Abstract
Simple SummaryThe thymus is the immune organ producing T lymphocytes that are essential to create immunity after encountering pathogens or vaccination. This review summarizes the thymus localization and histological studies, cell composition, and function in teleost fishes. We also describe how seasonal changes, photoperiod, water temperature fluctuations, and hormones can affect thymus development in fish species. Overall, the information helps identify future studies needed to understand thymus function in fish species and the immune system’s evolutionary origins. Since fish are exposed to pathogens, especially under aquaculture conditions, knowledge about the fish thymus and T lymphocyte can also help improve fish farming protocols, considering intrinsic and environmental conditions that can contribute to achieving the best vaccine responsiveness for disease resistance.The thymus in vertebrates plays a critical role in producing functionally competent T-lymphocytes. Phylogenetically, the thymus emerges early during evolution in jawed cartilaginous fish, and it is usually a bilateral organ placed subcutaneously at the dorsal commissure of the operculum. In this review, we summarize the current understanding of the thymus localization, histology studies, cell composition, and function in teleost fishes. Furthermore, we consider environmental factors that affect thymus development, such as seasonal changes, photoperiod, water temperature fluctuations and hormones. Further analysis of the thymus cell distribution and function will help us understand how key stages for developing functional T cells occur in fish, and how thymus dynamics can be modulated by external factors like photoperiod. Overall, the information presented here helps identify the knowledge gaps and future steps needed for a better understanding of the immunobiology of fish thymus.
Highlights
In mammals, the thymus is the primary site of T lymphocyte development, and it is found as a bi-lobed organ located above the heart
Epithelial cells in the medulla have not been well characterized, this population may be one of the cell types expressing Aire in medaka [56]. Both CCL19-like and CCL21 were found in Atlantic salmon and zebrafish, respectively [57,58,59,60] suggesting thymic epithelial cells (TECs) may produce these chemokines to play a role in the teleost thymocyte migration
double negative thymocytes (DN) implies these cells are negative for the cell-surface T cell receptor complex (TCR), at the late DN stage, thymocytes undergo the V, D, J rearrangement directed by the RAG1/RAG2 recombinase [69]
Summary
The thymus is the primary site of T lymphocyte development, and it is found as a bi-lobed organ located above the heart. Each lobe is multilobulated and each lobule has two compartments: the cortex, where there is a higher density of immature T cells called thymocytes, and the medulla, which contains a lower density of thymocytes Both the cortex and the medulla are crossed by a three-dimensional network of stromal cells made up of epithelial cells, dendritic cells and macrophages. The thymus function sustains the development and selection of the T cell repertoire that plays a central role in adaptive immunity. This primary lymphoid organ has been found in all of the remaining jawed vertebrate group, such as bony and cartilaginous fishes, amphibians, reptiles and birds [2]. We highlight how environmental and physiological factors (seasonal changes, aging, hormones, temperature and stress) influence the development and function of this organ in fish
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