Calsequestrin (CASQ) is the major component of the sarcoplasmic reticulum (SR) 
lumen in skeletal and cardiac muscles. This calcium-binding protein localizes to 
the junctional SR (jSR) cisternae, where it is responsible for the storage of 
large amounts of Ca(2+), whereas it is usually absent, at least in its 
polymerized form, in the free SR. The retention of CASQ inside the jSR is due 
partly to its association with other jSR proteins, such as junctin and triadin, 
and partly to its ability to polymerize, in a high Ca(2+) environment, into an 
intricate gel that holds the protein in place. In this work, we shed some light 
on the still poorly described in situ structure of polymerized CASQ using 
detailed EM images from thin sections, with and without tilting, and from 
deep-etched rotary-shadowed replicas. The latter directly illustrate the 
fundamental network nature of polymerized CASQ, revealing repeated nodal points 
connecting short segments of the linear polymer.