The best answer to the question: The Z disc forms the end____ of a sarcomere, would be: the end border, or the point where sarcomeres join, within a muscle fiber.
Explanation:
Concretely, a Z disc is the boundary line that divides sarcomeres within a muscle fiber. They are the bordes between sarcomeres and they contain the most important molecule, actin, which forms the thin filaments essential for muscle tissue movement. This actin molecule, present along the Z discs, under the correct stimulation and circumstances, will bind to myosin and thus movement will be possible.
