CHITIN CRYSTALS IN ARTHROPOD CUTICLES REVEALED BY DIFFRACTION CONTRAST TRANSMISSION ELECTRON-MICROSCOPY

The unambiguous location of chitin in arthropod cuticles is obtained by diffraction contrast transmission electron microscopy (DCTEM) operated in the bright-field mode. This technique was applied to the study of cross sections of the ovipositor of the fly Rhyssa persuosaria and of the cuticle of the crab Carcinus maenas. With both specimens, DCTEM revealed the chitin crystals as dark units embedded in a white nondiffracting matrix. This is opposed to the conventional imaging of stained sections of these specimens where the matrix is stained in dark whereas the chitin crystals remain unstained. In the crab cuticle a uniformity of the chitin crystal diameters is observed in all layers whether observed by DCTEM or by imaging after staining. In the ovipositor, chitin rods of 2 nm in diameter are observed by diffraction contrast as opposed to rods of 3 nm by conventional staining. This discrepancy, as the overall morphology of chitin in the crab cuticle is discussed. DCTEM appears to be a general and straightforward technique to visualize chitin crystals in a variety of specimens. © 1990.