Developing Mouse Embryo Brains Affected; Connection to Autism Remains Unclear

Prolonged and frequent use of ultrasound waves on pregnant mice caused abnormalities in the brain of the developing mouse embryo, Yale School of Medicine researchers reported in a issue of the Proceedings of the National Academy of Science. While the authors did not specifically address a connection to autism, the abnormalities occurred as neurons (nerve cells) were in the process of moving to their proper positions in the outer layers of the brain's cerebral cortex—the “grey matter” that is the center of “thinking”, memory, perceptual awareness, language and consciousness.

The Inside-to-Outside Journey of the Nerve Cells

In the early fetal development of mammals, the nerve cells originate and multiply in “proliferative” zones of the developing brain, according to the researchers. They describe how the nerve cells must then find their way with the aid of helper cells to their final destinations in the outer layers of the cerebral cortex of the brain.

When this inside-to-outside migration of the nerve cells progresses normally, the cells take their place in a well-ordered grid of horizontal and vertical layers of the cortex. As described by the neuroscientists, Pasko Rakic and Paul J. Lombroso, the earliest born nerve cells settle in the deepest cortical layers while the later born cells move on to the outer layers.

The accuracy of their placement is crucial to normal development because the positions of the nerve cells define their connectivity and later role in brain function.

When certain genetic or environmental factors intervene, the migration may be interrupted, with many of the nerve cells left scattered in inappropriate layers or below in the “white matter” of the cortex, the researchers state. It has been shown that repeated exposure of the rodent or primate fetal brain to alcohol, drugs, certain viruses and ionizing irradiation causes misplacement of nerve cells and behavioral deficiencies in the offspring.

Although the Yale study did not generate behavioral data, previous studies in rodents and primates indicate that prenatal exposure to ultrasound waves (USW) may affect higher brain function in their offspring.

While the implications of the current study for humans are unknown, numerous human neuropsychiatric disorders are thought to be the result of misplacement of cells as a consequence of abnormal neuronal migration. Published studies have shown evidence that frequent exposure of the human fetus to USW is associated with prenatal growth restriction, higher risk of left-handedness and delayed speech.

Nerve Cells are Tracked to Their Final Locations

On the 16th day of embryonic development — at a late stage when the pathways to the cortical layer are the longest — the Yale team injected pregnant mice with a special marker to track the location of the nerve cells. Within the next three days of embryonic development in which cell migration was continuing, the mice were exposed to multiple sessions of ultrasound waves with total exposure ranging from 5 to 420 minutes. A control group of mice completed an identical procedure except for exposure to USW.

In exposures totaling from 30 minutes to 420 minutes, results showed a consistently smaller number of the nerve cells had arrived in their proper places in the layers of the cortex, with the number of misplaced nerve cells increasing as the frequency and length of exposure to USW increased. Doubling exposure time from 15 minutes to 30 minutes increased the number of misplaced cells from 5% to 9%. At a total of 420 minutes exposure, investigators found that 19% of the nerve cells failed to find their correct positions.

In the group exposed to USW for just 15 and 5 minutes, the effect was not significant, but when all the data were pooled together, across all durations of exposure, the results showed a consistent overall pattern of a smaller number of the labeled nerve cells reaching the upper cortical layers where they belonged.

The role of stress due to the experimental procedure was tracked in the control group, which did not receive USW. The results showed no significant effect of stress up to 210 minutes, but at durations of 420 minutes, the stress of the long procedure led to increased cell misplacement.

Caveats/Discussion

The authors have noted that clinical studies have suggested that ultrasound may affect the birth weight, left handedness and speech development in humans. Delay in speech development is a symptom of both autism and autism spectrum disorder. Therefore, while the authors of the study do not specifically associate autism and ASD with ultrasound, and ultrasound is acknowledged to be a valuable diagnostic tool, in light of its use for nonessential commercial prenatal ultrasound videos that are outside the normal clinical use, there may be grounds for further clinical studies.

As reported by the researchers, the principal ultrasound parameters used in the study were well within clinical norms for fetal exams. Although the frequency used in the present study was slightly above standard obstetrical clinical practice, the latest ultrasound equipment with three-dimensional reconstruction and tissue harmonic imaging often employs even higher frequencies. In the study's exposures, the ultrasound parameters and total exposure time are comparable with or below those used by nonessential commercial prenatal ultrasound videos.

The study showed that exposure of the embryonic mouse to USW can affect nerve cell migration and prevent some neurons from attaining their designated position in the cerebral cortex. However, the Yale team found that the effect is relatively small and it is not possible to predict how many neurons will miss their mark in any single case. The linear arrangement of the embryos in the U-shaped mouse uterus may have resulted in some mice actually receiving more USW than others. Effects such as temperature changes could also play a role in the results. USW has a predictable effect only when the large population of exposed animals is compared with unexposed controls and only at total exposure durations of 30 minutes or longer.

Because there are huge differences in the number of nerve cells and the size of the cerebral cortex between the mouse and humans, and because nerve cells are produced and migrate over a much longer time, an exposure of 30 minutes represents a much smaller proportion of time than the same processes in a mouse and could have a lesser overall effect in humans.

On the other hand, the scientists believe there are reasons to think that USW may have a similar or even greater impact on nerve cell migration in the human fetal brain. Migrating nerve cells in the human forebrain are only slightly larger than in the mouse; the migratory pathway in the human brain is more complex and longer; the settling pattern is more precise, and the tolerance for misplaced nerve cells may be smaller.

While it is not possible to replicate this study in humans, the authors call for further investigation in the larger and slower-developing brains of non-human primates and continued scrutiny of unnecessarily long prenatal ultrasound exposure.

T.S. Wiley notes, "Autism was first described as a unique disorder in the 1940s. In the early 1990s, autism diagnoses began to soar. In the 10 years between 1993 and 2003, the number of American schoolchildren with autism diagnoses increased by over 800%. In 2006 a study showed that 1 in every 166 births will be diagnosed with Austism and one could conjecture as mammals that we would be subject to the same harm as the mice in the study.  Also, remember there are only two things that babies are born afraid of, falling and loud noises.  Loud noises sent into the baby thru the embryonic fluid and/or water would be extremely annoying to the fetus since water conducts sound better than air.  It's considered biological noise at the cellular lever. Forty years ago, when the ultrasounds were first used on pregnant women and only used when medically indicated, many healthy pregnancies did not require an ultrasound. Today, ultrasounds are well over used (5+ per pregnancy) and no one knows the dangers as a result."

Autism Speaks / T.S. Wiley

Department of Neurobiology and Kavil Institute for Neuroscience, Yale Medical School, and Sonic Tech, Inc.