An inner ring according to the invention or an inner ring segment according to the invention for an adjustable guide vane assembly comprises a plurality of uptakes, each for the bearing of an adjustable guide vane. The uptakes here each form an essentially ellipsoidally shaped uptake space for a guide vane head of the respective guide vane. A guide vane according to the invention for an adjustable guide vane assembly has a blade body and a guide vane head. The guide vane head is essentially ellipsoidal in shape and is equipped to be accommodated pivotably in an uptake of an inner ring or inner ring segment of the guide vane assembly.