Figure 1 Development profile of thymus in: (A) lizards; (B) snakes and (C) turtles the year round.

pheral lymphoid organ. Histogenesis of the spleen commences with a thickening of the dorsal meso-gastrium. The splenic rudiment then buds into the body cavity; later, blood sinuses develop as separate cavities in the mesenchyme, and spleen acquires its characteristic form by development of the capsule. The spleen in lizards, snakes and tortoises becomes lymphopoietic much later than the thymus, which is fully differentiated before any lymphocytes appear in the splenic rudiment. However, thereafter the spleen shows intense hematopoietic and lymphopoietic activity. After hatching and in adult life the spleen usually reveals well-developed and clearly defined white pulp, arranged around central arterioles, and red pulp, composed of blood sinusoids and cell cords. In the white pulp the framework of reticular cells contains macrophages, dendritic cells predominantly located in the periphery of the white pulp, and numerous densely packed lymphocytes. There is evidence to indicate that 50-70% of reptilian splenic lymphocytes are thymus derived (T-like cells) and are located in thymus-dependent juxta-arteriolar areas. The remaining lymphocytes are surface immunoglobulin (slg) bearing, homologous to the B cells of aves and mammals. In lizards and tortoises, 25-50% of splenic lymphocytes are sIgM+, and 5-20% slgY1. The sIgY+ lymphocytes coexpress slgM. Germinal centers and nodules typical of mammalian secondary lymphoid organs are lacking in reptiles. As in the thymus, the "ideal' spleen architecture is only observed in reptiles living in optimal conditions. Otherwise, the splenic lymphoid aggregates regress in size, and the number of T-like cells sharply diminishes. A significant proportion of slg1 lymphocytes survive even in animals exposed to rigorous environmental conditions (Figure 2). This suggests that reptilian T- and B-like cells differ in their sensitivity to environmental factors. The drastic depletion of splenic T-like cells is, however, reversible (Figure 2). It is indeed now established that the lymphoid elements of adult reptiles atrophy and regenerate, in a periodic manner, with the cycle of the species-specific adverse and optimal seasons.

Gut-associated lymphoid tissue (GALT) develops only after hatching in lizards, snakes and chelonians. Although reptiles do not possess tonsils, Peyer's patches or an appendix, numerous lymphoid aggregates occur in the mucosa and submucosa along the gut in most reptiles and consist of lymphocyte populations which exhibit differential sensitivity to seasonal conditions. In the lizard Chalcides ocellatus, esophageal lymphoid aggregates diminish considerably in size and number 1) during autumn and winter as compared with spring and summer, 2) after adult thymectomy, and 3) following hydrocortisone treat

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