Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 30

Fish lack lymph nodes, Peyer's patches, and bone marrow. Hematopoiesis in bony fish occurs primarily in the spleen and the cranial kidney with some activity in the liver, thymus, and pericardium in some species (Roberts and Ellis 2012). Additional lymphoid activity occurs in the mucosa‐associated lymphoid tissues (MALT). The gastrointestinal tract, gills, and skin act as pathogen barriers and contain local leukocyte populations. Another cell, the melanomacrophage, is present ubiquitously and increases in the presence of antigen stimulation. Pigmented macrophage aggregates (PMA), also known as melanomacrophage centers (MMC), are solid foci of these cells and found in high numbers in the liver, spleen, and cranial kidney (Roberts and Ellis 2012). They can be seen on wet mounts of these organs examined under direct microscopy and can increase with chronic inflammatory diseases.

The spleen is dark red to black and has sharp edges. There are multiple spleens in some fish and no distinct spleen in hagfish and lampreys (Agnatha).

The thymus can be very hard to find. Presence and size vary greatly among teleosts and it does not follow the mammalian involution pattern (Roberts and Ellis 2012). It may be found subcutaneously at the dorsal edge of the operculum, at the base of the gill arches, associated with pharyngeal epithelium, or within the cranial kidney (Stoskopf 1993). One pair of organs is typical, but some fish (e.g. clingfish, Gobiesocidae) have two pairs (Bowden et al. 2005).