OK, this outputs 2ma to 5ma, handles up to 20kohm tissue sample, and provides a place to monitor the current with a scope. If you want to reverse the current, reverse the probes. You can do it manually or with a DPDT switch or relay.
The MPSA43 will dissipate up to 600mw instantaneously, but if you keep the frequency and the duty cycle as designed, the average power is only about 200mw max. If you slow down the frequency and/or increase the duty cycle, you might need a heat sink.
The 120v supply does not need to be regulated, and a few volts of ripple won't hurt anything. Remember that 120 volts can be LETHAL!