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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.
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