Tannic acid staining of ‘microtubules’ in venom-secreting cells of a stingray

The venom-secreting cells of the integumentary sheath of the stingray Dasytis sabina were examined in thin sections after glutaraldehyde-tannic acid fixation. This technique provides negatively ‘stained’ images of subunit disposition in sections of conventional cytoplasmic microtubules and permits more accurate measurement of the diameters of these structures. The membrane limited microtubular aggregates in these stingray cells are of smaller diameter (Mean = 209 A cf. ca. 240 A) than cytoplasmic microtubules and include an electron-lucent core 60–80 A in diameter. Unlike tubulin polymers, in which the protofilament number is very constant for a designated cell type, the tubular structures within the venom-secreting gland display orbitals of 6–12+ within a single vacuole. The possiblity that the structures within stingray cells represent polymers of protein(s) other than tubulin is discussed in the context of the chemistry of stingray venom.