is a significant excitatory Y-type input to area 17. In a substantial proportion of area 17 neurons, this input converges onto the cells which receive also non-Y-type inputs. In one respect, velocity sensitivity, removal of the Y input had a weak but significant effect. In particular, C (but not S) cells when activated via the normal eye responded optimally at slightly higher stimulus velocities than when activated via the Y-blocked eye. These results suggest that the Y input makes a distinct contribution to velocity sensitivity in area 17 but only in C-type neurons. Overall, our results lead us to the conclusion that the Y-type input to the striate cortex of the cat makes a significant contribution to the strength of the excitatory response of many neurons in this area. However, the contributions of Y-type input to the mechanism(s) underlying many of the receptive-field properties of neurons in this area are not distinguishable from those of the non-Y-type visual inputs.

Animals↗

PubMed1992

Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in area 18 of the visual cortex of the cat.

Recordings were made from single neurons in area 18 of anesthetized cats (N2O/O2 mixture supplemented by continuous intravenous infusion of barbiturate) in which one optic nerve had been pressure blocked to selectively block conduction in the largest (Y-type) fibers. Cortical neurons were stimulated visually via the normal eye or via the eye with the pressure-blocked optic nerve ("Y-blocked eye"). Several properties of the receptive fields such as their spatial organization (S or C cells), orientation tuning, and the presence and strength of end-zone inhibition appear to be unaffected by removal of the Y input. By contrast, the removal of the Y input resulted in a small but significant reduction in the size of the discharge field and in the direction-selectivity index. In three respects, peak response discharge rate, eye dominance, and velocity sensitivity, removal of the Y input had strong and highly significant effects. Thus, the mean peak discharge frequency of responses evoked by the stimulation of binocular neurons via the Y-blocked eye was significantly lower than that of responses evoked by the stimulation via the normal eye. Accordingly, the eye-dominance histogram was shifted markedly towards the normal eye (more so than in the homologous experiment conducted on area 17-Burke et al., 1992). Finally, the mean preferred velocity of responses of cells activated via the normal eye was in the vicinity of 145 deg/s, whereas for cells activated via the Y-blocked eye the value was about 35 deg/s. Overall, the results of the present study imply that (1) apart from Y-type excitatory input there are significant excitatory non-Y-inputs to area 18; these inputs at least partially consist of indirect X-type input relayed via area 17; (2) in neurons of area 18 that receive both Y-type and non-Y-type excitatory inputs, the Y-type input has a major influence on strength of the response and velocity sensitivity and a lesser influence on the direction selectivity and size of the discharge fields; and (3) area 18 contains mechanisms determining such receptive-field properties as S- or C-type organization, orientation tuning, and direction selectivity which can be accessed either by the Y input or by non-Y input.

Afferent Pathways↗

PubMed1991

Prosencephalic connections of striate and extrastriate areas of rat visual cortex.

Afferent connections of rat primary visual cortex (area 17 or V1 area) and the rostral and caudal parts of areas 18a and 18b were studied, by placing in each of the areas, small electrophoretic injections of enzyme horseradish peroxidase (HRP) or wheat germ agglutinated-HRP. The results indicate that: 1) each of the areas has a distinct pattern of distribution of afferent neurons in the ipsilateral visual thalamus - area 17 receives its principal thalamic input from the dorsal lateral geniculate nucleus, the caudal parts of areas 18a and 18b receive a major thalamic input from the lateral posterior nucleus and a minor input from the posterior nucleus, while the rostral parts of areas 18a and 18b receive a major input from the posterior nucleus, and a minor projection from the lateral posterior nucleus; 2) the rostral and caudal parts of areas 18a and 18b each receive an associational input from area 17; 3) the rostral parts of areas 18a and 18b each receive associational input from three different extrastriate regions, the caudal part of the same extrastriate area, and the rostral and caudal parts of the other extrastriate area, whereas the caudal parts of areas 18a and 18b receive associational inputs only from one or two extrastriate regions; 4) area 17, area 18b and rostral area 18a each receive a substantial associational input from lamina V of the caudal part of the frontal eye field (FEF) in the motor cortex; however the input from the FEF to caudal area 18a (if present) is very small; 5) The extrastriate areas studied receive associational input from the restrosplenial cingulate area 29d; however, the input from area 29d to area 17 appears to be very small. The distinct patterns of distribution of prosencephalic afferents suggest to us that multiple retinotopically organized areas described previously in the rat cortex (cf Montero 1981; Espinoza and Thomas 1983) represent functionally distinct areas.

Afferent Pathways↗

PubMed1991

Interactions between callosal, thalamic and associational projections to the visual cortex of the developing rat.

The patterns of callosal interconnections between the visual cortices of rats display considerable plasticity in response to various neonatal manipulations. In the present study, many neurones in the principal visual thalamic relay nuclei, the dorsal lateral geniculate nucleus (DLG) and to a lesser extent those in the lateral posterior nucleus (LP) were destroyed by injections of the neurotoxin - kainic acid - on the first day of postnatal life. Four weeks later, as demonstrated with the anterograde and retrograde transport of the enzyme horseradish peroxidase (HRP) injected into the occipital lobe of one hemisphere, callosally projecting neurones and terminals were distributed more widely in the retinotopically organized areas 17, 18a and 18b of the visual cortex ipsilateral to the lesioned visual thalamus than in unoperated control animals of the same age. By contrast, in the visual cortex contralateral to the lesioned visual thalamus the areal distribution of callosally projecting neurones and terminals was similar to that of the controls, that is, largely but not exclusively restricted to the common border of areas 17 and 18a. Both in unoperated and operated animals, cells in lamina V of several cytoarchitectonically defined areas that are not retinotopically organized (area 8 in the frontal lobe, area 29d in the retrosplenial limbic cortex and perirhinal areas 35/13 in the temporal lobe) also project to contralateral visual cortices. In areas 8 and 29d, the total numbers, laminar distributions and densities of labelled callosal cells both ipsilateral and contralateral to the kainate-injected visual thalamus were similar to those in the controls. However, in the temporal lobe, the areal distribution of the labelled callosal neurones was more extensive than that in the controls and labelled cells in areas 35/13 of the cortex contralateral to the kainate-lesioned visual thalamus merged with those in the neighbouring areas 20 and 36. By contrast, the areal distribution of associational neurones in area 18a and in nonretinotopically organized areas projecting to area 17 were very similar in controls and in operated animals (neonatal kainate lesion of the visual thalamus, neonatal section of the corpus callosum or both procedures combined). However, in operated animals, the labelled associational neurones projecting from the supragranular laminae (II/III) of area 18a to area 17 constituted a higher proportion of all cells than did those in the unoperated control animals. Thus, overall the number of associational neurones projecting from area 18a to area 17 was slightly increased by the experimental manipulations performed.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1991

4B2: a monoclonal antibody to ganglion and bipolar cells in the retina of the cat, generated by intrasplenic immunization.

Using tissue from the inner layers of the retina as the immunogen, we have prepared a monoclonal antibody which is selective for two classes of neuron in the cat retina. The antibody, termed 4B2, was generated by intrasplenic injection, demonstrating that neuron-specific antibodies can be elicited by this technique, which requires only micrograms of immunogen. The 4B2 binds to the somas, dendrites and axons of ganglion cells. Double labelling with 4B2 and with a retrograde tracer injected into the retino-recipient nuclei of the brain demonstrates that, of the cell classes in the ganglion cell layer, 4B2 labels only ganglion cells, and that, amongst ganglion cells, 4B2 is strongly selective for large (alpha-) and medium-sized (beta- and perhaps gamma-) cells. Double labelling with 4B2 and with markers for amacrine cells confirms tht 4B2 does not label amacrine cells. Double labelling with 4B2 and with anti-GFAP confirms that 4B2 does not label the macroglia of the retina. The 4B2 also labels bipolar cells, showing their somas, dendrites and axons. The axons of the labelled cells terminate in large boutons, in the innermost part of the inner plexiform layer and in the ganglion cell layer. This level of termination suggests that 4B2 labels rod bipolars, and perhaps a subgroup of cone bipolars. Double labelling with 4B2 and with markers for amacrine, horizontal and Müller cells indicates that other cell classes with somas in the inner nuclear layer remain unlabelled. In addition, we have examined the ontogeny of 4B2 labelling in the cat retina. The developmental sequence of labelling with 4B2 follows the sequence in which the cells are born, first ganglion and then bipolar cells.

Animals↗

PubMed1990

Neurochemical correlates of gamma-aminobutyrate (GABA) inhibition in cat visual cortex.

High affinity binding of [3H]gamma-aminobutyric acid (GABA) to neuronal membranes from different parts of cat visual cortex was tested for sensitivity to GABA(A) agonists isoguvacine and THIP, GABA(A) antagonist SR95531 and GABA(B) agonist baclofen. Some of the GABA(A)-binding sites were found to have a very low affinity for THIP, suggesting the presence and, possibly, uneven distribution of "non-synaptic" GABA(A) receptors in cat visual cortex. There were no differences in Km and Vmax values of high affinity uptake of GABA and in the potency of K(+)-stimulated release of GABA, between primary and association cortices. Consequently, the present results indicate that despite the anatomical and physiological differences between the primary and association feline visual cortices the neurochemical characteristics of GABAergic inhibition are very similar in the two regions.

Animals↗

PubMed1990

The visual pathways of eutherian mammals and marsupials develop according to a common timetable.

Data from cats, ferrets, hamsters, macaques, mice, rabbits and rats concerning the time of occurrence of 26 developmental events involved in the establishment of the retinofugal, geniculocortical, corticogeniculate and corticocollicular pathways were analysed. For each species the timing of developmental events was expressed as a proportion of the period between conception and eye opening ('caecal period'). For 6 of these events, the values for all species fell within a range of 1-5% of the caecal period, the values for 11 other events fell within a range of 6-10% of the caecal period, 4 events had ranges of 11-15% of the caecal period, and only 5 events were spread over more than 15% of the caecal period. The 26 events had an overall mean variation of about 11% of the caecal period, suggesting that the visual pathways of eutherian mammals develop according to a common 'timetable' that is related to the duration of the caecal period. One of the roles of this common timetable may be to assist the establishment of orderly interconnections between the visual centres. Relatively few data are available concerning the timing of developmental events in marsupial visual pathways. However, it is apparent that, during the first half of the caecal period, most events occur much earlier in marsupials than in eutherians, whereas during the second half of the caecal period most events occur at the same stages of the caecal period in both marsupials and eutherians. The accelerated rate of visual development during the first half of the caecal period in marsupials may be related to the precocious development that they undergo prior to their very early birth.

Animals↗

PubMed1989

Developmental changes in the pattern of retinal projections in pigmented and albino rabbits.

The distribution of retinal axons and/or terminals in the retino-recipient nuclei of pigmented and albino rabbits varying in age from the 24th postconceptional day (24PCD) to adulthood was examined following unilateral intraocular injections of the enzyme horseradish peroxidase. Both in pigmented and albino rabbits contralateral retinal axons and/or terminals in the dorsal and ventral lateral geniculate nuclei (DLG and VLG), superior colliculi (SC), pretecta (PT) and accessory optic tract nuclei (AON) were already present on 24PCD. In the period 26-30PCD the contralateral projection occupied the entire volume of the DLG, VLG and SC. Although 32PCD (the day of birth) the proportions of the volumes of DLG and VLG occupied by the contralateral projections were slightly reduced, their volume continued to increase in absolute terms up to adulthood. In pigmented rabbits the ipsilateral projections to all retino-recipient nuclei were most dense and extensive on 26PCD. From 26PCD, the relative extent of the ipsilateral projections was gradually reduced, but a reduction in their absolute extent did not become evident until 32PCD. By 32PCD the ipsilateral projection to the AON had disappeared completely. The distribution of ipsilateral axons and/or terminals and the relative proportion of the nuclei occupied by the ipsilateral projection in all other retino-recipient nuclei had become adult-like by 34PCD. In albino rabbits only a sparse ipsilateral projection to the presumptive superficial collicular layers was present on 24PCD. In the remaining retino-recipient nuclei an ipsilateral projection was present on 26PCD. From 26PCD the relative extent and from 30PCD the absolute extent of ipsilateral retinal axons and/or terminals was gradually reduced. The relative extent of the ipsilateral projection had become almost adult-like by 34PCD. Throughout development ipsilateral projections in albinos were consistently less dense and less extensive than those in pigmented rabbits, and unlike in pigmented rabbits, the ipsilateral projections to the VLG and PT were only transient. The differences between the two strains in the pattern of retinofugal projections were further enhanced during the period of segregation of the ipsilateral and contralateral projections. Considering the fact that in both strains there is a partial correspondence between the period in which the spatial extent of the ipsilateral projections is reduced and the period of retinal ganglion cell (RGC) death, it is likely that RGC death plays a role in the process of segregation of the retinal afferents into ocular domains. However, our data suggest that other mechanism(s) also play an important role in the process.

Aging↗

PubMed1989

Nonuniform retinal expansion during the formation of the rabbit's visual streak: implications for the ontogeny of mammalian retinal topography.

We have studied the distribution of retinal ganglion cells (RGCs) which have been retrogradely labeled from massive bilateral injections of the enzyme horseradish peroxidase into the retino-recipient nuclei of foetal and postnatal albino rabbits aged from the 24th postconceptional day (24PCD) to adulthood. The number of labeled RGCs increases from about 447,000 on the 24PCD to a peak of about 525,000 on the 27PCD. From the 29PCD to birth (31/32PCD), the number of RGCs rapidly declines to about 375,000. During the next 20 d, the number of RGCs stabilizes at about 335,000. After the 51PCD, the number of RGCs gradually declines to the adult value of about 280,000. Retinal area steadily increases from about 40 mm2 on the 24PCD to about 500 mm2 in the adult, while RGC density decreases. However, the reduction in RGC density is nonuniform: RGC density in the visual streak drops from 18,600 RGCs mm2 on the 24PCD to 4700 RGCs/mm2 in the adult, whereas RGC densities at the superior and inferior edges of the retina decreases proportionally much more (from 9300 to 105 RGCs/mm2 and from 12,000 to 170 RGCs/mm2, respectively). As a result of this differential reduction in RGC density, the streak/inferior edge ratio changes from 1.6:1 to about 28:1. In the periods from the 24PCD to the 29PCD and from the 32PCD to adulthood, the proportional increases in the streak/superior edge and streak/inferior edge RGC density ratios are linearly related to the proportional increases in retinal area. However, between the 29PCD and 32PCD, the RGC density ratios increase at a greater rate than retinal area. We conclude that (1) the centro-peripheral difference in RGC density that is already present on the 24PCD might be attributable to differential RGC generation; (2) the redistribution of RGCs between the 24PCD and adulthood is mainly due to nonuniform expansion of the retina, with minimal expansion of the visual streak and maximal expansion at the superior and inferior retinal edges; and (3) a small component of the increase in the centro-peripheral RGC density ratio, which becomes apparent between the 29PCD and 32PCD, is probably due to differential RGC loss. We discuss the pattern of retinal expansion in the rabbit and the factors which might contribute to it.

Animals↗