An apparatus for accessing bone marrow inside a bone is provided. The apparatus may include a penetrator assembly having a tissue penetrator and a hub. The tissue penetrator my have a hollow cannula disposed in fixed relation to the hub. The apparatus may also have a driver configured to insert a portion of the tissue penetrator into the bone and bone marrow, and a depth control mechanism configured to control the depth of penetration of the tissue penetrator into the bone and bone marrow. The depth control mechanism may have a sensor configured to detect a position of the bone and bone marrow. The depth control mechanism may also have a mechanical stop.
