(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.

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