Everyone probably remembers the recent "monster" being created inside Mayuri's laboratory in secret, if not, I'm talking about this:
as you can see, even people used to Mayuri's creepiness and the like are shocked and disgusted by what he does, so this is not something they normally do and they normally blow up and cut apart people, shinigami, hollow, humans, quincy, Zanpakutou and other, so this might really be something, though it's probably another hype .
Most people more or less know from where the connection between Mayuri and Sayzel comes and it is this:
yep, those are special "specimen" found in Sayzel's underground vault, which picked Kurotsuchi's interests, kukukuku.
Now this was the easy part, now comes the reason why and how, from where and for what, were they stored ?
Let's stat with the fact that almost none in HM liked Aizen's rule, so certainly most people/arrancar were planing something or doing something in order to kill him in the future, Sayzel was no different. Yes, his experiment, super Arrancer, was in order to beat Aizen (indifference to Renji that trained to battle Aizen, this thing was meant in order to own him and that was way before Mayuri put his hands on it, so who knows just what might come up, kukukuku).
You probably wonder why "super" and why "arrancer", well that is quite simple, because those specimen are a part of the privaron espada.
Let's start with the first clue:
who send them you ask ? Well looks like Aizen also wants to know:
the culprit comes out clean quite fast:
unfortunately, you are not that smart Sayzel and certainly not the first one, you are nothing more than another Gin or Ichigo, nothing more than a meat for Aizen to devour, yep he knows "all":
yep, the mustacho wind kicker is likely the specimen on the right.
Now, the one on the left is another unfortunate soul:
Aizen already knows, so they don't need to hide their intentions anymore. On the spoiler below you can notice the things on her legs are the same as those on the specimen on the left:
So, at the very least there are 2 ex-espada specimen there, but let's not forget that there are at least 7 ex-espada (since Sado battle espada 107 and the 3-digits are all ex-espada) and probably other specimen, like Noitora that had to "pay" for Sayzel's help in taking down Nel somehow and probably Nel herself and who knows else ? Perhaps they are already somewhat fused, or something, not to mention that Sayzel beside analyzing equipment possessed cloning powers, so there might have been really something in there, kukukuku. But he died before he completed his research, but thankfully Mayuri garbed what was left behind .
You probably wonder if giving such a monster to SS might not be a little too much ? Not at all, after all what greater target/pray to show the next emperor's power could be, if not something that were meant to take down Aizen himself that is recognized as one of the war potentials ?
This monster might be quite a hurdle for Quincy, since Mayuri's battle methods and analysis are as below (a few pages):
So not only he might know his enemies really well, he might even know where they are and if he is really angry, he might drop that thing on them, before anyone else realizes it. That is assuming that Uryu hasn't purged those bacterias from himself, or that other Quincies haven't been infected when they attacked SS before.