AFTER EMBRYONIC DAY 17, DISTRIBUTION OF CELLS ON SURFACE OF PRIMARY MUSCLE-FIBERS IN MOUSE IS NONRANDOM

During the formation of skeletal muscle, secondary fibres form, by cel l fusion, on the surface of primary fibres, Three-dimensional reconstr uctions of primary fibres with the secondary fibres and cells on their surfaces were produced from spaced serial transmission electron micro graphs, Reconstructions were made of fibres from embryonic day (E) 17, E19, and E21 of the Extensor Digitorum muscle of Balb/c mice. Cell di stribution was analysed in two ways, Firstly, nearest neighbour analys is was used to see if cells were randomly arranged or clustered, Secon dly, the association of cells and secondary fibres was tested by measu ring the distances between cells and each secondary fibre. Cells were found to be randomly distributed on the surface of primaries at E17 bu t significantly clustered, and associated with smaller secondary fibre s at E19 and E21, Cells were not associated with the ends of secondary fibres, Cells associated with secondary fibres lay in the groove form ed by contact between adjacent primary and secondary fibres, This appa rent response to the topography of the surface on which the cells are lying has previously been mimicked in vitro by growing cells on groove d surfaces, It is likely that cells associated with secondaries will f use laterally with these fibres and their response to topography is pa rt of the process of bringing them into correct alignment with the fib re before fusion. (C) 1996 Wiley-Liss, Inc.