The use of the scanning electron microscope (SEM) to reconstruct the ultrastructure of sporoderm

Abstract

Scanning electron microscope (SEM) data can help in the interpretation of transmission electron microscope (TEM) ultrathin sections and reconstruction of the three-dimensional inner structure of large palynological objects like megaspores. For a SEM study of the inner structure of fossil megaspores, we tried three variants of embedding media: a water solution of glycerine and gum arabic, a water solution of sucrose and polyvinylpyrrolidone, and a mixture of epoxy resins. Semithin sections of fossil megaspores were made, the embedding medium was removed from the sections and they were observed under SEM. Epoxy mixture as an embedding medium and Maxwell's solution as a solvent turned out to be the most appropriate for our purposes. The most suitable way of processing is to embed the object, cut it by turns in semithin and ultrathin sections, and study them with SEM and TEM correspondingly. A combination of SEM and TEM data results in a more profound reconstruction of the inner structure of sporoderms. We used as test objects dispersed megaspores of a supposed lycopsid affinity identified as Maexisporites rugulaeferus Karasev et Turnau 2015 and Otynisporites tuberculatus Fuglewicz 1977. The materials studied are from the Lower Triassic and Upper Permian of the Russian Platform.

Keywords:

• SEM
• semithin sections
• sporoderm ultrastructure
• 3-D reconstructions
• fossil megaspores
• TEM

Acknowledgements

We are thankful to the late Prof. Lugardon (Toulouse University, France) for the discussion of ultrastructural studies of megaspores, the late Prof. Polyakov (Lomonosov Moscow State University, Moscow) for his advice in the course of our attempts with PVP-sucrose medium, Dr. Svetlana Polevova (Lomonosov Moscow State University, Moscow) and Dr. Maria Tekleva (A.A. Borissiak Paleontological Institute, Moscow) for discussion of the relevant literature and earlier versions of our manuscript, Dr. Roman Rakitov (A.A. Borissiak Paleontological Institute, Moscow) for assistance with SEM, and the head of the Laboratory of Electron Microscopy of Lomonosov Moscow State University (Moscow), Georgii Davidovich, for the assistance with TEM. Comments by Dr. David Batten and the anonymous reviewer helped us to improve the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The study was supported by the Russian Foundation for Basic Research [grant number 14-04-00044].

Notes on contributors

Natalia Zavialova

NATALIA ZAVIALOVA is the head of the Laboratory of Palaeobotany at the A.A. Borissiak Palaeontological Institute, Russian Academy of Science. Natalia graduated from the M.V. Lomonosov Moscow State University, where she defended her thesis Morphology and ultrastructure of fossil pollen grains from the upper Permian deposits of Viatka River Upstream and the Lower Jurassic deposits of Western Siberia. Her palynological interests include the morphology and ultrastructure of fossil exines from various intervals, such as Late Palaeozoic and the Mesozoic.

Eugeny Karasev

EUGENY KARASEV was appointed as a senior researcher in the Laboratory of Palaeobotany at the A.A. Borissiak Palaeontological Institute, Russian Academy of Science in 2005. Eugeny received his PhD for a thesis entitled The change of plant assemblages in the transitional stratigraphic interval at the Permian-Triassic boundary of the Moscow syneclise. His research focuses on the macroflora, dispersed leaf cuticles and megaspores from the Late Permian and Early Triassic of the Northern Hemisphere as well as palaeoclimate and plant biodiversity changes associated with the Permian–Triassic boundary. He administers the web site of the laboratory (http://www.paleobotany.ru/).