The spiral arms are actually regions of star formation characterized by bright blue giants and supergiants (O and B stars), and massive HII regions. The closer matter is to the center of a galaxy, the faster it revolves around the center of the galaxy. This would tend to soon wrap the spiral arms tighter than we see them, if the arms were constantly composed of the same matter.
Since star formation and HII regions correspond to regions of higher-than-average interstellar gas density, this suggests a way to explain what causes the spiral arms.
As explained by Lin and Shu in 1963, the spiral structure results from density waves in the interstellar gas, analogous to higher-density regions of cars in sections of highway undergoing construction (Shu, p. 275).
The galaxies seen here have random orientations to our line of sight - some are seen face-on (M 33), some edge-on (M 104), and most somewhere in between.
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