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LYMPHATIC SYSTEM
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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. |
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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). |
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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). |
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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. |
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SHARK |
GAR |
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FROG |
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CHICKEN |
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OPOSSUM |
GOAT |
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COW |
PIG |
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pig |
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CAT |
MONKEY |
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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. 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). |
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Leukocytes
are present in the following images of fish blood and |
FROG BLOOD |
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TURTLE BLOOD |
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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). Human white blood cells
are represented in the following images.
Neutrophil: |
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EOSINOPHIL |
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MONOCYTE |
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LYMPHOCYTE |
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While white blood cells are found in the blood (as in the above pictures), the vast majority of them are found in other tissues such as the: | |||||||||||||
THYMUS |
TONSILS![]() |
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SPLEEN |
APPENDIX |
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PEYER'S PATCH |
LYMPH NODE |
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