Abstract

The manatee, which belongs to the order Sirenia, has been found to be closely related to members of the order Proboscidea (elephants). To date, the evidence for the close phylogenetic relationship of the Sirenia and Proboscidea consists of a combination of similarities across a wide range of characteristics, such as nasolacrimal reinvention, chromosomal painting, mitochondrial rRNA sequences, as well as dental, taxepodial and other skeletal features.^1,5,6,7,8 The aim of this study is to morphologically define the mammary gland and teat in the Florida manatee and Asian elephant with an emphasis on associated lymphoid tissue. Lactation is an important part of mammalian postnatal development and reproductive biology. A greater understanding of the various aspects of the reproduction in these species is especially important because of necessary decisions for conservation.

Currently, in the manatee and elephant, maternal immunity has not been defined. In elephants, it is presumed that the calf acquires maternal immune protection entirely after birth via consumption of colostrum and milk; however, this remains to be confirmed. In manatees, this remains to be investigated. For endangered animals such as the Asian elephant and Florida manatee, overall health and management to ensure survival are of the utmost concern for the species. This study aims to further define the immune system of these species via analysis of the mammary gland and teat; a component of the mucosal immune system that is responsible for postnatal transfer of immunoglobulins from mother to calf. This would be of particular importance in evaluating immune development in calves, further aiding in resources available for successful neonatal care and hand rearing in these species.^2,3,4,9 In addition, comparative analysis between the species may provide insight into the evolution of their immune systems.

Mammary samples from six Florida manatees, lactating and non-lactating, were collected from the Marine Mammal Pathobiology Laboratory in St. Petersburg, Florida. Samples were also collected from one Asian elephant that had given birth to several calves over her lifetime but was not currently lactating. Samples were placed in 10% buffered formalin, embedded in paraffin, and sectioned at 5 µm. Sections were stained using hematoxylin and eosin, Masson's trichrome or periodic acid Schiff (PAS).

This study provides an overview of teat and mammary gland morphology, investigates similarities between the manatee and elephant, and compares previously described morphology in the African elephant.¹⁰ The teats contained several teat canals with varying epithelium. Similar to other eutherians, the lumens of the ducts and alveoli were enlarged in both lactating manatees and elephants. Lymphocytic cells were found lining the ducts, within the epithelium, and between distended alveoli, with reduced cell counts in non-lactating animals. Cell populations were investigated immunohistochemically for the presence of IgG positive cells. Macrophage presence and distribution was also investigated. Cell numbers were found to vary among individuals, however remained consistent with regard to distribution. Variation in cell numbers could reflect stage of lactation, however further research is needed to establish reference cell population ranges.

Acknowledgements

This work was supported by a grant from the Florida Fish and Wildlife Conservation Commission.

References

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