The concept of Midi-chlorians were introduced to a galaxy far, far away in Star Wars: Episode I — The Phantom Menace, when Jedi Master Qui-Gon Jinn instructed his Padawan, Obi-Wan Kenobi, to analyze a blood sample drawn from a young Anakin Skywalker — a slave boy from desert planet Tatooine.
"The reading is off the chart," Obi-Wan said of Anakin's Midi-chlorian count. "Over 20,000. Even Master Yoda doesn't have a Midi-chlorian count that high."
"No Jedi has," Qui-Gon said, concerned at the discovery.
Later, Qui-Gon informed Anakin that Midi-chlorians are "a microscopic life-form that resides within all living cells," and that living beings are symbionts with them.
"Life forms living together for mutual advantage," Qui-Gon explained.
"Without the midichlorians, life cannot exist. And we would have no knowledge of the Force. They continually speak to us, telling us the will of the Force. When you learn to quiet your mind, you'll hear them speaking to you."
The scientific exposition stood in direct contrast to the simpler and more spiritual explanation offered by an older Obi-Wan to a wide-eyed Luke Skywalker in A New Hope, where the aged wizard described the ancient aura.
"The Force is what gives a Jedi his power," Obi-Wan explained. "It's an energy field, created by all living things. It surrounds us, and penetrates us — it binds the galaxy together."
As reasoned by The Phantom Menace, the Force effectively communicated through Midi-chlorians, allowing certain sensitive individuals to tap into this mystical energy field.
Measuring the quantity of Midi-chlorians in the cells determined a being's Force potential, or the strength of the connection between that being and the Force, meaning one greatly attuned to the Force — like newcomer Rey, who has exhibited raw and powerful Force abilities despite a lack of training — would boast a greater Midi-chlorian count.
But Star Wars: The Last Jedi forgoes this idea completely, instead returning to the metaphysical interpretation of the Force.
Star Wars creator George Lucas once explained to Return of the Jedi director Richard Marquand that anyone can hook into the Force, not just the Jedi, saying the esoteric monks were the only ones to "take the time to do it."
“Like Yoga. If you want to take the time to do it, you can do it; but the ones that really want to do it are the ones who are into that kind of thing,” Lucas said. “Also like karate.”
Luke's instruction is more in line with that of his teachers — Obi-Wan and Yoda — with Luke stressing the intangible nature of the all-powerful energy field, encouraging the already highly-sensitive Rey to reach out and feel the Force around her.
The Last Jedi doesn't erase Midi-chlorians — they're still canon, after all, and Rey's Midi-chlorian count could potentially near even that of Anakin Skywalker, who had the highest-recorded Midi-chlorian count in the galaxy — but the concept of the microscopic life-forms and their place in the Force is actively ignored in both The Force Awakens and The Last Jedi.
The Midi-chlorians aren't retconned, per se — and the little organisms themselves are retcons, coming more than 20 years after Lucas introduced the concept of the Force in 1977 — but it's clear The Last Jedi isn't interested in exploring blood counts, siding with the understanding of the Force offered in the Original Trilogy versus the more technical explanations brought up by the Prequel Trilogy.
Star Wars: The Last Jedi is now playing.