What is a tissue?

The human body is made up of millions and trillions of cells. A group of cells that have a similar shape and function forms a tissue. There are four types of tissues in humans:

  1. Epithelial Tissue: Epithelial tissue or epithelium forms the outer covering of the skin and lines the body cavity.
  2. Connective Tissue: Helps in maintaining the form of the body and its organs and provide cohesion and internal support.
  3. Nervous Tissue: The function of nervous tissue is to form the nervous system's communication network by conducting electric signals across the tissue.

Importance of tissues samples in research

  1. Tissue research provides information that may help prevent, diagnose, and treat cancer patients in the future- for example, finding which patients respond better to or have fewer side effects from a particular drug.
  2. The study of tissues has also led to the discovery of biomarkers. Biomarkers are biological substances in a cell that can help predict the disease progress, prognosis, and treatment effectiveness.

How is the tissue sample obtained from a patient?

  1. Autopsy: Removing extensive tissue or organ from a dead body (post-mortem).

What is tissue processing?

Leica RM2235 Microtome (https://www.leicabiosystems.com/histology-equipment/microtomes/products/leica-rm2235/)
Automatic Tissue Processor by AMIS Medical (https://www.amismedical.com/product/automatic-tissue-processor/)

Tissue Processing steps

  1. Obtaining fresh tissue sample (fresh tissue): Fresh tissue sample can be obtained by various methods (post-mortem/dissection/surgery). Fresh tissue samples can get damaged easily, and hence it is vital to handle the sample appropriately.
  2. Fixation: In the fixation step, the specimen is placed in a fixative such as formaldehyde solution (formalin). The purpose of fixation is to keep the tissue sample as life-like as possible to preserve them permanently by preventing or arresting the degenerative processes which commence as soon as the tissue is deprived of its blood supply. Fixation should be done immediately after obtaining the fresh tissue sample to avoid autolysis.
  3. Dehydration: Dehydration is the process of removing water from tissues using alcohol (such as ethanol). Removal of water from tissues is essential as paraffin is insoluble in water. Dehydration is complete when less than 3–4% of water remains in the tissues.
  4. Clearing: Involves removal of dehydrating agents and replacing it with a solution/fluid that is soluble in wax. Examples of clearing agents are Toluene, Xylene, Chloroform, etc. After this step, we get transparent tissue with a higher refractive index. This makes the tissue easily visible under a microtome.
  5. Infiltration and Embedding:

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