Hybrid skeletal muscle fibres: a rare or common phenomenon?

1. The main aim of the present review is to raise awareness of the molecular complexity of single skeletal muscle fibres from "normal" and "transforming" muscles, in recognition of the many types of hybrids that have been observed in vertebrate skeletal muscle. The data used to illustrate various points made in the review were taken from studies on mammalian (mostly rat) and amphibian muscles. 2. The review provides a brief overview of the pattern and extent of molecular heterogeneity in hybrid muscle fibres and of the methodological problems encountered when attempting to identify and characterize such fibres. Particular attention is given to four types of skeletal muscle hybrids: (i) myosin heavy chain (MHC) hybrids; (ii) mismatched MHC-myosin light chains (MLC) hybrids; (iii) mismatched MHC-regulatory protein hybrids; and (iv) hybrids containing mismatched MHC-sarcoplasmic reticulum protein isoforms. 3. Some of the current ideas regarding the functional significance, origin and cognitive value of hybrid fibres are examined critically.