Nuclear pore structure in quiescent buds of Tilia europaea

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Abstract
Quiescent shoot apical tissue from buds of the linden tree (Tilia europaea L.), collected during winter months, was frozen without pretreatment and freeze-fractured for electron microscopy. A proportion of the nuclear envelopes bore unusually large nuclear pores, having diameters up to 175 nm. Results from cross-fractured nuclear envelopes were consistent with results from envelopes fractured in the membrane plane, indicating that the larger pores are unlikely to be artefacts of fracturing. In these large pores, it was not possible to determine the relationship between the annulus (the material contained within the membranous pore) and the membranous periphery of the pore. Particularly, it could not be determined whether the annulus expanded concomitantly with the pore. It is proposed that variations in nuclear pore diameter may arise as a result of pulsatory activity occurring in all pores, rather than representing static differences between individual pores.