The placenta is an essential organ that develops during pregnancy. It forms in the uterus and serves as a link between the mother and the developing fetus. The placenta’s main functions include supplying oxygen and nutrients to the fetus, removing waste products, and producing hormones that help sustain the pregnancy. It typically attaches to the uterine wall and connects to the fetus via the umbilical cord. The placenta plays a crucial role in supporting fetal growth and development throughout pregnancy.


Endometrial Changes in Pregnancy

The formation of decidua and the decidual reaction are critical processes in the early stages of pregnancy that involve significant changes in the endometrial lining (endometrium) of the mother’s uterus. These changes are essential for the successful implantation and development of the embryo. Let’s explore these changes in detail:

  1. Menstrual Cycle Preparation: Before discussing decidua and the decidual reaction, it’s important to understand that the endometrium undergoes cyclical changes throughout the menstrual cycle. During the menstrual phase, the superficial layer of the endometrium is shed in response to hormonal signals.
  2. Proliferative Phase: Following menstruation, the proliferative phase begins under the influence of estrogen. During this phase, the endometrium thickens and regenerates, preparing for potential embryo implantation.
  3. Secretory Phase: After ovulation, during the secretory phase, the endometrium becomes more vascularized and glandular under the influence of both estrogen and progesterone. This phase is when the endometrium is most receptive to embryo implantation.
  4. Formation of Decidua: Once a fertilized egg (zygote) successfully implants into the thickened and receptive endometrial lining, changes occur to support and protect the developing embryo. These changes lead to the formation of decidua, which is a specialized type of endometrial tissue.
    • Decidua Basalis: This forms at the site of embryo implantation, and it is crucial for anchoring the embryo to the uterine wall. It develops from the maternal endometrial tissue.
    • Decidua Capsularis: This surrounds and covers the embryo, providing a protective layer. It develops from the maternal endometrial tissue.
    • Decidua Parietalis or Decidua Vera: This is the rest of the endometrial lining outside the immediate implantation site and plays a role in supporting the growing pregnancy.
  5. Decidual Reaction: The decidual reaction refers to the structural and functional changes in the decidua in response to pregnancy. This includes:
    • Hypertrophy and Hyperplasia: Decidual cells enlarge and multiply, increasing in size and number to support the growing embryo.
    • Increased Vascularity: Blood vessels in the decidua become more prominent to ensure an adequate blood supply to the developing placenta.
    • Secretory Activity: Decidual cells become secretory in nature, producing substances that nourish and protect the embryo, such as glycogen and lipids.
    • Immune Tolerance: The decidual tissue helps create an immune-privileged environment, preventing the mother’s immune system from attacking the developing embryo as it contains both maternal and paternal genetic material.

In summary, the endometrial changes leading to the formation of decidua and the decidual reaction are crucial for a successful pregnancy. These changes create a nurturing and protective environment for the developing embryo, ensuring its implantation and early development are supported while maintaining immune tolerance to prevent rejection by the mother’s immune system.


Types of Chorionic Villi

Chorionic villi are finger-like structures that form in the placenta during pregnancy. They play a crucial role in exchanging nutrients and oxygen between the mother and fetus. There are two main types of chorionic villi:

  1. Primary Chorionic Villi: These are the earliest structures to develop in the placenta. They begin to form around the end of the first week after fertilization. Primary chorionic villi consist of an outer layer of syncytiotrophoblast cells and an inner core of cytotrophoblast cells. They penetrate into the uterine wall and establish connections with the maternal blood vessels to facilitate nutrient and gas exchange.
  2. Secondary Chorionic Villi: Secondary chorionic villi develop from the primary villi around the end of the second week of pregnancy. They contain an additional layer of mesodermal tissue, which gives rise to blood vessels and connective tissue. This mesodermal core is surrounded by the same syncytiotrophoblast and cytotrophoblast layers as the primary villi. Secondary chorionic villi enhance the exchange of nutrients and waste products between the maternal and fetal circulations due to the presence of blood vessels.

The development of chorionic villi is a dynamic process, and they continue to evolve throughout pregnancy. As pregnancy progresses, tertiary chorionic villi can develop, which are characterized by more complex branching patterns of the villous tree. These structures further increase the surface area for efficient nutrient and gas exchange.

It’s important to note that the proper development and function of chorionic villi are essential for a healthy pregnancy, as any abnormalities or complications in their formation can lead to pregnancy-related issues or fetal development problems.


Placenta Development: Fetal & Maternal

The development of the placenta is a complex and crucial process in pregnancy, consisting of both fetal and maternal components. Here’s a detailed overview of the development of each part:

Fetal Part of the Placenta:

  1. Chorionic Villi Formation: The development of the fetal part of the placenta begins shortly after fertilization. After the blastocyst implants into the uterine wall, the outer layer of cells, known as the trophoblast, forms finger-like projections called chorionic villi.
  2. Placental Disk Formation: Over the next few weeks, the chorionic villi multiply and differentiate into two main types: cytotrophoblasts and syncytiotrophoblasts. The syncytiotrophoblast layer eventually forms the outermost covering of the chorionic villi and plays a crucial role in nutrient and gas exchange.
  3. Blood Vessel Development: The fetal circulatory system develops, including the umbilical arteries and vein. These vessels extend into the chorionic villi, connecting the developing fetus to the placenta.

Maternal Part of the Placenta:

  1. Decidua Formation: Concurrently with the fetal development, the maternal part of the placenta begins to form. The uterine lining undergoes changes and is referred to as the decidua. The decidua forms the maternal side of the placenta.
  2. Uteroplacental Circulation: As the chorionic villi continue to develop, they come into close proximity with the maternal blood vessels within the decidua. Specialized structures called spiral arteries in the decidua supply maternal blood to the intervillous spaces (spaces between the chorionic villi).
  3. Exchange of Nutrients and Gases: The syncytiotrophoblasts of the fetal chorionic villi facilitate the exchange of oxygen, nutrients, and waste products between the fetal and maternal bloodstreams through a process called diffusion. This ensures that the developing fetus receives the necessary nutrients and oxygen while eliminating waste products.
  4. Hormone Production: The placenta also produces hormones, including human chorionic gonadotropin (hCG), human placental lactogen (hPL), and progesterone, which are vital for maintaining pregnancy and supporting fetal development.

Throughout pregnancy, the placenta continues to grow and adapt to the increasing needs of the developing fetus. It plays a critical role in nourishing and protecting the fetus while also serving as an endocrine organ that helps regulate various aspects of pregnancy. After childbirth, the placenta is expelled from the uterus, marking the end of its temporary but vital role in supporting fetal development.


Functions of placenta


Placental Circulation

Placental circulation is a crucial aspect of fetal development during pregnancy, as it ensures the exchange of nutrients, oxygen, and waste products between the mother and the developing fetus. It involves a complex network of blood vessels and structures within the placenta, a temporary organ that develops in the uterus during pregnancy.

Here is a detailed description of placental circulation:

  1. Placenta Structure:
    • The placenta is a disc-shaped organ that attaches to the uterine wall, providing a direct connection between the maternal and fetal circulatory systems.
    • It consists of maternal and fetal components. The maternal side, called the decidua basalis, interfaces with the mother’s uterine tissue, while the fetal side, the chorionic plate, interfaces with the developing fetus.
  2. Maternal Blood Supply:
    • Maternal blood is supplied to the placenta through the uterine arteries, which branch out and penetrate into the placental tissue.
    • Once inside the placenta, maternal blood flows into specialized structures called intervillous spaces, which are surrounded by finger-like projections known as chorionic villi.
  3. Fetal Blood Supply:
    • The fetal circulatory system includes the umbilical cord, which contains two umbilical arteries and one umbilical vein.
    • The umbilical arteries carry deoxygenated blood and waste products away from the fetus and into the placenta.
    • The umbilical vein carries oxygenated blood and nutrients from the placenta to the fetus.
  4. Exchange of Substances:
    • Within the placental villi, there is a thin membrane separating maternal and fetal blood, allowing for the exchange of substances.
    • Oxygen and nutrients from the maternal blood diffuse into the fetal blood through this membrane, while carbon dioxide and waste products from the fetus diffuse into the maternal blood.
    • This exchange ensures that the fetus receives the oxygen and nutrients it needs for growth and development while removing waste products.
  5. Oxygenation:
    • Oxygenated maternal blood from the uterine arteries is delivered to the placenta, where it releases oxygen to be taken up by the fetal blood.
    • Deoxygenated fetal blood, rich in carbon dioxide, is transported to the placenta and releases carbon dioxide into the maternal blood.
  6. Hormonal Regulation:
    • Hormones like human chorionic gonadotropin (hCG) and placental lactogen play a role in maintaining the placental circulation and supporting pregnancy.
    • These hormones help ensure a steady blood supply to the placenta and facilitate the maternal-fetal exchange of substances.

In summary, placental circulation is a dynamic system that enables the exchange of oxygen, nutrients, and waste products between the maternal and fetal bloodstreams. This exchange is vital for the well-being and growth of the developing fetus throughout pregnancy.