Читать книгу Genetic Disorders and the Fetus - Группа авторов - Страница 142

Evaluating the placenta requires an understanding of its unique structure and development. The chorionic villi that compose the placenta are organized into 50–70 distinct tree‐like structures that grow in a clonal manner outwards from the chorionic plate into the basal plate (which interface with the maternal decidua).¹ These villi are bathed in maternal blood, from which they sponge up nutrients important for fetal growth. The maternal blood is in direct contact with the outer trophoblast bilayer of the chorionic villi. This bilayer is made up of a multinucleated syncytium derived by fusion of the cytotrophoblast cells that form a single‐cell layer below the syncytium. In addition, some cytotrophoblasts form columns that migrate into and anchor the placenta to the uterine wall. Invasive cells that detach from these columns are termed extravillous trophoblasts (EVTs) and include the interstitial cytotrophoblasts (iCTBs) found in the decidual stroma and those that remodel maternal blood vessels, termed endovascular cytotrophoblasts (eCTBs).¹ The inner core of the villi is the chorionic mesenchyme, which includes structural components, and a mix of cells including fetal blood vessels, fibroblasts, pericytes, and Hofbauer cells (placental macrophages). These extraembryonic cells derive from the epiblast of the blastocyst, from which the fetus is also derived.

Placental size is strongly correlated with fetal size; however, there is considerable variation in placental size for any given birthweight.² The efficiency of the placenta depends on the surface area for exchange, thickness, and density of transporter proteins,³ and birthweight is highly associated with placental weight.⁴ Interestingly, mean placental size can vary between populations and even within a population over time because of changes in maternal nutrition or other environmental conditions.⁵