Novel insights into the glia limitans of the olfactory nervous system

Olfactory ensheathing cells (OECs) are often described as being present in both the peripheral and the central nervous systems (PNS and CNS). Furthermore, the olfactory nervous system glia limitans (the glial layer defining the PNS-CNS border) is considered unique as it consists of intermingling OECs and astrocytes. In contrast, the glia limitans of the rest of the nervous system consists solely of astrocytes which create a distinct barrier to Schwann cells (peripheral glia). The ability of OECs to interact with astrocytes is one reason why OECs are believed to be superior to Schwann cells for transplantation therapies to treat CNS injuries. We have used transgenic reporter mice in which glial cells express DsRed fluorescent protein to study the cellular constituents of the glia limitans. We found that the glia limitans layer of the olfactory nervous system is morphologically similar to elsewhere in the nervous system, with a similar low degree of intermingling between peripheral glia and astrocytes. We found that the astrocytic layer of the olfactory bulb is a distinct barrier to bacterial infection, suggesting that this layer constitutes the PNS-CNS immunological barrier. We also found that OECs interact with astrocytes in a similar fashion as Schwann cells in vitro. When cultured in three dimensions, however, there were subtle differences between OECs and Schwann cells in their interactions with astrocytes. We therefore suggest that glial fibrillary acidic protein-reactive astrocyte layer of the olfactory bulb constitutes the glia limitans of the olfactory nervous system and that OECs are primarily "PNS glia."

The M6 cell: A small-field bistratified photosensitive retinal ganglion cell

We have identified a novel, sixth type of intrinsically photosensitive retinal ganglion cell (ipRGC) in the mouse-the M6 cell. Its spiny, highly branched dendritic arbor is bistratified, with dendrites restricted to the inner and outer margins of the inner plexiform layer, co-stratifying with the processes of other ipRGC types. We show that M6 cells are by far the most abundant ganglion cell type labeled in adult pigmented Cdh3-GFP BAC transgenic mice. A few M5 ipRGCs are also labeled, but no other RGC types were encountered. Several distinct subnuclei in the geniculate complex and the pretectum contain labeled retinofugal axons in the Cdh3-GFP mouse. These are presumably the principle central targets of M6 cells (as well as M5 cells). Projections from M6 cells to the dorsal lateral geniculate nucleus were confirmed by retrograde tracing, suggesting they contribute to pattern vision. M6 cells have low levels of melanopsin expression and relatively weak melanopsin-dependent light responses. They also exhibit strong synaptically driven light responses. Their dendritic fields are the smallest and most abundantly branched of all ipRGCs. They have small receptive fields and strong antagonistic surrounds. Despite deploying dendrites partly in the OFF sublamina, M6 cells appear to be driven exclusively by the ON pathway, suggesting that their OFF arbor, like those of certain other ipRGCs, may receive ectopic input from passing ON bipolar cells axons in the OFF sublayer.