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Here we have investigated the mechanisms that control astrocyte differentiation within the developing rat optic nerve. Astrocytes are normally generated by astrocyte precursor cells within the embryonic optic nerve. We show that there is a close temporal and spatial correlation between endothelial and astrocyte differentiation. We tested the potential role of endothelial cells in inducing astrocyte differentiation by developing an immunopanning method to highly purify endothelial cells from developing optic nerves. We show that the purified endothelial cells, but not other embryonic optic nerve cell types, strongly induce the differentiation of purified astrocyte precursor cells into astrocytes in vitro. Leukemia inhibitory factor (LIF) and LIF receptors have been implicated previously in astrocyte differentiation in vivo. We show that purified endothelial cells express LIF mRNA and that their ability to induce astrocyte differentiation is prevented by a neutralizing anti-LIF, but not anti-ciliary neurotrophic factor, antiserum. These findings demonstrate a role for endothelial cells in inducing astrocyte differentiation. The induction of astrocyte differentiation by endothelial cells makes sense phylogenetically, anatomically, and functionally, because astrocytes evolved concurrently with brain vasculature and ensheathe capillaries throughout the brain. The ability to purify and culture astrocytes and endothelial cells should provide an excellent model system for future studies of bloodbrain barrier development. Key words: glial development; vasculature; capillaries; astrocyte precursor cells; leukemia inhibitory factor (LIF); endothelial cells; blood-brain barrierTo better understand the cell-cell interactions that control astrocyte development, we have been focusing on the development of glial cells within the rat optic nerve. The optic nerve is part of central white matter and, in addition to the axons of retinal ganglion cells, contains the same glial cell types found in white matter throughout the CNS. Previous studies have found that two different glial lineages develop within the nerve Miller et al., 1985Miller et al., , 1989. Type-1 astrocytes develop in the embryonic optic nerve from astrocyte precursor cells (APCs) Miller et al., 1985Miller et al., , 1989Mi and Barres, 1999), whereas oligodendrocytes develop in the postnatal optic nerve from oligodendrocyte precursor cells (Raff et al., 1983;Miller et al., 1985).In the developing brain, it has been found previously that astrocytes are generated by neural stem cells located in the ventricular zone and subventricular zone (Levison and Goldman, 1993;Goldman, 1996;Barres, 1999). The generation of astrocytes from multipotent neural stem cells in culture is induced by bone morphogenetic factors, ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), basic FGF (bFGF), and Notch signaling Lillien et al., 1988;Yoshida et al., 1993;Nakagaito et al., 1995;Gross et al., 1996;Johe et al., 1996;Richards et al., 1996;Bonni et al., 1997;Qian et al., 19...

Primary afferent somatosensory neurons mediate our sense of touch in response to changes in ambient pressure. Molecules that detect and transduce thermal stimuli have been recently identified, but mechanisms underlying mechanosensation, particularly in vertebrate organisms, remain enigmatic. Traditionally, mechanically evoked responses in somatosensory neurons have been assessed one cell at a time by recording membrane currents in response to application of focal pressure, suction, or osmotic challenge. Here, we used radial stretch in combination with live-cell calcium imaging to gain a broad overview of mechanosensitive neuronal subpopulations. We found that different stretch intensities activate distinct subsets of sensory neurons as defined by size, molecular markers, or pharmacological attributes. In all subsets, stretchevoked responses required extracellular calcium, indicating that mechanical force triggers calcium influx. This approach extends the repertoire of stimulus paradigms that can be used to examine mechanotransduction in mammalian sensory neurons, facilitating future physiological and pharmacological studies.mechanotransduction ͉ sensory signaling ͉ somatosensation ͉ touch