Abstract

As a part of the lymphatic system, the thymus is considered a primary organ due to its central role as being the core of development of T cells, which then disperse throughout the body to direct and assist with immunity. While these cells are produced through most of life, the mammalian thymus usually undergoes atresia and begins to involute around the time of young adulthood (pubescence). Connective and adipose tissue invade as the thymic parenchyma becomes reduced. Stressors, such as cold and pregnancy, can cause the thymus to involute more severely. The Florida manatee (Trichechus manatus latirostris) has been previously described to be resilient to disease with mortality being attributed mostly to prolonged cold exposure, red tide algal blooms and watercraft strikes. The roles that the immune system plays in defending this species against the invasion of microorganisms have been little defined. The microanatomy of the components of the immune system including the thymus has not been described. We have begun to examine the thymus of the Florida manatee histologically, using histochemical and immunohistochemical tools to characterize this essential organ of the lymphatic system. Formalin-fixed paraffin embedded sections of the thymus from animals of several age groups (calves, juveniles and adults) and causes of death (acute and chronic boat strikes, cold stress, and red tide exposure) were stained with hematoxylin & eosin, Gomorri's tri-chrome and Perl's Prussian Blue stains in order to delineate microanatomy of the thymus. Immunohistochemistry was performed for macrophages using a monoclonal antibody, AM-3K. Overall patterns of involution appear to be most accentuated in cold stress animals. Loss of parenchyma in relatively healthy adults (those that died by acute boat strike) appears to be minimal when compared to young (calves). Morphometric measurements detailing specific cell loss, macrophage presence and activity, and changes in the stromal compartment are being made. The purpose of these studies is to describe, in detail, the similarities and differences of the manatee thymus with regards to previously described mammals, and to determine involution from age and from stress.