What is a tissue?

  1. Muscle Tissue: It is composed of cells with the special ability to shorten or contract to produce movement of the body parts.
  2. Epithelial Tissue: Epithelial tissue or epithelium forms the outer covering of the skin and lines the body cavity.
  3. Connective Tissue: Helps in maintaining the form of the body and its organs and provide cohesion and internal support.
  4. 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. 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. Biopsy: Removing a bit of tissue from a living being.
  2. Autopsy: Removing extensive tissue or organ from a dead body (post-mortem).

What is tissue processing?

Tissue Processing is a procedure to remove water from cells and replace it with a medium that solidifies, allowing thin sections to be cut on a microtome.

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. 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.
  2. 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.
  3. 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.
  4. Infiltration and Embedding:

Infiltration is the saturation of tissue cavities and cells by a supporting substance, which is generally the medium in which they are finally embedded. The most common agent of choice is “paraffin wax” which is molten when hot and solid when cold.

Embedding is the standard method used to produce blocks of tissue for section cutting. In this method, the tissue is surrounded by paraffin wax to give it external support and harden it for proper section cutting.

6. Section cutting: It is the process of cutting tissue samples into skinny slices using a microtome.

After sectioning, the section is put in a water bath with less than the melting wax temperature. This is done to remove the fold in section, and then eventually, paraffin wax is removed.

7. Staining: The purpose of staining is to highlight the tissue’s essential features and enhance the tissue contrasts to be used in the medical diagnosis of tumors.

8. Mounting: To preserve and support a stained section for light microscopy, it is mounted on a clear glass slide and covered with a thin glass coverslip. A mounting medium is used to adhere the coverslip to the slide.

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