Successful conception requires that the embryo implant in the uterus. Implantation occurs about six to seven days after fertilization, when the embryo arrives at the uterus. During its transit in the Fallopian tube, the early embryo remains contained within the zona pellucida. The zona pellucida protects the early embryo and prevents implantation in the Fallopian tube. An important step that allows implantation is the release of the embryo from the zona pellucida or “zona hatching”.
Before implantation, the embryo undergoes cleavage, that is, cell divisions without growth. The implantation stage embryo is known as a blastocyst. A blastocyst consists of roughly 70-100 cells, which have differentiated into two cell types:
The trophoblast will give rise to extra-embryonic structures, notably the chorion, which is the fetal part of the placenta. The inner cell mass will give rise to the embryo.
Once the embryo is released from the zona pellucida, it very readily implants because the trophoblast is quite sticky. Implantation is initiated when the trophoblast adheres to the surface of the endometrium. This stimulates proliferation of the trophoblast cells, which will divide into two parts: a syncytial trophoblast and a cellular trophoblast. The syncytial trophoblast is a syncytium, meaning there are multiple nuclei but no cell membranes.
The syncytial trophoblast is invasive, secreting proteolytic enzymes that allow the embryo to penetrate into the endometrium. Digestion of the endometrial tissue creates spaces known as lacunae. The proliferating cellular trophoblast forms branched structures, known as chorionic villi (singular: chorionic villus). Note that the developing embryo pulls away from the wall of the proliferating disc to create a new space. This will eventually develop into the amniotic cavity, a fluid-filled space that will surround the developing fetus.
The syncytial trophoblast produces the hormone chorionic gonadotropin. Detection of this hormone in the urine is the basis for pregnancy tests. Chorionic gonadotropin is an analogue of luteinizing hormone (LH), meaning it binds and stimulates the LH receptor. Chorionic gonadotropin is necessary to stimulate and maintain the corpus luteum, which is the primary source of estrogen and progesterone in the first trimester of pregnancy.
The placenta forms from the trophoblast and the endometrium (during pregnancy the endometrium is known as the decidua). The placenta becomes established roughly 5 weeks after implantation. The fetal part of the placenta, or chorion, is made up of the chorionic villi, which become highly branched structures containing blood vessels just under the trophoblast layer. The villi protrude into large spaces (lacunae) in the decidua. Maternal blood flows into the lacunae and washes over the villi, allowing for the exchange of nutrients, wastes, and blood gases between the mother and fetus. The placenta also makes hormones, becoming the source of estrogen and progesterone after 8 weeks of pregnancy.