Calretinin immunoreactivity in the monkey and cat cerebellum: cellular localisation and modular distribution

We have studied the morphology and distribution of calretinin-containing neurons in the cat and monkey cerebellum immunocytochemically. Immunoreactivity is detectable in cortex and white matter, with cell bodies mainly in the granular layer, the commonest of which are granule cells. Labelled Lugaro cells and a type of bipolar neuron are also occasionally seen. Other larger cells in the granular layer are strongly labelled. They are characterised by a spherical cell body giving rise to a thick primary dendrite with a spray of terminal appendages, like the unipolar brush cells (UBC) described in several species. They are distributed according to a mediolateral gradient of numerical density, with most in parasagittally congruent cell clusters in the vermis and the paravermal parts of the hemispheres. The axons of UBCs originate from the soma or primary dendrite, and are a major source of labelled fibres in the granular layer. Positively stained climbing fibres are radially oriented in the white matter and cortex, and form longitudinal bands, especially in the vermis. Climbing fibre bands are congruent with UBC clusters in the vermis but less so in the hemispheres. Both mossy and climbing fibres seem likely to synapse on UBSs. We postulate that a pathway from afferent fibres to UBCs to Purkinje cells may represent a short circuit for rapid cerebellar reflexes without passing through much of the multisynaptic circuits of the cortex. Calretinin may be involved in signal processing in the cerebellum, perhaps regulating the amount and form of intracellular calcium.