I think this really depends on the students' interests and the strengths of the individual institution. For instance, here at Pitt we have a greater number in Neuroscience, Bioengineering, and Immunology. But in theory we can do our PhD in any Dept at Pitt or CMU, even non-science ones.
Personally my PhD is in Pathology. Our Dept has the 5th highest NIH funding in the country, but only a handful of MSTP students (and we're a pretty student-friendly dept too!). Go figure 🙂