Oct 29 (Reuters) - The S&P 500 and the Nasdaq indexes dropped from record highs at open on Friday, as results from mega-cap firms Apple and Amazon.com reignited concerns of labor and supply shortages that have been at the forefront of this quarterly earnings season. The Dow Jones Industrial Average fell 18.20 points, or 0.05%, at the open to 35,712.28. The S&P 500 opened lower by 23.55 points, or 0.51%, at 4,572.87, while the Nasdaq Composite dropped 124.83 points, or 0.81%, to 15,323.29 at the opening bell. (Reporting by Devik Jain in Bengaluru; Editing by Maju Samuel)