How placentas are created by retrovirus

Lodging in viruses

The greatest thing that can come from hate is love. But, l don't think you will ever love it. You so hate it, because all it does is cause pain, discomfort, and misery. But, not this time around. I may sound like a Shakespeare but without it, you wouldn't have been here. Imagine trekking to a place and not getting room to rest. How awful, so it is for a baby without a placenta. But, did you know that the placenta is created by a virus? Yeah, the virus you so hate.


The placenta is an organ that develops in the uterus during pregnancy. It's developed around eight to ten days after a fertile sperm fertilizes an egg. It attaches to the wall of the uterus and connects the baby to the mother through the umbilical cord. The placenta starts to develop as a few cells and grows larger and longer.  The placenta is the most diverse tissue across all mammals and without viruses, there will be no placentas.


Viruses as well known are microscopic entities that have either RNA or DNA as their genetic material and can only replicate in a living cell. The genetic material is covered by a protein called a capsid. Around the capsid is a spike that enables viruses to bind to the host and enters its cells. These viruses are called envelope viruses. Viruses replicate through attachment to the host cell, penetration, break down of the viral particles, replication, translation, assembly, and transportation of virions. But, there are classes of viruses called retroviruses that replicate the inverse-way of viruses.‌‌

Retroviruses are types of viruses that have RNA as their genetic material instead of DNA.

Retroviruses have genetically modified us by inserting viral genes into our chromosomes and expressed by fetal cells causing placentas to grow to allow extended gestation.


‌When infected with a retrovirus, a cell modifies the retroviral RNA into DNA, which is then inserted into the DNA of the host cell. The cell then produces additional retroviruses, which infect other cells. Now, the additional retrovirus produced becomes a footprint of the previous retroviral infection and is termed fossil viruses. This retrovirus becomes part of the human genome without performing any special function so they are termed " junk genes".

Again, the retrovirus is inherited by generation since it has become a part of the human genome. Mind you it's a virus, so the offspring of the proceeding generation naturally has the virus without retroviral infection. And we termed "endogenous retroviruses" which means retroviruses from within. An example is the human endogenous retroviruses( HERVs).

What we use to think of as junk genes are modulating our development unknowingly. Why do l say so? About eight percent of the human genome comes from ancient viruses which form part of the human endogenous retroviruses. The percentage of the human genome that does not encode for protein is termed junk genes/ DNA. It might be true that this junk gene has no function in the human system but how valid is it? What if is the one responsible for placental development? First of all, human endogenous retrovirus (HERVs) are ancient infectious viruses that affected the primate's genome long ago, over 100 million years, and became permanent elements at the interface between self and foreign DNA. The co-evolution of a human endogenous retrovirus with the host led to the activities of the retrovirus life cycle, providing novel cellular roles.‌‌For example, specified human endogenous retrovirus envelope proteins have been co-opted for pregnancy-related purposes.

The Origin of the placenta.

‌‌Once a month, female mammals release an egg from an ovary into a fallopian tube to be fertilized by sperm. After sexual intercourse, the fertilized egg divides continually as it moves to the uterus. First, the fertilized egg in the uterus becomes a solid ball of cells and then transforms into a hollow ball of cells called a blastocyst. Inside the uterus, the blastocyst ties up to the lining of the uterus - the process known as implantation. The cell wall of the blastocyst is very thick except in one area. The interior cells in the thickened region develop into the embryo and the outer cells carve into the wall of the uterus and develop into the placenta.

According to research, the placenta- is a defining characteristic of live-bearing mammals-rodents, cats and primates, etc. And before the placenta, you had to lay eggs if you wanted to reproduce. A baby had to be in an eggshell which is walled off with all the nutrients needed for the baby similar to a chicken egg, the yolk is a maintenance package that has to last the embryo till it's bred. So, the evolution of the placenta involved losing the eggshell and replacing that with some sort of organ that attaches to the mother's uterus during development.

However, it's challenging to lose that shell but since the maternal and fetal bloodstreams remain separate, the separation of the bloodstreams is done through a cell layer called the syncytiotrophoblast.‌‌The syncytiotrophoblast is a layer of cells that is fused to the uterus forming a wall. The protein that allows the cells to fuse to the wall of the uterus is called syncytin. And this syncytin didn't look like its human DNA but rather look more like HIV. According to research, this protein came from an ancient retrovirus.

Now, this is how it happened, long ago, some retrovirus infected an egg-laying vertebrate. The virus then settled into the animal's egg cells and got fertilized by a sperm. The baby that was hatched had copies of the virus's DNA in all its cells.

Fortunately, the virus didn't kill the baby but gave this offspring a premium feature. We all know that viruses fuse with things to infect them. So the viral DNA of the offspring makes a protein that fuses things.‌‌Once a viral protein, the virus evolved into what we now know as a syncytin-a protein that gives babies the placenta that connects their mother and them and keeps them separate too.

‌‌In a nutshell, the virus helped the mammalian ancestors survive by giving them a better placenta. Also, the piece of DNA was passed on to the next generation and eventually spread into the population. This is a process we know to be the molecular domestication of an ancient retrovirus element. And this continues to be essential for placental development in human beings.


1. "How the placenta evolved from an ancient virus — WHYY" retrieved from

2. "Stages of Development of the Fetus - Women's Health Issues - MSD Manual Consumer Version"



I am an artless lady who seeks to improve the health care of children in deprived areas with little knowledge l have. I love to write, motivate, and inspire people with academic challenges.