Ultrastructure of Somatic Muscle

Somatic muscle cells of Ascaris lumbricoides consist of three differently specialized components referred to as the fiber, which contains the contractile apparatus (described previously), the belly, and the arm. The belly is shown to be a sac of glycogen, which is depleted during starvation of the animal. The arm extends to a nerve cord where it establishes a myoneural junction characterized by giant mitochondria and clusters of vesicles in the nerve fibers and by a 500 A neuromuscular gap. The arms, which have been shown to be "electrically interconnected" in the vicinity of the nerve cord, form " t ight junctions" with one another in just this region. At high magnification, these junctions can be resolved into several types. In some there is fusion of the outer leaflets of the membranes with formation of an intermediate line. Others resemble septate desmosomcs in that a residual ¢xtracellular space ~ 2 0 A in width remains between the membranes, but the outer leaflets are interconnected across the gap. I t is suggested that the term "t ight junct ion" encompasses a variety of structures distinguishable only at high magnification and that the different variations are not necessarily equivalent functionally. I N T R O D U C T I O N Multiple specialization in metazoan cells is particularly well demonstrated by the body muscle cells of the parasitic nematode, Ascaris. Each of these enormous cells is divided into three morphologically distinct and spatially separate parts subserving different functions. The spindly fiber contains the obliquely striated contractile apparatus (28, 29); the sac-like belly houses the nucleus, and, as will be shown, serves in addition as a glycogen storage depot; the elongated arm, or innervation process, conducts signals from a distant myoneural junction to the contractile part of the cell (5). So extreme are the cytological differences among the several parts of the cell that one would scarcely suspect that the fiber, belly, and arm are interconnected at all. The structural intricacies of the contractile apparatus in the fiber have already been presented in a separate report, and the present paper will therefore be concerned only with the remaining constituents of the cell, the belly and the arm, and with the neuromuscular junction. 1 The innervation processes are of special interest because of physiological data indicating that they 1 These findings were presented as a demonstration at the third annual meeting of the American Society for Cell Biology held in November 1963 (28). 579 on S etem er 1, 2017 jcb.rress.org D ow nladed fom are "e lec t r ica l ly i n t e r c o n n e c t e d " (5); i.e. ionic cur rents a p p a r e n t l y flow readi ly f rom one to the next in a m a n n e r co r respond ing to " e p h a p t i c t ransmiss ion" be tween inve r t eb ra te nerve cells, as descr ibed by Arvani tak i m o r e than 20 years ago (1), and to "e lect r ica l t ransmiss ion" or "electr ical coup l ing" as descr ibed m o r e recent ly (3, 12, 14). In the case of the Ascaris muscle arms, the physiological d a t a suggest tha t the in te rconnec t ions occur a t the distal ext remit ies of the a rms in the region w h e r e they converge on the nerve cord (5). This pa r t i cu la r region was therefore s tudied wi th special interest in an effort to ascer ta in the s t ructural basis for the p h e n o m e n o n in this an imal in comparison wi th others. M A T E R I A L S A N D M E T H O D S Specimens of Ascaris lumbricoides, var. suum, obtained from a local slaughterhouse, were fixed with csmium tetroxide either immediately or after 6 days at 37°C. in Kronecker 's solution (5). Details of fixation, dissection, and further preparat ion fer electron microsscopy are described elsewhere (29). Electron microFmunE 1 Diagram of muscle cells and myoneural junctions in transverse section. The belly (B) containing the nucleus of the muscle cell is continuous with the core of the striated fiber (F) and with the elongated arm (A). The arm subdivides as it approaches the nerve cord (N). The individual axons comprising the nerve cord are embedded in a trough-like extension of the hypodermis (H) which underlies the animal's cuticle (C). 580 THE JOURNAL OF CELL BIOLOGY • VOLUME 96, 1965 on S etem er 1, 2017 jcb.rress.org D ow nladed fom