Revisiting the Teleost Thymus: Current Knowledge and Future Perspectives

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.

Adenosine A1, A2a, A2b, and A3 Receptors in Hematopoiesis. 1. Expression of Receptor mRNA in Four Mouse Hematopoietic Precursor Cells

Four mouse bone marrow or thymus cell populations, namely granulopoietic/monocytopoietic, erythropoietic, B-lymphopoietic, and T-lymphopoietic precursor cells have been assayed by RTPCR technique for the presence and relative amounts of adenosine A1, A2a, A2b, and A3 receptor mRNA. It has been found that (i) all four populations studied express all four adenosine receptor subtypes, (ii) the A1 receptor is the least expressed in all populations studied, (iii) the A3 receptor is markedly expressed in the populations of granulopoietic/monocytopoietic and erythropoietic cells, (iv) the A2a receptor is markedly expressed in the populations of B-lymphopoietic and T-lymphopoietic cells, and v) the A2b receptor does not predominate in any of the precursor cells studied. Our data offer a new possibility for the assessment of the readiness of these cells to respond, by receptor-mediated mechanisms, to adenosine or its analogs present in the tissues as a result of endogenous processes and/or following their administration.