Rhythmic motor activity and interlimb co-ordination in the developing pouch young of a wallaby (Macropus eugenii)

1. The forelimb motor behaviour of developing wallaby was studied. A clock-like alternating movement was reactivated whenever the animal was removed from the pouch. 2. Forelimb stepping frequency increased during the first 3 weeks of development, while the phase relationship remained constant. Forelimb activity could be affected by altering the afferent feedback from the contralateral limb, or an increase in ambient temperature. 3. In vitro experiments were performed using an isolated brainstem-spinal cord preparation from animals up to 6 weeks postnatal. Fictive locomotor activity could be evoked by electrical stimulation or bath-applied NMDA (< 10 microM). 4. Bath-applied strychnine (10-25 microM) and bicuculline (10-50 microM) disrupted the phase relationship between motor pools, while rhythmic motor discharge remained in the absence of these inhibitory pathways. 5. The present findings indicate that the pattern generator that underlies the robust forelimb movement during the first journey to the pouch is retained for different motor functions during in-pouch development. The neural network that underlies such behaviour can be divided into two major components, a rhythm generator within each hemicord, and a pattern co-ordinating pathway which involve both glycinergic and GABAergic interneurones.

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.

Development of the lymphoid tissues of the tammar wallaby Macropus eugenii

A study has been made of the development of four lymphoid tissues from birth to maturity in the tammar wallaby Macropus eugenii--the cervical and thoracic thymus, lymph nodes and gut-associated lymphoid tissue (GALT). The development of these tissues in the tammar wallaby is similar to that in two other marsupials, the quokka Setonix brachyurus and the Virginian opossum Didelphis virginiana. Lymphocytes were first detected in the cervical thymus of the tammar at Day 2 post partum and in the thoracic thymus at Day 6. They were subsequently detected in lymph nodes at Day 4 and in the spleen by Day 12 but were not apparent in the GALT until around Day 90 post partum. By Day 21, the cervical thymus had developed distinct areas of cortex and medulla and Hassall's corpuscles were apparent. The maturation of other tissues followed with Hassall's corpuscles in the thoracic thymus by Day 30 and nodules and germinal centres in the lymph nodes by Day 90. Measurement of immunoglobulin G concentrations in the serum of young animals indicated a rise in titre around Day 90 post partum, correlating with the apparent maturation of the lymphoid tissues.

Retinal pigment epithelium and photoreceptor maturation in a wallaby, the quokka

Cell generation and the early stages of maturation of the retinal pigment epithelium (RPE) and photoreceptors were examined in a marsupial, the quokka, Setonix brachyurus. Results are presented for animals aged up to postnatal day (P)250. RPE cell generation was studied by analysis of cell number from wholemounted retinae and by tritiated thymidine (3HThy) autoradiography in sectioned material. For 3HThy autoradiography, quokkas aged P1-P200 were injected with 3HThy and killed either 6-20 hours later (pulse-kill) or at P100 or P250 (pulse-leave). The extent of pigmentation of the RPE sheet was examined from sections of embryonic and early postnatal stages. Retinae from animals aged P5 to P160 were also examined at the electron microscope. By P100, RPE cell number is within the range found in adults. New RPE cells are generated in a peripheral band which moves outwards as cells leave the cell cycle in more central locations. RPE cells thus complete their last cell division in a centre-to-periphery wave centred about the optic nerve head. At any given retinal location, RPE cells complete their last cell division earlier than the overlying layers of the neural retina. Cells of the RPE rapidly develop a mature morphology. For example, melanin granules are observed at P5 and Verhoeff's membrane (the terminal bar complex) is evident by P25. By contrast, photoreceptor development in this species is protracted; cone inner segments are observed by P40, whilst the first rod inner segments are observed at P60. Despite being generated earlier, morphological maturation of the cones appears retarded and prolonged compared with that of the rods. The last stages of RPE cell maturation occur late in development, in synchrony with the generation of rods.

Anterior commissure of the wallaby (Macropus eugenii): adult morphology and development

In metatheria, all neo- and paleo-cortical commissural connections are made by the anterior commissure. We have examined the adult morphology of this commissure and its development in a diprotodontid metatherian, the wallaby (Macropus eugenii), at both the light and electron microscope level. The total number of axons in the adult anterior commissure was 21.7 million, of which 55-62% were myelinated. The dorsal two thirds of the commissure, containing neocortical commissural axons, showed a higher percentage of larger, myelinated axons than the ventral one third, which contains paleocortical commissural axons. The commissure also showed a topographical gradient, with cells in the dorsal cortex projecting through the dorsal region of the commissure, the fasciculus aberrans. In the rostrocaudal axis, axons from the frontal cortex tended to pass more anteriorly through the commissure and those from the occipital more posteriorly, but there was extensive overlap of projections from different areas. The gestation length of this wallaby is 28.3 days, and all commissural development occurs postnatally. The anterior commissure first appeared at P (postnatal day) 14, at which time commissural fibres were apparently derived from the external capsule exclusively. Commissural fibres passing through the internal capsule, and joining the anterior commissure via the fasciculus aberrans, were first noted at P18. By that age there were 94,000 to 161,000 axons. Peak axon counts of 50 to 63 million occurred between P100 and P150. The number of growth cones in a single midline section peaked at approximately P114 (480,000) and dropped to 0 by P170. The distribution of growth cones was analysed during the early stages of anterior commissure development (P18, P30, P82). At P18, growth cones were concentrated in the dorsal parts of the commissural bundle, suggesting a ventrodorsal sequence of addition of axons. There was no apparent preferential association of growth cones with the periphery of the commissure or glial structures at any of the three ages examined. The results show that axonal overproduction and regression in cortical commissural connections are features of development in diprotodontid metatheria, as in eutheria.

Development of the blood-forming tissues of the tammar wallaby Macropus eugenii

The development of the haematopoietic tissues of the tammar wallaby Macropus eugenii follows a similar pattern to that observed in eutherian and other metatherian mammals. At birth, the liver appears to be the only site of haematopoiesis with significant numbers of neutrophils and stem cells present in the circulation. By Day 3, the spleen shows limited haematopoietic activity and by Day 12 contains areas of erythroid and myeloid cells. At two weeks after birth, the haematopoetic activity in the liver declines and small areas of haematopoiesis are apparent in the bone marrow. By the end of the first month, the bone marrow appears to be the major site of haematopoiesis.

Ontogeny of the projection tracts and commissural fibres in the forebrain of the tammar wallaby (Macropus eugenii): timing in comparison with other mammals

The sequence of appearance of major forebrain projection and commissural fibre bundles in the tammar wallaby (Macropus eugenii) during development was examined with the aid of silver and haematoxylin stained material. At the time of birth (P0), the cerebral cortex is unformed, but two prominent fibre bundles are apparent in the forebrain: the medial forebrain bundle and the stria medullaris thalami. There is also an unidentified tract (possibly thalamostriate or striothalamic), which appears to be transient, in that it cannot be identified at P8. By P2 the posterior commissure, fasciculus retroflexus and mammillothalamic tract have appeared. Fibres of the fornix were first visible at P8. Cortical projection fibres (internal and external capsular fibres) were first noted at P10 and the anterior commissure at P14. It was not until P18 that the cortical commissural bundle unique to diprotodontid metatherians, namely the fasciculus aberrans, was first seen. The hippocampal commissure was seen to develop relatively late, at P35. The sequence and tempo of development of these tracts has been compared in metatherian and eutherian forebrains. The sequence is similar in the two groups of mammals with one exception: isocortical commissural connections appear to develop considerably earlier in diprotodontid metatherians than in eutherians. With regard to the tempo of forebrain tract development, mammals with r selection reproductive patterns (large litter sizes, many litters per reproductive lifetime, rapid development of offspring, e.g. polyprotodontid metatherians, rodents) appear to have forebrain tract development occupying a relatively greater proportion of the period from conception to the attainment of behavioural autonomy than do those animals with K selection reproductive patterns (few offspring per reproductive lifetime, relatively prolonged development of offspring, e.g. diprotodontid metatherians, primates). This difference is irrespective of whether a mammal is metatherian or eutherian, independent of encephalization, and probably reflects the greater time allocated to aspects of brain development occurring after initial tract formation (elaboration of cortical and forebrain circuitry, dendritic tree growth, synapse overproduction and elimination) among selection mammals.

Development of the chiasm of a marsupial, the quokka wallaby

We have previously shown that the mature optic chiasm of a marsupial is divided morphologically into three regions, two lateral regions in which ipsilaterally projecting axons are confined and a central region containing only contralaterally projecting axons. By contrast, in the chiasms of eutherian (placental) mammals studied to date, there is no tripartite configuration. Ipsilaterally and contralaterally projecting axons from each eye are mixed in the caudal nerve and in each hemichiasm and encounter axons from the opposite eye near the midline of the chiasm. Here, we show that, unlike eutherians, marsupials have astrocytic processes in high concentrations in lateral regions of the nerve and rostral chiasm. Early in development, during the period when optic axons are growing through the chiasm, many intrachiasmatic cells are seen with densities five to eight times higher in lateral than in central chiasmatic regions. Such cells continue to be added to all chiasmatic regions; later in development, considerably more are added centrally, as the chiasm increases in volume. In the mature chiasm, cell densities are similar in all regions. By contrast to the marsupial, cell addition in the chiasm of a placental mammal, the ferret, is almost entirely restricted to later developmental stages, after axons have grown through the chiasm, and there are no obvious spatial variations in the distribution of cells during the period examined. During development, similar to the adult marsupial, ipsilaterally projecting axons do not approach the chiasmatic midline but remain confined laterally. We propose that the cells generated early and seen in high densities in the lateral chiasmatic regions of the marsupial may play a role in guiding retinal axons through this region of pathway selection. These data suggest that there is not a common pattern of developmental mechanisms that control the path of axons through the chiasm of different mammals.

Neural responses to free-field auditory stimulation in the superior colliculus of the wallaby (Macropus eugenii)

Auditory responses to free-field broad band stimulation from different directions were recorded from clusters of neurones in the superior colliculus (SC) of the anaesthetized tammar wallaby. The auditory responses were found approximately 2 mm beneath the first recording of visually evoked responses in the superficial layers, the vast majority being solely auditory in nature; only one recording responded to both auditory and visual stimulation. Responses to suprathreshold intensities displayed sharp spatial tuning to sound in the contralateral hemifield. Those from the rostral pole of the SC disclosed a preference for auditory stimuli in the azimuthal anterior field, whereas those in the caudal SC preferentially responded to sounds in the posterior field. A continuum of directionally tuned responses was seen along the rostrocaudal axis of the SC so that the entire azimuthal contralateral auditory hemifield was represented in the SC. Furthermore, tight spatial alignment was evident between the best position of the visual responses in the superficial layers in azimuth and the peak angle of the auditory response in the deeper layers.

Timecourse of development of the wallaby trigeminal pathway. I. Periphery to brainstem

The development of the vibrissae and their innervation and the maturation of the brainstem trigeminal sensory nuclei have been studied in the wallaby, Macropus eugenii, from birth to adulthood. At birth, developing vibrissal follicles consist of solid epidermal pegs surrounded by dermal condensations. The developing follicles and adjacent skin are innervated by trigeminal afferents. Ten days after birth the follicle contains a dermal papilla and the deep vibrissal nerve can be recognised. A hair cone is present at postnatal day (P) 30 and hairs are apparent on the skin surface by P35. By P63 the deep vibrissal nerve can be seen innervating Merkel cells in the outer root sheath; in addition, the first signs of the blood sinus can be recognised. Innervation of the inner conical body and lanceolate and lamellated receptors supplying the mesenchymal sheath and waist region are not seen until P119, when the follicle resembles that seen in the adult. At birth, central processes of the trigeminal ganglion cells have entered the trigeminal tract and extend from the rostral pons to the upper cervical cord. Labelling with a carbocyanine dye at P0 shows afferents extending medially from the tract into the trigeminal subnuclei at all levels. At this stage the trigeminal nuclei appear as areas of increased cell density in the lateral brainstem. By P30-40 the four subnuclei can be distinguished on the basis of shape, cytoarchitecture, and succinic dehydrogenase reactivity. Adult morphology is not fully established until P210. In mature animals, nucleus principalis contains closely packed, polymorphic cells, frequently aligned parallel to thick fibre bundles that traverse the nucleus obliquely. Subnuclei oralis and interpolaris contain sparsely distributed, medium to large cells, randomly oriented, as well as prominent rostrocaudally directed fibre bundles. Subnucleus caudalis consists of the marginal layer, substantia gelatinosa, and magnocellular layers as described in other species. Patches of increased succinic dehydrogenase or cytochrome oxidase reactivity, presumably corresponding to the vibrissae, are present in subnuclei principalis, interpolaris, and caudalis in developing and adult animals, although the pattern is less clear than in rats. The brainstem patches are first seen at P40, approximately 6 weeks before the corresponding vibrissal-related pattern develops in the cortex. This suggests that the onset of patch formation may be regulated independently at different levels of the pathway.

Pubertal development of the pouch and teats in the marsupial Macropus eugenii

In the female tammar wallaby, Macropus eugenii, which has a highly seasonal breeding pattern, teat eversion and enlargement of the pouch occur at puberty, about 40 weeks after birth. The most obvious sign of puberty is teat eversion: 22 of 23 wild caught, and 23 of 24 captive postpubertal animals had fully everted or everting teats. Full eversion of the teats took on average two to three weeks after puberty. The pouch opening enlarged at puberty, and the rate of enlargement from 2 weeks before puberty to 2 weeks after puberty was significantly greater than the rate before puberty. In a group of pouch young ovariectomized at 5-10 weeks of age, no such changes in either teats or pouch were observed by 46 weeks of age. However, after treatment with oestradiol (0.5 microgram kg-1 body mass), four of five young showed teat eversion within 3-4 weeks. Progesterone (2 mg kg-1) had no effect on inverted teats. In these ovariectomized females oestradiol treatment caused a significant increase in the rate of growth of the pouch opening. During progesterone injections the size of the pouch remained the same. Thus, at puberty the teats and pouch of the tammar wallaby undergo rapid developmental changes and growth. Ovariectomy at an early stage of gonadal differentiation disrupts these normal changes, but treatment of these animals with physiological doses of oestradiol at the age when puberty would normally have occurred can restore teat and pouch maturation. Teat eversion and pouch enlargement can therefore be used as markers for puberty.(ABSTRACT TRUNCATED AT 250 WORDS)

Two stages in the development of a mammalian retinocollicular projection

The retinocollicular projection in the marsupial mammal the wallaby Macropus eugenii, has been investigated anatomically to determine the order in the developing projection and electrophysiologically to determine the time of onset of synaptic transmission by recording evoked potentials in the colliculus in response to stimulation of the optic nerve. There are two clear stages: a protracted period when retinal axons grow into the colliculus in coarse retinotopic order with no recordable electrical activity followed by the formation of terminal zones in retinotopically correct positions, the loss of more widely distributed axons and the onset of evoked potentials. The two stages are not seen in non-mammalian vertebrates where the projection is functional from the beginning.

Geometry of the projection of the visual field onto the superior colliculus of the wallaby (Macropus eugenii). II. Stability of the projection after prolonged rearing with rotational squint

At about the time of eye opening, one eye of seven tammar wallaby pouch young was surgically rotated about the optic axis by approximately 90 degrees. In adulthood the projection of the visual field through the rotated eye onto the contralateral superior colliculus was mapped electrophysiologically. Although apparently distorted, the projection could be coherently re-rotated mathematically to a reasonable copy of the normal projection from the opposite eye of the same animal, including details such as regional variations of the magnification factor. The same was true of three adult animals in which the eye rotation was done after anaesthesia immediately before the electrophysiological mapping. In animals in which the visual field seen through one eye, the other being normal, was rotated for the entire period of visual experience, there was no sign of compensation or rearrangement of the topographic map. Retinocollicular synaptic connections appear unmoved by such discordant visual experience.

Energy metabolism in potoroine marsupials

Fasting and fed metabolic rates were measured in three species of potoroine marsupials, the rufous rat-kangaroo (Aepyprymnus rufescens), the long-nosed potoroo (Potorous tridactylus) and the brush-tailed bettong (Bettongia penicillata). There were no significant differences among potoroine species in fasting metabolic rate. The lowest fasting heat production for each species was 11-20% less than the interspecific value of 295 kJ.kg-0.75.day-1 for basal metabolism of mature, non-reproductive eutherian homeotherms. The respiratory quotient of all species was reduced significantly as starvation proceeded, but only for B. penicillata was there a significant effect of starvation duration on fasting heat production. The night-time activity of P. tridactylus and B. penicillata doubled their daytime fasting heat production; the corresponding increase for A. rufescens was only 25%. The calorimetric measurement of fed animals showed no differences in digestible energy or metabolisable energy between species. Nevertheless, P. tridactylus and B. penicillata produced more heat per unit metabolic body mass. The maintenance energy requirements (kJ.kg-0.75.day-1) were 479, 494 and 345 for P. tridactylus, B. penicillata and A. rufescens, respectively. The net availability of metabolisable energy was about 0.70 in the three species. The combined heat production of fed female A. rufescens and their pouch young stayed relatively constant for the first two-thirds of pouch life, after which it rose sharply (20%) in response to the rapid growth of the young. Only during the last week of pouch life did the female enter negative energy balance. There was no indication that the metabolism of the female increased in response to the presence of a pouch young. The presence of pouch young did not alter the efficiency of utilisation of metabolisable energy. The daily energy requirement for maintenance was 0.83 MJ.day-1 or 0.36 MJ.kg-0.75.day-1.

Development of hindlimb muscle spindles in the marsupial Macropus eugenii (tammar wallaby)

The development of muscle spindles was studied in the hindlimb of pouch young of the marsupial Macropus eugenii (tammar wallaby) from birth to 100 days. Primary myogenesis was evident in hindlimb muscles at birth. Presumptive muscle spindles were identified as early as day 30 postnatally. The intrafusal muscle fibres developed sequentially, to form the full juvenile complement by day 50, followed by development of the periaxial space. The formation of muscle spindles in the marsupial's hindlimb after birth was in contrast to the mainly prenatal development of spindles noted in placental mammals.

Development of whisker representation in the cortex of the tammar wallaby Macropus eugenii

The somatosensory cortex associated with the whiskers has been studied in adult tammar wallabies (Macropus eugenii) and in pouch young from 60-120 days of pouch life. The time course of anatomical changes examined with succinic dehydrogenase (SDH) histochemistry and Nissl staining has been correlated with the maturation of electrically evoked cortical responses to stimulation of the whisker follicles. The earliest signs of aggregates of SDH reaction product in layer IV of the cortex were seen at 85 days, coincident with the first recordings of an immature cortical evoked potential. Aggregates, in a pattern corresponding to that of the facial whiskers, were most clearly seen from 90-140 days. At later stages, and in the adult, they were present but their arrangement was less clearly seen. By 186 days the electrical activity resembled the mature pattern. Patches of SDH activity in layer IV were not associated with changes in soma density characteristic of true barrels.

Thyroid hormones during development of a marsupial, the tammar wallaby, Macropus eugenii

The levels of thyroid hormones in the plasma and the activities of 5'-deiodinase activity in liver and kidney were determined in the tammar wallaby, Macropus eugenii, from early pouch life to adulthood. The total concentration of plasma thyroxine (T4) was below 15 nmol/l before day 75 of pouch life, rose to about 75 nmol/l at day 160, and then decreased to about 12 nmol/l in the adult. The total concentration of plasma tri-iodothyronine (T3) was below 0.4 nmol/l before day 120, increased to 3 nmol/l by about day 220 and then decreased to 1.0 nmol/l in adults. Concentrations of free T4 and free T3 followed a similar pattern but peaked at 45 and 160 pmol/l respectively. Concentrations of reverse T3 (rT3) were extremely variable, ranging from 0 to 1 nmol/l at day 100, and from 0 to greater than 2 nmol/l at day 180. After about day 230, rT3 levels fell rapidly and were below 0.3 nmol/l in adults. Liver and kidney 5'-deiodinase activities, which were undetectable before day 80, reached adult levels by day 220. Half-maximal activity of both these enzymes occurred at about day 205, mid-way between the peaks of T4 and T3. These findings suggest that the systems supporting synthesis and release of hormones from the thyroid gland are probably mature by about day 160 of pouch life in the tammar, while peripheral deiodinase activity, which is a major factor in the production of T3 in the plasma, matures by about day 220. These events thus precede the development of physiological independence of the young tammar from its mother.

Development of connections to and from the visual cortex in the wallaby (Macropus eugenii)

The time course of the development of connections between the visual cortex and the main subcortical visual structures, as well as intrahemispheric and interhemispheric connections, has been studied in the marsupial wallaby (Macropus eugenii) to compare its development with that of placental mammals. Pouch young are born prior to retinal innervation of the primary visual centers and spend a protracted period of development in the pouch, making them ideal for visual, developmental studies. Horseradish peroxidase conjugated to wheatgerm agglutinin was injected into either the presumptive visual cortex or the superior colliculus in young of varying ages. Thalamocortical projections from the dorsal lateral geniculate and lateral posterior nuclei reach the presumptive visual cortex between 12 and 15 days after birth. Descending cortical connections form later. Corticogeniculate axons are first detected in the geniculate and lateral posterior nucleus at 48 days after birth, while corticocollicular axons first reach the superior colliculus at 71 days and, by 81 days, have innervated the superficial layers. Intrahemispheric and interhemispheric connections form even later. By 99 days intrahemispheric axons from area 17 have accumulated in visual association areas but are yet to invade layers III and IV, their major termination zones in adult, while axons projecting back to area 17 have also reached their target area. At this time interhemispheric axons from area 17 have begun to accumulate in the opposite visual cortex, although they have not invaded the cortical layers. By 111 days cortical cells projecting to the opposite visual cortex are first labelled. These have a more widespread distribution in area 17 at 111 and 122 days compared to the adult, where they are confined to the 17/18 border. The results show that the marsupial wallaby has a timetable of similar sequence, but different relative timing, in the formation of cortical connections compared to that of placental mammals. In the first half of the period between conception and eye opening, the timing in the wallaby precedes considerably that in placental mammals. Ascending connections from the thalamus develop relatively earlier in the wallaby but descending collicular connections are delayed until the same relative time that they appear in placental mammals.

Development of retinotopy in projections from the eye to the dorsal lateral geniculate nucleus and superior colliculus of the wallaby (Macropus eugenii)

The development of retinotopy in projections from the eye to the dorsal lateral geniculate (dLGN) and superior colliculus (SC) has been studied in the marsupial wallaby. Discrete retinal lesions were made and the remaining retinal projections were traced with horseradish peroxidase in animals at stages ranging from just after optic innervation of the dLGN and SC to the time when the projections are mature. Topographically organised projections could be recognized a few weeks after axons first reached the dLGN and SC with a topographically discrete projection from nasoventral retinal recognized later than from dorsal, dorso-temporal, temporal, and temporoventral retina. Over time there was an increase in precision of the retinotopy as judged by an increase in sharpness of the borders of filling defects in the projection labelled with horseradish peroxidase. Refinement of the projection from temporal retina preceded that from nasal retina in both the dLGN and SC and in the former occurred concomitantly with the segregation of eye-specific terminal bands. Refinement was complete 16 weeks after birth, prior to eye opening at around 20 weeks after birth. Inequalities in retinal representations in both nuclei were present from the time retinotopy could first be detected. This was before the inequalities in retinal ganglion cell distribution, which underly these representations in the adult, were obvious. Retinotopy and inequalities in retinal representation characteristic of the adult are present from a very early stage in the protracted development of visual projections in the wallaby. Refinement may involve death of inappropriately projecting cells, pruning of inappropriately projecting axon arborizations or could be achieved by growth of the retinorecipient neuropil. Temporonasal differences in the time course of refinement may reflect gradients of maturation in the retina.

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.

Expanded retinofugal projections to the dorsal lateral geniculate nucleus and superior colliculus after unilateral enucleation in the wallaby Setonix brachyurus, quokka

We removed one eye of quokkas either neonatally, before retinal innervation of visual centres, or at 35-40 days postnatal, when projections overlap bilaterally and are more widespread than in the adult. Retinal projections to the dorsal lateral geniculate nucleus and superior colliculus at postnatal day 100 were demonstrated following anterograde transport of horseradish peroxidase. There were significant reductions in the size of the dorsal lateral geniculate nucleus and superior colliculus ipsilateral to the remaining eye. However, the extent of retinofugal projections was markedly expanded in comparison to the normal input from one eye. Unexpectedly, projections were expanded to similar extents in the two series of enucleated animals although ipsilateral labelling appeared more dense after neonatal enucleation. In controls, label was restricted to eye-specific regions but in enucleated animals there were no label-free zones. Nevertheless the alpha laminae remained distinct in the dorsal lateral geniculate nucleus of enucleated animals. Our findings suggest that binocular interactions are necessary for the segregation and refinement of visual projections but not for the formation of the alpha laminae.

Retinal ganglion cell number is unchanged in the remaining eye following early unilateral eye removal in the wallaby Setonix brachyurus, quokka

The expanded visual projections which develop after unilateral eye removal have been associated in some studies, but not in others, with the survival of more ganglion cells than normal in the remaining eye. We have addressed this issue using the small wallaby Setonix brachyurus, quokka. Moreover to determine whether more ganglion cells survive when the eye is removed at a very early stage, we have compared the effect of enucleations at two ages. These were within 3 days of birth, before optic fibres innervate visual centres, and at 35-40 days postnatal, when visual projections are exuberant. At 100 days postnatal, retinal ganglion cells were retrogradely labelled from primary visual centres and tracts with horseradish peroxidase, allowing 24 h for transport. Numbers of ganglion cells were similar between animals enucleated as neonates (X = 231,000, n = 3) and at 35-40 days postnatal (X = 218,000, n = 4). These results were comparable to those of controls (X = 227,000, n = 5). Distributions of ganglion cells were also essentially similar in experimental and control series. However, mean ganglion cell soma diameter was significantly greater than normal in both the area centralis and temporal retina after neonatal enucleation. Our results indicate that in enucleated quokkas increased ganglion cell numbers do not underlie enhanced retinofugal projections.

Histological and electron microscopic milestones in the development of the retina of a marsupial wallaby, Macropus eugenii

Retinal differentiation in the pouch young of the wallaby Macropus eugenii was characterized microscopically and morphometrically. Mitosis occurs until early in the second month in the central retina, and until early in the fourth month, peripherally. Separation of the neuroblast layer by the outer plexiform layer did not immediately halt cell division. The retinal surface continued to expand well past the time of cessation of proliferation. Cell death in the ganglion cell layer continued through the fourth month centrally and to nearly five months in the periphery. The major period of cell death was coincident with the segregation of retinal afferents and the refinement of topography in the superior colliculus and dorsal lateral geniculate nucleus. Beginning in the third month retinal thickness, measured between the outer limiting membrane and nerve fiber layer, declined equally in peripheral and central regions. At all stages the combined thicknesses of the outer and inner nuclear layer in the retinal periphery was greater than that in the center. Together with a late thickening of the inner plexiform layer, the data are consistent with the suggestion that expansion of peripheral non-ganglion cell elements may play a role in development of center to periphery differences in ganglion cell distributions. Retinal differentiation of the wallaby follows the pattern of most mammals. The onset of development of key milestones for the acquisition of retinal function occurred in the sequence: conventional synapse formation prior to ribbon synapse formation in the inner plexiform layer, and photoreceptor outer segment differentiation prior to terminal triad synapse formation.

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.