Placental Structure And Function
The human placenta is the sole interface between the mother and her developing embryo/fetus. Humans differ from most other mammals in that maternal blood comes into direct contact with fetally derived placental tissues. This organization characterizes the hemochorial placenta through which all maternal nutrients and fetal wastes must pass. The placenta is a very active organ that has specialized mechanisms to promote fetal growth and survival. These include, but are not limited to, efficient gas exchange, active transport of energy substrates, immunologic tolerance of the fetal allograft and fetal acquisition of maternal immunity.
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.