An ultrastructural study of the spleen of the ranid frog Rana perezi.

The spleen of Rana perezi is encapsulated by connective tissue and shows by light microscopy two areas with no obvious border: the white pulp and the red pulp. The white pulp-lymphoid clusters are scattered throughout the organ and contain lymphocytes, reticular cells, and some plasma cells. The red pulp displays two different portions. The predominant region consists of reticular cells, lymphocytes, a variety of other leucocytes, and cells undergoing division. This area possibly performs a haemopoietic function. The smaller portion of the red pulp is characterized by reticular-phagocytic cells and may be haemocaretic in its function. Macrophages and pigmented cells occur in both white and red pulp. The organization of the spleen of R. perezi can be considered as a transitional or intermediate state between the primitive condition seen in certain fishes and amphibians and the more complex organ of ammiotes.