Microscopy in Pharmacognosy

Abstract

Microscopy is useful for the study of the internal structure, constitution, and inclusions of plant and animal cells or other objects in detail. It is necessary for the detection of adulterants and contaminants of the herbal preparations and thus provides means for assessing the authenticity and quality of herbal drugs. Size, shape, relative position of different cells and tissues as well as the chemical nature of the cell walls, and the form and nature of cell contents are considered during microscopic analysis of crude drugs. Electron microscope uses electron beam to illuminate a specimen and thus has greater resolving power than a light microscope which uses visible light. Depending on the number of eyepieces or ocular lenses, a microscope may be mono-, bi-, and trinocular, and bright-field, dark-field, phase-contrast, florescence microscope, etc., are light microscopes while transmission, scanning, reflection, scanning transmission, low-voltage electron, etc., are electron microscopes. Botanical microscopic atlas uses the characteristics of botanically authenticated multiple samples that have been compared and cross-checked against other microscopic characterizations for consistency and completeness. Microscopic evaluation of botanical drugs may be of both qualitative and quantitative. Qualitative microscopy includes studies of the transverse sections of leaf, root bark, as well as longitudinal section of root bark under photomicrograph with or without staining. In case of powder microscopy, different staining reagents such as iodine for detection of starch grains and calcium oxalate crystals while phloroglucinol for detection of lignified components are used. Quantitative microscopy of some pharmacognostic parameters like vein-islet number, vein termination number, stomatal number, stomatal index, and palisade ratio are used for identification, purity determination, and evaluation of crude leafy drugs. Drawing of morphological and histological structures of plant and animal organs and various other minute structures (e.g., trichomes, glands, stomata, calcium oxalate crystals) is also used for quantitative microanalysis of admixed or adulterated powdered drugs. Plant sections or powders of the drug are mounted in water or dilute glycerol for light microscopic examination. Color and clearing, bleaching and defatting reagents are used to stain and clear prior to microscopic examinations. Tissues are macerated by using chemicals to disintegrate the middle lamella and isolation of tissues for study. The characteristic microscopic features include trichomes, palisade and spongy parenchyma, collenchyma, stomatal frequency, their index, vein-islet, vein termination number, palisade ratio, shape and size, as well as vascular bundles, xylem and phloem cells, inclusions, etc., and their physical constants for leafy drugs while cork cambium, primary cortex, phloem fibers, medullary rays, endodermis, pericycle, vascular bundles, etc., in the transverse and longitudinal sections, and their physical constants stand as characteristic microscopic features of drugs from root, stem, etc. Micrometry and camera lucida drawing to scale of tissues, cells, cellular elements, cell inclusions, and other minute structures are of significant value in the examination of crude drugs for quality assessment in presence of adulterants. With the worldwide increase in popularity and acceptance of herbal medicines, the classical tool like microscopy is urgently needed for the assessment and quality control of plant products such as crude herbal drugs, registered herbal medicinal products, over-the-counter herbal products, or health foods. Modern pharmacopoeias offer new monographs on herbal drugs, including their microscopic characterization.