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SeqMatic Forum: Finding the Right Histopathology Pipeline

The histopathology pipeline, a critical component of the preclinical evaluation of medical devices, encompasses a series of meticulous steps designed to analyze the biological response to implanted materials. This process begins with necropsy, targeting both tissues intended for clinical implantation (target tissue) and those not directly impacted by the device (non-target tissue). Non-target tissues, including regional lymph nodes, the five major organs, and all organs specified in annex E of ISO 10993-11, are collected to meet regulatory requirements and to observe any potential systemic effects or abnormalities. Preparation for a perfusion may also be considered at this stage if deemed necessary.

The next step, grossing, involves the trimming or “breadloafing” of the collected tissues, which may also undergo decalcification if the implant is in contact with bone, utilizing one of several agents such as formic acid, hydrochloric acid, or EDTA. This step ensures that the tissues are properly prepared for subsequent processing and analysis.

histopathological staining and microscopic imaging of the implant site then follow, employing one of three main embedding methods: paraffin, which involves alcohol dehydration and xylene clearing before sectioning with a microtome; frozen, utilizing cryoprotectants and OCT (optimal cutting temperature compound) for sectioning in a cryostat; and plastic embedding, where resin is used and sections are cut with an ultramicrotome for ultrastructural analysis. Each method is selected based on the specific requirements of the study and the nature of the tissues and devices being examined.

The final step involves staining the prepared sections for visualization under a microscope. Hematoxylin and eosin (H&E) staining is commonly used for general tissue morphology, while special stains and immunohistochemistry (IHC) provide detailed insights into specific tissue components and cellular responses. Examples of special stains include Trichrome for connective tissue and Luxol Fast Blue for myelin, and IHC stains such as GFAP for astrocytes and Iba-1 for microglia, offering a deeper understanding of the tissue’s response to the implanted device (see Figure 1). Through this comprehensive histopathology pipeline, researchers can obtain a detailed view of the device-tissue interaction, guiding the development of safer and more effective medical technologies.

Figure 1: This figure presents an immunofluorescent image of a brain sample implanted with different neural electrodes, stained for markers Iba-1 and GFAP, to analyze the immune response elicited by each electrode type. By comparing the intensity and distribution of Iba-1, a microglia/macrophage marker, and GFAP, an astrocyte marker, the figure highlights variations in glial activation and neuroinflammatory responses associated with each electrode. The analysis aims to determine which electrode design is less reactive to the immune system, providing insights into developing neural interfaces with minimized gliosis and enhanced biocompatibility1.

Citations:

  1. Shen, W., Das, S., Vitale, F. et al. Microfabricated intracortical extracellular matrix-microelectrodes for improving neural interfaces. Microsyst Nanoeng 4, 30 (2018). https://doi.org/10.1038/s41378-018-0030-5

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