The present invention relates to a placeholder, especially for vertebrae or vertebral discs, with a tubular body (4). Along a jacket surface (10) of the placeholder, a plurality of breakthroughs or openings (9) for over-growth with adjacent tissue are disposed. The placeholder includes at least a second tubular body (2, 3) provided with a plurality of breakthroughs and openings (9) at least partially inside the first tubular body. The first and second tubular bodies can have different cross-sectional shapes, can be arranged inside one another by press fit or force fit or can be connected to each other via connecting pins or retaining plates. Several second bodies can be arranged side by side to each other in the first body. The invention also refers to a modular system comprising: - a plurality of tubular bodies (2, 3, 4), each of which has a plurality of openings (9) or breakthroughs at their jacket surface (10) and different dimensions, such that at least two different tubular bodies may be arranged at least partially inside one another, and - connecting means (8, 13, 14) for connecting the tubular bodies.