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Presence of cholesterol clefts in apical periodontitis lesions is a common histopathological observation. However, a potential aetiological association of cholesterol crystals to non‐resolving apical periodontitis lesions after endodontic treatment has been suggested only recently. This article is an attempt to explain the biological basis for the inability of body cells to eliminate the local accumulation of cholesterol crystals, and to consolidate available clinical and experimental data in support of the view that massive accumulation of cholesterol crystals in inflamed periapical tissues can interfere with the periapical healing after conventional root canal treatment. As the irritating cholesterol crystals and certain other aetiological agents of failed endodontic treatment exist outside the root canal system, it is concluded that re‐treatment alone, of such cases, is unlikely to resolve the factors that sustain the lesion. Apical surgery is indicated for successful management of such cases.

Lymphoid organs are highly organized structures made up of different tissue compartments, each with its own specific cell populations. However, the cellular elements of the lingual tonsil, which forms a significant part of Waldeyer's pharyngeal ring, are not yet documented. This study, therefore, describes the fine structure and tissue organization of tonsilla lingualis in Macaca fascicularis. Ten selected crypto-lymphatic units originating from five perfusion-fixed animals were analysed ultrastructurally. Based on the fine-structural elements contained within, the lymphoid tissue of tonsillar units could be subdivided into follicular (germinal centre) and parafollicular areas. The latter contained predominantly small lymphocytes, lymphoblasts resembling T-blasts, plasma cells, macrophages, occasional neutrophils and many reticular cells resembling fibroblasts. A distinct feature of the parafollicular area was the presence of numerous high endothelial (HEV)or postcapillary venules (PCV). The follicular areas contained many small and large lymphoid cells, mitotic cells, plasmablasts, macrophages and specialised reticular cells resembling follicular dendritic cells (FDC) with distinct desmosomal junctions. These observations show that the crypto-lymphatic units of the lingual tonsil are, in fact, organised into distinct B- and T-cell compartments with their own specific lymphoid and accessory cells.

Previous light-microscopic and ultra-immunohistochemical tracer studies revealed the existence of an independent local immune response of the simian oral mucosa. This local response is attributed to the presence of minor salivary gland (MSG) duct-related lymphoid tissue. Semithin sections from a total of 263 Epon-embedded tissue blocks from the labial and buccal mucosae of seven monkeys, Macaca fascicularis, were analysed light-microscopically, and 10 suitable MSG duct/follicle assemblies were investigated ultrastructurally. These duct/follicle assemblies include follicular and parafollicular compartments with distinct fine-structural elements. The follicular area or germinal centre contains numerous small and large lymphoid cells, mitotic figures, plasmablasts, macrophages, and cells resembling the follicular dendritic cells with distinct desmosomal junctions. The parafollicular area, which includes the heavily infiltrated duct wall, contains numerous small lymphocytes, T-lymphoblasts, plasma cells and reticular cells resembling fibroblasts. A distinct feature of this compartment is the presence of high endothelial venules (HEV). The presence of HEV and numerous blast cells, resembling blast-forming T-lymphocytes activated in vitro, in a specific area of the duct/follicle assembly strongly suggests that this area is structurally and physiologically identical to the thymus-dependent area of other lymphoid tissues. In other words, the duct/follicle assemblies of simian MSG contain the various specific fine-structural elements that are suitable for antigen recognition and processing. These elements are distributed in discrete compartments comparable to the B- and T-cell areas of “classical” lymphoid tissue.

A crypto-lymphatic unit was observed at the left lateral aspect of the uvula of a mature female monkey, Macaca fascicularis. A light- and transmission electron-microscopic investigation revealed that the lumen of the crypt was filled with bacteria, desquamated epithelial cells, lymphocytes and neutrophils. The non-keratinized stratified squamous epithelium of the crypt was fragmented and showed heavy mononuclear cell infiltration and surface discontinuities, exposing lymphoid cells to foreign material. The lymphatic parenchyma consisted of organized lymphatic tissue including germinal centres. The resident cell population included lymphocytes of varying size, blastforming B- and T-lymphocytes and two types of reticular cells resembling the fibroblastic reticulum cell and the follicular dendritic cell, respectively. Occasionally granulocytes were encountered. At its base and laterally the crypto-lymphatic unit was ensheathed by a thin connective tissue capsule. Three other monkeys of the same species failed to reveal similar structures at the same site.

Condensation of the appropriate 4-hydroxyquinolin-2-ones with 3-methylbut-2-enal gave pyrano[3,2-c]quinolin-5-ones which were further elaborated to the three alkaloids named in the title.