Microscopic study of the thymus of Guiana dolphin and Humpback whale F. M. O. Silva 1,2,3,* , J. P. Guimarães 1,2,4 , M. N. Rodrigues 1 , J. E. Vergara-Parente 2 , V. L. Carvalho 5 , A. C. O. Meirelles 5 , B. S. P. O. Stefanis 3 , S. M. Santos 3 and M. A. Miglino 1 1 Departamento de Cirurgia, Faculdade de Medicina Veterinária e Zootecnia, Universidade de São Paulo, Av. Prof. Dr. Orlando Marques de Paiva, 87, Cidade Universitária, 05508-270, Sao Paulo, SP, Brazil 2 Núcleo de Estudos dos Efeitos Antropogênicos nos Recursos Marinhos, Fundação Mamíferos Aquáticos, Av. Tancredo Neves, 5655, Jabotiana, 49095-000, Aracaju, SE, Brazil 3 Instituto Biota de Conservação. R. Santa Joana, 196, Riacho Doce, 57039-290, Maceió, AL, Brazil 4 Pós-graduação em Sustentabilidade de Ecossistemas Costeiros e Marinhos, Universidade Santa Cecília, R. Oswaldo Cruz, 266, Boqueirão, 11045-907, Santos, SP, Brazil 5 Programa de Mamíferos Marinhos, Associação de Pesquisa e Preservação de Ecossistemas Aquáticos. Av. José de Alencar, 150, Praia de Iparana, SESC Iparana, 61627-010, Caucaia, CE, Brazil * Corresponding author; e-mail: fernanda_fmos@hotmail.com Studies regarding thymus in cetaceans are scarce, with no report to the species occurring in Brazil. Due to an increasing concern about marine mammals’ immunological status and how it is affected by the environment, this study aimed to describe histological characteristics of the thymus in two cetacean species with high occurrence in the Brazilian coast: the Odontoceti Guiana dolphin (Sotalia guianensis) and the Mysticeti Humpback whale (Megaptera novaeangliae). Guiana dolphins (3 calves, 1 young and 2 adults) and Humpback whales (2 calves, 1 young and 1 adult) were used and thymus samples were analyzed by light, scanning and transmission electron microscopy (SEM and TEM, respectively). The thymus was a lobulated lymphoid organ, anatomically located in the anterior superior mediastinum, in front of the heart. It was enclosed by a connective tissue capsule from which several connective tissue trabeculae arise and under which there was a cortex with an extensive network of interconnecting spaces. These spaces resembled a bee hive and were colonized by immature T-lymphocyte. There were several blood vessels in the capsule and trabeculae. The trabeculae extend into the interior of the organ, consisted of a dark-staining outer cortex and a light-staining inner medulla. The cortex contained densely packed lymphocytes, without the formation of lymphatic nodules. The medulla had fewer lymphocytes and more epithelial reticular cells that form the thymic (Hassall’s) corpuscles. These corpuscles were round/oval structures consisting of spherical aggregations (whorls) of flattened epithelial cells and exhibiting calcification or degeneration centers. No adipose tissue replacing the thymic tissue was found in young animals. There were no differences between the species for any histological features. However, with regard to age, Hassall’s corpuscles were more prominent in young animals, as the result of a decline in the lymphocyte production with age. However, this structure function remains uncertain. Thymus from cetaceans was similar to the observed in domestic mammals with involution related to age. The data presented in this study may contribute to a better understanding of the physiological processes related to the immunity of Odontoceti and Mysticeti during their growth. Keywords: cetaceans; histology; immune system 1. Introduction The cetacean thymus has a typical mammalian organization, including cortex, medulla and Hassall’s corpuscles [1, 2]. This organ has been observed in several cetaceans’ species such as the Bottlenose dolphin (Tursiops truncatus) and the Amazon River dolphin (Inia geoffrensis) [3]. However, little information on these typical features is known, especially regarding animals’ age and gender, with most reports of thymic structure based on young animals [4]. The thymus involutes as it loses lymphocytes progressively with increasing age. Thymus lobules may display fat infiltration and/or involution, this due to Hassall’s corpuscles degeneration or condensation of the thymus epithelial reticulum. Cysts development can also be observed, in the thymic tissue, completely replacing it in Tursiops truncatus [2]. However, the thymus can be easily visualized macroscopically in adult individuals, with some remnants being observed in senile Harbor porpoises and bottlenose dolphins. Such findings suggest that thymic involution is a slow and progressive age-related process in cetaceans [4, 5]. Studies regarding thymus in cetaceans are scarce, with no report to the species occurring in Brazil. This, allied to the fact that there is an increasing concern about marine mammals’ immunological status and how it is affected by the environment, this study aimed to describe histological characteristics of the thymus in two cetacean coastal species with high occurrence and more susceptible to anthropic impacts in the Brazilian coast: the Odontoceti Guiana dolphin (Sotalia guianensis) and the Mysticeti Humpback whale (Megaptera novaeangliae). 2. Material and methods Ten cetaceans, 6 Guiana dolphins (3 calves, 1 young and 2 adults) and 4 Humpback whales (2 calves, 1 young and 1 adult), stranded on the Northeast coast of Brazil (Sisbio Permit 37369-1, Animal Bioethics Protocol 2571/2012) were Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.) © FORMATEX 2014 __________________________________________________________________ 465