The schematics for the inside of a Sperm Cell. Of particular note is that their wedding rings either come in X or Y shaped gemstones, representing the chromosome they contribute, and their offspring's potential gender.
The entirety of episode 3 has the narrator and cast describing the process of an erection and causes of erectile dysfuction with a straight face. Even if the goal is to potentially give new life to the world, you know there's likely a blooper reel for this alone.
RBC: If we run now, who's going to trigger the erection?
The English dub of episode 3 is chock full of dick jokes, considering the subject matter.
The cheerful outro, in which the two secondary White Blood Cells each take a dramatic, hammy pose in turn, and the primary one raises her arms halfheartedly, before the Hepatocyte pops up happily in front of the camera.