6 Tissue processing
Incorporating
Principles of tissue processing
Stages of tissue processing
• Fixation – stabilizes and hardens tissue with minimal distortion of cells.
• Dehydration – removal of water and fixative from the tissue.
• Clearing – removal of dehydrating solutions, making the tissue components receptive to the infiltrating medium.
• Infiltrating – permeating the tissue with a support medium.
• Embedding – orienting the tissue sample in a support medium and allowing it to solidify.
Dehydration
Clearing
The criteria for choosing a suitable clearing agent are:
• rapid penetration of tissues
• rapid removal of dehydrating agent
• ease of removal by melted paraffin wax
Infiltrating and embedding reagents
Alternative embedding media
• Processing reagents remove or destroy tissue components that are the object of investigation, e.g. lipids
• Sections are required to be thinner, e.g. lymph nodes
• The use of heat may adversely affect tissues or enzymes
• The infiltrating medium is not sufficiently hard to support the tissue
Resin
Resin is used exclusively as the embedding medium for electron microscopy (see Chapter 22), ultra-thin sectioning for high resolution and also for undecalcified bone (see Chapter 16).
Orientation of tissues
Tissues requiring special orientation include:
• Tubular structures: cross section of the wall and lumen should be visible; arteries, veins, fallopian tube and vas deferens samples.
• Skin biopsies; shave punch or excisions, cross section of the epidermis, dermis and subcutaneous layers must be visible.
• Intestine, gallbladder, and other epithelial biopsies: cut in a plane at right angles to the surface, and oriented so the epithelial surface is cut last, minimizing compression and distortion of the epithelial layer.
• Muscle biopsies: sections containing both transverse and longitudinal planes.
• Multiple pieces of a tissue are oriented side by side with the epithelial surface facing in the same direction.
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