Histology

Details of histology with microscopic appearance of the drug in sectional view and powdered form are given in monographs of respective drugs in pharmacopoeia.

a. Plant parts

• Histology refers to the arrangement and individual characters of the numerous tissues that make up the plant parts.
• The histological studies are done with very thin transverse or longitudinal sections properly mounted in suitable stains or reagents or mounting media.

b. Powdered drugs

• Since the complete structure is not available, powdered drug should be reduced to not less than No. 40 powder.
• The cells are mostly broken except those with lignified walls.
• The cell contents: calcium oxalate crystals, starch, aleurone, oil, gum, resin etc. remain scattered in the powder. They are the evidence of the source.
• Proper mounting and reagents can demonstrate cell walls, blast-fibres, stone-cell, leaf-epidermal tissues.
• The study of stomatal index also helps to determine the identity of the plant.

Microchemistry

• It comprises the study of the constituents of drugs by application of chemical methods (and physical methods) to small quantities of the powdered drug or to the histological section of the drug.
• By these methods, various constituents of drugs can be isolated and identified.
• Outline of the principle techniques:

A. Isolation of constituents of drugs by

(a) Micro-sublimation

• Small quantity of the drug is heated to evaporation
• The vapours of this are collected in crystallized form by condensation.
• When these crystals are formed, they represent the characteristic pattern of structure of the pure constituent.
• Crystallography will determine the identity.
• e.g. red droplets of cinchona bark.

(b) Chemical solvents

• (i) micro-extraction — separation of the constituents from a small quantity of drugs
• (ii) micro-filtration
• (iii) micro-crystallization

B. Identification of constituents of drugs by

• (a) Crystallography: It is the science of the forms, structures, properties and classification of the crystals that help in identification of the constituents of the drugs.
• (b) Melting point determination.
• (c) Confirmative tests:
  • i. Physical test
  • ii. Chemical test — Colour reaction test (These are very valuable aids for the identification; though by themselves they may not be authoritative, some total tests will decide upon the purity)