These PNS glia include Schwann cells in nerves and satellite glial cells in ganglia. Gliophilic migration is the migration of glia along glial fibres. These glia include the oligodendrocytes, ependymal cells, and astrocytes. RGPs also produce certain lineages of glia, including astrocytes and oligodendrocytes. Tanycytes share some features with radial glia cells and astrocytes. These cells can differentiate into neurons, glia, keratinocytes, smooth muscle cells and melanocytes in vitro. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. The type VI intermediate filament protein nestin is expressed in developing neurons and glia. This progression leads to the neurons and glia that populate cortical layers. Differentiation begins with the retinal ganglion cells and concludes with production of the Muller glia. Included in these layers are stellates, radial glia, and pyramidal neurons. This progression leads to the generation of neurons and glia that populate cortical layers. The neurons then extend along the radial glia to reach the cortical zone. The signaling between glia and neurons in the developing nervous system is reciprocal. In all, a neuroblast is capable of generating an indefinite number of neurons or glia.