U. SHEFFIELD / U. LEEDS (UK) — Compared to those of other mammals, human hearts are disorganized jumbles of tissue until relatively late in pregnancy, new research shows.
Researchers have created the first comprehensive model of human heart development using observations of living fetal hearts. The results show surprising differences from existing animal models.
Although scientists saw four clearly defined chambers in the fetal heart from the eighth week of pregnancy, they did not find organized muscle tissue until the 20th week—much later than expected.
Developing an accurate, computerized simulation of the fetal heart is critical to understanding normal heart development in the womb and, eventually, to opening new ways of detecting and dealing with some functional abnormalities early in pregnancy.
Studies of early heart development have previously been largely based on other mammals such as mice or pigs, adult hearts, and dead human samples.
The research team, led by scientists at the University of Leeds is using scans of healthy fetuses in the womb. One mother volunteered to have detailed weekly ECG (electrocardiography) scans from 18 weeks until just before delivery.
This functional data is incorporated into a 3D computerized model built up using information about the structure, shape, and size of the different components of the heart from two types of MRI (Magnetic Resonance Imaging) scans of fetuses’ hearts.
Early results are already suggesting that the human heart may develop on a different timeline from other mammals.
While the tissue in the walls of a pig heart develops a highly organized structure at a relatively early stage of a fetus’s development, the paper published in the Journal of the Royal Society Interface Focus reports that the there is little organization of the human heart’s cells until 20 weeks into pregnancy.
A pig’s pregnancy lasts about three months and the organized structure of the walls of the heart emerge in the first month of pregnancy. The new study only detected similar organized structures well into the second trimester of the human pregnancy. Human fetuses have a regular heartbeat from about 22 days.
Eleftheria Pervolaraki, visiting research fellow at the University of Leeds School of Biomedical Sciences, says: “For a heart to be beating effectively, we thought you needed a smoothly changing orientation of the muscle cells through the walls of the heart chambers. Such an organization is seen in the hearts of all healthy adult mammals.
“Fetal hearts in other mammals such as pigs, which we have been using as models, show such an organization even early in gestation, with a smooth change in cell orientation going through the heart wall. But what we actually found is that such organization was not detectable in the human fetus before 20 weeks,” she says.
Professor Arun Holden, also from University of Leeds, adds: “The development of the fetal human heart is on a totally different timeline, a slower timeline, from the model that was being used before.
“This upsets our assumptions and raises new questions. Since the wall of the heart is structurally disorganized, we might expect to find arrhythmias, which are a bad sign in an adult.
“It may well be that in the early stages of development of the heart arrhythmias are not necessarily pathological and that there is no need to panic if we find them. Alternatively, we could find that the disorganization in the tissue does not actually lead to arrhythmia.”
Researchers from the University of Sheffield, University of Leeds, the University of Edinburgh, the University of Nottingham, and the University of Manchester contributed to the research.