WASHINGTON (AP) — Lawmakers clashed over science, ethics and politics at a House hearing Thursday on using fetal tissue in critically important medical research, as the Trump administration reviews the government's ongoing support for such studies.

Research fields in which fetal tissue is used include HIV, childhood cancers, treatments that enlist the body's immune system to battle cancer, and the hunt for a vaccine against the Zika virus, a cause of birth defects.

Republicans said alternatives to fetal tissue are available and should be used instead. Democrats said that view is at odds with science. Each side called on expert witnesses.

"Most of my constituents don't understand when you harvest baby parts, why that is OK," said Rep. Mark Meadows, R-N.C., who chaired the hearing by the Oversight and Government Reform Committee. Meadows, leader of the conservative House Freedom Caucus, is one of President Donald Trump's biggest allies in Congress.

But Rep. Gerry Connolly, D-Va., responded, "The consensus in the scientific community is that there is currently no adequate substitute for fetal tissue in all of the cutting-edge research for which it is used today."

The government has funded research using fetal tissue for decades, under administrations of both political parties. Trump has gone out of his way to court social and religious conservatives among his staunchest supporters.

The administration's new review of whether taxpayer dollars are being properly spent on fetal tissue research has raised alarms among medical investigators, who fear their work will be stopped to satisfy anti-abortion activists.

Under Secretary Alex Azar, the Health and Human Services Department says it is trying to balance "pro-life" and "pro-science" imperatives in its ongoing audit of fetal tissue research.

Azar's office said in a statement that the National Institutes of Health put a pause on procurement of new human fetal tissue in the fall, after the audit was announced. The department says research with fetal tissue already on hand was allowed to proceed, and that it never intended to stop research. The HHS statement left open the possibility of procuring new fetal tissue to prevent research projects from being interrupted.

Research involving fetal tissue accounted for $98 million in NIH grants and projects during the 2017 fiscal year, a small fraction of the agency's overall research budget. NIH said that $98 million figure represents the entire budget for the grants at issue, even if only a portion of a particular grant was devoted to fetal tissue research.

HHS has not announced a timeline for completing its audit.

Fetal tissue is used to produce research mice that model how the human immune system works. The tissue, from elective abortions, would otherwise be discarded.

Biochemist Tara Sander Lee told the committee that alternatives to fetal tissue are available and can be used.

"We do not need fetal body parts from aborted babies to achieve future scientific and medical advancements," said Sander Lee, with the Charlotte Lozier Institute, which is opposed to abortion. Tissues from infants who have to undergo heart surgery are among the alternatives, she said.

But neuroscientist Sally Temple, testifying on behalf of the International Society for Stem Cell Research, told lawmakers that alternatives to fetal tissue are simply not suitable for every disease and condition being studied.

"The consensus opinion is that those alternatives are not sufficient," she said.

Temple explained that tissue samples from different stages of the life cycle are not interchangeable. "It is not the same material," she said. "It is a different developmental stage. It has unique properties."

Temple said researchers would readily use alternatives to fetal tissue if that was suitable.