A hair follicle is a part of the skin that grows hair by grouping old cells together.
In 2151, Ah'len told Commander Charles Tucker III that the hair follicles on his face were growing. Tucker replied that he hadn't had the opportunity to shave since he had boarded the Xyrillian vessel to help repair it. (ENT: "Unexpected")
In 2153, Degra, designer of the Xindi superweapon, was captured by the crew of Enterprise NX-01. He refused to reveal where the weapon to destroy Earth was being built, so Captain Jonathan Archer concocted a story where he and Degra had spent three years in prison together and escaped. To give the impression that years had passed, Archer had Dr. Phlox temporarily erase his memory and stimulated his hair follicles, so his hair would be of a longer length. (ENT: "Stratagem")
In 2365, Dr. Katherine Pulaski greatly advanced in age after contracting an aging virus from David, a genetically engineered child from Gagarin IV. Before she nearly died of old age, a hair follicle of Pulaski's was used to take a sample of her genetic code so she would be reverted back to her original age, via the USS Enterprise-D's transporter. (TNG: "Unnatural Selection")
In 2369, a Bajoran named Ibudan was found murdered in one of Deep Space 9's holosuites. Dr. Julian Bashir scanned the holosuite for hair follicles, hoping to find the culprit of Ibudan's murder. (DS9: "A Man Alone")
Later that year, Dr. Julian Bashir asked Lieutenant Commander Data how he could control the growth of his artificial hair follicles while they were working together in engineering on the Enterprise-D. (TNG: "Birthright, Part I")
In 2371, while massaging Kes' feet as part of the rolissisin ritual, The Doctor noticed a similarity between the Ocampan process of reproduction with a species called the Gree. The difference between the two species was that they stimulated follicles, rather than feet. (VOY: "Elogium")