The development of the myo-tendinous junction in the upper extremity of mouse embryos (days 15 p.c.--1 p.p.).

The development of the myo-tendinous junction in the upper extremity of mouse embryos of days 15 p.c. to 1 p.p. was studied electron microscopically. In the region of the myo-tendinous junction two cell types can be differentiated (muscle cells, fibroblasts) which undergo characteristic changes during the course of their development: The muscle cells show an increase in contractile material, the fibroblasts show a massive collagen production. The myo-tendinous junction is characterised by the appearance of four structures: 1. Irregular pattern of the plasma membrane resulting from intracellular membrane vesiculations in defined areas under the cell membrane on the longitudinal side as well as the end of the muscle fibre. These membrane vesiculations can break through to the cell membrane, thus leading to cell invaginations appearing in large amounts up to day 1 p.p. 2. Densifications of the plasma membrane towards the cell centre. 3. Insertion of the myofilaments of the muscle fibre at the desinfications of the plasma membrane (hemidesmosomes). 4. Insertion of collagen filaments in the extracellular space into the basement membrane which then also appears densified. The mechanical stress transmission seems therefore to be dependent upon the adhesion between myofibrils, plasma membrane, basement membrane and collagenous filaments.