Histology and ultrastructure of the thymus during development in tilapia, Oreochromis niloticus

Abstract

The thymus in teleost fishes plays an important role in producing functionally competent T-lymphocytes. However, the thymus in tilapia is not well known, which greatly hampers investigations into the immune responses of tilapia infected by aquatic pathogens. The histological structure and ultrastructure of the thymus in Oreochromis niloticus, including embryos and larvae at different developmental stages, juveniles, and adult fish, were systematically investigated using whole mount in situ hybridization (WISH), and light and transmission electron microscopy (TEM). The position of the thymus primordium was first labeled in the embryo at 2 days post-fertilization (dpf) with the thymus marker gene recombination activating gene 1 (Rag1), when the water temperature was 27 °C. Obvious structures of the thymus were easily observed in 4-dpf embryos. At this stage, the thymus was filled with stem cells. At 6 dpf, the thymus differentiated into the cortex and medulla. The shape of the thymus was 'broad bean'-like during the early stages from 4 to 10 dpf, and became wedge-shaped in fish larvae at 20 dpf. At 6 months post-fertilization (mpf), the thymus differentiated into the peripheral zone, central zone, and inner zone. During this stage, myoid cells and adipocytes appeared in the inner zone following thymus degeneration. Then, the thymus displayed more advanced degeneration by 1 year post-fertilization (ypf), and the separation of cortex and medulla was not observed at this stage. The thymic trabecula and lobule were absent during the entire course of development. However, the typical Hassall's corpuscle was present and underwent degeneration. Additionally, TEM showed that the thymic tissues contained a wide variety of cell types, namely lymphocytes, macrophages, epithelial cells, fibroblasts, and mastocytes.

Histology of some major immune system organs in Lake Van fish Alburnus tarichi (Güldenstädt, 1814) (Cyprinidae)

“…The epithelial capsule lining the apical surface facing the gill chamber contained mucous cells, in which the secretory material was stained positively with PAS and AB. Similar to the observations made herein, the thymic epithelium was also observed to comprise a large number of mucous cells in clingfish ( Sicyases sanguineus ) (Gorgollon, 1983), rainbow trout (Chilmonczyk, 1992), mandarin fish ( Siniperca chuatsi ) (Xie et al, 2006) and tilapia ( Oreochromis niloticus ) (Cao et al., 2017). The abundant mucous cells in the surface epithelium of thymus could constitute an immune barrier and protect the organ against microbial attacks and infections sourced from aquatic environments (Lazado & Caipang, 2014).…”

“…It was proposed that those supporting cells synthesise factors that provide thymocyte maturation and the acquisition of thymocyte immune characteristics (Chilmonczyk, 1992). As determined in this study, myoid cells have been reported in some fish, including Rutilus rutilus (Zapata, 1981a), rainbow trout (Chilmonczyk, 1983), seabream ( Diplodus puntazzo ) (Romano et al., 1999), juvenile Australian lungfish ( Neoceratodus forsteri ) (Mohammad et al., 2007), tilapia ( Oreochromis niloticus ) (Attia et al., 2010; Cao et al., 2017) and flounder ( Paralichthys olivaceus ) (Liu et al., 2004). Rimmer (1980) discussed the role that myoid cells might play as a local source of antigenic for self‐recognition, which increased the number of those cells and was related to the autoimmune disease myasthenia gravis.…”

Post-Aire Medullary Thymic Epithelial Cells and Hassall’s Corpuscles as Inducers of Tonic Pro-Inflammatory Microenvironment

“…In addition to the conventional mTECs described above, the thymic medulla contains unique structures called Hassall’s corpuscles (HCs). These structures, firstly characterized in 1846 by Arthur Hill Hassall ( 38 ) in human thymi have, after their initial description, been found in several other mammals ( 39 ) but as well as in bird and fish species ( 40 , 41 ). In humans, the HCs appear as a concentric merged cluster of unnucleated cells with a typical diameter of 20-100 μm and are present in large quantities already by the 28 th week of prenatal development ( 42 ).…”

Maternal immunity of tilapia broodstock vaccinated with polyvalent vaccine and resistance of their offspring againstStreptococcus agalactiae

“…RAG1 is the first immune gene expressed at the levels of embryo and larvae in vertebrate animals and expressed by the thymus organ when B cells and T cells were immature (Corripio-Miyar, Bird, Treasurer, & Secombes, 2007;Hsu, 2009;Willett, Cherry, & Steiner, 1997). Thymus organs contain various of cells including lymphocytes, macrophages, epithelial cells, fibroblasts and mastos (Cao, Chen, Lu, Hu, & Wang, 2017;Chun, Schatz, Oettinger, Jaenisch, & Baltimore, 1991;Greenhalgh, Olesen, & Steiner, 1993), and RAG1 expression in mammals was detected higher in thymus organs than that in spleen, liver and bone marrow organs.…”