CNN  — 

Republican presidential candidate Ben Carson on Thursday defended his past use of tissue from aborted fetuses for medical research even as he continued to criticize Planned Parenthood.

The retired neurosurgeon said his research, which took place in 1992, does not conflict with his call to defund Planned Parenthood after several undercover videos surfaced that purportedly show officials with the organization working with research companies using tissue from aborted fetuses.

Jen Gunter, an obstetrician-gynecologist, wrote on her blog that Carson had co-authored an academic paper published in Hum Pathol, in which he described working with material “from two fetuses aborted in the ninth and 17th week of gestation.”

In her blog post, Gunter sharply criticized the GOP presidential candidate.

“As a neurosurgeon Dr. Ben Carson knows full well that fetal tissue is essential for medical research,” wrote Gunter, author of “The Preemie Primer,” a guide for parents of premature babies.

“His discipline would have a hard time being where it is today without that kind of work. What is even more egregious than dismissing the multitude of researchers whose work allowed him to become a neurosurgeon is the hypocrisy of actually having done that research himself while spouting off about its supposed worthlessness.”

Gunter wrote that fetal tissue research has recently helped develop a vaccine against Ebola and is currently being used to develop treatments for blindness, HIV and other illnesses.

On the campaign trail in Manchester, New Hampshire, Carson told CNN his research simply used the tissue from aborted fetuses that was made available to him.

“We have banked material in the pathology lab from people from every age – from day 1 of concept to 120 years told. Those specimens are available for people who want to do comparisons,” Carson said. “To not use the tissue that is in a tissue bank, regardless of where it comes from, would be foolish. Why would anybody not do that?”

Carson also issued a statement, saying, “There is absolutely no contradiction between the research I worked on in 1992 and my pro-life views. The issue of fetal tissue has everything to do with how the tissue is acquired. My primary responsibility in that research was operating on people to obtain diseased tissue for comparison to banked tissue samples. Killing babies and harvesting tissue for sale is very different than taking a dead specimen and keeping a record of it, which is exactly the source of the tissue used in our research.”

And later Thursday, Carson went on Facebook to further defend his work.

“I, nor any of the doctors involved with this study, had anything to do with abortion or what Planned Parenthood has been doing,” he said in a post. “Research hospitals across the country have microscope slides of all kinds of tissue to compare and contrast. The fetal tissue that was viewed in this study by others was not collected for this study.”

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Carson had previously told CNN that organs and tissue from aborted fetuses aren’t needed for medical research.

“Virtually everything that can be attributed to progress by using fetal tissue can also use other types of tissue,” he said. “If it were the only way to do something and there was no other way, there might be an argument. But under these circumstances, there isn’t a legitimate argument.”

Planned Parenthood has denied that it has broken any laws and said that it donates the tissue for medical research.

READ: How exactly fetal tissue is used for medicine