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The apical ectodermal ridge (AER) in Neoceratodus develops after an initial period of mesenchymal proliferation and outgrowth of the fin bud and persists until chondrogenesis of the stylopod and zeugopod is initiated. At this time, the lateral margins of the AER convert to the fin fold leading to subsequent development of the dermal fin skeleton. Thorogood's (1991 ) fin fold model predicts that the AER should persist longer in Neoceratodus than it does in actinopterygians because of the comparatively extensive endochondral skeleton in lungfish. While the AER does persist into early chondrogenesis and is extended compared to actinopterygians (lost before fin radial chondrogenesis) it does not persist into further stages of chondrogenesis, providing partial support for Thorogood's model. Fgf8 appears in the lungfish fin epithelium during the initial period of fin outgrowth before a physical AER forms, when Fgf8 is restricted to the AER plus the preaxial and postaxial epithelium immediately adjacent to the AER. Fgf8 is no longer detected after the AER is replaced by a fin fold. Neoceratodus appears to provide a halfway point between ray fins and limbs during very early development as Thorogood proposed, but not precisely for the reasons his model suggests. [ABSTRACT FROM AUTHOR]

The anatomy, histology and ultrastructure of the thymus of a dipnoan, the Australian lungfish, Neoceratodus forsteri, was studied by light and transmission electron microscopy. The thymic tissue showed clear demarcation into a cortex and medulla with ample vascularization. Large cells including foamy and giant multinucleated cells with periodic acid Schiff/Alcian blue positive staining properties were localized mainly in the medulla. The major cellular components were epithelial cells and lymphoid cells. The epithelial cells were classified by location and ultrastructure into six sub-populations: capsular cells, cortical and medullary reticular cells, perivascular endothelial cells, intermediate cells, nurse-like cells and Hassall-like corpuscles. Myoid cells were found mainly in the cortico-medullary boundary and medulla. Macrophages and secretory-like cells were also present. These findings will provide a base of knowledge about the cellular immune system of lungfish. [ABSTRACT FROM AUTHOR]