Pollen-wall formation in Arum alpinum.

BACKGROUND AND AIMS: Arum alpinum has a quite uncommon pollen wall. A sporopolleninous ektexine is missing. The outermost pollen wall layer is formed by the endexine which is covered by polysaccharidic ornamentation elements. An ontogenetical investigation was accomplished to clarify pollen-wall development, with special reference to callose and pollen-wall development. METHODS: Plants of Arum alpinum grown in their natural habitat were collected once a week within the vegetative period and processed for semi- and ultra-thin sectioning. KEY RESULTS: At any stage of pollen-wall formation callose is missing. Microspores are released from the tetrad by invagination of the amoeboid tapetum. The polysaccharidic wall ornamentations are formed by the tapetum. CONCLUSIONS: There appears to be no truth in the dogma that callose is essential for microspore separation and release from the tetrad. The lack of callose does not influence fertility but could be the reason for the uncommon pollen wall, where a sporopolleninous ektexine is missing.

Ultrahigh resolution optical coherence tomography of the monkey fovea. Identification of retinal sublayers by correlation with semithin histology sections.

Optical coherence tomography (OCT) has become an established diagnostic tool for the clinical assessment of retinal pathology but correlation of acquired signals with retinal substructures has often been ambiguous. In the monkey retina we have now obtained ultrahigh resolution (UHR) OCT images with 1.4 microm axial x 3 microm transverse resolution from perfusion-fixed eye cups of Macaca fascicularis and optimized the identification of retinal anatomy by correction of spatial artefacts in correlated histology. After resin embedding, serial semithin sections were obtained that corresponded to OCT transects. The direct overlay of features identified in histological sections with corresponding OCT locations was limited by non-linear tissue shrinkage due to dehydration and sectioning stress. In the present study, these misalignments were further corrected by using polygonal spline morphing based on corresponding unequivocal landmarks. The geometric normalization then allowed detailed comparison of both profiles including delicate sublayers of photoreceptor inner- and outer segments. Such correlation will facilitate the extraction of structural information from in vivo ultrahigh resolution OCT images in clinical and experimental applications.