Fig. 5.

The process of implantation (including invasive implantation with decidualization and non-invasive implantation of non-decidualizing species) a: Zygote; b: 2 cells; c: 4 cells; d: 8 cells; e: Morula; f: Blastocysts; g: Endometrium; h: Uterine cavity; i: Trophoblast cells; j: Microvilli. Estradiol (E2) produced by the developing ovarian follicles interacts with progesterone produced by the CL to prepare the endometrium receptivity necessary for embryo implantation. The meeting of the oocyte and sperm and subsequent fertilization occur in the ampulla of the oviduct, followed by early embryo development within the oviduct, and the morula migrates to the uterus, where implantation occurs. The appearance of a fluid-filled inner cavity (blastocoel) is accompanied by cellular differentiation: the surface cells become the trophoblast and give rise to the extra-embryonic tissues, including the placenta, while the inner cell mass gives rise to the embryo and finally shedding of the zona pellucida, followed by orientation, apposition, attachment and adhesion of the blastocyst to the endometrium. If the blastocyst was not present, the CL would regress, and the uterus would start the cycle again. The time and chronological events of implantation differ among mammalian species irrespective of the length of gestation. In contrast to humans, horses, primates and rodents, in which implantation occurs shortly after the hatching of the blastocyst, the blastocyst in domestic ruminants and pigs elongates before implantation (the time to implantation: in pigs, the 14th day; in sheep, the 16th day; and in cattle, the 18th day), and this unique developmental event does not occur in the laboratory or in rodents or humans