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This study focused on the hippocampus, considered to be the seat of learning and memory, whose shrinkage in Alzheimer's disease causes steadily worsening symptoms. The study's authors targeted a key player in the hippocampal "learning system," which includes the hippocampus itself, the subiculum (the major output structure connected to the cortex, the self-aware "thinking" part of the brain), and the adjacent entorhinal cortex.Previously, these scientists had demonstrated that damage to the subiculum in rats led to deterioration of the hippocampus, and problems with learning. The next question was obvious: Could researchers do the opposite, repair the hippocampus and restore the memory functions? ...First, the scientists injected a neuron-destroying chemical into the subiculum area of 48 adult rats. Next, again using precise micro-injections, the scientists transplanted hippocampal cells that had been taken from newborn transgenic mice and cultured in an incubator into the hippocampi of about half the rats. These special cells had a green fluorescent protein used to "label" and track them after transplantation. (Transgenic mice are bred with a little extra DNA that allows their cells to be grown in glass plates in incubators.)Two months later, the scientists measured how well both the transplant and non-transplant rats learned and remembered, using two well-established maze tests of spatial learning. The rats given cell transplants had recovered completely: On both mazes, they performed as well as those rats which had not had their subiculums damaged. The rats without transplants did not recover: They had many problems learning their way through the mazes. After studying behavior, the scientists examined what happened in the brain. Under the microscope, it appeared that the transplanted cells had settled mainly in a sub-area of the hippocampus called the dentate gyrus. There, the transplants appeared to promote the secretion of two types of growth factors, namely brain-derived neurotrophic factor and fibroblast growth factor, which boost the growth and survival of the cells that give rise to neurons. In the hippocampi of rats with cell transplants, the expression of brain-derived growth factor went up threefold.