Involution of lymphoid organs in bottlenose dolphins (Tursiops truncatus) from the western Gulf of Mexico: implications for life in an aquatic environment.

Involution of lymphoid tissues in relation to age has not been defined for bottlenose dolphins (Tursiops truncatus). Twenty-five bottlenose dolphins from the coast of Texas and western Louisiana were examined and complete necropsies were performed with histological samples taken of nearly all tissues. Ages ranged from several days to 27 years. The histology of four lymphoid organs-thymus, pharyngeal tonsil, mucosa-associated lymphoid tissue (MALT) of the colon, and anal tonsil-was assessed. Numerical scores were assigned to specific morphological features, thus creating an involution score. Definable and scorable features of each organ were selected for evaluation and determination of loss of lymphoid elements. Neonatal dolphins were recorded as the reference standard for no involution. The highest score for each organ represented the greatest amount of retention of tissue elements. Thus, the lower the score, the greater degree of involution. Comparing involution scores to tooth age permitted an assessment of involution over time. The greatest degree of involution was found in the MALT of the colon. The MALT of the colon declined dramatically so that after age 10 it was absent from 4 of 14 animals and minimally present in 8 others. Thymic tissue also suffered a precipitous drop in volume after about age 5, but was found in animals up to 24 years of age. Involution was moderate and variable in both pharyngeal and anal tonsils. In some animals, these tissues were reduced in volume early, and prominent in others well into adult life (over 20 years).