Figure 3.

Preimplantation period. (1) After interacting with GH, most likely secreted from the oviduct, the GHR is translocated to the nucleus of the zygote, promoting the transcription of genes involved in cells proliferation. GH induces the expression of the glucose transporter Glut 1, which leads to glucose uptake by the zygote therefore providing the substrate needed for the production of the energy necessary for beginning proliferation processes shown in (2) It is unclear if IGF-I is secreted from the oviduct in 1 or 2. (3) While the newly formed embryo is progressing along the oviduct to the uterus, there is a clear secretion of GH and IGF-I from the oviduct walls; IGF-I receptor appears now together with GHR in the membrane of cells constituting the morula, facilitating cell proliferation. (4) In a further step, the blastocyst is formed and their cells begin to express GH, GHR, IGF-I, and IGF-I R. This precedes the implantation in the uterus.