Microtubules with different diameter, protofilament number and protofilament spacing in Ornithogalum umbellatum ovary epidermis cells.

Microtubules present in the epidermis of Ornithogalum umbellatum ovary in the area of lipotubuloids (i.e. aggregates of lipid bodies surrounded by microtubules) are 25-51 nm in diameter. They consist mainly of 10 and 11, sometimes 9 and 12 protofilaments. An average diameter of microtubule consisting of 9 subunits is about 32 nm, of 10-35 nm, of 11-38 nm and of 12-43 nm, however, individual microtubules in each category significantly vary in size. These differences result from varying distance between protofilaments in microtubule walls and diameters of protofilaments: in thin microtubules they are densely packed and smaller while in thicker ones they are loosely arranged and bigger. A hypothesis has been put forward that changes in microtubule diameter depend on structural changes associated with their functional status and are executed by modifications of protofilament arrangement density and their diameters in microtubule wall. The above hypothesis seems to be in agreement with the opinion formed on the basis of in vitro image of microtubules, that lateral contact between tubulin subunits in neighboring protofilaments indicates some flexibility and changeability during microtubule function.