Fig. 2. The major components of the myotomal basement membrane are produced in Myf5^nlacZ/nlacZ embryos, but fail to assemble into a continuous matrix. Whole-mount Myf5^nlacZ/+ (A) and Myf5^nlacZ/nLacZ (B) embryos at E9.5 (seen through the dermomyotome towards the sclerotome), transverse sections of E10.0 Myf5^nlacZ/+ (D,F,H) and Myf5^nlacZ/nLacZ embryos (E,E',G,G',I,I') and longitudinal section of a Myf5^nlacZ/nLacZ embryo at E10.5 (C), immunolabelled for ß-gal and laminin (A-E,E'), type IV collagen (F,G,G') or perlecan (H,I,I'). (A-E') Laminin immunoreactivity in Myf5^nlacZ/+ embryos shows that laminin is organised into a sheet-like matrix between the ß-gal-positive myotomal cells and the mesenchymal sclerotome (arrows in D). In Myf5^nlacZ/nlacZ embryos, laminin-immunoreactivity appears absent at low magnification (B); at higher magnification, this immunoreactivity is limited to dots and lines (arrows in C,E,E'). (F-I') The myotomal basement membrane of Myf5^nlacZ/+ embryos also contains collagen type IV (F) and perlecan (H). In Myf5^nlacZ/nlacZ embryos, collagen type IV (arrows in G) and perlecan (arrows in I) immunoreactivity presents a patchy organisation, but ß-gal-positive cells are not delimited by this matrix. Patches of collagen IV and perlecan, are also visible near the dermomyotome (arrows in G',I'). ep, epaxial; hyp, hypaxial; nt, neural tube. Scale bars: 100 µm in A-B,D-I'; 50 µm in C.