Mitochondria in pigeon breast muscle are composed of two protein gels: a fibrous gel, in the form of a folded ribbon, enclosed within a non-fibrous matrix. An external limiting "membrane" is not demonstrated, and there is poor demarcation between the mitochondria and adjacent structures or sarcoplasm.
No internal structure has been determined for sarcosomes. These structures, however, are symplasmic with mitochondria and usually are located within mitochondrial depressions. They apparently have a high lipid content.
The myofibrils also have no external limiting "membranes" and the mitochondria and sarcosomes are symplasmic with them. The mitochondria normally lie in the intermyofibrillar sarcoplasm oriented with their longitudinal axes parallel to the myofibrils. Cross-sections show that the myofilaments are oriented into well defined sheets.