HUMAN MUSCLE
A microscopic view of lab-grown human muscle bundles stained to show patterns made by basic muscle units and their associated proteins (red), which are a hallmark of the human muscle.Duke University

Researchers have grown in the lab human skeletal muscle that contracts and responds just like real tissue to drugs and external stimuli.

Besides providing personalised medicine to patients, these tissues could serve as a test bed for clinical trials in a dish given that taking muscle biopsies are often difficult.

They could eventually replace animals in tests.

It was a few months ago that the team in collaboration with other researchers had grown strong, self-healing muscle tissue in the lab.

The latest study published in the open-access journal eLife was led by Nenad Bursac, associate professor of biomedical engineering at Duke University, and Lauran Madden, a postdoctoral researcher.

The team started with a small sample of human cells that had already progressed beyond stem cells but not yet become muscle tissue.

They expanded these muscle tissue precursors by more than a 1000-fold and then put them into a scaffolding with a nourishing gel to form muscle fibres.

In what is a first time in the lab, the muscles formed contracted robustly in response to electrical stimuli. The signalling pathways allowing nerves to activate the muscle were intact and functional.

The response of the muscle to a variety of drugs, including statins used to lower cholesterol and clenbuterol, a performance enhancer drug, matched those seen in human patients.

It has been a challenge to come up with muscles that are as strong and responsive as the real tissue.

"We are working to test drugs' efficacy and safety without jeopardizing a patient's health and also to reproduce the functional and biochemical signals of diseases—especially rare ones and those that make taking muscle biopsies difficult."

The Duke researchers had shown last year that the muscles they built in the lab integrated into mice quickly, and was able to self heal, both inside the laboratory and inside an animal.

Muscle fibres present a challenge unlike other human parts like ear, heart, liver and vagina that have been grown and even implanted into humans.

These fibres need to have the right structure and density to be strong, and yet flexible enough to contract. Structure aside, connections to blood supply and nerves are critical for muscle functionality.

Skeletal muscles are good at repairing themselves due to the many adult stem cells they carry. But when scar tissues form in injuries the muscle stops being able to regenerate, and can lead to even paralysis.

Growing human muscle in the lab from the patient's own stem cells could be a first step towards personalised treatment.