EXPRESSION OF LAMININ SUBUNITS IN HUMAN FETAL SKELETAL-MUSCLE

The laminin variant of adult skeletal muscle fibres and Schwann cells is known as merosin, and is composed of M-B1-B2 chains. Blood vessels and immature fibres express the A chain in association with B1 or S, a nd B2. The importance of merosin has recently been shown by its absenc e in one form of congenital muscular dystrophy and in the mutant dy/dy mouse, and by its partial deficiency in Fukuyama congenital muscular dystrophy. We have examined the immunocytochemical localization of the M, A, B1 and B2 laminin chains in human fetal muscle from 7 to 40 wee ks' gestation to ascertain their developmental expression. The B1 and B2 chains were detected on muscle fibres at 7 weeks, but only traces o f the A or M chain were seen. By 21 weeks maximal levels of all four s ubunits were observed on all fibres. Tl;is suggests that the basement membrane is still being assembled until this stage of development. Exp ression of the A chain on muscle fibres was not reduced until 34 weeks and low levels persisted at birth. The concomitant expression of the M and A chains at early stages may indicate a laminin variant, in addi tion to merosin, that is highly expressed in fetal muscle. Merosin was seen in intramuscular nerves at 11 weeks. B1 and B2 subunits were det ected in blood vessels from 7 weeks' gestation and the A chain from 11 weeks. The capillary network, however, is not fully established in fe tal muscle. Merosin is therefore detected early during human fetal mus cle development, and this should be taken into account when assessing aborted fetuses at risk for congenital muscular dystrophy.