Haemoglobin is a well-described gas transport protein commonly found in erythrocytes, however, non-erythroid tissues, such as cancer cells, also express haemoglobin mRNA and protein. We previously published that granulosa and cumulus cells from murine ovarian antral follicles express haemoglobin mRNA and protein, which are hormonally regulated over the peri-ovulatory period. In this study, we investigated the gene expression of haemoglobin subunits and mediators of oxygen-carrying capacity in the early embryo, comparing in vivo to in vitro development.
Pre-pubertal CBAF1 female mice were treated with 5IU eCG/5IU hCG and mated. Embryos were collected 44, 54, 86 and 92 h post-hCG treatment, corresponding to the 2-cell, 4-cell, morula and blastocyst stage. For in vitro experiments, cumulus-oocyte complexes were collected 16 h post-hCG, and in vitro fertilisation and embryo culture carried out. Embryos were collected 20, 42, 55, 92 h post-in vitro culture, corresponding to the same stages.
RT-PCR revealed, for the first time, high expression of Hba-a1 and Hbb at the 2-cell stage in vivo compared to in vitro expression, which increased at the 4-cell stage, and declined to near undetectable levels by the morula stage; suggesting Hba-a1 may be switched on at the 4-cell stage and degraded at the morula stage. Haptoglobin (Hp) and 2,3-bisphosphoglycerate mutase (Bpgm) were virtually undetectable in vivo and in vitro. The function of haemoglobin within in vivo embryos remains unknown, but we propose that sequestering gases, particularly oxygen, could allow the embryo to survive in the low oxygen environment of the female reproductive tract.