Helen- Nothing I read about the creation of the Pieta indicated that he had help from apprentices, I think since he was a young and unknown artist and it took him two years to complete that is possible he worked alone. However it was common for master artists during the era to use apprentices with their work and I'm certain apprentices help him with the Sistine chapel. 

tjust- I think this is the one of the only pieces that he did sign and I am sure he didn't sign any after. Michaelangelo was an unknown artist in his 20's when he completed this, I read during my research that when the Pieta was being unveiled he was in the crowd and overheard someone give another, more well known artist credit for the masterpiece. so in the middle of the night, he came back and chiseled his name across Virgin Mary's sash. Later he felt foolish for this and vowed never to sign a piece of work again.

Jmpham- Honestly, I can"t think of any modern art that compares to the religious imagery of Michelangelo's Pieta, but I know its completely subjective.  Max Beckmann was an expressionist artist who I think put a lot of emotion and traditional elements into his figures. I compare him because I find his style very personal and visually potent.

Alliemt- Michelangelo was a brilliant artist that was influenced by many apects of life, humanism, science, anatomy, etc. Lorenzo Ghibertia and Bertoldo di Giovanni, were early renaissance sculptors who influenced Michelangelo, he was also highly influenced by the highly respected Medici family.  As Im sure many more.

Lara- that's a very good question and upon my search I found that most of Michelangelo's pieces were religious based or influenced, especially his famous pieces. some sculptures that I found that were not are: The Crouching boy, Victory, and and Dying Slave. However I think at least one of these were created to be part of a tomb.