Immunogenetics (2021) 73:53–63 https://doi.org/10.1007/s00251-020-01196-0 REVIEW Anatomy of teleost fish immune structures and organs Håvard Bjørgen1 · Erling Olaf Koppang1 Received: 1 September 2020 / Accepted: 23 November 2020 / Published online: 11 January 2021 © The Author(s) 2021 Abstract The function of a tissue is determined by its construction and cellular composition. The action of diferent genes can thus only be understood properly when seen in the context of the environment in which they are expressed and function. We now experience a renaissance in morphological research in fsh, not only because, surprisingly enough, large structures have remained un-described until recently, but also because improved methods for studying morphological characteristics in combination with expression analysis are at hand. In this review, we address anatomical features of teleost immune tissues. There are approximately 30,000 known teleost fsh species and only a minor portion of them have been studied. We aim our review at the Atlantic salmon (Salmo salar) and other salmonids, but when applicable, we also present information from other species. Our focus is the anatomy of the kidney, thymus, spleen, the interbranchial lymphoid tissue (ILT), the newly discovered salmonid cloacal bursa and the naso-pharynx associated lymphoid tissue (NALT). Keywords Bursa · Fish · Histology · ILT · Immune organ · Kidney · Lymphoid tissue · Morphology · Spleen · Thymus Introduction of similarly specialised cells that together perform certain special functions” (Studdert et al. 2012). Most tissues contain In 2000, Zapata and Amemiya (2000) published a review lymphoid cells, but that does not necessarily make them paper addressing the ontogeny of immune structures in lymphoid. However, if the tissues predominantly contain vertebrates. Anatomy is one of the most ancient disciplines lymphoid cells, they are frequently arranged in lymphoid in medicine, and the old saying goes: “Anatomia medicinae organs. The precise defnitions are in other words not clear- fundamentum est”. Therefore, by all standards, one would cut. Following the terminology presented above, it is not have thought that major lymphoid tissues in fsh should easy to defne a lymphoid tissue if it cannot be visualised have been discovered and described by 2000. However, as an aggregate of lymphoid cells, e.g. the mammalian this was not the case. Since then, large lymphoid structures, Peyer’s patches, which although being an integrated part of previously un-recognised, have been identified in fish, the abdominal wall also are defned as secondary lymphoid and in addition, continuous research eforts have provided organs (Pabst et al. 2007). To refer to difuse lymphoid complementary information regarding the tissues already tissue, which is common in piscine immune research, seems known. alien according to traditional anatomical terminology. In What is a lymphoid organ? An organ, lymphoid or not, our opinion, the term “lymphoid tissue” outside a well- may be defined as a “somewhat independent body part defned localization predominantly consisting of lymphoid that performs a specifc function or functions” (Studdert cells is confusing and should be avoided. For instance, it et al. 2012). Organs are again made up of tissues and a is common to describe lymphoid cells in the intestines of good and useful defnition of a tissue is “a group or layer fsh as gut-associated lymphoid tissue and that fsh have a “difuse lymphoid tissue” at this location. To have a “difuse This article is part of Topical Collection on Fish Immunology tissue” is contradictory with respect to the given defnition of a tissue. The intestinal lymphocytes are indeed part of a * Erling Olaf Koppang tissue (the intestinal mucosa), but they do not form a tissue [email protected] in its own right. A more precise terminology describing 1 Section of Anatomy, The Faculty of Veterinary Medicine, the existence of leukocytes in the fsh intestines would be Norwegian University of Life Sciences, Ullevålsveien 72, to report what is objectively observable, namely intestinal Oslo, Norway Vol.:(0123456789)1 3 54 Immunogenetics (2021) 73:53–63 mucosa infltrated with in most cases scattered leukocytes. fact and have frequently been reporting about the lymphatics The mucosa is divided into two efector compartments, i.e. of fsh. This is especially so for researchers using zebra fsh as the epithelium and the lamina propria, which are separated model organisms. Apparently, their swimming human has no by the basal membrane. The basal membrane is permeable solutions by its own. This negligence of available information and allows trafc of leukocytes from one compartment to the prompted Vogel (2010) to publish an excellent comment other. This trafc occurs through fenestrations of the basal regarding this topic. In a striking comment, he stated that membrane which are more prominent at sites with mucosa- “Given that all actinopterygian fsh studied thoroughly have associated lymphoid organs (MALTs) (Takeuchi and Gonda been shown to have a secondary vascular system, it would be 2004). In mammalian anatomy, there is a clear distinction extremely surprising if zebrafsh were the only exception.” between the ordinary mucosal surface and that containing Other researchers have taken this information into account MALT structures, which only are found at certain locations. when presenting their results and arguing for the existence of As emphasised by Smith et al. (2013), these locations are lymphatic vessels in teleost fsh (Hellberg et al. 2013). And not to be confused. So, although well-established in the that may well be so. Based on available information, it is a fair piscine immunology community, we therefore argue for not assumption that teleost fsh possess a secondary vascular system using a terminology implying that a “difuse tissue” is a which by no means is equivalent to a lymphatic vessel system, tissue. In the following, we will use the term “lymphoid but that fsh in addition may have lymphatic vessels, most tissue” as an area where predominantly lymphoid cells are probably related to the intestines (Hellberg et al. 2013). This is observed within defned structural boundaries, and such sites just one example of many where scarce anatomical information are normally confned to lymphoid organs. This review will has a great impact on our perception on the function of the therefore not cover mucosa-associated lymphoid cells which immune system, and where additional morphological studies recently have been reviewed separately (Salinas and Miller are highly warranted. 2015; Bjørgen et al. 2020), but with the exception of discrete Our scope is to give a broad overview of teleost immune lymphoid structures. tissues in an anatomical context focusing on salmonids (Fig. 1). In our opinion, morphological studies of the fsh immune Other tissues not mentioned in this review contain varying systems seem somewhat forgotten in the age of genomics and amounts of immune cells. In the salmon industry, where fsh proteomics. Researchers dealing with zebrafsh have a tendency is intraperitoneally vaccinated, chronic infammatory intra- of regarding this fsh as a swimming human, or at the best, as abdominal changes have been frequently reported. The reader a swimming human embryo, with no biological rights in its is referred to special literature with respect to these issues. The own. But teleost fsh evolved millions of years ago (Zapata and work by Zapata and Amemiya (2000) serves as a comprehensive Amemiya 2000), and they are fnely tuned to their lifestyle and fundament for our review. Other excellent reviews are also habitat. They have not simply been frozen in their evolution at available, for instance Press and Evensen (1999). These works a given developmental stage but have rather over these years cover several fundamental aspects related to kidney, thymus and developed their own solutions. This fact seems frequently spleen. With respect to the nasopharynx-associated lymphoid to have been neglected by researches only regarding fsh as tissue, a recent and excellent review has been published (Das model organisms. A stunning example of the discrepancy is and Salinas 2020), and this tissue will therefore only briefy the debate, or even lack of debate, regarding the existence of be presented here. However, the recently discovered immune a lymphatic vessel system in fsh. It has long been established structures interbranchial lymphoid tissue (ILT) (Haugarvoll et and proven that fsh have a secondary vascular system. This al. 2008) and the salmonid bursa (Løken et al. 2020) need to secondary vascular system may appear as lymphatic vessels, be presented as lymphatic structures in the context of other fsh but it is not. However, some researchers seem to ignore this immune organs and will therefore be of focus in this review. Fig. 1 Schematic topography of immune organs in Atlantic salmon. A Thymus, B head kidney, C trunk kidney, D spleen, E gills with the interbranchial lymphoid tissue (ILT), F salmonid bursa, G olfactory organ with the nasopharynx-associated lym- phoid tissue (NALT) 1 3 Immunogenetics (2021) 73:53–63 55 Thymus cortex/medulla organization as known from mammals. This compartmentalization is important, because it provides The thymus of teleosts is a paired organ placed in a dorsal specialised immunologic niches that are imperative for T projection in the epithelium of the opercular cavity. It is cell development. In the cortex, there is proliferation and covered by the pharyngeal epithelium which forms a barrier recombination activity (positive selection) of T cells. In towards the external milieu (Castillo et al. 1998). The organ the medulla, negatively selected T cells undergo apoptosis. consists mainly of reticulated epithelial cells forming Surviving and mature T cells may leave the organ through the niches in which T cells are embedded (Chilmonczyk 1983; medullary vasculature. In salmonids, it has been difcult to Castillo et al. 1990). The organogenesis involves thymic identify medullary and cortical organization. Rather, the trout bud formation from pharyngeal pouches (Chilmonczyk thymus may be divided into an outer capsular zone (C) followed 1992).