the OPA335 can work with input voltages from 100mV below negative supply (V-)...
here is an in-amp circuit:
(the ground symbol is the reference voltage, which should be about 200mV at a gain of 50... u would translate the range [-500uV..+50mV] to [+175mV..+2700mV])
(the resistors might be like these: R1=R2=100kR and Rgain=4.17kR: http://www.wolframalpha.com/input/?i=solve+for+g%3A+50*j%3D(1%2B100%2F100%2B2*100%2Fg)*j )