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What Is the Placenta? - Definition, Development & Function

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  • 0:01 Definition of the Placenta
  • 0:20 The First Six Days
  • 1:58 Development: Placental…
  • 3:25 Function of the Placenta
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor: Derrick Arrington

Derrick has taught biology and chemistry at both the high school and college level. He has a master's degree in science education.

As a new life forms inside a mother it must have a means of obtaining nourishment. In this lesson we will examine the development and function of the placenta, which provides that nourishment. Our goal is to gain an understanding of this crucial factor in reproduction.

Definition of the Placenta

The placenta is a flattened circular organ in the uterus of pregnant mammals that nourishes and maintains the fetus through the umbilical cord. This cord is the main link from the fetus to the placenta. Through it, the placenta provides oxygen and nutrients to the growing baby and removes waste products.

The First Six Days

Shortly after an egg is fertilized by a sperm cell, a zygote is formed. This zygote results from the union of the nuclei of the sperm and egg. It automatically begins a process of internal cellular division producing a hollow ball of cells known as a morula. The morula moves from the place of fertilization through the fallopian tube to the uterus. The process from fertilization to implantation normally takes three days.

Once in the uterus, the morula begins filling with fluid and becomes known as a blastocyst, an embryo consisting of approximately 200 cells. The blastocyst consist of three main parts. The outer layer of cells of the blastocyst are known as trophoblast cells and will form the placenta and fetal membranes. The mass on the other end of the blastocyst will form the embryo, and the center area becomes a fluid filled cavity.

After another day, the cells begin to multiply quickly and the fluid cavity enlarges. This process is often referred to as hatching. During this time, the mass is bathed in secretions from the uterus that provide the growing mass with oxygen and other metabolic minerals. These secretions are only able to sustain the mass for approximately twenty-four hours.

At this point, the blastocyst moves to an optimal place in the uterus and attaches to the lining of the uterus. This happens approximately six days after fertilization. The uterus lining provides the blastocyst with access to other substrates needed for growth, such as stroma cells that are filled with nutrient rich glycogen.

Development: Placental Stem Cells

With the beginning of the process of gestation, placental stem cells are formed. These are known as progenitor cytotrophoblast cells. These cells differentiate, or change, to form the placenta. Some of the cells form the outer layer of the placenta while others form the inner layer. The outer layer cells turn into a form of epithelium, almost like skin cells, that function to transport gases, nutrients, and wastes.

They also make hormones that guide fetal, placental, and maternal systems throughout the pregnancy. The cells begin penetrating the lining of the uterus to connect the blood supply of the mother. The inner layer of stem cells turn into arteries and muscle cells that later form uteroplacental arteries and veins that will connect the blood supply of the mother to that of the fetus. By three weeks after fertilization, the first evidence of blood circulation from the mother to the fetus can be detected.

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