Figure 1. Phylogenetic relationships among Sauropsidae. Reptiles and Birds are grouped in a taxon designated as Sauropsidae. The early branch of reptiles, the Chelonia (tortoises and turtles), has retained numerous characteristics of the most ancient reptiles like the amphibious habits. The Rhynchocephalia, represented by the unique species Sphenodon punctatum, have survived with little change from the Triassic period. The Squamata (lizards and snakes) are a modern group descending from forms related to the Rhynchocephalia. The Crocodilia are closely related to birds and both form the Archaosauria group of the living amniotes. Divergence is based on paleontologic evidence and molecular comparisons.

Figure 2. Lymphoid organs of Sauropsidae. (a) snake, (b) chicken. In snake, lymphoid organs on this figure correspond to thymus and spleen.

Figure 3. Ontogeny of haematopoietic organs. Developmental schedule of haematopoietic rudiments in the avian embryo. Scale in the middle: embryonic days of development. Above the scale: localization of the sites of haematopoiesis; in blue, the yolk sac; red, the aorta; green, the allantois. The cones represent the haematopoietic production. The yolk sac is the first site to undergo haematopoiesis, but its production was shown to be transient during embryonic life. The aorta and allantois are secondary sites of progenitor commitment. The aorta was shown to produce definitive haematopoiesis and to be a site where HSC emerge. The chick aorta displays two sequential activities of haematopoietic production. One between the 1st and 4th day designated as intraaortic clusters, the second between the 6th and 9th day designated as paraaortic foci. These two aspect are developmentally linked i.e. intraaortic clusters give rise to paraaortic foci. Aorta-borne cells colonize the definitive haematopoietic organs according to the time schedule summarized below the scale. Under the scale. Colonization schedule of the definitive haematopoietic organs. Orange, cyclic phases of thymic colonization; red unique phase of spleen colonization; blue, bone marrow colonization; pink, bursal colonization.

Figure 4. Migration routes of the T-cell lineage. T-cell progenitors of the first wave of colonization are derived from the paraaortic foci, whereas progenitors of the second and third waves originate from the bone marrow. A direct contribution of allantois HSC may occur. For all three waves the generation of T cells takes 9 days and for T cells takes 12 days. Thymus colonization in waves and T-cell differentiation kinetics lead to successive waves of and T cells that leave the thymus sequentially, bound for peripheral organs such spleen and gut. The stage of development is indicated as numbers of days.

Figure 5. Immunoglobulin and T-cell receptor loci of Sauropsidae. (a) Genomic organization of Chicken and turtle Pseudemis scripta IgH genes. The chicken Ig chain loci possess only a single functional V (variable) gene downstream of clusters of V pseudogenes, many D (diversity) segments and one J (joining) segment. The 3¢ end of the locus contain genes encoding the constant (C) region of the IgH chains. The upper scheme corresponds to unrearranged IgH locus. In developing B cells, the only functional V gene rearrange with one D segment and the only J segment (middle scheme). Diversity is thence introduced into the rearranged V(D)J segments by gene conversion using V pseudogenes as schematically described by an arrow. In the turtle Pseudemis scripta, the IgH locus consist on many V, D and J segments, the generation of IgH diversity is obtained by the random combinatorial rearrangement of these genomic segments. (b) Genomic organization of the chicken TcR, , and genes.