Placentation: Videos & Practice Problems

The placenta is a vital temporary organ that facilitates the exchange of oxygen, nutrients, and waste between a mother and her developing fetus. It is formed from both maternal and fetal tissues and is expelled after childbirth. Throughout pregnancy, the placenta plays a crucial role by producing several hormones essential for maintaining the gestational period.

One of the first hormones produced by the placenta is human chorionic gonadotropin (HCG), which begins to be released around 10 to 12 days post-conception. This hormone is significant as it can be detected in modern pregnancy tests, indicating the presence of a developing embryo. In addition to HCG, the placenta also produces estrogen, progesterone, placental lactogen, and relaxin. The production of these hormones starts more gradually and their levels fluctuate throughout pregnancy, contributing to various physiological changes in the mother.

Placentation refers to the process of forming the placenta, which begins during the implantation phase and continues throughout the fetal development period. By the time of delivery, typically around 40 weeks, the placenta can weigh between 1 to 2 pounds, indicating its substantial growth. An interesting aspect of the placenta is its appearance; it is rich in blood vessels, giving it a deep red color similar to that of a liver. This organ remains connected to the baby via the umbilical cord until it is cut after birth.

The placenta plays a crucial role during pregnancy, primarily facilitating the exchange of nutrients and gases between the mother and the developing fetus, known as the conceptus. This organ is not a permanent structure; rather, it is temporary and is expelled after childbirth. Unlike the amniotic sac, which encapsulates the fetus, the placenta attaches to the uterine wall, ensuring that the fetus is nourished throughout the pregnancy. It remains intact during the entire fetal period, without dissolving or diminishing in function during the trimesters. Understanding the placenta's functions is essential for grasping the complexities of prenatal development and maternal-fetal interactions.

During the early stages of placentation, specifically around weeks 2 to 3, significant developments occur as the embryo implants into the uterine wall. The syncytiotrophoblast, a layer of cells from the embryo, plays a crucial role by releasing digestive enzymes that break down the endometrium, creating pools of maternal blood known as lacunae or intervillous spaces. This process is essential for establishing a connection between the mother and the developing embryo.

As implantation progresses, cells from the embryonic disc proliferate to form a layer of extraembryonic mesoderm. The term "extraembryonic" refers to structures that are outside the embryo itself. The extraembryonic mesoderm consists of mesodermal cells that surround the embryo, contributing to the formation of vital structures. The syncytiotrophoblast continues to erode the endometrium, facilitating the formation of these lacunae filled with maternal blood.

Over time, the extraembryonic mesoderm and syncytiotrophoblast develop into a structure called the chorion. The chorion acts as a pre-placental structure that encases the conceptus, eventually becoming the outer layer of the embryonic sac. This structure is critical for the embryo's development as it will later form projections known as chorionic villi.

The chorionic villi extend into the lacunae and serve as the primary site for nutrient, gas, and waste exchange between the mother and the conceptus. These projections are essential for the embryo's growth and development, ensuring that it receives the necessary resources while eliminating waste products. As development continues, more mature forms of these chorionic villi will be observed, further enhancing the efficiency of placentation.

The primary structure responsible for nutrient and gas exchange between the mother and the fetus is the chorionic villi, which are part of the placental structure. The placenta facilitates the exchange of essential substances, including nutrients, waste, and gases, between the maternal blood and the developing fetus. While the chorion serves as the outer layer of the embryonic sac, it is the chorionic villi that extend into the maternal blood pools, enabling this critical exchange. Therefore, the chorionic villi are essential for supporting fetal development by ensuring that the fetus receives the necessary nutrients and oxygen while also removing waste products.

During weeks 4 to 12 of embryonic development, significant changes occur in placentation, primarily involving the development of the chorionic villi and the endometrium. By week 4, the chorionic villi begin to extend and develop blood vessels that connect to the umbilical arteries and veins, establishing a crucial link between the embryo and the maternal blood supply.

As the pregnancy progresses from weeks 5 to 12, the endometrium undergoes extensive cellular changes to support the developing embryo. It is important to understand that the endometrium beneath the embryo contributes to the maternal portion of the placenta, while the endometrium facing the uterine cavity surrounds the embryo and the amniotic sac. This distinction is vital for understanding the structure and function of the placenta.

In week 4, the embryo is fully implanted within the uterine wall, with the amniotic sac and early umbilical cord forming. The umbilical cord contains two umbilical arteries and one umbilical vein, which are essential for nutrient and gas exchange. The chorionic villi, which contain networks of blood vessels, are embedded in intervillous spaces filled with maternal blood, facilitating the exchange of nutrients, waste, and gases between the mother and the conceptus.

As the embryo grows, the chorionic villi facing the uterine cavity become smaller and flatter due to pressure from the expanding amniotic sac. By week 12, the placenta is fully formed, serving as the primary source of nutrients and oxygen for the developing fetus while also removing waste products. The placenta's structure at this stage shows a flattened appearance on the side facing the uterine cavity, with the endometrial tissue completely surrounding the embryo and amniotic sac.

This intricate relationship between the maternal and fetal tissues is essential for a successful pregnancy, highlighting the importance of the placenta in supporting embryonic development.

The placenta plays a crucial role in fetal development, serving as the primary organ for nutrient, gas, and waste exchange between the mother and the fetus. By approximately 12 weeks post-conception, the placenta is expected to be fully functional, effectively supporting the fetus's needs. During the earlier weeks, specifically between 6 to 8 weeks, the placenta undergoes significant development, including the growth of chorionic villi and the formation of blood vessels within these villi. This maturation process is essential for establishing a robust connection between the mother and the developing conceptus. Therefore, the key takeaway is that by 12 weeks, the placenta is well-developed and capable of fulfilling its vital functions.