While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Osiris Therapeutics (NASDAQ:OSIR) closed up a big 12% yesterday after Piper Jaffray upgraded the stock from "neutral" to "overweight."

So what: Along with the upgrade, analyst Edward Tenthoff reaffirmed his price target of $21 per share, representing about 12% worth of upside to where the stock sits now. Osiris skyrocketed in July on positive interim results on Grafix in treatment of diabetic foot ulcers, or DFUs, but Tenthoff's expectation of even more positive data, coupled with the stock's recent pullback, gives investors some decent upside to work with.

Now what: Osiris is set to present full DFU data at the Desert Foot Conference in November.

"We anticipate these results will facilitate Grafix's reimbursement in DFU, contributing to significant Biosurgery sales growth in 2014 and beyond," Tenthoff said. "OSIR shares have pulled back ... thus presenting the opportunity to upgrade back to Overweight with our current $21 price target."

So while Osiris might still be too speculative for average investors, Grafix's positive outlook, coupled with the fact that the shares remain well off their 52-week high, make the stock an interesting pick for biotech-savvy Fools.

Fool contributor Brian Pacampara has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.