Pre-embedding "Stains"

Also known as "en bloc" staining.
All will enhance electron contrast of specific components.
Staining can occur during fixation and/or dehydration or after embedding.

Generally staining after embedding is better because:
     Sometimes pre-embedded stain can make tissue hard and difficult to section
     The reagent may affect the tissue and introduce artifacts
     The reagent may interfere with subsequent cytochemical rxns.

Alcian Blue
Sufficiently electron dense
Is used during aldehyde fixation at physiological pH – causes reactive areas to become osmiophilic
Binds extracellular glycoproteins, mucins.

Lead
Avoid exposure to CO2 or precipitation occurs
OsO4 is a mordant for lead products.
 
Lead Citrate
Mixture with form/glut/osmium in phosphate buffer and kept at 4 C
Lead Aspartate
Fix with glut and osmium, then in stoppered vial for 30-60 min at 70 C
high temp to stabilize.

Uranyl Acetate
Reacts strongly with Cacodylate buffer and phosphate buffers, so these should be rinsed extensively prior to exposure.
Incubate either before or after osmium fix.
usually aqueous, but some claim methanolic UA better at penetration.

Tannic Acid
Improves fixation with aldehydes of proteins and reacts with OsO4
Mix the glutaraldehyde with TA
Must be prepared fresh and filtered before use.