Fig. 3. Gross structure of the iridescent portion of anterior cardiac nerve (acn) and electron microscopy of its central core. (A–C) Light micrographs of Toluidine Blue-stained sections of the acn. These micrographs show that the iridescent portion of the acn can be divided into two parts, a central core containing large diameter axons (indicated numerically in A–C) and a peripheral region (see Figs 4 and 5). At the proximal end of the acn (A), five axons (labeled 1–5) are present. Regardless of preparation or fixation protocol, these axons are approximately 10 µm in major cross-sectional diameter. As one moves distally through the nerve (B), four of the five axons terminate into numerous smaller diameter axons (asterisks). The remaining axon (labeled 1 in B and C) maintains a constant diameter through the medial portion of the acn, ultimately exiting the iridescent portion of nerve (C). (D) Transmission electron micrograph of one of the five axons present in the central core of the acn. While taken from a distal section of an acn segment, the ultrastructure of this axon is typical of the ultrastructure of all axons present in the entire iridescent portion of the acn. Like all acn axons, the axon shown in this panel contains filamentous axoplasm (ap), mitochondria (m) and occasionally dense-core (DCV) and electronlucent vesicles. In this micrograph, a single DCV is evident. This, and all other acn axons, is ensheathed by a thick glial wrap (gw). The glia contain a relatively homogeneous cytoplasm, often with mitochondria present. In this example, structures within the axon are labeled with black lettering while those associated with the glial wrap are labeled with white lettering. A, B and C are taken from different preparations. D is taken from the same preparation as C. Scale bars, 30 µm (A–C); 1 µm (D). It should be noted that the difference in appearance of the tissue in A versus B and C is due to the type of plastic used for embedding, namely LRWhite and EMBed, respectively.