Environmental lighting controls temporal changes for organisms that live on earth. In mammals, the major biological clock is located in the suprachiasmatic nucleus (SCN). This major clock drives other minor biological clocks that are located in the peripheral tissues. In the dark phase of the circadian rhythm, the SCN signals the pineal gland to release indoleamine melatonin (N-acetyl-5 methoxytriptamine). During pregnancy, maternal melatonin crosses the placenta, signalling the day length for the fetus, and modulates fetal development [1–4].
In development, melatonin acts by means of the MT1 and MT2 membrane receptors (Mel1a and Mel1b for avian), mainly by inhibiting the adenylate cyclase enzyme . Due to the lipophilic nature of melatonin, it is able to function by binding to receptor in the nucleus , a cytosolic binding site the calmodulin protein  and also by modulating the cytosolic detoxifying enzyme NRH: Quinone Oxireductase 2 (NQO2). It does so through the MT3 melatonin binding site, which was showed to be NQO2 . Furthermore, it has been observed that melatonin modulates the myeloperoxidase enzyme . However, the most primitive mechanism of melatonin action is the direct scavenger function on reactive oxygen species (ROS), such as hydrogen peroxide (H2O2) and the super-oxide anion (O2-). This neurohormone also has an indirect antioxidant role thought to be triggered by membrane receptors and binding sites, mainly resulting in the up regulation of the antioxidant enzymes (glutathione peroxidase, glutathione reductase, gamma-glutamyl cisteine synthetase and glucose 6-phosphate deydrogenase) and the down regulation of the oxidant enzymes (NO synthase and lipoxygenases) [9, 10].
Furthermore, melatonin modulates mammalian reproduction [11, 12]. In the ovary, this hormone protects and stimulates folliculogenesis . Its synthesis takes place in both cumulus cells and oocytes, where it reaches high levels. In fact, ovarian follicular fluid contains melatonin in higher concentrations than in plasma. At these concentrations, melatonin has a protective action by acting as a direct free radical scavenger [14, 15]. Additionally, several effects of melatonin on the ovary are triggered by its membrane receptors that are also underlying its effects on photoperiodism, and on other events related to reproduction [16–21].
The direct free radical scavenger action of melatonin is very useful to the development of an embryo produced in vitro. It is important to note that species-specific concentration of the melatonin included in oocyte in vitro maturation (IVM) protocols increases the production of in vitro fertilized embryos in mice [22, 23], pig  and buffalo . The addition of melatonin is very useful as a sperm cryoprotective agent as well .
The addition of melatonin in IVM medium is justified because in vitro production (IVP) is strongly influenced by events before in vitro fertilization (IVF), particularly during IVM [27, 28]. The maturation process is where the oocyte acquires “competence” to assure proper zygotic development until the blastocyst stage . It has been showed that melatonin has a long-term effect on embryos, which is observed in cleavage and blastocysts formation rates. The efficacy of melatonin depends on culture conditions, such as O2 tension . This dependence of the O2 tension further suggests that melatonin influences in vitro embryogenesis by acting as a radical free scavenger. Others constituents of the IVM culture medium (i.e. FSH and LH)  can also influence the melatonin efficacy on oocyte maturation as well. Beside animal reproduction, melatonin has been used in human assisted reproductive technologies. To increase fertilization and pregnancy rates for female infertility due to poor oocyte quality, oocytes are treated with melatonin. This results from melatonin reduction of the toxic products caused by oxidative stress during oocyte maturation .
The aim of the study was to evaluate the effects of melatonin on bovine oocyte IVM by measuring the cleavage rates and blastocysts formation rates, as well as investigating the presence of the NQO2 enzyme, and MT1 and MT2 melatonin membrane receptors in bovine oocytes and blastocysts.