We are our brains, and so there isn't much room for outright replacement with new tissue as a strategy for regeneration anywhere inside the skull. Thus it is very important that researchers develop ways to repair the brain in situ. Fortunately, it looks as though this goal will be achievable sooner rather than later, with comparatively early stage stem cell therapies: "Neuron transplants have repaired brain circuitry and substantially normalized function in mice with a brain disorder, an advance indicating that key areas of the mammalian brain are more reparable than was widely believed. ... [Researchers] transplanted normally functioning embryonic neurons at a carefully selected stage of their development into the hypothalamus of mice unable to respond to leptin, a hormone that regulates metabolism and controls body weight. These mutant mice usually become morbidly obese, but the neuron transplants repaired defective brain circuits, enabling them to respond to leptin and thus experience substantially less weight gain. Repair at the cellular-level of the hypothalamus - a critical and complex region of the brain that regulates phenomena such as hunger, metabolism, body temperature, and basic behaviors such as sex and aggression - indicates the possibility of new therapeutic approaches to even higher level conditions such as spinal cord injury, autism, epilepsy, ALS (Lou Gehrig's disease), Parkinson's disease, and Huntington's disease. ... There are only two areas of the brain that are known to normally undergo ongoing large-scale neuronal replacement during adulthood on a cellular level - so-called 'neurogenesis,' or the birth of new neurons - the olfactory bulb and the subregion of the hippocampus called the dentate gyrus, with emerging evidence of lower level ongoing neurogenesis in the hypothalamus. The neurons that are added during adulthood in both regions are generally smallish and are thought to act a bit like volume controls over specific signaling. Here we've rewired a high-level system of brain circuitry that does not naturally experience neurogenesis, and this restored substantially normal function."