The human embryonic haemoglobins were discovered in 1961. These include Hb-Gower 1, consisting of 2 zeta chains and 2 epsilon chains, and Hb-Gower 2, which consists of 2 αlpha-chains and 2 epsilon-chains, the zeta and epsilon chains being the embryonic haemoglobin chains. Embryonic hemoglobin is a

tetramer A tetramer () ('' tetra-'', "four" + '' -mer'', "parts") is an oligomer formed from four monomers or subunits. The associated property is called ''tetramery''. An example from inorganic chemistry is titanium methoxide with the empirical formula ...

produced in the blood islands in the embryonic

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is ...

during the mesoblastic stage (from 3rd week of pregnancy until 3 months). The protein is commonly referred to as hemoglobin ε.

Chromosomal abnormalities A chromosomal abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder, is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities, where the ...

can lead to a delay in switching from embryonic hemoglobin.

Hemoglobin Gower 1

Hemoglobin Gower 1 (also referred to as ζ₂ε₂ or HbE Gower-1) is a form of hemoglobin existing only during embryonic life, and is the primary embryonic hemoglobin. It is composed of two zeta chains and two epsilon chains, and is relatively unstable, breaking down easily.

Hemoglobin Gower 2

Hemoglobin Gower 2 (also referred to as α₂ε₂ or HbE Gower-2) is a form of hemoglobin existing at low levels during embryonic and fetal life. It is composed of two alpha chains and two epsilon chains, and is somewhat unstable, though not as much as hemoglobin Gower 1. Due to its relative stability compared to hemoglobin Gower 1 and

hemoglobin S Sickle cell disease (SCD) is a group of blood disorders typically inherited from a person's parents. The most common type is known as sickle cell anaemia. It results in an abnormality in the oxygen-carrying protein haemoglobin found in red blo ...

, it has been proposed as a subject for reactivation in the adult in cases of severe β thalassemia and hemoglobinopathies in subjects for which the reactivation of

hemoglobin F Fetal hemoglobin, or foetal haemoglobin (also hemoglobin F, HbF, or α2γ2) is the main oxygen carrier protein in the human fetus. Hemoglobin F is found in fetal red blood cells, and is involved in transporting oxygen from the mother's bloodstream ...

is contraindicated due to toxicity concerns.

Hemoglobin Portland I

Hemoglobin Portland I (also referred to as ζ₂γ₂ or HbE Portland-1) is a form of hemoglobin existing at low levels during embryonic and fetal life, composed of two zeta chains and two gamma chains.

Hemoglobin Portland II

Hemoglobin Portland II (also referred to as ζ₂β₂ or HbE Portland-2) is a form of hemoglobin existing at low levels during embryonic and fetal life, composed of two zeta chains and two beta chains. It is quite unstable, more so than even hemoglobin Gower 1, and breaks down very rapidly under stress. Despite this, it has been proposed as a candidate for reactivation in cases of severe α thalassemia or hemoglobinopathies afflicting the alpha chain.

Table

References

Hemoglobins {{protein-stub