Introduction:

Tannic acid was introduced as a secondary fixative mixture with aldehydes for biological tissues, and also as a stain. Specimens treated with Tannic acid show increased contrast and more delineation of cell membranes.

A low molecular weight, Tannic acid (LMGG), galloylglucose (C14H1009)n, provides and overcomes the previous problems of unsatisfactory penetration, extraction, and precipitation when high molecular weight Tannic acid (C76H52O46) was utilised.

It works primarily as a mordant between osmicated structures and Lead citrate of the post-staining, revealing additional ultra-cellular structures and details better delineated.

Procedure:

The procedure reported by Simionescu (1976) involves the following steps:
(Sodium Cacodylate buffer is preferred.):

  1. Fix tissue in Glutaraldehyde and Osmium tetroxide.
  2. Rinse in 0.1M buffer (pH 7.2) 3 times for five minutes each time at room temperature.
  3. Treat with 1% LMGG in 0.5M buffer for 30 minutes.
  4. Rinse in the same buffer containing 1% Sodium sulphate for 5 - 10 minutes.
  5. Dehydrate in ethanol followed by Propylene oxide: leave over night in a 1:1 EMbed 812 and Propylene Oxide mixture at room temperature; embed the next day.
  6. Stain thin sections in Lead citrate for 3 - 5 minutes.

Solution: 1% LMGG Tannic acid (C14H1009) in 0.005M Sodium Cacodylate buffer, freshly prepared.

Concentration of LMGG can be adjusted in a range of 0.25% to 2.0% according to the nature of the tissue and section thickness.

Reference:

N. Simionescu and M. Simionescu, J. Cell Biology (1976-70), 608-621.