The human body is a marvelous thing. It has 78 organs working together and five vital organs that are essential for survival. One of the least understood organs that plays an important role in development is a temporary organ, the placenta. The placenta develops in a women’s uterus during pregnancy. It provides oxygen and nutrients to the growing baby and removes waste products from the baby’s blood. Although the placenta works to protect the mother and baby during pregnancy, new research from the Wellcome Sanger Institute, the University of Cambridge, and their collaborators have found evidence to support the theory of the placenta being a depot for genetic defects, whereas the fetus corrects or avoids these errors.

Their evidence has been published in the journal Nature, in an article titled, “Inherent mosaicism and extensive mutation of human placentas.” The findings will bring rise to further studies related to genetic aberrations and birth outcomes to better understand premature birth and stillbirth-related issues.

“Placentas can exhibit chromosomal aberrations that are absent from the fetus,” the researchers wrote. “The basis of this genetic segregation, which is known as confined placental mosaicism, remains unknown. Here we investigated the phylogeny of human placental cells as reconstructed from somatic mutations, using whole-genome sequencing of 86 bulk placental samples (with a median weight of 28 mg) and of 106 microdissections of placental tissue.”

The researchers discovered that each one of these biopsies was a genetically distinct clonal expansion.

“Our study confirms for the first time that the placenta is organized differently to every other human organ, and in fact resembles a patchwork of tumors,” explained Steve Charnock-Jones, PhD, a senior author of the study and professor at the University of Cambridge. “The rates and patterns of genetic mutations were also incredibly high compared to other healthy human tissues.”

The study is the first high-resolution survey of the genomic design of the human placenta.

“To our knowledge, unlike any other healthy human tissue studied so far, the placental genomes often contained changes in copy number,” the researchers wrote. “We reconstructed phylogenetic relationships between tissues from the same pregnancy, which revealed that developmental bottlenecks genetically isolate placental tissues by separating trophectodermal lineages from lineages derived from the inner cell mass. Notably, there were some cases with full segregation—within a few cell divisions of the zygote—of placental lineages and lineages derived from the inner cell mass.”

In one biopsy, the researchers observed three copies of chromosome 10 in each cell, two from the mother and one from the father, instead of one copy from each parent. Other biopsies from the same placenta and from the fetus carried two copies of chromosome 10, both from the mother.

“It was fascinating to observe how such a serious genetic flaw as a chromosomal copy number error was ironed out by the baby but not by the placenta,” noted Gordon Smith, PhD, a senior author of the study and professor at the University of Cambridge. “This error would have been present in the fertilized egg. Yet derivative cell populations, and most importantly those that went on to form the child, had the correct number of copies of chromosome 10, whereas parts of the placenta failed to make this correction. The placenta also provided a clue that the baby had inherited both copies of the chromosome from one parent, which can itself be associated with problems.”

These findings demonstrate the need for further studies using larger sample sizes to help reveal the causes of complications and diseases that arise during pregnancy.

“Our findings reveal extensive mutagenesis in placental tissues and suggest that mosaicism is a typical feature of placental development,” concluded the researchers.

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