SmI cortical barrels in an Australian marsupial, Trichosurus vulpecula (brush-tailed possum): structural organization, patterned distribution, and somatotopic relationships.

This study reports on the cerebral cortex of an Australian marsupial, Trichosurus vulpecula (brush-tailed possum). It consists of an analysis of layer IV of somatosensory cortex in tangential sections of flattened specimens and in oblique radial sections stained to show Nissl substance or myelin, or tested for succinic dehydrogenase. It includes results of electrophysiological mapping experiments that ascertained the somatotopic significance of the cytoarchitecture of this cortical region. Layer IV has two interlocking cytoarchitectural fields: one granular (the barrelfield, comprising cell-dense barrels 150 to 500 microns in diameter) and one dysgranular. Only neurons within the granular field responded to light cutaneous stimulation. In the barrelfield cell-sparse septa (about 100 microns wide), low in succinic dehydrogenase activity and containing many radial myelinated axons, separate adjoining barrels. Possum barrels are "solid," lacking the prominent hollows characteristic of most rodent barrels. In some specimens three to five small neuronal "lobules" may constitute each large barrel. In tangential sections the size, shape, and arrangement of barrels combine to form a histological caricature of the possum's body, especially of the face and forepaw. Six rows of "mystacial barrels" are homeomorphic to the six rows of large mystacial vibrissae, and "forepaw barrels" are homeomorphic to the glabrous palmar and apical digital pads. Correlating cortical recording sites and receptive fields confirmed that individual barrels represent specific cutaneous regions. These results show that the cortical barrels of brush-tailed possums are remarkably similar to those of rodents, in structure, arrangement, and functional significance.

How thick should a retina be? A comparative study of mammalian species with and without intraretinal vasculature.

In this study we describe a measurement method which closely approximates in vivo retinal thickness. Using this method we examined the laminar organisation of vascular and avascular retinae from placental and marsupial mammals. Thickness measurements on retinal wholemounts show that the avascular retinae of the placental guinea pig (140 microns) and the marsupial brushtail possum (170 microns) are thinner and show less centroperipheral taper than do the vascular retinae of placental cats (250 microns), rats (220 microns) and marsupial quolls (220 microns). In general, limitation in thickness of avascular retinae is borne by most retinal layers, but most particularly by the inner plexiform layer, the synaptic region farthest removed from the choroidal blood supply. Except for the absence of blood vessels, the histological organisation of the brushtail possum's retina resembles closely that of its fellow marsupial, the quoll's. In contrast, intraretinal organisation differs amongst the two avascular retinal species with the guinea pig displaying a much coarser photoreceptor grain.

Gradients of neurogenesis in possum neocortex.

Gradients of neurogenesis in neocortex of the brushtailed possum were determined with [3H]thymidine autoradiography, using data taken from 16 possums injected with [3H]thymidine between postnatal (P) days 5-68, and allowed to survive until brain cytoarchitecture was mature. Gradients of neurogenesis shown in neocortex were: (a) an inside-out radial gradient within cortical areas with neurogenesis beginning around P5 in layer 6 and concluding between 1 and 2 months later in layer 2, and (b) a single, simple, regional gradient across all neocortex. On the lateral surface of the hemisphere neurogenesis runs anteroventrolateral to posterodorsomedial, and on the medial surface neurogenesis runs anteroventral to posterodorsal. When plotted for the whole neocortex, the two regional gradients from the lateral and medial surfaces of the hemisphere are seen as parts of the one simple pattern. The time duration of neurogenesis in possum cortex is 1-2 months, and is similar to that seen in monkeys.

Neurogenesis in a marsupial: the brush-tailed possum (Trichosurus vulpecula). II. Sensorimotor pathways.

The times of origin of neurons in sensorimotor pathways of the marsupial brush-tailed possum were determined with 3H-thymidine autoradiography. A series of 20 possums were injected with 3H-thymidine from postnatal days 5-95 and were normally allowed to survive until the brain cytoarchitecture was mature. Brain stem and spinal sensorimotor regions were not labelled in our study and presumably form before birth in order to enable the newborn young to make the journey from the birth canal to the pouch on its own. Neurogenesis in thalamic sensorimotor nuclei probably begins about the time of birth and continues into the 2nd week of postnatal life. Formation of neurons in the sensorimotor cortex and the basal ganglia occurs during the first 2 months of postnatal life and in the cerebellum during the first 3 months. This protracted postnatal development of telencephalic and cerebellar sensorimotor regions offers great advantages for developmental studies.

The relative effectiveness of some chelating agents as antidotes in acute cadmium poisoning.

An examination has been made of the relative effectiveness of three chelating agents in reducing lethality in acute cadmium poisoning. The chelating agents used were sodium 2,3 dimercaptosulfonate (DMPS), N-Acetyl d,1 penicillamine (NAPA), and 2,3 dimercaptosuccinic acid (DMSA). For cadmium acetate administered ip at a level of 16.9 mg/kg, the oral administration of the chelating agents at a 20:1 mole ratio in three doses subsequent to the cadmium acetate at 20 minutes, 90 minutes, and 210 minutes resulted in a significant enhancement of the survival rate for all three compounds. At this level none of the control animals survived beyond 2 days. On the basis of the experiments carried out here the survival rates decreased in the order sodium 2,3 dimercaptopropane-sulfonate greater than 2,3 dimercaptosuccinic acid greater than N-Acetyl d,l penicillamine. In a study of the effect of the mole ratio of the chelating agent sodium 2,3 dimercaptopropanesulfonate to cadmium acetate on the survival rate, it was found that the survival rate attained a maximum for values between 20 and 60 and dropped off at both higher and lower values. It was also found that for at least one set of conditions, younger animals were better able to survive the administration of cadmium acetate and the antidote than older animals.