(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.

Photos: Ben Carson's career in politics Photos: Ben Carson's career in politics Ben Carson attends the National Action Network (NAN) national convention at the Sheraton New York Times Square Hotel on April 8, 2015, in New York City. Hide Caption 1 of 13 Photos: Ben Carson's career in politics Carson speaks during the 41st annual Conservative Political Action Conference (CPAC) at the Gaylord International Hotel and Conference Center on March 8, 2014, in National Harbor, Maryland. Hide Caption 2 of 13 Photos: Ben Carson's career in politics Carson speaks to guests at the Iowa Freedom Summit on January 24, 2015, in Des Moines, Iowa. Hide Caption 3 of 13 Photos: Ben Carson's career in politics Carson is surrounded by supporters as he waits to be interviewed at the annual Conservative Political Action Conference (CPAC) at National Harbor, Maryland, outside Washington on February 26, 2015. Hide Caption 4 of 13 Photos: Ben Carson's career in politics Carson speaks at the South Carolina Tea Party Coalition convention on January 18, 2015, in Myrtle Beach, South Carolina. A variety of conservative presidential hopefuls spoke at the gathering on the second day of a three-day event. Hide Caption 5 of 13 Photos: Ben Carson's career in politics Carson delivers the keynote address at the Wake Up America gala event on September 5, 2014, in Scottsdale, Arizona. Hide Caption 6 of 13 Photos: Ben Carson's career in politics Carson speaks during the 41st annual Conservative Political Action Conference (CPAC) at the Gaylord International Hotel and Conference Center on March 8, 2014, in National Harbor, Maryland. Hide Caption 7 of 13 Photos: Ben Carson's career in politics Carson speaks during the National Prayer Breakfast at the Washington Hilton on February 7, 2013, in Washington. Hide Caption 8 of 13 Photos: Ben Carson's career in politics Honoree and director of pediatric neurosurgery at Johns Hopkins University, Carson poses with actor James Pickens Jr. at the Jackie Robinson Foundation Annual Awards Dinner on March 16, 2009, in New York City. Hide Caption 9 of 13 Photos: Ben Carson's career in politics Before his jump into conservative politics, Carson was known for his work as a neurosurgeon. Carson was awarded the Presidential Medal of Freedom by then-President George W. Bush on June 19, 2008. At that time, he was the director of pediatric surgery at Johns Hopkins Hospital in Baltimore, Maryland. Hide Caption 10 of 13 Photos: Ben Carson's career in politics In a story that garnered international attention, Carson was ready to separate a pair of 10-year-old Indian girls, Saba and Farah Shakeel, who are joined at the head in New Delhi, India. Here, he addresses a press conference at the Indraprashtra Apollo Hospital on October 4, 2005. Hide Caption 11 of 13 Photos: Ben Carson's career in politics Carson and a team of 20 specialists approved the procedure after studying the girls' brains; however, their parents were worried about their daughters' lives and did not give doctors permission to operate. The surgery did not happen. Hide Caption 12 of 13 Photos: Ben Carson's career in politics Carson observes the start of neurosurgery proceedings at the Raffles Hospital in Singapore on July 6, 2003. Carson and Dr. Keith Goh, left, performed a complex operation that was unsuccessful to separate 29-year-old twins Ladan And Laleh Bijani, who were joined at the head. Hide Caption 13 of 13

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."

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.