Lymphocyte formation in the thymus of the embryonic chick

Abstract

Abstract Embryonic thymus provides a distinct advantage over the definitive thymus because the earliest formation of lymphocyte and lymphocytic precursors can be observed in a relatively simple and uncomplicated situation. Special cytological techniques combined with light, phase, and electron microscopy have been performed in this investigation involving over 150 chick embryos between 5 and 18 days of embryonic development. Sequential cytological changes in the development of the thymus into an active lymphocytopoietic organ indicate that lymphoblasts develop by the gradual proliferation and transformation of “undifferentiated” epithelial cells comprising the primordial thymus. Lymphoblastic transformation begins on the seventh day of embryonic development in the chick and is characterized by increased cytoplasmic and nucleolar basophilia and chromatin condensation. “Undifferentiated” epithelial cells undergo two distinct lines of differentiation between the seventh and tenth days: into lymphoblasts and into stellate reticular‐epithelial cells which constitute the organ parenchyma. All stages of lymphocytic maturation may be observed by 10–11 days as the thymus assumes a predominately lymphocytic character. Absence of lymphocytes or lymphocytic precursors in the connective tissue surrounding the embryonic thymus before and during the period of initial lymphoblastic transformation; presence of a continuous basement membrane surrounding the developing thymus; and absence of cells passing through this membrane during this phase of development indicate that the lymphocytic elements appearing in the embryonic thymus parenchyma are of epithelial rather than mesenchymal derivation. Although a contribution of mesenchymal elements to the lymphocytic population via vascular invasion and lobular formation in later stages of thymic development is unlikely, this question cannot be answered at this time. In spite of functional immunological distinctions between the lymphocytes of the chick thymus and bursa of Fabricius, the similarity of origin of the lymphocytic elements of these lympho‐epithelial organs is apparent.