Abstract

In continuing to examine components of the immune system of the Florida manatee (Trichechus manatus latirostris), we have been studying one of its primary organs, the thymus, which plays a central role, being the center for development of T cells, which then disperse throughout the body to direct and assist with immunity. Formalin-fixed (10%) paraffin embedded sections and glutaraldehyde-fixed (2%) semi-thin and ultra-thin plastic sections of the thymus from young animals (calves) were stained with hematoxylin & eosin, toluidine blue and uranyl acetate & lead citrate and then evaluated by light and transmission electron microscopy, respectively.¹ Specimens originated from animals that had died from natural causes and were collected at the Marine Mammal Pathobiology Laboratory in St. Petersburg, Florida.

The cytostructure of the thymus of the young Florida manatee enables the distinction of specific cell types based solely on morphology within parenchymatous and non-parenchymatous components. The parenchyma of the mammalian thymus consists of an outer cortex and inner medulla that together comprise incomplete lobules with partial septae. In the Florida manatee, these regions tend to be small and intimate in the young individual, winding in a cord-like manner with variable amounts of connective tissue. Within the cortical parenchyma, the primary cell population consists of thymocytes (lymphocytes), which easily defines this region by the heavy density of this cell type from the adjacent medullary portion, which shares this latter region with a number of other cell types (Figure 1). Ultrastructurally, cortical thymocytes possess generally rounded to occasionally indented and irregularly shaped nuclei with centrally placed nucleoli and clumped peripherally placed heterochromatin (Figure 2). These cells contrast sharply with the cells that form the epithelial reticulum (epithelioreticular cells), which possess variably shaped nuclei that generally are much larger and more euchromatic than the thymocyte nuclei.

Within the corticomedullary junction, relatively thin-walled pericyte-lined blood vessels form conduits for incoming lymphocytes and differ in connective tissue construction from those of nearby septae. Monocytes and heterophils are seen within the lumen of these blood vessels. The appearance of the thymocytes in this region can vary as many undergo stages of degeneration (Figure 3). The thymocytes are contained within an intraparenchymal compartment of both the cortex and the medulla and separated from direct connection with the vasculature by a specialized epithelial reticulum, consisting of epithelioreticular cells. The epithelioreticular cells are usually divided into multiple types among mammals. In the Florida manatee, at least three types can be morphologically distinguished on the basis of size, organization of the cytoplasm, and appearance of the nucleus as well as association with one another and adjacent connective tissue elements (Figure 4). Overall, the manatee thymus is similar to that of most mammals with some interesting variations.

The purpose of these studies is to continue to describe, in detail, the similarities and differences of the manatee thymus with regards to that previously described in other mammals and to compare changes that may result from age, injury and disease.

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Acknowledgements

This work was supported by a grant from the Florida Fish and Wildlife Commission. We thank Martine DeWitt and the staff of the Marine Mammal Pathobiology Laboratory at St. Petersburg, Florida for their support and provision of the specimens for the study.

Reference

1.  Hatfield JR, Natiello M, Lewis PJ, Chisholm M, Samuelson DA 2003. Iridocorneal angle structure of the West Indian manatee (Trichechus manatus), short-finned pilot whale (Globicephelus macrorhynchus), hippopotamus (Hippopotamus amphibius), and African elephant (Loxodonta africana). Vet Ophthalmol 6:35-43.