Recent trends in tapetum research. A cytological and methodological review

Abstract

Cytological, developmental, and functional aspects of tapetum cells have been extensively studied in the past using classical light-and electron microscopy (LM, EM). Recently, some advanced preparation methods and analytical techniques have come of age; they already have been applied with some success for the investigation of pollen and tapetum development. Analytical transmission EM techniques (ESI, EELS) as well as immunocytochemistry and fluorescence microscopy have become invaluable tools for the localization of substances. Laser scanning microscopy and video-enhanced microscopy have brought about a revival of LM, the latter method permitting the study of dynamic cellular processes with high spatial and temporal resolution. Hence, so far static notions of cellular ultrastructure based on the interpretation of classical TEM micrographs can now be completed by a more dynamic concept. In TEM, the application of freeze fixation and freeze substitution as well as sophisticated chemical fixation protocols have considerably improved ultrastructure preservation thereby revealing, for instance, that tapetum senescence starts much later than generally assumed. Some patterns of cytoplasmic disorganization which are usually interpreted as tapetum degeneration were found to result from specimen preparation. Besides presenting recent advances in the fields mentioned above, this paper also reviews literature on aspects of primary/secondary tapetum metabolisms and the regulation of normal/abnormal tapetum (and microspore) development.