The Placenta—an Unappreciated Organ
Of all the body’s organs, perhaps the one that best exemplifies selfless service is the placenta. Most people give little thought to the placenta, and few appreciate the marvelous complexity and profound importance of this organ that is discarded and forgotten after birth.
After the egg is fertilized, the placenta is the very first organ to develop. Recent studies show that when the fertilized egg divides to form the first two cells, one is already destined to form the placenta, while the other becomes the baby.
An Important Hormone-Producing Gland
As soon as three days after fertilization—weeks before the mother normally suspects she is pregnant—cells of the developing placenta, called trophoblasts, begin to produce hormones. These hormones ensure that the lining of the uterus—the endometrium—will be ready to receive the embryo’s implantation. Over the next few weeks, the developing placenta begins to make hormones that control the mother’s physiology in a way that ensures the proper supply of nutrients and oxygen, which are essential to the baby’s growth.
By about five days after fertilization the trophoblast cells, surrounding the developing embryo, begin to fuse together to form one giant cell with many nuclei (see Figure 1). This cell is called the syncytial trophoblast. One of the first functions of this placental giant cell is to invade the uterine wall of the mother in an amazing process called implantation (see Figure 2).
Prevents the Rejection of the Baby as a Foreign Graft
Although the developing placenta and baby implant within the thick nutrient-filled wall of the mother’s uterus, they are not actually part of the mother’s body. One of the placenta’s important roles is to protect the developing baby from an attack by the mother’s immune system, since the baby and the placenta are genetically unique and distinctly different from the mother.
It is still a mystery how the placenta prevents the mother from rejecting it and the baby as a foreign graft without shutting down her immune system.
After implantation, the placental giant cell “invades” the walls of several uterine arteries and veins, causing the mother’s blood to flow through channels within the cell (see Figure 3). When the baby develops its own blood and blood vessels, the mother’s blood and the blood of the developing baby come into close association, but they never mix or come into direct contact. The syncytial trophoblast forms a thin, seamless, and selective barrier between maternal blood and fetal blood. All the critically important nutrients, gases, hormones, electrolytes, and antibodies that pass from mother’s blood to the baby’s blood must travel across this seamless and selective filter. Waste products in the baby’s blood must, in turn, pass across this filter to the mother’s blood.
The Placenta Does It All!
In order to appreciate the marvelous work of the placenta, consider this: while the unborn baby’s vital organs are developing and maturing, they (with the exception of the heart) are essentially useless. The placenta serves the functions of these organs by working in association with the mother. With the help of the mother’s blood, the placenta must function as the baby’s lungs, kidneys, digestive system, liver, and immune system. The placenta does this so well that a baby can actually survive until birth even when one or more of these vital organs sadly fail to develop in its own body.
During the later part of pregnancy, the flow of maternal blood through the placenta reaches a rate of about one pint (.5 liter) per minute. To allow an adequate surface area for exchange between mother’s blood and baby’s blood, the interface between the two is folded and amplified in a complex way that resembles the trunks, branches, and twigs of trees (see Figure 4). There are typically about 20 of these tree-like structures (called cotyledons) in the mature placenta. The baby’s blood flows in vessels inside these cotyledons, whereas the mother’s blood flows all around the outside, like air blowing through a small grove of trees. The entire surface of all the tree-like cotyledons is covered by syncytial trophoblast, forming a seamless covering, which comprises a single cell with millions of nuclei (see Figure 5). This means that the entire surface of the placenta is covered by one giant cell, which has a surface area of over 100 square feet (9.3 square meters).
The Zygote:
This single cell is now either male or female. This human is unique, i.e., never before in the history of the world has this exact individual human existed. Never again in history will another exactly like this human exist.
This being is complete, i.e., nothing else — no bits or pieces — will be added from this time until the old man or woman dies — nothing but nutrition and oxygen. This being is programmed from within, moving for-ward in a self-controlled, ongoing process of growth, development, and replacement of his or her own dying cells.