Q:

How do I build a 10uA current source/limiter?

I need a constant current of 10uA through a variable resistor in the 100k values. I need to inject the current into that resistor and I need the resistor to be connected to Ground such that I can measure the voltage across it with a microcontroller(writing this I figured I could just read the voltages in both points and substract them, but oh well). So how do I do this with discrete components? I heard that current mirrors don't work with discrete components. I don't know if using an operational amplifier and a sense resistor, I would be able to source/sink such a small current. Is a current limiting circuit better? I don't really need to inject the current, I just need constant 10uA.
What's a reliable, stable and fairly exact 10uA current source/sink that I can build with discrete components and without any IC's as I don't want to wait for themto ship to me?
Edit: I'm working with supply voltages in the range 5v-12v
Edit 2: 1% accuracy is just fine. Heck, even 5% accuracy is fine, anything below 10% is fine. My load is a 220k resistor at it's maximum (it's actually a 100k thermistor, but that's as high as it will get). And my supply would be 5V, so in order to get 10uA through it, I would need to supply about 2.2V, so no problem there. 

A:

Something like this will give you a fairly constant current. It's only (as in not quite as) stable as your 12V supply, but it has quite good compliance & output resistance and reasonable temperature stability for normal room temperature type changes (maybe +/- a couple % for +/- 10°C change). Ideally, put the BJTs face-to-face with a touch of thermal grease between them and shrink wrap around them. 
One can do much better with an op-amp and a proper reference, but this will work and meets your stated requirement of using only discrete components and no ICs. 

simulate this circuit – Schematic created using CircuitLab

A:

There are many ways to do this.  Since you didn't specify accuracy or compliance range, this will do:

Let's say you want to keep about 2 V on R1, which would make it around 200 kΩ.  That would make the zener diode about 2.6 V.  This will have some temperature dependence due to the B-E drop being part of the reference voltage.  But, with only 4 cheap and small parts it's simplicity, size, and cost is very good.  Size R2 to keep just enough current thru the zener for it to regulate its volage well.  1 mA works well enough with most zeners, but see the datasheet.
A more complicated but more accurate circuit is:

The opamp actively regulates the voltage across R1 to match the zener voltage.  With 5.6 V across R1, 10 µA will always flow thru it.  This same current is also the load current, which is how the load current is regulated.
Now that you've said 12 V is available, we can use that to get some idea of compliance range.  Q1 can be just about any low voltage P-channel MOSFET.  It's on resistance will be so low compared to 560 kΩ as to be irrelevant.  This current source can therefore drive the output up to the supply minus the current sense resistor voltage, or 12 V - 5.6 V = 6.4 V.

