Capillary configuration and fiber shortening in muscles of the rat hindlimb: correlation between corrosion casts and stereological measurements.

It has been a matter of discussion whether the dramatic increase in capillary tortuosity visualized in shortened muscles by vascular cast represents in vivo situations. In this study, we combined vascular cast and stereological methods in the same samples, in order to obtain (1) measurements of sarcomere length in the same muscles from which corrosion casts were prepared, and (2) scanning electron micrographs of the three-dimensional arrangement of capillaries in the same muscles where capillary anisotrophy was estimated by morphometry. Various rat skeletal muscles (soleus, gastrocnemius, and gracilis) were examined at lengths ranging from full shortening to full extension. We found a very good correlation between capillary geometry in material prepared for vascular casts and in muscles perfusion-fixed in situ. All muscles, cast and noncast, showed the same progressive curvilinear decrease in capillary anisotropy with decreasing sarcomere length. Capillary tortuosity visualized by corrosion casts in shortened muscles is a consequence of fiber shortening, within physiological sarcomere lengths; it does not represent an artifact related to the casting procedure.