It has been difficult to establish whether we are limited to the heart muscle cells we are born with or if cardiomyocytes are generated also later in life. We have taken advantage of the integration of 14C, generated by nuclear bomb tests during the Cold War, into DNA to establish the age of cardiomyocytes in humans. We report that cardiomyocytes renew, with a gradual decrease from 1% turning over annually at the age of 20 to 0.3% at the age of 75. Less than 50% of cardiomyocytes are exchanged during a normal lifespan. The capacity to generate cardiomyocytes in the adult human heart suggests that it may be rational to work towards the development of therapeutic strategies aiming to stimulate this process in cardiac pathologies.

We describe here the isolation of stem cells from juvenile and adult rodent skin. These cells derive from the dermis, and clones of individual cells can proliferate and differentiate in culture to produce neurons, glia, smooth muscle cells and adipocytes. Similar precursors that produce neuron-specific proteins upon differentiation can be isolated from adult human scalp. Because these cells (termed SKPs for skin-derived precursors) generate both neural and mesodermal progeny, we propose that they represent a novel multipotent adult stem cell and suggest that skin may provide an accessible, autologous source of stem cells for transplantation.

Spinal cord injury often results in permanent functional impairment. Neural stem cells present in the adult spinal cord can be expanded in vitro and improve recovery when transplanted to the injured spinal cord, demonstrating the presence of cells that can promote regeneration but that normally fail to do so efficiently. Using genetic fate mapping, we show that close to all in vitro neural stem cell potential in the adult spinal cord resides within the population of ependymal cells lining the central canal. These cells are recruited by spinal cord injury and produce not only scar-forming glial cells, but also, to a lesser degree, oligodendrocytes. Modulating the fate of ependymal progeny after spinal cord injury may offer an alternative to cell transplantation for cell replacement therapies in spinal cord injury.

A fundamental question in stem cell research is whether cultured multipotent adult stem cells represent endogenous multipotent precursor cells. Here we address this question, focusing on SKPs, a cultured adult stem cell from the dermis that generates both neural and mesodermal progeny. We show that SKPs derive from endogenous adult dermal precursors that exhibit properties similar to embryonic neural-crest stem cells. We demonstrate that these endogenous SKPs can first be isolated from skin during embryogenesis and that they persist into adulthood, with a niche in the papillae of hair and whisker follicles. Furthermore, lineage analysis indicates that both hair and whisker follicle dermal papillae contain neural-crest-derived cells, and that SKPs from the whisker pad are of neural-crest origin. We propose that SKPs represent an endogenous embryonic precursor cell that arises in peripheral tissues such as skin during development and maintains multipotency into adulthood.

Several distinct cell types in the adult central nervous system have been suggested to act as stem or progenitor cells generating new cells under physiological or pathological conditions. We have assessed the origin of new cells in the adult mouse spinal cord by genetic fate mapping. Oligodendrocyte progenitors self-renew, give rise to new mature oligodendrocytes, and constitute the dominating proliferating cell population in the intact adult spinal cord. In contrast, astrocytes and ependymal cells, which are restricted to limited self-duplication in the intact spinal cord, generate the largest number of cells after spinal cord injury. Only ependymal cells generate progeny of multiple fates, and neural stem cell activity in the intact and injured adult spinal cord is confined to this cell population. We provide an integrated view of how several distinct cell types contribute in complementary ways to cell maintenance and the reaction to injury.

Neurons are continuously generated from stem cells in discrete regions in the adult mammalian brain. We found that ependymal cells lining the lateral ventricles were quiescent and did not contribute to adult neurogenesis under normal conditions in mice but instead gave rise to neuroblasts and astrocytes in response to stroke. Ependymal cell quiescence was actively maintained by canonical Notch signaling. Inhibition of this pathway in uninjured animals allowed ependymal cells to enter the cell cycle and produce olfactory bulb neurons, whereas forced Notch signaling was sufficient to block the ependymal cell response to stroke. Ependymal cells were depleted by stroke and failed to self-renew sufficiently to maintain their own population. Thus, although ependymal cells act as primary cells in the neural lineage to produce neurons and glial cells after stroke, they do not fulfill defining criteria for stem cells under these conditions and instead serve as a reservoir that is recruited by injury.

Precursor cells of the embryonic cortex sequentially generate neurons and then glial cells, but the mechanisms regulating this neurogenic-to-gliogenic transition are unclear. Using cortical precursor cultures, which temporally mimic this in vivo differentiation pattern, we demonstrate that cortical neurons synthesize and secrete the neurotrophic cytokine cardiotrophin-1, which activates the gp130-JAK-STAT pathway and is essential for the timed genesis of astrocytes in vitro. Our data indicate that a similar phenomenon also occurs in vivo. In utero electroporation of neurotrophic cytokines in the environment of embryonic cortical precursors causes premature gliogenesis, while acute perturbation of gp130 in cortical precursors delays the normal timed appearance of astrocytes. Moreover, the neonatal cardiotrophin-1-/- cortex contains fewer astrocytes. Together, these results describe a neural feedback mechanism; newly born neurons produce cardiotrophin-1, which instructs multipotent cortical precursors to generate astrocytes, thereby ensuring that gliogenesis does not occur until neurogenesis is largely complete.

Cultured embryonic cortical progenitor cells will mimic the temporal differentiation pattern observed in vivo, producing neurons first and then glia. Here, we investigated the role of two endogenously produced growth factors, the neurotrophins brain-derived neurotrophic factor and neurotrophin-3 (NT-3), in the early progenitor-to-neuron transition. Cultured cortical progenitors express BDNF and NT-3, as well as their receptors TrkB (tyrosine kinase receptor B) and TrkC. Inhibition of these endogenously expressed neurotrophins using function-blocking antibodies resulted in a marked decrease in the survival of cortical progenitors, accompanied by decreased proliferation and inhibition of neurogenesis. Inhibition of neurotrophin function also suppressed the downstream Trk receptor signaling pathways, PI3-kinase (phosphatidyl inositol-3-kinase) and MEK-ERK (MAP kinase kinase-extracellular signal-regulated kinase), indicating the presence of autocrine-paracrine neurotrophin:Trk receptor signaling in these cells. Moreover, specific inhibition of these two Trk signaling pathways led to distinct biological effects; inhibition of PI3-kinase decreased progenitor cell survival, whereas inhibition of MEK selectively blocked the generation of neurons, with no effects on survival or proliferation. Thus, neurotrophins made by cortical progenitor cells themselves signal through the TrkB and TrkC receptors to mediate cortical progenitor cell survival and neurogenesis via two distinct downstream signaling pathways.