Embryonic haemglobins are haemoglobins produced in the yolk sac stage of erythropoiesis. There are three embryonic haemoglobins Gower I, Gower II and Portland. Embryonic haemoglobins like their adult counterparts are tetramers made of two α-like (α and ζ)chains and two β-like chains(γ and β). The α-chain is a part of haemoglobins from the embryonic stage and complete loss of α-globin leads to anaemia from early gestation. This not the case with the β chain.
The earliest evidence of erythropoiesis is found in the extra-embryonic yolk sac in the form of blood islands. The erythrocytes produced by the yolk sac are larger, nucleated and contain embryonic haemoglobins (see N Engl J Med 1999; 340-617 for images of embryonic erythropoiesis). The yolk sac is the site for haematopoiesis from 19 days through week 8 of gestation.
The embryonic haemoglobin show co-operative oxygen binding but to a lesser extent than the adult haemoglobin. This results in a high affinity which reflect in a lower P50. The P50 of haemoglobin Gower I is 4mm, Gower II is 12mm and Portland is 6mm. The values for foetal haemoglobin is 19mm and adult haemoglobin is 26mm.
Deletions of ζ and ε gene causing thalassaemia have been described. These deletions include the α (in case of ζ gene) or β (in case of ε gene). Symptoms associated with these deletions are attributable to deletions of either α or β genes. There are no symptoms attributable to the ζ and ε genes.
Detection of ζ chains in the blood has a role in diagnosis of α thalassaemia.An ELISA using monoclonal antibodies against the ζ-globin chain for detection of southeast asian type of Hb-Bart’s hydrops foetalis has been described (Am J Clin Pathol 2008;129:309-315)