Double-membrane arrays in type II fibers of mouse extraocular muscle.

Double-membrane arrays were observed in singly innervated type II fibers of the global region of mouse superior rectus. These arrays were selectively confined to the endplate region of such cells. Close associations and continuities between the cisternae of the arrays with those of the intermyofibrillar sarcoplasmic reticulum suggest derivation of the former from the latter. These proliferations of the cellular smooth membranes might be related to the specialized functional requirements of the innervation site.