Lizard and Snake Haemopoietic System
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Haemopoiesis is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells.
Blood cells form in the bone marrow (predominant site for erythrocytes and granulocytes), spleen and, in early life, the liver. The myeloid stem cells are multipotential cells, giving rise to all the cell types found in the bone marrow.
Bone Marrow and Haemopoietic tissue
Haematopoietic tissue is found widely in the bones of reptiles, including the vertebral bodies and ribs of snakes. This tissue is similar to that of birds and mammals. Extramedullary haemopoiesis may occur in the spleen and liver, with some erythrocytes dividing mitotically in the bloodstream or after recirculation through the bone marrow, in order to create more erythrocytes during periods of chronic anaemia.
Spleen
In reptiles, the spleen lies in the dorsal mesentery, close to the stomach and pancreas, as in mammals. However, the spleen-to-bodyweight ratio is the smallest of any vertebrates except birds (Tanaka, 1998). In some species, it may be a separate organ but found combined with the pancreas as splenopancreas (e.g. some snakes, such as the milk snake). In boid snakes, the spleen is multilobulate.
The splenic artery, arrising from the coeliac artery, supplies the spleen with blood. It is composed of mesenchymal tissue enclosed in a fibrous capsule. The white pulp is well developed and the red pulp is absent in snakes but present in other reptiles. Arteriovenous connections are generally of the closed type, unlike the more common open type seen in mammals. It appears to be primarily lymphopoietic and no blood storage capacity has been reported in reptiles. Futhermore, the spleen is a site of possible extramedullary haemopoiesis, as is the liver. The predominant splenic lymphocyte is the T cell.
In lizards, the spleen is spindle-shaped and located just off the greater curvature of the stomach.
Thymus
Located in the cervical region near the carotid arteries, it may have parathyroid glands attached and is often found adjacent to the ultimobranchial bodies. In snakes and lizards the thymus is lobulated.
It produces T lymphocytes as in mammals. It may involute irreversibly under nutritional stress.
Other lymphoid deposits
Some reptiles possess bursa-like structures; lymphoid deposits can also be found in the liver, kidneys, and GI tract (e.g. oesophageal tonsils and Peyer's-like patches in the small intestine). The significance is that multiple organs may be involved in conditions such as lymphoma in reptiles, increasing the variability of clinical signs.
References
- Girling, S.J. and Raiti, P. (2004). BSAVA Manual of Reptiles. British Small Animal Veterinary Association, 2nd Edition. pp. 350. ISBN 978-0905214757