Development of Ig-synthesizing cells in the bursa of chick embryo was studied by immunohistochemical staining method as well as by in vitro incorporation of leucine-3H into Ig. Ig-synthesizing cells are first detected in the bursa of a 14 day old chick embryo and increase with the maturation of the embryo. Acrylamide gel analysis of leucine-3H-labeled Ig shows that synthesis of nonsecretory IgM-H0 precedes that of secretory IgM-H, reflecting an ontogenetic sequence of development of lymphoid cells synthesizing IgM.
Since IgM-H0 is not secreted, we further studied biochemical differences between two heavy chains. The difference is attributable to lack of galactose attachment to H0 chains. It is proposed that during differentiation of lymphoid cells synthesizing and secreting Ig, attachment of galactose may play an obligatory role in the development of cellular capacity for Ig secretion.