Device-to-device (D2D) communications can enhance the overall system capacity by reusing cellular spectrum, which at the same time leads to severe interference to cellular connections and vice versa. In practice, even when two wireless connections share the same spectrum, the interference between them may not always exist if there is no conflict at packet-level transmissions. Thus, in this paper, we establish a cross-layer model for D2D communications underlaying cellular network to characterize the realistic interference scenario. The closed-form stable throughput region between one D2D pair and one cellular link is deduced. Subsequently, our model is extended to a generalized scenario where multiple D2D pairs share the same resources with one cellular link, in which the upper and lower bounds of the stable throughput region are derived to characterize the resources reusing among multiple interacting queues. As a consequence, the stable throughput regions are significantly enlarged via the proposed model.