Excitability and isometric contractile properties of mammalian skeletal muscle constructs engineered in vitro

Our purpose was to engineer three-dimensional skeletal muscle tissue constr ucts from primary cultures of adult rat myogenic precursor cells, and to me asure their excitability and isometric contractile properties. The construc ts, termed myooids, were muscle-like in appearance, excitability, and contr actile function. The myooids were 12 mm long and ranged in diameter from 0. 1 to 1 mm. The myooids were engineered with synthetic tendons at each end t o permit the measurement of isometric contractile properties. Within each m yooid the myotubes and fibroblasts were supported by an extracellular matri x generated by the cells themselves, and did not require a preexisting scaf fold to define the size, shape, and general mechanical properties of the re sulting structure. Once formed, the myooids contracted spontaneously at app roximately 1 Hz, with peak-to-peak force amplitudes ranging from 3 to 30 mu N. When stimulated electrically the myooids contracted to produce force. T he myooids (n = 14) had the following mean values: diameter of 0.49 mm, rhe obase of 1.0 V/mm, chronaxie of 0.45 ms, twitch force of 215 mu N, maximum isometric force of 440 mu N, resting baseline force of 181 mu N, and specif ic force of 2.9 kN/m(2). The mean specific force was approximately 1% of th e specific force generated by control adult rat muscle. Based on the functi onal data, the myotubes in the myooids appear to remain arrested in an earl y developmental state due to the absence of signals to promote expression o f adult myosin isoforms.