It can be hard to decide how much detail to include in the Introduction. In your excitement about your work, it is all too easy to accidentally write a review of the current state of your field. At the other extreme, you can forget to slow down and explain to readers why you did what you did the way that you did it.

Here is a simple list of questions to keep in mind when writing the Introduction:
Why this topic?
Why this aspect of this topic?
Why this experimental approach?
Why this organism/model?

How to decide if something should be included in the Introduction:
Does it answer one of the above questions?
If so, include it!
If not, think hard about why you are considering including it. It probably does not belong.

How to decide if your Introduction is complete:
Are all of those questions answered?
If so, you are ready to move on!
If not, go back and add what’s missing.

The Introduction is not a place to review all that is known about a field. It is also not the place to describe every experimental approach previously used. So, unless the details of an earlier paper address one of the four questions above, leave them out and just state the conclusion.

For example:
“Bumble and Bumble (2010) used yeast two-hybrid and in vitro pull-down assays to show that HA-tagged HAIRSPRAY1 interacts with BIGBANGS3. They also found reconstituted fluorescence in BiFC assays that indicated interaction when the proteins were transiently expressed as fusion proteins in tobacco leaves.”

could instead be:
“HAIRSPRAY1 and BIGBANGS3 interact directly (Bumble and Bumble, 2010).”

What about information that will help readers interpret your results? The usual advice is to include EVERY bit of information that your readers will need to understand your work. Accordingly, you might decide to bring it up in the Introduction, but if the topic is only distantly related to the original topic of study, it might be better to leave it for the Discussion, when your results have provided/explained the connection to your original topic. Use your judgment — if you can work it in without interrupting the logic of the Introduction, then do so!