Sentence:  Since the launch early last year of Udacity and Coursera, two Silicon Valley <WSD>start-ups</WSD> offering free education through MOOCs, massive open online courses, the ivory towers of academia have been shaken to their foundations. University brands built in some cases over centuries have been forced to contemplate the possibility that information technology will rapidly make their existing business model obsolete. 
Meaning of the word marked with <WSD> tags:  A new company or organization or business venture designed for rapid growth.