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LYMPHATIC SYSTEM

In animals with open circulatory systems, there is no distinction between blood and lymph and therefore there is no separate lymphatic system. There are undoubtedly many possible ways in which organisms could organize their microbe-fighting tissues. Some invertebrates possess “lymphoid tissues”, “white bodies”, or “branchial spleens” which produce cells which function in immune responses as do those cells of vertebrate lymphatic tissues but are not homologous with vertebrate lymphatic tissues (Hoar, 1983). Higher vertebrates do possess a separate lymphatic system which returns fluid to the circulatory system, transports lipids from the digestive tract, and is essential in combating disease.

In humans, not all of the fluid which leaves the arteriole end of a capillary reenters the venule end of the capillary. This fluid enters a system of lymphatic capillaries which form larger and larger lymphatic vessels until they form the thoracic and right lymphatic ducts which return the fluid to the circulatory system at the brachiocephalic vein. It is generally held that jawless fish and cartilaginous fish lack a separate set of lymphatic vessels although lymphatic functions are performed in the thin-walled sinuses which empty into the veins (Dehal, 2002; Torrey, 1979; Romer). Bony fishes and tetrapods have a separate set of lymphatic vessels which typically travel alongside the veins. The lymphatic ducts of the opossum below are equivalent to those found in humans.

Lymph can be propelled by skeletal muscle contractions and even by contraction of regions referred to as “lymph hearts”. These lymph hearts consist of smooth muscle surrounding the regions where lymphatic vessels enter veins and are known in every group of vertebrates except mammals and cartilaginous fish (Webster, 1974). The lymph hearts of embryonic birds usually become nonfunctional later in life, although they persist in some birds. These contractile lymph hearts may even possess separate chambers (Hoar, 1983; Torrey, 1979).

Non-mammalian vertebrates typically lack large numbers of lymph nodes (Torrey, 1979). Reptiles have lymphatic cisterns or lymphatic sites at same sites as true lymph nodes in mammals and some water birds (Kardong, 484).

Reptiles, birds and some mammals possess pharyngeal tonsils (referred to as adenoids when enlarged in humans). Amphibians may have tissue homologous to pharyngeal tonsils. Palatine and lingual tonsils are only found in mammals. (Weichert, 1970, p. 247).

CAT

The only lymphatic tissue identified in lampreys to date is gut-associated lymphatic tissue (GALT) which is also present in higher vertebrates (Flajnik, 2007; Varner, 1991). In jawless fish, the spleen is not a separate organ, but rather diffuse tissue associated with the digestive tract (Torrey, 1979). In hagfish, the liver seems to sequester red blood cells, which may indicate that hagfish livers perform a function observed in the spleen of higher vertebrates (Forster, 2001). The gnathostome spleen is a single structure caudal to the stomach. The spleens of a number of animals are pictured below.

SHARK

GAR

GAR SPLEEN

FROG

CHICKEN

OPOSSUM

GOAT

COW

PIG

pig

CAT

MONKEY

All mammals have a spleen, thymus, and appendix in addition to mucosa associated lymphatic tissues (MALT) in homologous body regions. The thymus has the same histological structure in all therian mammals. Monotremes possess lymphatic nodules (similar to amphibian jugular bodies) in the sites where therian mammals possess lymph nodes. There is evidence to suggest that second immune responses are greater in therians than in monotremes (Belov, 2003; Vernersson, 2004).

Jawless fish do possess cells which resemble lymphocytes and accumulations of these cells in the branchial region have even been called a “thymus”. This region is more developed in lampreys than in hagfish (Shintani, 1999; Weichert, 1970; Romer, p. 448). Jawed vertebrates were the first to develop a true thymus, MHC self-recognition system, and acquired immunity. The thymus not only allows the development of lymphocytes, it is also a site where self-reacting lymphocytes are reacted against (Rolff, 2007). All gnathostomes possess a thymus which is derived from pharyngeal pouch epithelium (Hoar, 1983). In gnathostomes, the thymus develops from several pharyngeal pouches rather from each pouch as in lampreys (except in caecilians). In all vertebrates except mammals, the thymus forms from the dorsal portions of the pharyngeal pouches. In mammals the thymus forms from the ventral portions of pharyngeal pouches 3 and 4. (Weichert, 1970, p. 245-6)

Liver endothelial cells (LEC) eliminate physiological and foreign waste products (including all four major classes of biomolecules) through receptor mediated endocytosis from the endothelia of liver sinusoid capillaries. These cells contribute to innate immune defenses are part of the reticuloendothelial system. While these scavenger endothelial cells are located in the liver in bony fish and tetrapods, they are located in specialized gill arteries in hagfish, lampreys, and cartilaginous fish. In insects pericardial cells perform a similar function to these scavenger endothelial cells, while insect haemocytes perform a function similar to that of macrophages (Seternes, 2002).

WHITE BLOOD CELLS

1) LEUKOCYTES AND INNATE IMMUNITY

Vertebrate white blood cells are not unique in their ability to consume microbes through phagocytosis. This ability is even possessed by many microorganisms, such as such as amoeba. All animals possess amoeba-like cells (similar to white blood cells which perform phagocytosis) which float in the fluid around the body cells. Such phagocytes are even known in the most primitive animal groups, such as sponges and starfish, and are present in invertebrates which lack a true circulatory system (Hoar, 1983). These ameobocytes, like white blood cells, may be full of inclusions, some of which are phagosomes (Harrison, Vol. 2, p.47). A number of invertebrates, including tunicates, possess cytotoxic cells which have been compared to natural killer cells (Parrinello, 1996).

In hemichordates, blood flows through spaces in connective tissue in which amoeba-like blood cells travel. The amoebocyte blood cells typically possess kidney-shaped nuclei. (Benito, form Harrison 1997, p. 61). Ascidian blood includes macrophages, different kinds of granular amoebocytes with odd-shaped nuclei which perform phagocytosis, cytotoxic cells, and a number of other cell types (Parrinello, 1996;Burighel, from Harrison, 1997, p. 269).

Vertebrate white blood cells are classified in two categories, granulocytes and agranulocytes, each of which contains additional subclasses of cells (neutrophils, eosinophils, and basophils are granulocytes; monocytes and lymphocytes are agranulocytes). These leukocytes and the specific molecules they use to function are not unique to humans. Neutrophils are typically the most common white blood cell in vertebrates and basophils the least common (Torrey). In lungfish, heterophils have from 1 to 3 lobes. The large number of immature blast cells in lungfish blood suggests that some hematopoeisis may occur in circulation, unlike the situation in mammals where it occurs primarily in the bone marrow (Hine, 1990a).

A number of local hormones are involved in human immune reactions and homologs of these signals exist in simpler animals. Starfish possess molecules similar to interleukin-1. Worms and tunicates possess interleukin-1 and TNF; hornworms produce interleukin-1 and interleukin-6. Three kinds of cytokines are known from invertebrates.

Mammalian leukocytes are more similar to those of lungfish than to those of teleosts (Hine, 1990). The enzyme eosinophil peroxidase is not known in jawless fish, its activity is weak or absent in cartilaginous fish, and it is present in at least some of the members of all higher groups of vertebrates. The enzyme alkaline phosphatase found in neutrophils is known from jawless fish and all higher groups (Hine, 1990).

Leukocytes are present in the following images of fish blood and

FROG BLOOD

TURTLE BLOOD

In lower vertebrates, the liver, spleen, submucosa of the intestine, and intertubular tissue of the kidney continue to produce blood throughout the life of the adult. In adult reptiles, the spleen and red bone marrow produce both red and white blood cells. In adult mammals and birds, the red bone marrow is the site of red blood cell and granulocyte synthesis while sites such as the thymus and spleen are important for the maturation and proliferation of agranulocytes (Torrey). There are also differences among vertebrates with regard to where immune reactions occur. Large numbers of white blood cells, particularly lymphocytes and macrophages, are present in the central nervous system of fish. The density of lymphocytes in the spinal cord, for example, is more than 5000 times the density observed in mice (Dowding, 1993).