Lymphoid organs and cells

The yolk sac is the first and major hematopoietic organ in reptilian embryos. Later in development, blood cell formation occurs in fetal spleen and fetal liver. Hematopoiesis in bone marrow starts near the end of embryonic development and persists throughout life. The thymus, the first organ to become lymphoid during reptilian ontogeny, is a bilateral organ lying on either side of the neck in lizards and immediately anterior to the heart in snakes; it is usually multilobulated in turtles. The thymic primor-dium originates as dorsal outgrowths of pharyngeal pouches II and III in lizards, IV and V in snakes and III and IV in turtles that detach from the pharyngeal wall and appear as a condensation of epithelial cells surrounded by mesenchyme, with no capsular device yet formed. Thymic lobes increase in size and fibroblasts condense around the organ to form a distinct capsule. The epithelial thymic rudiment is then invaded by bloodborne large, basophilic, undifferentiated cells, which divide and mature into large, medium and small lymphocytes. A complete demarcation into cortex and medulla is apparent just before hatching (or birth). Later on, the thymus lobes show a peripheral cortex loaded with lymphocytes, and central medulla with fewer thymocytes so that the epithelial groundwork is easily discernible. Numerous macrophages and large and small, round, oval or elongate myoid cells are seen in the medulla and corticomedullary junction; cross-striations similar to those of skeletal muscle are evident in some myoid cells. Dendritic-like cells, with numerous labyrinthine processes forming gap-like junctions with the neighboring lymphocytes, are frequent in the medulla of the turtle Mauremys caspica. These cells lack Birbeck granules and the extensive membrane-membrane contacts like those reported for mature interdigitating cells of mammalian lymphoid organs, but often contain abundant cell debris and dense bodies, suggesting a certain degree of phagocytic activity. It is important to note that the thymus appears so well developed only in reptiles collected from the field, or maintained in the laboratory under suitable conditions of temperature, photoperiod and feeding, during optimal, species-specific, seasonal conditions (Figure 1). Otherwise, the thymus shows various degrees of degeneration, whereby the number of lymphocytes in the cortex and medulla greatly decreases and numerous myoid cells and epithelial cysts are observed. Reptilian thymic cysts range from minute intracellular structures to formations over 0.1 mm in diameter. They probably represent foci of cellular degradation segregated from surrounding thymic parenchyma by a continuous layer of epithelial cells. Involution continues with persistence of the adverse environmental conditions. Cortical epithelial cells degenerate and dendritic cells are absent. Eventually, the thymus may transform into fibrous tissue and become scarcely detectable. This involution is totally reversible, as in suitable seasonal and other ambient conditions rebuilding of the thymus begins and progressively proceeds to a mammalianlike picture and organization. The issue of whether the degenerated epithelial cells found in involuted thymus are eliminated and substituted by a new cell population, or recover with the advent of optimal seasonal conditions, remains unresolved as yet; nor have the waves of thymocyte production in the rejuvenating thymus been investigated.

The spleen in reptiles is the most important peri-

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