Ultrastructural features of the classic, well-differentiated epithelial mesothelioma:
(1) Microvilli are abundant, wavy, long and slender, with a high length-to-diameter ratio.
(2) Microvilli tend to be on all free cell surfaces.
(3) Numerous bundles of tonofilaments may be present.
Ultrastructural features of adenocarcinomas:
(1) Microvilli tend to be few and short.
(2) Microvilli usually present on the apical surface of the cell wall.
(3) Some adenocarcinomas may have rootlets and/or lamellar inclusion bodies (Warhol et al).
length-to-diameter ratio =
= (length of microvillus in nanometers) / (diameter of microvillus in nanometers)
where:
• Burns used the mean for the ratio of the 10 longest microvilli to compare tumors.
• nanometers are 10^(-9) meters = 10 Angstrom units = 1,000 microns.
Interpretation:
• Burns found the mean ratio was 11.9 for mesotheliomas and 5.28 for adenocarcinomas. There were no adenocarcinomas with a ratio > 10. Mesotheliomas can have a ratio <= 5.
• Battifora felt that a ratio >= 15 was required to positively identify a mesothelioma. A ratio less than this could not reliably distinguish between the tumors.
Limitations:
• Some types of mesotheliomas (poorly differentiated, sarcomatoid) may have short microvilli or lack them entirely.
• Ultrastructural features of mesotheliomas are inconstant and should not be used to exclude the diagnosis if other features favor the diagnosis.
• There is a significant grey zone in the ratio where distinction cannot be made.
• Ultrastructure cannot separate malignant mesothelial cells from normal or irritated mesothelial cells.
