The Structure of the Sarcolemma of the Frog Skeletal Muscle Fiber

The classical paper of Bowman (1) 1 provides an excellent description of the delicate tubular structure which invests each striated muscle fiber, and is especially replete with elaborate sketches of various muscle fibers from a comparative point of view. Subsequently many papers have been published using the light microscope in conjunction with various histological techniques. In recent years both replica and sectioning methods have provided electron micrographic evidence which has further elucidated the ultrastructure of this region of the fiber. In this brief report we shall attempt to present both light and electron micro-graphs of the frog muscle fiber so as to bring together in one paper the various levels of observation with regard to the structure of this region which is of interest to both cytologists and physiologists. MATERIAL AND METHODS Specimens were obtained from the tibialis anticus muscle of the frog (Rana pipiens). Single fibers were prepared by microdissection with the aid of a dissecting microscope. Laidlaw's silver impregnation method for connective tissue was used in photomicro-graphic studies. For electron micrographic studies the conventional fixation procedure was used with a 1 per cent solution of OsO4 buffered at pH 7.8 with veronal acetate. Most of the specimens were embedded in methacrylate. Some material was embedded in epon-mixture B of epon 812 as described by Finck (2)-and sectioned at 60°F. OBSERVATIONS AND DISCUSSION So as not to become involved in fruitless discussion of "proper" nomenclature, we shall proceed at the 1 Bowman in his paper (1) acknowledges a previous description of this structure by Schwann (cf. footnote p. 475). outset, as did Bowman (1) in his paper of 1840, to consider a view of the freshly dissected fiber. In Fig. 1 there is seen a typical dissected fiber from the tibialis anticus of the frog in which a "retrac-tion clot" (cf. Speidel (3)) has been induced by mild compression with a pair of dissecting forceps. The empty tube (also referred to by some authors as "husk" or "shell," "casing," etc.) was described by Bowman (1) as follows: "I allude to a tubular membranaceous sheath of the most exquisite delicacy, investing every fasciculus from end to end, and isolating its fibrillae from all the surrounding structures." (474) "... I have been in the habit of styling it Sarco-lemma, a term descriptive of its nature." (475) It is precisely the various levels of this structure, the "sarcolemma," that will be the …