Bergmann Glial Cells Form Distinct Morphological Structures To Interact With Cerebellar Neurons Jens Grosche, 1 Helmut Kettenmann, 2 and Andreas Reichenbach 1 * 1 Paul Flechsig Institute for Brain Research, Leipzig, University, Leipzig, Germany 2 Cellular Neurosciences, Max Delbru ¨ck Center for Molecular Medicine, Berlin, Germany It is well established that Bergmann glial cells closely interact with neuronal elements in the molecular layer of the cerebellum. We reconstructed dye-labeled Berg- mann glial cells from electron microscopic serial sections and identified their contact sites with neurons as “glial microdomains“ (Grosche et al. [1999] Nature Neurosci. 2:139 –143). In the present paper we describe these structures in more detail, and show that 1) immature Bergmann fibers up to postnatal day 7 are smooth and lack appendages but contain several large mitochondria at sites where the first indications of growing side branches are observed; 2) Bergmann fibers from cere- bella at postnatal day 30 form two types of outgrowths, short simple thorns and longer complex appendages; 3) each of the latter (i.e., a glial microdomain) is in contact with only a few synapses and nonsynaptic neuronal ex- crescences; 4) every given region of the neuropil is oc- cupied by (at least) two interdigitating glial microdo- mains; 5) the synaptic clefts are entirely surrounded by glial protrusions, whereas the extrasynaptic surfaces and small axons are only partially covered; and 6) many small neuronal excrescenses without vesicles are completely ensheathed by glial caps, representing novel glial- neuronal structures of unknown function (glial thimbles). Computational modelling of the microdomains indicates that each is electrotonically independent of the stem process from which it arises, as well as of neighbouring domains. We assume that the glial microdomain is a morphological unit to compartimentalize ensembles of synapses, serving to synchronize local synaptic activity. © 2002 Wiley-Liss, Inc. Key words: Bergmann glial cells; cerebellar neurons The radially aligned Bergmann glial cell processes (Bergmann fibers) have been known for more than a century (Bergmann, 1875). They span the entire thickness of the molecular layer of the cerebellum in all vertebrate species studied so far, whereas their somata are aligned within the Purkinje cell layer. The Bergmann fibers play an important role as guidelines for the migration of young granule cells away from the external granular layer (Rakic, 1971; Gregory et al., 1988; Hatten, 1990; Hartmann et al., 1998). Bergmann glial cells are thought to play important roles, in the cerebellum of adult animals, in the mainte- nance of neuronal survival and functioning, similar to astrocytes elsewhere in the brain. Recently, the experi- mentally induced ablation of Bergmann glial cells in trans- genic mice was shown to cause severe defects of cerebellar neurons as well as motor discoordination (Cui et al., 2001). According to this view, the Bergmann fibers are covered with many complex side branches, which may constitute the sites of glial-neuronal interactions. The principal counterparts for such interactions are the gluta- matergic, i.e., excitatory, synapses between the parallel fibers and the spines of the Purkinje cell dendrites. Thus, it has been speculated that Bergmann glial cells may serve homeostatic functions for the extracellular spaces around these synapses, such as K + clearance (Reichenbach et al., 1995; Mu ¨ller and Kettenmann, 1995) or glutamate uptake (Ottersen et al., 1990; Ruiz and Ortega, 1995; Chaudhry et al., 1995). It has been demonstrated that Bergmann cells possess functional glutamate receptors (Blackstone et al., 1992; Mu ¨ller et al., 1992, 1993; Luque and Richards, 1995; Kirischuk et al., 1996; Meguro et al., 1999), which may be (co-)activated during neuronal synaptic transmis- sion. Despite many morphological studies on the cerebel- lum and even the Bergmann glia, the structural basis for such interactions is not known in much detail. Both light and electron microscopically, the lateral appendages dis- play a very complex appearance. There have been a few reports of estimated surface-to-volume ratios which, on average, were about 13–15 m –1 (Grab et al., 1983; Grosche et al., 1999) but may be as high as 25 m –1 in complex appendages (Grosche et al., 1999). This com- plexity has caused the impression that glial appendages are irregular, randomly shaped structures. It was only recently that, by performing serial electron microscopy and three- *Correspondence to: Prof. Dr. Andreas Reichenbach, Paul Flechsig Insti- tute for Brain Research, Leipzig University, Jahnallee 59, D-04109 Leipzig, Germany. E-mail: reia@server3.medizin,uni-leipzig.de Received 8 November 2001; Revised 20 December 2001; Accepted 29 December 2001 Published online 27 February 2002 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jnr.10197 Journal of Neuroscience Research 68:138 –149 (2002) © 2002 Wiley-Liss, Inc.