Placental morphology

After it enters the uterus, the human blastocyst resides within the uterine cavity for 2–3 days prior to implantation into the decidualized uterine endometrium (Chapter 16). Implantation can be divided into three distinct processes: apposition of the blastocyst to the endometrial epithelium at the site of implantation, a brief period of stable adhesion of the blastocyst to this epithelium and invasion of the developing embryo into the uterine decidua. The signals governing these processes are complex and involve active maternal and fetal participation. Apposition requires the secretion of soluble mediators by uterine epithelia and the blastocyst that include interleukins, prostaglandins and leukemia inhibiting factor (LIF). Adhesion is promoted by the expression of ligands on the surface of the developing embryo that specifically bind to receptors expressed on the uterine lining at the site of implantation. One receptor–ligand pair that has been implicated in embryo adhesion is heparin-binding epidermal growth factor and heparin sulfate proteoglycans. Also important in the adhesion process is a family of adhesion molecules expressed on uterine epithelia in a time-specific and hormone-dependent fashion: the integrins. Invasion of the blastocyst into the maternal uterine decidua requires an alteration in the expression of embryonic surface molecules, from those promoting adhesion to the endometrium to others that stimulate invasion of vascular structures. During invasion, the embryo also begins to secrete proteases that digest between the cells of the decidua and allow invasion to areas deep within the uterine lining.

The blastocyst is comprised of two populations of cells (see Fig. 16.1): one will become the fetus, the other, the placenta. At the blastocyst stage, the embryo is characterized by a fluid-filled cavity (the blastocele) surrounded by a layer of trophectoderm cells. The trophectoderm will develop into the placenta. Within the trophectoderm shell is a collection of cells called the inner cell mass. All nonplacental fetal tissues will arise from the inner cell mass.

During implantation, trophectoderm cells begin to differentiate into cellular subtypes that will characterize the mature placenta. The mature placenta is comprised of a mass of tree-like placental cotyledons called villae, which are bathed in maternal blood (Fig. 17.1). Blood enters the space between the villae through low-resistance, high-flow vessels that branch from the maternal uterine spiral arteries. Fetal vessels are located within the core of each placental cotyledon. Loose connective tissue and layers of trophoblast cells surround each fetal vessel. The inner layers of the trophoblast shell around the fetal vessels are comprised of cytotrophoblast cells. The outer layer of the placental villae is coated by a multinucleated syncytium of fused cytotrophoblast cells called the syncytiotrophoblast. The syncytiotrophoblast is directly bathed in maternal blood in the mature placenta.

The vast majority of placental cotyledons/villae float freely in the intervillous space. Here, they are surrounded by maternal blood. A subset of villae completely cross the intervillous space and attach to the maternal uterine decidua. These are called anchoring villae. At the maternal end of the anchoring villae, a subset of cytotrophoblast cells change their phenotype and invade deep into the uterine lining. These extravillous cytotrophoblast cells come into direct contact with the maternal decidual immune cells that include a small number of B lymphocytes, a moderate number of T lymphocytes and macrophages, and a voluminous number of natural killer cells. A subset of extravillous cytotrophoblast cells will invade directly into the maternal vasculature. These are the endovascular cytotrophoblast cells. They replace the endothelial cells of maternal vessels and transform the decidual vessels into the low-resistance, high-flow structures characteristic of a healthy, mature placenta.

Like the fetus, the placenta grows in size throughout pregnancy. At term, it is a large, beefy structure. Its maternal surface is covered with numerous cotyledons containing millions of villae. The fetal surface is covered with a smooth, glistening membrane called the amnion. The umbilical cord inserts almost perpendicularly into the fetal surface of the placenta.