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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 dermal bone of the chelonian shell and 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
(liz) Site of possible extramedullary haemopoiesis. Spindle-shaped and located just off the greater curvature of the stomach.
In reptiles, the spleen lies in the dorsal mesentery, close to the stomach and pancreas, as in mammals. However, the splee-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. many chelonians and some snakes, such as the milk snake). In boid snakes, the spleen is multilobulate.
References
- Tanaka, Y. (1998). Structure of the reptilian spleen. Biology of the Reptilia 19, 533-586.