Embryonic Haemoglobins

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.

Haemoglobin is a tetramer of two α-like and two β-like globin chains. The genes of the α-like globins are found on chromosome 16 (16p13.3) and those for the β-like genes are found on chromosome 11 (11p15.5). The α-like has seven genes ζ2, Ψζ1, μ, Ψα, α2, α1, θ. ζ2 encodes for the ζ chain found in embryonic haemoglobins , α2, α1 encode for alpha chains, Ψζ1 and  Ψα are pseudogenes and the function of μ and θ is unknown. the β-like gene cluster consists of six genes ε, Gγ, Aγ, Ψβ, δ and β. ε gene encodes for the ε chain of haemoglobin Gower I and Gower II, Gγ and Aγ encode for γ chains of HbF, Ψβ is a pseudogene and δ and β encode fro chains found in HbA2 and HbA respectively (α2δ2 is HbA2 and not Hb Gower I that is erroneously shown in the diagram).

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)

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