THE OCCURRENCE OF FOETAL ERYTHROPOIESIS AFTER INFANCY BY

In a previous study the change from foetal to adult erythropoiesis, as reflected in the peripheral blood, was followed up to the age of 32 weeks (Fraser and Raper, 1962). Before that age chemi-cally-estimated foetal haemoglobin (Hb F) had reached the adult level, and after that age foetal cells, demonstrated by their partial resistance to an acid buffer, had fallen to less than 1 % of the red cells. An explanation was offered in terms of a progressive maturation of gene-controlled enzyme systems, whereby production of the y-polypeptide chains of Hb F was superseded by that of the,B-chains of adult haemoglobin (Hb A). But it was recognized that the process was not synchronous in all the erythropoietic cells. After early infancy, Hb F can be found in the blood in a variety of circumstances: (1) In the presence of a clearly inherited abnormality, the 'high-F ' gene (Conley, Weatherall, Richardson, Shepard and Charache, 1963): this will not be further considered here, except in comparison with other conditions. (2) In certain anaemias that begin in early infancy. (3) In severe acquired disturbances of haemopoiesis, leukaemia, aplasia, myelosclerosis, and megaloblastic anaemias (Beaven, Ellis and White, 1960). Amongst the anaemias that begin in early infancy, a raised Hb F content of the blood is most often found in the haemoglobino-pathies, especially those that are caused by an abnor-mality in the,-polypeptide chain of the haemo-globin molecule, such as thalassaemia, sickle-cell anaemia, and some of their variants. It is a reason-able hypothesis that Hb F appears in these anaemias because they begin concurrently with the normal change from foetal to adult erythropoiesis. Stated in chemical terms the suggestion is that, especially when haemoglobin turnover is greatly increased, the limiting factor is the supply of P-polypeptide chains, and that the deficit can be made good by a continued use of the y-chain pathway; and it must be supposed that this pathway will be more effectively used if it is kept open from infancy onwards, rather than if it is required to open again in adult life (Zuelzer an