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==Intrroduction==

==Intrroduction==

'''Haemopoiesis''' is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells.

'''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 [[Lizard_and_Snake_Cell_Identification#Erythrocytes|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.  

Blood cells form in the bone marrow (predominant site for [[Lizard_and_Snake_Blood Cell_Identification#Erythrocytes|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==

==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 [[Lizard_and_Snake_Cell_Identification#Erythrocytes|erythrocytes]] dividing mitotically in the bloodstream or after recirculation through the bone marrow, in order to create more erythrocytes during periods of chronic anaemia.

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 [[Lizard_and_Snake_Blood Cell_Identification#Erythrocytes|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==

==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.

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 [[Lizard_and_Snake_Cell_Identification#Lymphocytes|lymphocyte]] is the T cell.

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 [[Lizard_and_Snake_Blood Cell_Identification#Lymphocytes|lymphocyte]] is the T cell.

In lizards, the spleen is spindle-shaped and located just off the greater curvature of the stomach.

In lizards, the spleen is spindle-shaped and located just off the greater curvature of the stomach.

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.  

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 [[Lizard_and_Snake_Cell_Identification#Lymphocytes|lymphocytes]] as in mammals. It may involute irreversibly under nutritional stress.

It produces T [[Lizard_and_Snake_Blood Cell_Identification#Lymphocytes|lymphocytes]] as in mammals. It may involute irreversibly under nutritional stress.

==Other lymphoid deposits==

==Other lymphoid deposits==