Development and cell generation in the hippocampus of a marsupial, the quokka wallaby (Setonix brachyurus)

Development and cell generation in the hippocampus of the marsupial, the quokka wallaby, has been examined. Cells in this brain region are similar in morphology to those in eutherian species, with predominantly pyramidal and granule cells. In the quokka, development of the hippocampus takes place postnatally; this region is first seen just after birth on postnatal day 1 (P1) as an out-pouching of the medial cortical wall into the lateral ventricle. The cornu ammonis (CA) region first appears at P20 as a line of denser cells and by P30, CA3 and the granule cell layer of the dentate gyrus (DG) can be defined. A specific region of the ventricle, near to the developing fimbria, produces the granule cells destined for the dentate gyrus. These cells initially migrate in a curved trajectory into the hilus, following the path of thick, vimentin-positive glial fibres. Cells are generated in the hippocampus from around P5 until at least P85 when some cells in the hilus and also glial cells are labelled with [3H]thymidine. In the cell sparse region around the hippocampal fissure there is a peak of neuron production before P20 followed by a decline and subsequent increase in the production of probably glial cells after P60. The peak of cell generation in the CA region and the granule cell layer of the DG is around P40. Cells continue to be produced in the hilus of the DG much later, with numbers still high at P85, presumably these cells are destined to reach the granule cell layer later in development.