Analysis of the development of the lizard, Calotes versicolor. II. Histogenesis of the thymus

Abstract

The origin, histogenesis and lymphopoiesis of the thymus have been studied in the lizard, Calotes versicolor. The thymic rudiments arise as bilateral epithelial thickenings from the second and third pharyngeal pouches. Stem cells, characterized by their basophilic cytoplasm and prominent nucleoli, were first identified in the yolk sac blood islands and in the circulation of the lizard embryo at stage 28. Later, stem cells were observed in the epithelial thymic rudiments at stage 35. Large and then small lymphocytes appeared in the thymus at stage 36 and 37. Differentiation into cortex and medulla began by the accumulation of lymphocytes in the peripheral region of the organ (the cortex) from stage 38 onwards. Lymphocytes are drained from the thymus after lymphatic vessels form at stage 38. The sequential events in the developing cellularity of the thymic primordium, suggest the origin of thymic lymphocytes from blood borne stem cells. These findings are discussed from a phylogenetic perspective.