Embedding Susmita Chakrabarty

It is the orientation of tissue in melted paraffin which when solidified provides a firm medium for keeping intact all parts of the tissue when sections are cut.

Types of moulds:

a) Leuckhart's L pieces - These are two 'L' which are resting metal usually brass, which are resting on a flat metal or glass plate.

Leuckhart's L pieces

b) Compound embedding units - consists of square shaped brass or metal plates in a series of interlocking plates.

Compound embedding units

c) Others like plastic embedding blocks (tissue Tek system).

Techniques of casting:

  1. Molten paraffin wax which is heated at a temperature 2-3° above the melting point is poured into the mould to an adequate depth so as to cover the thickest tissue block.
  2. The wax touching the mould will quickly form a thin semi solid layers, Now introduce the tissue with a pre warmed forceps to prevent the wax to stick to it. The tissue is pressed in this semisolid wax to orient it at the bottom of mould in a correct plane.
  3. Fix the label in position by pressing one edge against solidifying wax usually sides of the mould are preferred.
  4. As soon as a film of solid wax is formed on the surface, the whole block with mould are submerged in cold water at 20°C. If this is not done there will be crystallization of wax, using ice water to do initial cooling will also cause the block to crack.
  5. When blocks are set hard they are removed from mould. The tissue surface towards the mould base is from where the sections are to be cut this surface should be trimmed lightly with a scalpel so as to expose the tissue.

Following points must be taken care off during casting :

  1. Paraffin should not be allowed to cool around the tissue to be blocked for this before introducing the tissue in the mould it should be kept in heated wax or in cassette placed over thermostatic hot plate.
  2. To prevent excess of wax solidifying on the bottom of the block during winter pre warmed moulds may be used.
  3. The cutting surface of the tissue should be facing at the bottom of the mould.
  4. If 2 or more tissues have to be casted remember to keep them both at the same depth.
  5. If small biopsy fragments have to be casted, the largest piece should be first blocked and other pieces should be as near it as possible.
  6. All four corners of the block should be in one horizontal plane.
  7. The tissue should have at least 2 mm wax around its edges.
  8. Smear mineral or machine oil on the inner surface of the mould for facilitating easy removal of block.
  9. Whitish areas around tissue in block denotes crystallization which may be due to moisture or due to incomplete removal of clearing agent. Most tissue sections are cut from the largest area but some tissue needs special mention such as:.
  • Tissue of tubular nature are cut transversely so should be embedded vertically.
  • Skin is cut in a plane at right angles to the surface so should be embedded at right angles to the bottom.
  • Muscle biopsy should be sectioned in both transverse and longitudinal planes.