Final answer:
Other substances found in the α granules of platelets include adenosine diphosphate (ADP) and serotonin, in addition to previously mentioned substances. ADP encourages further platelet adhesion, while serotonin helps maintain constriction of blood vessels.
Step-by-step explanation:
The α granules of platelets contain a variety of substances that are vital for the blood clotting process. Apart from von Willebrand factor (vWF), fibrinogen, and fibronectin, which you've mentioned, they also include platelet factor 4 (PF4) and platelet-derived growth factor (PDGF).
However, the substances you are meant to identify from the provided options that are found in α granules are adenosine diphosphate (ADP) and serotonin. ADP helps additional platelets to adhere to the injury site, while serotonin is crucial in maintaining vasoconstriction, which reduces blood flow to the area and minimizes blood loss.
Other contents of α granules include various coagulation factors and compounds that are involved in wound healing and cell attraction. These granules do not contain collagen, elastin, heparin, histamine, thrombin, or plasin, as they are involved in different aspects of the clotting process or are present elsewhere in the body.