Samples of the thymus and head-kidney of domestic carp (Cyprinus carpio) were studied in order to evaluate the process of functional and cytological change during ageing. We examined larvae from 6 days post-fertilization, 1- to 10-year-old fish from fishponds, and experimentally stunted 15-year-old fish that had attained no more than 100-120 mm total length (TL) and 18-20 g weight. The primordium of the thymus appears 6 days after fertilization as a group of nonspecialized cells 0.8 mm(3) in volume. Differentiation of cell types, especially thymocytes, starts in fry of 6 mm TL, and the thymus continues to enlarge, attaining 200 mm(3) (+/- 3.4 mm(3)) in 8-year-old fish of 550 mm TL. Throughout its development the thymus remains active, containing various forms of reticulocytes and thymocytes. The head-kidney appears in pre-hatching embryos on either side of the pharynx as renal units, each with a single glomerulus and a short renal tubule. During development the protonephridia pass through three functional stages: (1) excretory, as the primordial kidney, (2) in a mixed renal and glandular function, producing blood cells and adrenergic cells, and (3) in young fish of 80 mm TL and above, they continue to function as blood producing and endocrine organs. At this last stage the renal elements become degraded. The largest head-kidney measured had a volume of 8 cm(3) (+/- 0.8 cm(3)) in an 8-year-old fish weighing 6.5 kg. Retention of thymal cytological integrity in reproductive older carp and other fish would indicate that ageing in fish, and possibly also in other exothermic vertebrates, is not simply a time-related phenomenon, thereby differing greatly from similar processes in endothermic vertebrate.
