Purkinje cell axonal anatomy: quantifying morphometric changes in essential tremor versus control brains

Abstract

Growing clinical, neuro-imaging and post-mortem data have implicated the cerebellum as playing an important role in the pathogenesis of essential tremor. Aside from a modest reduction of Purkinje cells in some post-mortem studies, Purkinje cell axonal swellings (torpedoes) are present to a greater degree in essential tremor cases than controls. Yet a detailed study of more subtle morphometric changes in the Purkinje cell axonal compartment has not been undertaken. We performed a detailed morphological analysis of the Purkinje cell axonal compartment in 49 essential tremor and 39 control brains, using calbindin D28k immunohistochemistry on 100-µm cerebellar cortical vibratome tissue sections. Changes in axonal shape [thickened axonal profiles (P = 0.006), torpedoes (P = 0.038)] and changes in axonal connectivity [axonal recurrent collaterals (P < 0.001), axonal branching (P < 0.001), terminal axonal sprouting (P < 0.001)] were all present to an increased degree in essential tremor cases versus controls. The changes in shape and connectivity were significantly correlated [e.g. correlation between thickened axonal profiles and recurrent collaterals (r = 0.405, P < 0.001)] and were correlated with tremor duration among essential tremor cases with age of onset >40 years. In essential tremor cases, thickened axonal profiles, axonal recurrent collaterals and branched axons were 3- to 5-fold more frequently seen on the axons of Purkinje cells with torpedoes versus Purkinje cells without torpedoes. We document a range of changes in the Purkinje cell axonal compartment in essential tremor. Several of these are likely to be compensatory changes in response to Purkinje cell injury, thus illustrating an important feature of Purkinje cells, which is that they are relatively resistant to damage and capable of mobilizing a broad range of axonal responses to injury. The extent to which this plasticity of the Purkinje cell axon is partially neuroprotective or ultimately ineffective at slowing further cellular changes and cell death deserves further study in essential tremor.

Keywords: Purkinje cell; axon; essential tremor; neurodegenerative; recurrent collateral.

Figure 1

Calbindin immunohistochemistry on 100-µm cerebellar neocortex sections, ×100. (A) Normal control brain with thin visible axonal profiles. (B) Three torpedoes (arrow heads), two of which are on axons with recurrent collaterals (carets). Several thickened axonal profiles are shown (short arrows). (C) Terminal axonal sprouting (thick arrow). (D) Arciform axon profile; profile is also thickened. (E) Torpedoes (arrow heads) with axonal recurrent collaterals (carets) and axonal branching (long arrows and inset). (F) A visible portion of the recurrent collateral plexus at the Purkinje cell layer (boxed). (G) Thickened axonal profiles (short arrows).

Figure 2

Comparison of axonal pathologies between essential tremor cases and controls. Bars represent one standard error. *P < 0.05. **The extent of plexus is expressed as the per cent of the length of the Purkinje cell layer covered by a visible recurrent collateral plexus; it is divided by 10 for the purposes of graphical display.