The fine structure of fibrillar components and plasma membrane contacts in esophageal myoepithelium of Ascaris lumbricoides (var. suum)

Esophageal myoepithelium of Ascaris consists of syncytially arranged cells penetrated by deep plasmalemmal infoldings. Myofibrillae are transversely oriented to the long axis of the esophagus as bundles of thin (35–75) Å and thick (140–175) Å myofilaments. Nonmyofibrillar regions of the cytoplasm contain mitochondria, endoplasmic reticulum, and bundles of tightly packed, filaments 75–100 Å thick. The myoepithelial plasmalemma is deeply invaginated and forms extensive cavernous, membrane-bounded spaces within the cytoplasm. The invaginated plasmalemmae come into “close” junctional contact forming two, morphologically distinct types of contact in the myofibrillar regions. One is oriented transverse to the long axis of the myofibrils and is similar in morphology to intercalated disks of cardiac muscle. The other is oriented longitudinally with respect to the long axis of the myofibrils and is similar in morphology to “tight junctions” of electrotonic transmitting synapses and between some vertebrate smooth muscle cells. The significance of the above findings is discussed in terms of the function of the two types of fibrillar components and membrane contacts found in the myoepithelium.