Tracing of cells of the avian thymus through embryonic life in interspecific chimeras.

Abstract

Differences in the structure of the interphase nucleus between two species of birds, the Japanese quail (Coturnix coturnix japonica) and the chick (Gallus gallus) has been used to distinguish cells from different origins in interspecies combinations. This biological cell marking technique was applied to thymus histogenesis. Using various combinations between components of quail and chick thymic rudiments, the respective contribution of endodermal epithelium, mesenchyme, and blood-borne extrinsic elements to the histogenesis of thymus was analyzed. It was demonstrated that the whole lymphoid population of the thymus is derived from immigrant blood-borne stem cells which are chemically attracted by the endoderm of the 3rd and 4th pharyngeal pouch. The latter is determined to differentiate into thymic epithelial reticulum as soon as the 15-somite stage, and is able to attract blood stem cells even when transplanted in an heterotopic position such as the ventral body wall of the embryo. It was shown that the thymic mesenchyme originates from the neural crest mesectoderm which colonizes early the 3rd and 4th branchial arches. It participates in the formation of perivascular mesenchyme, but does not give rise to lymphocytes. From heterospecific transplantations of quail thymuses into chick embryo (and inversely) at various stages of development is appeared that the thymic rudiment becomes attractive for lymphoid stem cells at a precise stage of its evolution for each species. The attractivity period lasts about 24 h for the quail and 36 h for the chick. Then, the inflow of stem cells becomes very low until the end of the incubation period. At this time, a second wave of lymphocytoblasts invades the thymus and the primitive embryonic lymphoid population is completely renewed around the hatching time. Competent thymic stem cells are present in the blood before and after the period of physiological thymic attractivity. The identity of basophilic cells appearing in the thymus during its histogenesis and lymphoid stem cells has been demonstrated from the analysis of quail-chick chimeric thymuses.

At the crossroads between tolerance and aggression

“…1 This model was supported by a series of observations, made first in avian species 2,3 and later in the mouse, [4][5][6][7][8][9][10] that revealed the existence of unique waves of lymphocyte production during fetal and neonatal development.…”

“…2,3 The final wave occurred around hatching time and was observed to almost completely replace the earlier waves of precursors. However, despite the elegant demonstration that distinct layers of thymopoiesis can be observed in this model, the functional significance of these observations remained largely unknown.…”

Kinetics of thymocyte developmental process in fetal and neonatal mice

“…In this process, developing cells proliferate, differentiate and are selected for the generation of immunocompetent T cells. During fetal development, common lymphoid progenitors (CLPs) colonize the thymus epithelial rudiment and develop into thymocytes [1].…”

L’épithélium thymique, un passé dans la dualité et un présent unifié

“…Il a été suggéré que le cortex se développe à partir d'une cellule souche d'origine ectodermique tandis que la médulla dérive de l'endoderme [5]. D'autres travaux suggèrent que les précurseurs de ces deux types cellulaires ont pour origine le seul endoderme [6]. Les travaux de Gill et al, en Australie, ont permis une avancée significative en identifiant un progéniteur épithé-lial foetal, exprimant la glycoprotéine MTS24, capable de générer les lignées corticale et médullaire [7].…”