Spleen of the snake (Elaphe climacophora) and intrasplenic vascular architecture

The spleen of the blue-green snake (Elaphe climacophora) lies at the head of the pancreas and is separated from it by a fibrous capsule. A hilus is not clearly distinguished. Arteries and veins enter or leave the spleen through the capsule, but no collateral relationship is observed between these vessels. Histologically, the spleen consists of the capsule-septal tissue, lymphoid tissue (the white pulp), and a fibrous zone (the perilymphoid fibrous zone: PLFZ) around the lymphoid tissue that includes many small veins. The PLFZ probably represents the involuted red pulp. The border between the white pulp and PLFZ is unclear in routine histological sections due to diffuse infiltration of lymphocytes into the latter region, but the border could be distinguished clearly in silver-impregnated specimens. Arteries enter the spleen, run within the septa, and divide into terminal arteries in the lymphoid tissue that form end branches. There are no ellipsoids around the terminal arteries. The end branches communicate with veins of the PLFZ through transitional vessels within the lymphoid tissue (closed circulation). The veins of the PLFZ anastomose with drainage veins in the capsule. The snake spleen retains the basic histological characteristics of a spleen and is morphologically distinguishable from a lymph node. It may represent an extreme example of a spleen modified by the remodelling of the intrasplenic vasculatures during evolution. © 1995 Wiley-Liss, Inc.