The fine structure of the longitudinal layer of the tunica muscularis of the mouse jejunum was studied in various stages of mechanically stimulated contraction. The relaxed cell is long and narrow with smooth cytoplasmic and nuclear contours. As contraction progresses, the cell becomes ellipsoid and its borders exhibit invaginations at the points of myofilamentous attachment to the plasma membrane and vesicle-containing projections of the intervening membrane. These changes are interpreted as representing the deforming forces applied by the myofilaments to the plasma membrane. The nucleus of the contracted cell is shortened and widened, with convolution of its limiting membranes. This alteration, as well as progressive changes in the alignment of cytoplasmic organelles, is thought to be due to forces exerted on the internal structure of the cell by the contractile elements. The myofilaments form a network of oriented bundles during contraction. Aggregates of filaments of two different diameters are noted. The two sizes of filaments intermingle only in small areas of increased density. These dense areas increase in length and number during contraction. A model of the functional organization of the cell is proposed.

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